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 Post subject: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Fri 03, 2020 2:06 am 
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Joined: May Thu 04, 2017 1:08 pm
Posts: 361
Location: Raleigh, NC area
I hope this makes sense...kind of long, but hard to explain.

I've been working on debugging a Sencore LC53 meter. For the most part, it has gone well, despite the fact that I don't really understand the exact functionality of the schematic. But, I have a scope, and I know how the individual logic gates should perform. There are a few notes on the schematic, about clock pulses and rates, and control lines which are actuated via the front panel buttons. Those check out, and after replacing a few of the suspected bad ICs, the unit seems to operate properly now.

Of particular interest here is that fact that my board does not match the schematic. There are a number of wire adds, spidered parts, and even a dead-bug'ed IC on top of IC12. I obtained the operator's manual and the schematic/parts list from an online source, but the two don't agree 100%. So far, I've not had to deal with the parts of the circuit where changes were made.

Since the unit was then functioning as I expected, I began looking into the calibration. On p.36 of the operator's manual, there is instruction for balancing the capacitor and inductor lead zeroing. (In other words, obtaining a setting such that the lead capacitance and lead inductance is zeroed at the same time. Without the "balance," the front panel zeroing pot only zeros for capacitance OR inductance, not both.) The operator's manual says to use the R33 pot (schematic would be "R133" the way Sencore has these numbered), and the manual points to the part in a little diagram. It is the white vertical pot in the lower left corner of the photo below. However, that pot seems to have no effect on the lead zeroing. In fact, R33 (R133) does not appear on the schematic. So at some point, they changed the way this works.

On the back panel of my unit (and on the schematic: R9), there is a pot for balancing the L/C lead zeroing, and in fact that works just as R33 should have worked, and the lead zeroing is now balanced. So you are probably asking right now, "What is the problem, Dave?"

Well, I MOVED that R33 pot, and I didn't mark where it was (oops). And, I don't know what it is supposed to do. So, I started tracing the wires, and drawing out what I see, pondered it for an hour or two, and I STILL don't know what it is supposed to do, or how I might know where it should be adjusted.

R33 is connected via the two red wires to a spare gate in the IC27 package near the center of the board. That is a 4066 quad bilateral switch. The switch functions to short out the R33 pot, or not. I've drawn this out, and you can see my line of thinking in the notebook page which I added below. I'm sorry, it's a mess of notes... That 4066 switch is pulsed on at about a one second rate, only when the Capacitor Value button is pressed. When ON, that switch shorts out R33, which appears in series with the R8A pot (front panel lead zeroing), and the R9 pot (back panel L/C zero balancing).

There is a schematic note in the vicinity of this circuit, which says "CAP LEVEL DETECT." I don't know what that means. Auto-ranging? Voltage level for safety? Not sure. All I know is that R33 seems to have no effect on lead zeroing, as below:

1) Short the leads.
2) Press Inductor Value. The 4066 control pin is low, so R33 is in the circuit / not shorted out.
3) Use front panel Lead Zeroing control (R8A) to zero the display.
4) Open the leads.
5) Press Capacitor Value. R33 is being pulsed in and out of series with R8A/R9 at this point.
6) (Don't use R33 as instructions say.) Use back panel L/C balance pot (R9) to zero the display.

So, at Step 6, R33 is being pulsed in and out of series with R8A/R9 (100k/250k). R33 is a 100k pot, so it can have a large effect, I would think, but the pulsing isn't anywhere near 50% duty cycle--much shorter. When I complete Step 6, if I then rotate the R33 pot, I see no difference. The display remains balanced/nulled (reading zero).

Do you guys have any idea what R33 is supposed to be used for, and how I can determine where it should be adjusted?
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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Fri 03, 2020 4:08 am 
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Joined: Mar Thu 15, 2018 2:23 pm
Posts: 350
Just a suggestion... you should at least include the schematics, cal guide, screenshots, etc., of the areas that you seek help on. This way everyone who chooses to help can see the same diagrams your looking minimizing any confusion and save time. Not everyone has the time or desire to search, locate, and download documentation.

For Runs 23 - 30...
R133 is a lead zero adjustment. With test leads open, press capacitor value button and adjust until displayed value is "000". This pot "balances" the inductance and capacitance zero point to be the same as on the front panel lead zero.

