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 Post subject: Precision 10-40 Calibration?
PostPosted: May Wed 20, 2009 7:35 pm 
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Joined: Mar Thu 12, 2009 2:20 am
Posts: 1615
Location: Dayton, OH
Anyone have any recommendations for a calibration procedure for a Precision 10-40?

The procedure all over the web for the Precision 612 doesn't seem terribly applicable. At least I can't see how to apply it....

There's also a couple of extra variable resistors in the schematic, beyond the two that are fairly obviously labeled 'calibration control'. (R13 & R11 - Specifically)

The schematic in the back of the operators manual is fairly painful to read as well... :-(

Hints please???


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 Post subject:
PostPosted: May Thu 21, 2009 1:49 am 
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Joined: May Sun 06, 2007 2:52 am
Posts: 2863
Hi MR.Holland,This is going to get long winded folks,so here goes.

Yours is the$64,000 dollar question and not easy to answer.Precision never published calibration data for their tube testers.They wanted the units returned to factory for this service.

However,I have a model 912 which uses the same basic circuit as yours.I expect yours has a built in VOM,which may account for the extra calibration pots.

I will be glad to share with you what I have learned from a good deal of research about calibration of my rig.

There are only two adjustments,at least on mine.One to calibrate the line adjust control and one for the quality(merit) test.The usual accepted procedure for the line adjust is to rotate the line adjust knob on the panel until the meter needle lines up with the "line" mark.Then,take your voltmeter and measure the full secondary voltage on the power transformer.On a 912,it is 300 volts.Adjust the appropriate pot to get the correct reading.I can't be very specific,I don't have your schematic.

Interestingly,the Eico 667 uses the same set-up,but line-adjustment is made by a procedure measuring the primary voltage of the transformer.I had an Eico 667 and the resulting filament voltages would be dead on(with no load,anyway)..I wish I had tried both methods and compared results when I had my 912 opened-up.Maybe you could try both methods and report on your results.I will bet,they will be pretty close.

As far as the quality test goes,I used some deductive reasoning,but I think my conclusions are correct.

In my manual,and probably yours as well,you will find a paragraph or two entitled "tube brand variations".In this,they state what to do if you encounter a particular brand of tube that regularly tests high or low for merit(known good tubes),for a given tube number.They tell you to make up your own setting for these tubes by varying the position of the meter shunt control,or control"D" on a 912.The manual says to vary this control so that the needle rests in the center of the letter "G" of the word "GOOD" on the meter face.On a 912,this is at 66 of 100,or 2/3 scale.This tells me that this is where they feel that,on average, the needle should rest for a known good or new tube.Why would they pick that particular point?I tested a boat load of 12au7's and 6v6's,and this conclusion appears to be true.The average good tube consistantly falls in this area for the merit test.

The factory calibration has not been changed on my tester.It needed a lot of cleaning,but all of the resistors were still within spec.Hard to belive,but true.I changed the one paper cap,tested the rectifier in another rig and let it go at that.The secondary voltage measured about 308 volts,but I expected it to read slightly high with a DMM and high modern line voltage.I saw no reason to go any further and buttoned it up.I have had no reason not to believe the results it gives me and it has been quite a good tester..Other opinions can,and probably will,vary.Good luck.


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 Post subject:
PostPosted: May Thu 21, 2009 3:13 am 
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Joined: Mar Thu 12, 2009 2:20 am
Posts: 1615
Location: Dayton, OH
(Sorry, for got to sign my name at the end.)

Howdy, here's my long winded reply. (Its only taken me an hour to write it.)

(Someone's labeled all the non-obvious potentiometers w/ a magic marker internally, so apparently I'm not the first one to start down this rabbit hole)

Thanks for the input, but I'm not entirely certain its applicable......... ( I gladly hope I'm mistaken. )

Your procedure sounds similar to the 612.....

My questions lay around the fact that I have 3 (not 4, nor 2) potentiometers internally to adjust. (I started tracing schematics this evening. R13 is the Zero Adjust for the Gas test, and is not relevant.)

Fortunately Precision at least put their schematics is in the back of there operator manuals, for reference the schematic (a GIF) is here: (The schematic in the back of the operator manual is clearer btw.)

http://oldradios.50webs.com/precision/1 ... ematic.gif

Op manual here:

http://sh1.webring.com/people/gc/cghworth/10-40_op.pdf

R18, and R33 are both labeled calibration controls, there's also a R11. Based on your comments, and schematic comparison. R33 is the adjustment for "merit" gauge. I'm guessing R11 is the filament voltage adjustment, based on its rectifier proximity. (I'll mark its position, twiddle it, and confirm,)

That leads me to R18.. the last one....