For Runs 31 and over...
R133 is an adjustment for the 100pF range for the LC53 capacitor calibration. Grab a 100pF capacitor, connect the leads, and press the capacitor value button. Adjust R133 until the displayed value matches the capacitor value.


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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Fri 03, 2020 5:24 am 
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Joined: May Thu 04, 2017 1:08 pm
Posts: 361
Location: Raleigh, NC area
Awesome, I'll give it a try, thank you! There must be a different operator's manual for the version I have, because R33 isn't mentioned for calibration.

I didn't include the schematic because it did not match what actually appears on the board in that area. I redrew that part to hopefully have less confusion. However, from the point at which the control of the spare 4066 gate connects to the rest of the circuit, I can see how it would be needed for determining the reason for the pulsing of that control, assuming one understands the circuit operation (I don't, mostly).

The documents are a bit large. The screenshots might be detailed enough if high resolution. A link might be better:
http://ftb.ko4bb.com/getsimple/index.ph ... 53_Z-Meter

The first picture is the digital board where I've found bad parts (two flip-flop devices: IC122 & IC133; NAND IC132). There may be more bad ones; I've not tested all the ranges. The second picture is the analog board and the display board. Tracing through this thing has taken me hours and hours, back and forth between the boards, taking it apart and putting it back together at least four times now. I still understand only a fraction of how this works. Kind of a quest. Maybe, if I get familiar enough with it, I'll keep it instead of the LC102, which is more than what I need.

So far, I've fixed:
- minus sign stuck on
- lead zeroing unresponsive for capacitance
- capacitor value not reading at all
- inductor ringing not reading at all

Also replaced caps on 20V, 7V, -7V, and added an additional one for the 7V "island" I saw on the PCB. (The photo below was taken before I replaced those.)

What was working when I got it:
- lead zeroing for inductance, I think
- inductor value

Yeah, I see the screenshots look bad. The PDFs at the link above are much better.
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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Fri 03, 2020 3:34 pm 
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Joined: May Thu 04, 2017 1:08 pm
Posts: 361
Location: Raleigh, NC area
Presto, it worked! R33 pot does indeed adjust the 100pF range, and it's back into position. Whew! Thanks again mr.fabe.

I may be liking the LC53 better than the LC102, if only because I can easily see the LC53's bright red display. The overhead lighting in my garage makes it hard for me to read the LC102's LCD display. Would be better if it was backlit.


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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Sat 04, 2020 2:13 am 
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Joined: Mar Thu 15, 2018 2:23 pm
Posts: 350
Glad you got things working! Nice work.

Keep the LC102. More functionality and variable voltage up to 1000V. HV diode testing, zeners, etc. Add a battery and it becomes a portable unit that's easy to grab and use if you are limited in bench space or help others with repairs. Consider making a modification to illuminate the display and post it here. Be the first :lol:


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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Sun 05, 2020 5:16 am 
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Joined: May Thu 04, 2017 1:08 pm
Posts: 361
Location: Raleigh, NC area
A member requested some additional details of my debugging on this unit, so I'm adding it here. Perhaps it will be useful to someone. I think at least the "techniques" will be useful. It's not going to be written like a how-to manual, though, sorry...

I have over 10 pages of scribbles so far in a notebook, but of course they are specific to the issues I had with this unit. It would make no sense to post them as-is. I'll pull out tidbits to describe. There are others on ARF far more knowledgeable than I about these testers. I first read everything I could find on ARF. I looked elsewhere on the web, but the best information is right here on ARF. Some very bright (and friendly, and helpful) people hang out here, and there is such a wealth of experience to draw from. Amongst the posts, I found a thread started by one "dockbrn." He (or she?) managed to fix his LC53 eventually. What stuck with me was one of his final posts, in which he reported success, and that getting there meant replacing a handful of logic ICs.

This stuck with me because I have never seen ICs just "go bad" before, without something like lightning coming to play--not on a scale like that, with multiple ICs bad. Maybe the earlier ones did. I once had to replace a handful of monolithic multi transistor ICs in a Tektronix 7000-series scope's power supply. A bunch of them had gone bad. But never logic. Here on ARF, we usually see recommendations to clean the switches and pots, replace the electrotytic caps, etc. Debugging the LC53 probably requires a more determined effort.

I didn't even start with the electrolytics, because the power supply rails were all good enough (20V, -7V, 7V, and 5V). However, I did later replace the large caps, and even added another 1000uF cap at the "island" which connects to Pin 1 of IC30 uA78MG.

I started by printing 11x17" copies of the schematics, and studying them. Just poring over them. I do not claim to understand the schematics. What I was trying to do was "divide and conquer," i.e. try to determine what parts of the schematic were responsible for what functions. I soon found that I also needed to make a one page diagram of the ICs on the logic board, with just their IC numbers inside rectangular outlines, because I found myself searching too often for the parts on the board, and often forgetting where they were, then searching all over again.

I did not buy an IC tester as dockbrn did. My opinion is that there is not much point in doing that, because you need to have the patience of a saint and the hands of an artisan to desolder the ICs without damaging them or the traces on the board. I have neither. When I had finally convinced myself that an IC "must be bad," I cut its pins off with a pair of dikes and then desoldered the pins individually and cleaned up the pads for the new part. Many of the ICs are about 50 cents at DigiKey. A number of them are obsolete. A few more are available on eBay for nutty prices (like $8-$12). So, after I had an idea of the scale of possible bad parts, I made a list of parts to have on hand, even if I didn't need them. I've only had to use a few of them, but I only wanted to pay postage once. For multi-gate parts, with gates sometimes used amongst unrelated functions, it seemed prudent to have more than less. Here is the list, and my notes.

4001B: IC1, IC13 - Misc gates, multiple functions. Order 2.
4011B: IC2,8,19,25,26,32 - Misc gates, multiple functions. Order 4.
4013B: IC3,14,22,23,28,33 - Several suspected bad -- assume all bad. Order 6.
74LS90: IC4 - Seems ok because all clocks are ok. Don't order.
4042B: IC5,10 - possible 1 bad, but cannot order (DigiKey Obsolete).
4017B: IC6 - leakage control. Maybe ok. Order 1.
14553: IC7 - probably ok, but cannot order (DigiKey Obsolete).
4002B: IC9 - don't know. Order 1.
14518: IC11,12,15 - 2 of 3 are definitely ok, but cannot order (DigiKey Obsolete).
4016B: IC16,17,27 - probably ok (?), but cannot order (DigiKey Obsolete).
LM7805A: IC29 - OK, but a little low, maybe replace it. Order 1.

The three below were listed as "later if needed", and are expensive on eBay. Maybe can get from other surplus sources. Of particular note: these are suffix "AE". I do not know if "AE" is required, but the Sencore Parts List makes special note of the "AE", so it seems like they considered it a requirement:
4016AE: IC24
4001AE: IC18
4011AE: IC19

I don't recall if this list includes the few ICs located on the analog board. I didn't suspect issues on the analog board, but I confess I've spent far less time on that one and don't understand it very well. I DID have to go back and forth between the two boards, tracing signals across the schematics. I've had the boards apart at least five times now and remember all the cable connections, which fortunately, are robust and easy to remove and re-connect. Separating the boards is a little tricky--just do it a little at a time, on each side, back and forth.

Be careul of static discharge. Most of these are CMOS ICs, and it is winter time, so drier and easy to zap something, especially if you are wearing a fleece.

Something important to keep in mind:

The schematic and manual, available at the link I posted, do not actually go together. The manual refers to an earlier "Run" as noted by mr.fabe. I don't know what "Run" that schematic pertains to, but the posted owner's manual appears to be older than the schematic, because it refers to a pot (R33) for lead zeroing/balancing, and I cannot locate that pot on the schematic. Furthermore, mr.fabe noted that the R33 pot's function changed in later runs. The schematic refers to two other pots, one on the front panel and one on the back panel. For newer Runs, that R33 is apparently used in calibration of the 100pF capacitance range, as noted by mr.fabe (that does seem to be correct), but I cannot locate it on the schematic. It is definitely on the board, and as I traced it out, I found that my board does not match the schematic. The Run number on my unit is 41. With that, there are a number of "additions and changes" they made with "flywires" and "dead-bugged" parts on my board. These changes were not likely done by a user of the equipment, because they are done very neatly. So, if you have a suspect issue in an area where they made changes, it will make things more difficult.

Until I encountered the "R33" discrepancy, the schematic matched my board enough for me to get the functions to work (though I have not tested all ranges for all functions yet.)