It looks like its in the middle of some sort of bridge with the 12AU7 that's in the thing. Any what's up w/ that, and how to adjust it? (Googling "12AU7 Bridge" gives me lots of VTVM hits, and a Sencore MU140 hit, so maybe I'm on to something there.) I gather from the Sencore comments its for meter protection (?) Its late here in Dayton, so I'm off to bed for now.....

Can anyone confirm (or deny) I'm not entirely off into la-la land?

Thanks for your time... g-Night!

David


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 Post subject:
PostPosted: May Thu 21, 2009 10:50 pm 
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Joined: May Sun 06, 2007 2:52 am
Posts: 2863
Hi Dave,Well...I think if you looked hard at ther schematic for the 912,you would see that the info I gave you probably does apply.I could be wrong,but I have read countless times that the 9 series circuit is pretty much identical to the 10 series.They are both Dynamic testers,whilst the 612 is a basic emissions tester.

I couldn't get the GIF schematic to come up,and I don't have the patience for PDF's with a dial-up connection,so maybe someone else here will very if in fact the circuits are the same.

The only real question about this I keep seeing,and actually the only one I was trying to answer,is what values you would be after when adjusting the calibration pots,ie.,the filament voltage and the quality test.I probably jumped to conclusions,because this is what I thought you were asking for.I really can't help you with the questions as to what pot does what.

Considering the number of people on here who own testers like yours,and use them,I'm rather surprised at the silence.

Anyone....Anyone.....???


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 Post subject:
PostPosted: May Thu 21, 2009 11:06 pm 
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Joined: Mar Thu 12, 2009 2:20 am
Posts: 1615
Location: Dayton, OH
No, your data appears to be quite valid, and I appreciate the input.

Pretty certain your technique is correct. I think that last potentiometer is just the zero adjust for the bridge.

Bought a new 12AU7 (MCM Sells them), and a couple of new electrolytic caps, I'll swap'm out over the weekend (and glue the wooden case back together.)

I guess I'll mark/measure all the POTS and see how it goes if I don't hear nothin'..

Thanks,

David


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 Post subject:
PostPosted: Jun Tue 16, 2009 3:43 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 32
Location: Sao Paulo, SP, Brazil
Hi everyone, I'm glad about meeting people interested in such data.

Just finished doing some measurements with my newcomer, the Precision 10-40. Aesthetics far from prize winner, but circuitry amazingly preserved made it easy to figure out original calibration goals. Will post more detailed results soon. Those go in advance:

Found the misterious R18 to be effective only for the 0-20uA scale calibration (this is for gas test). R33 essential for quality test calibration, can be most effectively achieved with "D" control turned fully counterclockwise (minimum sensitivity). Should deflect full scale (120) at 25mA plate current when using range 4 (75mA when using range 1) of "A" control. Used silicon diode in series with power rheostat of several kilo ohms to limit and adjust current between plate and cathode pins. Just for checking, maximum sensitivity obtained with "D" control fully clockwise should be 1,5mA at 120 indication. Still didn't gather any data for in between points - only those extremes. Still hope this can be sort of useful.

Please write back, I'll keep the tester on the bench a few more days before packing up (I'm about to move) and will gladly get more data upon request. It would be much appreciated to exchange information with other owners.

Regards,

Flavio, from Brazil.


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 Post subject:
PostPosted: Jun Tue 16, 2009 6:12 pm 
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Joined: Mar Thu 12, 2009 2:20 am
Posts: 1615
Location: Dayton, OH
Howdy,

Interesting information. Yes its useful. I'll have to test mine and see how far out of calibration it is.

R11 was definitely the filament calibration control. The other resistors I decided to leave alone. They were sealed in place, and I wasn't having a particularly "good day" while working on the tester.

After going over it, and replacing the couple of caps I identified, The meter seemed to be always reading low. No matter what I set R11 to, it would not make 1/2 scale. Coudln't find any resistors that were out of spec either.

So I took the meter out, and hooked it up to the power supply, a resistor & trim pot, and the DMM. Its a 1ma meter, so I set the power supply/resistor to give me 0.5ma current through it, but it still read low. Started disassembling the meter to check for iron bits in the movement, and since I appear to be an idiot, and clumsy, broke the needle in the process. :cry:

After cussing for a while, I ended up replacing the meter w/ a digital panel meter. It ruins the looks, and will make reading certain scales very difficult but it does work.
(And wired in a wall-wart 9v supply for the DPM itself.)

(For future archival purposes: The meter is 160 ohms resistance, I used a 200mv meter in parallel with a 120 ohm resistor, w/ a 39 ohm, and 1 ohm in series with meter/resistor combination. The meter should read between 0, and 120. "Good" starts at 65. Line Adj, should be for 60. )

R11 then would gave me 1/2 "scale" properly, so I guess the meter was defective after all..... :? <I don't suppose anyone has a 10-40 meter they want to get rid of as an aside?>

I then decided to button the thing up, before I broke anything else. The tubes I've tested on it seem to give "intelligible" readings. (old tubes read low. new tubes read high.) Which I suppose is the point...