I started with something that I thought would be "easy." The minus sign was stuck on. Using a DMM and a scope, I just traced the logic backward from the display element driving the minus sign. That eventually got me back to IC22, which is a dual flip-flop. Bear in mind that I do not know how the circuit operates, but I do know how the logic gates themselves work, or how they are supposed to. In this case, IC22 was not operating as I expected. Its data input (Pin 5) was high, at 7V. I needed both channels of the scope to see the clock (Pin 3) and reset (Pin 4), to determine what, if anything, the Q output should be doing. I could see a clock pulse at Pin 3, followed by a reset pulse at Pin 4. That, with data Pin 5 tied high, should have created a high pulse on the Q output Pin 1. There was nothing there. It was stuck low. That IC22 needs to go high, because (I think) there is no other way for the Lead Zero and L/C Zero Balancing pots to have any effect at all, otherwise.

Eventually, after a lot of other probing (and I needed to wait for parts, anyway), I decided to cut out IC22 and replace it. That fixed a number of things all at once, including the minus sign. In particular, Capacitance Lead Zeroing, Capacitance Value measurement began working. Oddly, it didn't fix the Lead Zeroing. My notes also said that Inductor Ringing was working, but other checks I did of the logic in that area told me it probably was not working correctly.

I used this technique for a number of the flip flops, because I suspected there might be a "batch" problem with those ICs from that manufacturer. I also checked other gates for proper function. If I checked at a pin, and saw no activity (high or low pulses, or clocks), I would press the four front panel buttons individually as well, to see if I could see any activity. Sometimes that is what is needed.

The four front panel buttons actuate four main signals on the board:

The signal CAP CONTROL (not well labeled--see bottom edge, center of the logic board schematic) goes high when the Capcitor Value button is pressed.
The signal IND CONTROL goes high when the Inductor Value button is pressed.
The signal LEAKAGE CONTROL (labeled almost in the center of the logic board schematic) goes high when the Capacitor Leakage control is pressed.
The signal RINGER CONTROL (next to CAP CONTROL) goes high when the Inductor Ringing button is pressed.

Some of those main signal lines are inputs to the logic gates, so you can see that to observe the functionality of the gate, you would need to push the corresponding button while observing the other signals on the scope.

You need to be careful when looking for activity. There is a large range of pulse sizes, and frequencies. One time, I thought I had observed that there was no pulsing happening, but it was not true. I used my DMM and saw an intermediate voltage of about 4V, not 0 or 7V as expected. That told me there must be pulsing at roughtly 50% duty cycle, and I just had the incorrect time range on the scope and was missing it.

There are some signals which have very tiny pulses. Often the pulses are happening on a very long time scale, like once to twice a second, so very easy to miss.

This brings me to the clocks. You should probably, as a first step, go through all the clocks at the top right of the logic board schematic. The clock rates are all labeled. I used a frequency counter to check them, and also spot checked logic levels. For the most part, logic levels were fine. The gates either worked or didn't, in most cases.

Another gate I convinced myself was bad: IC32. This is down in the part of the circuit labeled Ringer Pulse Generator. I was not sure why IC32 appeared bad while the Inductor Ringing function seemed to be working. Looking back through my notes, I'm now pretty sure that IC32 was in fact intermittent.

At least one of the "issues" I had was caused by intermittent triggering (noisy controls) on my old Hitachi VC-6045 scope. I love and hate that scope. It won't break enough to justify replacing it, but it won't work well enough to make me happy. Well, now I have a couple of sort-of-working Tek 453's to play with as well...

Let's see... Oh yeah, another manual correction: Input Protection Relay Trip Point Adjustment. The instructions, both written and the diagram itself, are wrong, and in fact the written description doesn't even match their own diagram (the information in the manual is inconsistent with itself). The correct DVM measurement point is anywhere that the "downstream" side of the input relay ("top" in the schematic) connects to the switches. It is easiest to connect the DVM to the Ringing switch, 6th contact from the front of the unit, 2nd column from the right side of the chassis.

I'm sure I will find other things as I continue with this unit. For example, even though all functions appear to be working now, I have the following puzzlement: Over on the left side of the logic board schematic, at the IC20 comparator, Pin 2 is labeled "3.5V". Well, I measure 2.4V.