The tester I have appears to have a relatively complete set of printed factory tube data updates with it. I will probably scan it at some point, since the tube data available off the Chris/Precision tube data site doesn't seem to be entirely accurate for the 10-40.

thanks for the data..

David


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 Post subject:
PostPosted: Jun Wed 17, 2009 3:54 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 32
Location: Sao Paulo, SP, Brazil
Hi David,

It is nice to communicate here. I guess I found the right place to share opinions on such subjects.
Quote:
R11 was definitely the filament calibration control.

Some say to use the 300V winding instead as the reference for such alignment. It seems to exist some controversy here. I agree that the heater voltage is most important during any tube quality evaluation. Note that the "line adjust" voltage is derived from the 50 volts winding. I'm getting 6.8V (open circuit) from the 6.3V winding which sags down to 6.5V when testing a 6SN7, and only 6.2V with a 6DG6. Is this the same with you?
Quote:
I wasn't having a particularly "good day" while working on the tester.

All of us have occasionally have those days.
Quote:
<I don't suppose anyone has a 10-40 meter they want to get rid of as an aside?>

Perhaps the movement from another model could be installed inside the old meter casing using the same face plate. Probably the same manufacture (Pace)?
Quote:
I then decided to button the thing up, before I broke anything else. The tubes I've tested on it seem to give "intelligible" readings.

I didn't tamper with my adjustments too - only checked them. In the lack of reference standards, intercomparison is the way to go. If you placed a miliammeter in series with the plate circuit (as wisely indicated by Mr. Douglas in his book), we could compare our results. For now my calibration directions can only be regarded as "tentative data". From other 10-40 owners reports I expect to sketch a more solid procedure.
Quote:
The tester I have appears to have a relatively complete set of printed factory tube data updates with it.

My rollchart displays an "H2B" code - does this match with yours? I also have two booklet supplements (one "GK" followed by an "HB").
Quote:
... tube data available off the Chris/Precision tube data site doesn't seem to be entirely accurate for the 10-40.

Chris' page is wonderful, but test data information is misleading as for the 10-40 goes. One should be careful to note that data therein provided is strictly intended for the 10-12, 10-15, 10-20, 10-22 and 10-54 models. So data is useless for 10-40 owners? No! A first inspection revealed that only major difference resides on control "E" (filament voltage) settings. By merely using the following conversion table:

[10-12] - - > [10-40]
    6 - - > 8
    7 - - > 9
    8 - - > 10
    9 - - > 12
    10 - - > 13
    11 - - > 16
    12 - - > 18
    13 - - > 19
    14 - - > 20
    15 - - > 21
    17 - - > 22

One can safely export 10-12 test data to the 10-40 as filament voltage goes (1 through 5 remain unchanged). Please note that remaining settings may have been updated later (otherwise fully compatible).

Still looking forward any results regarding correlation between plate currents and meter readings at well defined settings of the control "D" so I can crosscheck them with mine (intercomparison project). :wink:

Cheers,

Flavio


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 Post subject:
PostPosted: Jun Wed 17, 2009 4:37 am 
Member

Joined: Jan Thu 01, 1970 1:00 am
Posts: 32
Location: Sao Paulo, SP, Brazil
Quote:
The schematic in the back of the operators manual is fairly painful to read as well... Sad

I totally agree, and I eventually suceeded in circumventing this by using plenty of hydrocolor pens from the children school stuff. Now I understood almost everything except a few things regarding gas test:
    :?: Why is the filament supply secondary disconnected during gas test for filament voltages 15V and up?
    :?: Why is cathode return (for gas and picture tube tests) disconnected for ranges 3 and 7 through 12 of the "F" control?
Also detected an error in the schematic which shows control "F" disconnecting C5 from R1 in position 12. Inspecting the wafer section shows that is impossible (they remain connected even in position 12). :?

Here goes a warning for everyone who intends to carry out voltage regulator tubes tests - DON'T BLOW THEM AWAY (and maybe even the tester itself) by turning control "A" through neither "GAS" nor "PICTURE" positions when testing a VR tube. That will apply full charge of C5 to the poor tube WITHOUT ANY LIMITING RESISTOR! :shock: It is unfortunate that the manufacturer trapped the VR and IR positions of control "A" between the other two. :x A separate control would have been far more adviseable, I think. Nevertheless, I still find a great feature of the 10-40, its capability of carrying out VR tubes tests the correct way. :) Unfortunately it is hard to reach 40mA when testing a VR150 (0D3) tube, as we have to push the line adjust too far, which seems quite a stress to the tester. Has anyone had problems with that? :roll:

Regards to everyone,

Flavio


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