There are things in the schematic layout that just make it hard to understand. For example, IC24D (a switch on the logic board) has both in and out going to the analog board. Over there, it serves one purpose: to "short out" C1008 between the negative input and the output of the LM324 op-amp (an integrator?)

Another one I added schematic notes for: The Leakage Range switch on the Analog schematic page. UP is "Low" leakage range. DOWN is "High" leakage range. The top switch (SW4A) "shorts out" R1094/C1023 when in the High range. (That R/C is near the bottom center of the analog board.) The bottom switch (SW4B):
- In the Low range, connects 181 to IC106 [J], via board-to-board pin [24].
- In the High range, connects 181 to IC106 [H], via board-to-board pin [33].

At least the schematics are readable, and printable on just two 11x17" pages. But goooolllly, they sure are hard to understand. This entire thing is logic-driven, with no microcontroller and no software. This is when logic designers were REAL logic designers, not like now when we write RTL for it all and stick it in an FPGA or CPLD.

There is probably more--no doubt there WILL be more as I continue with whatever calibration I am able to do--but I'm getting pretty tired.

If you are still awake, congratulations. As dockbrn noted for himself, getting this LC53 to work became a "quest" for me. I was certain that, given enough time, I could get it to work--as long as I didn't lose interest first. I don't think one could make money fixing these things, when the value of time is considered. I got this unit for "free" (part of a deal I made with another purchase.) So, aside from the cost of a bunch of replacement ICs, it's not a financial concern; it's mostly a diagnostic challenge to keep the gray matter humming.

- Dave


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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Sun 12, 2020 3:04 pm 
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Joined: May Thu 04, 2017 1:08 pm
Posts: 361
Location: Raleigh, NC area
[more pictures!]

I've done what I can for DVM calibration and inductance calibration, given available and cheap inductors with decent tolerance. I have no standard inductors except the 250uH coil that I made for my Q-meter, and an ARF member generously measured for me.

So now I'm looking at the Ringing Calibration for inductors. First to note: Ringing works. I'm just not convinced it works well, because the LC102 seems to work better. The LC53 calibratrion for this section SEEMS straightforward. But after a lot of debug, I'm not convinced the Ringing Test Calibration instructions are correct. I'm stuck right in the beginning, in the blue-outlined part:
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"Impedance Match" and "Blue Positions" is obvious to those familiar with the front panel. Basically, NOT the section for inductors and flybacks. The schematic section for this part is here:
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The "Blue" section of the Impedance Matching switch corresponds to the top two positions of SW3B and SW3C on the right side of the schematic. Those are the ones with 1M resistors to ground. Those connect to the gate of the JFET TR1015 (TR15 on the board), near the center of the schematic under the word, RINGER.

NOTE: There is a typo in the calibration instructions. The refer to "TR217," yet the PCB layout diagram they included points to TR1017 (TR17 on the board), and there is no TR217 in the schematic or the board:
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When I measure the collector of TR1017 (TR17 on the board), I get 0.098V, with or without the Ringer button pushed. It seems stuck low, but it isn't, as I'll show later. Leads are shorted together for this calibartion, as instructed. I did some poking around, with the DMM to try to understand the DC operating point, since that seems to be what the instructions are focusing on when they ask for 6V to 6.5VDC at the collector of TR17.

TR17 collector is a logic level input to the Delayed Ringer Control switch IC117 on the digital board (IC17 on that board, not shown in this post). Only to say that TR17 collector needs to be a CMOS logic level, and the calibration instructions for 6 to 6.5V would not be out of the question.

Following the signal from the input (leads shorted = GND), when the Ring Test button (bottom right) is pushed, the gate of JFET TR1015 (TR15) is grounded (0V). TR15 is an N-channel FET. Its source (bottom) measures (DMM) about 1V. So the G-S junction is reverse-biased (Gate negative with respect to the Source), so TR15 is off. This is with Ringing button pushed or not. Does TR15 enter into the picture here? I don't know, but it seems like if TR15 is always off, the circuit won't work.

Since I know that "Ringing works," at least partially, I thought to follow the logic signals from the other board, via board-to-board pin 26 (the input to the IC1001 NAND gate, left side of the schematic above.) I used a scope for this, because there is some very fast pulsing going on, with very low repetition rate. Scope persistance is no good here, but my Hitachi VC-6045 has a rudimentary digital storage mode, which I made use of.

In my notes, I drew little pictures of what I saw on the scope. The pictures below start at the output of the IC1001 NAND gate, and go eventually to the collector of TR17, where I'm supposed to see 6 to 6.5V DC. Here it is:
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Given that TR17 collector pulses high like that, at a low rate (like once ot twice a second), and the pulses are so brief, it seems there is NO way I could ever read 6 to 6.5V at that point. Furthermore, as I write in my notes, it seems like maybe it is working as designed.

Does anyone any have other calibration instructions which might be different from the one I have? I don't see how the ones I have could possibly be correct.

Gotta run... Thanks!


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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Sun 12, 2020 5:03 pm 
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Thanks for posting the additional information! The Sencore schematics are a bit hard to follow, as one has to "chase" the wiring from one sheet to another.

_________________
Tim KA3JRT


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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Tue 14, 2020 12:12 am 
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I guess I'll just keep adding debug notes. Up into the "cloud" they go, where they end up, nobody knows...

Answering my own question: No, the Ringer Calibration instruction cannot work as described, at least not on Run 41 versions of the instrument. The circuit on my board differs from that in the schematic. I noticed it when I was measuring DC values in the circuit and found one that was not possible.

Image below is the circuit as shown in the schematic (above) and as it is actually on my board (below). I do wish we had more versions of the schematics and calibration instructions available. It would make this much easier.
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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Wed 15, 2020 3:59 am 
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This is kind of cool, so I thought I'd add it to the thread.

As I noted last time, those Ringer Calibration instructions are no good for the newer units like this one. I looked all over the web for a different version, but no luck.

So, I decided to look at the circuit when it was NOT in calibration mode. For the measured signals below, I used a 6800uH inductor, which the LC-53 reported as having 57 rings. Sometimes 56, sometimes 58. For this discussion, you need the Riinger circuit I posted earlier, and the little section of two transistors in front of it (TR18 and TR19). Below are my notes. Don't pay any attention to "Picture #" in the notes. That was just to help me get the pics right for this post. I'll explain it all.
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So the Ringer takes one of the caps you select with Switch SW3B, and puts it in parallel with the inductor you want to Ring check. The digital part of the circuit delivers a 5.4us low pulse via the IC1001A NAND gate, into TR18/TR19. That pulse is shown below, along with the collector of TR19 (anode of CR25). There is a roughly 7V swing from the CMOS NAND gate, but a 14V swing seen at the anode of CR25!
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At first, I said, "How does that happen," but I think its because of CR26, which is connected to that point with its anode to -7V. So, the logic gate kicks that TR18/TR19 pair, and a BIG transition occurs at CR25. CR25 gets back to normal over a much larger period, with the RC response:
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Then it gets interesting. There is some kind of feedback mechanism here; I'm not very good with the analog stuff. Take a look then at the JFET (TR15) gate. This is over the same time period shown in the prior photo:
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And that results in this waveform at the collector of TR17, which then heads back over to the digital board to get its rings counted. This picture is in the same time period as the prior two. Sure enough, about 57 rings, before it decays below the logic high level:
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Sorry the scope pics are at an angle. The scope was on the floor and the garage lights were making it hard to get decent pictures.

Dave


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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Thu 16, 2020 7:32 am 
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Check your PM's... surprise inside


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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Fri 17, 2020 12:40 am 
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Hey, I was wrong!

R81 does indeed adjust the voltage on TR16 collector to the 6V to 6.5V range as indicated in the Ringer calibration instructions.

The problem on my unit was that R81 was (a) nowhere near where it should have been, and (b) very tight in the second half of its range. I thought I was moving it fully CW, but I was only moving it about halfway. That pot takes TR17 from being fully ON (CCW) to fully off (CW).

Now, I wonder how that affects the measurements I've done? :shock:


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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Fri 17, 2020 4:43 am 
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Looks like you fixed your issue. Nice!

BTW, I've located my LC53 schematics and they match your posting.


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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Sat 18, 2020 8:32 pm 
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Continuing with my documentation of the calibration... This post is for the second half of the Ringing calibration (steps 4-7 inside the yellow box). It is important to do steps 1-3 first. The calibration instructions look like a slightly updated version of what I had; perhaps re-written by someone. Thanks mr.fabe!
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So the second part in the instructions at step 7 talks about "counting." This is very ambiguous and even misleading. During this part, the meter display does not exhibit counting as for example 56, 57, 58, ... No, in fact I'm pretty sure what they mean is that the LOGIC itself is counting, and the display is just showing the final/total count (or some number), and updating that about once a second or a little faster or so.

In other words, if the display is showing changing numbers, updating around once a second--no matter what those number are--it is "counting." When it stops "counting," the display no longer updates (and in fact my display shows 000 at that point.) At least that's what I think, and here is why:

Since the calibration instruction's description of what I should see was cryptic at best, I decided to try to find a signal which would help me to watch the action on the scope, along with watching numbers on the display. I chose a location by now familiar to me: the collector of TR17. A convenient place to probe is at the board to board connector, Pin 44.

When R1054 (R54) is adjusted through its range, the signal at that point starts as about a 50% duty cycle square wave, and then changes to increasingly positive (narrower negative-going pulses), and eventually to a DC high signal. Somewhere in the middle of the range of R54, the waveform at Pin 44 looks like this (and the unit is still "counting"):
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Finally, when the "counting" stops and the display reads 000, the waveform at Pin 44 looks like this:
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R54 is described as "Ring Level," and I'm not entirely sure what we've adjusted in this context, other than the sensitivity of TR17 (signal level into its base).

That said, after this "calibration," a "poor" coil which I had previously measured (I think it was nine rings), now shows four (004). But a good coil shows 057, pretty much unchanged from before. In fact, with a good coil in place, adjusting R54 seemed to have little or no affect on the final value. I'm guessing the Ring Level adjustment only concerns those coils which ring poorly.

I'd love to hear if anyone else has had any experience with the Ringing calibration, either confirming or refuting my findings.


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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Sat 18, 2020 11:34 pm 
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I have never done any of this but am wondering if perhaps a simple Q measurement might be an accurate test.


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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Sun 19, 2020 12:23 am 
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Well, now, that is an interesting thought, Bob. It would not help with the inductor ringing calibration of the LC53, but it would be interesting to compare the ringing values of a set of coils, using both the LC53 and the LC102 (which presently do not agree), and see if one or both of them show a linear relationship to Q. I would have to normalize the test frequency between all three, I think.

I have a Heathkit QM-1 Q meter. While I have a standard coil for it, I wouldn't expect it to be much better than a relative measurement (just like the ringing test.) But probably good enough.

I may try it. But I don't want to vector too much right now, and there is precious little room on the bench, with the three new Sencore boxes. I really need a better work area than the shared garage workshop/table saw/electronics bench. But that won't happen any time soon. And I still want to build that AM transmitter... And work on the new old Tek 453's that I picked up...


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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Sun 19, 2020 2:16 am 
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I suspect my list of projects is longer than yours. I do have a couple of Boonton Q meters that I doubt I will ever need in light of the capabilities of the nanoVNA.

What would a ringing test of an inductor tell you that wouldn't shake out of a Q reading? With the Q meter you measure Q over a wide frequency range and even could find the self resonant frequency. The nanoVNA does all that as well.


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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Sun 19, 2020 3:24 am 
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Bob, I hear what you are saying, and I don't disagree. But the point of this thread wasn't to discuss the value of this instrument and its features, vs. any other. Sure, there are better ways to do some things.

This unit can apply a high voltage to a cap and measure leakage. (I would never expect the nanoVNA to be able to do that.) But that is why I bought this unit, and since it wasn't working, I got it for a song. The "Ringing test," regardless of its perceived or actual value, is simply one of this unit's features, that I'm testing along the way to fixing it up.

And maybe someday, someone else wih a broken LC53 will find the info useful.


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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Sun 19, 2020 11:55 pm 
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I have another "discovery" I thought to share today about this LC53 -- at least the version I have, which is "Run 41."

I finished looking at the Inductance Ringing calibration, fairly sure that I understood it, and that this part was properly calibrated. So I went on to look at Capacitance calibration. There are only three ranges to calibrate on earlier units:

8000 pF
0.8uF
800uF
And for Runs 31 and over, there is also a 100pF range.

I had decently-close capacitors to use to check the lower ranges:

100pF 1%
6800pF 1%
0.82uF 3%

For the high range, I had a number of high value electrolytics, but no way to know what was correct, since these have poor tolerance specs. I ended up leaving ALL cap calibration ranges alone on the LC53 because they looked pretty good, as compared to the spec'd parts I had.

But here's the rub: I had previously measured an "incorrect" voltage value in an area related to the capacitance measurement circuit. At IC20 on the Digital board, there is a schematic note saying "3.5V" at Pin 2. And the schematic says that all noted voltages are with all buttons OUT. (So, with NO measurement taking place.)

I measured 2.44V at IC20 Pin 2. So what is up with that? Is my board different here as well? Is the schematic note wrong? Well, I'll spoil the story and just tell you that mostly, the schematic note is wrong. But there is also a difference in the circuit values. Let's look at the schematics.

First, in the analog board schematic snip below, you can see the Capacitance calibration pots, R83, R88, and R90. (And on Run 30 and above, there is also R33 for the 100pF range--but that is not shown on the schematics available online.) You can see that as the transistors are turned on one by one (for auto-ranging), the current from the 5V supply passes through the series resistance, just as it says on page 6 of the manual. The collector current then goes off to the right side to the capacitor under test, i.e. to the "capacitor value" switch, SW100A, not shown in the snip. But the key here is the TL084 buffer, IC50. That is checking the voltage on the capacitor under test, for measuring the RC delay. More below...
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The output of that TL084 buffer goes through Pin 7 of the board to board connector, to the digital board. A snip of that part of the digital board schematic is shown below. You can see that it feeds Pin 3 of the IC20/21 comparator set. And THAT is the rub: How can I be sure that my capacitance ranges are calibrated properly, when the capacitance value is being measured vs. a "3.5V" reference which measures 2.44V? Well, we need to know what's going on there. Is the note wrong? The schematic? What's up? Yeah I already told you, but there is more to the story. More below...
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Now look at the snip from my notebook. I've re-drawn the voltage dividers from the comparator circuit above. If all the circuit values on the board match those on the schematic, I should indeed see 3.5V at IC20, Pin 2, provided that TR15 is turned off. More below...
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The first thing I did, was go check the circuit values, and I did find one on my board which didn't match the schematic: R74 on my board is 50k 1%, not 46k 1%. I also found that TR15 is normally turned ON! So the new voltage divider calculations show that the IC20 Pin 2 voltage should be...yep...2.44V, just as measured!

Great! But should TR15 be normally turned on? We need to know... Well, TR15's base is driven through the 330k resistor by Point (A) in the schematic, which I didn't copy for you, but that is the "zero" output on the IC6 decade counter. With all buttons OUT, IC6 pins are as follows:

0 output: high (which is driving Point (A) and therefore the base of TR15, turning it ON.
1-9 outputs: all low
Clock: 50% duty cycle, 5Hz clock (200ms)
Reset: Interesting...65ms low-going pulses, repeating at 400ms rate, but the pulses only driving from 7V down to 6V, being inhibited by something that I didn't track down.

But, when the Capacitance Value button is pushed (and a 6800pF cap in the leads), that Reset pin starts getting full swing pulses, at a different rate and duty cycle. And that makes the "zero" output pulse, which pulses the base of TR15, and that makes the voltage at IC20, Pin 2 alternate between 2.44 and 3.5V. (And it's a 1.5Hz rate, if anyone cares.)

So the schematic note should actually say "2.44V" at IC20, Pin 2, when all buttons are OUT, as the schematic notes dictate. I'm not sure what the earlier Runs look like, though. (Perhaps on those units, TR15 is normally OFF, which would make the schematic note correct.

At any rate, one more piece of the LC53 puzzle is now in the cloud, as long as the cloud is there. :shock:

Dave


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 Post subject: Re: Sencore LC53 Debug / what does this pot do?
PostPosted: Jan Mon 20, 2020 5:12 am 
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Posts: 350
You can use any capacitor that is close to the range needed. You can also use multiple caps to "hit" the range your looking for. You will just to to validate the capacitance measured on another LCR meter and adjust the trimmer to the value of the cap(s).

There was an instrument test by Electronic Servicing on the LC53. Despite the non-standard methods that Sencore used, the results were in the ballpark. It seemed to me that Sencore sought out different measurement techniques to register a patent and generate market fever for their specialized line of equipment. For me, it's a good go/no go tester with a multitude of features that work fast and easy. If I wanted accuracy, a LCR bridge would be the only other option for measurement not testing.
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