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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Mon 12, 2011 10:25 pm 
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Did you hang your scope probe on the DC line to see what the ripple looks like?

I did that recently when I recapped the Viking Ranger I got at NearFest. By doing so, I discovered I forgot the tie the negative end of the high voltage caps to ground. :) It worked a whole lot better once I did that.

Anyway, it sounds like a cap is charging up to a point where it breaks down and then conducts to ground until it discharges enough to allow it charge up again and then repeats the cycle about 250,000 times a second.

arnie


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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Tue 13, 2011 5:09 am 
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Arnie-AE wrote:
Did you hang your scope probe on the DC line to see what the ripple looks like?

I did that recently when I recapped the Viking Ranger I got at NearFest. By doing so, I discovered I forgot the tie the negative end of the high voltage caps to ground. :) It worked a whole lot better once I did that.

Anyway, it sounds like a cap is charging up to a point where it breaks down and then conducts to ground until it discharges enough to allow it charge up again and then repeats the cycle about 250,000 times a second.

arnie


D'oh! (slaps self in forehead). Seems pretty obvious when you mention it. No, I hadn't gotten around to that yet. First thing in the morning...

I'm working on getting my junk box refilled, so far as those boat anchor type electrolytics. My stock of the HV sizes had dwindled to nearly nil, just a couple of axials and a couple of can types. I'll replace the 20uF 600V output cap ASAP; I'm about to place an order. The two 40uF 350V caps in the FW doubler have been replaced. They were 1960's originals that were shot and starting to vent.

Haven't really thought much yet about the R's and C's implications of that 250KHz oscillation. Maybe this is some decoupling cap somewhere in those stages, or maybe the power supply. There is so much solder rework to do in that radio. I'll probably end up systematically replacing a lot of perfectly good disks. But one's confidence level is low upon finding non-soldered connections which may trace back to the original kit build, and a lot of awful looking solder joints. Will probably end up redoing every single switch lug connection and a majority of other connections.

The present wiring differs significantly from the original Knight design. Too many hands have been under that chassis. It's so bad, I think I am going to have to hand draw a partial schematic of what I really have in there. Then I can compare it to the manual and work through the differences. Right now, I look at that chassis, then at the schematic, and it makes my head swim. LOTS of components have migrated to different locations. I've found at least one incorrect component value in the B+ distribution.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Tue 13, 2011 7:06 am 
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Clutter,
I was thinking about you're T-60 earlier. Any way you can lose the screen voltage on the final and look back at earlier waveforms; osc, driver, etc? It may give an indication of what stage the problem is in. With what you've described, there is also the possibility that some acid core solder got into the radio at some point. Now you've got your hands full. I would hate to think of a complete tear-down and a baking soda bath. CB

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Wed 14, 2011 10:06 am 
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starstation wrote:
Clutter,
I was thinking about you're T-60 earlier. Any way you can lose the screen voltage on the final and look back at earlier waveforms; osc, driver, etc? It may give an indication of what stage the problem is in. With what you've described, there is also the possibility that some acid core solder got into the radio at some point. Now you've got your hands full. I would hate to think of a complete tear-down and a baking soda bath. CB


I have seen photos of radios trashed by acid core solder. They stand out like the proverbial sore thumb. To the best of my knowledge, I have never personally seen that first hand. I think this radio is OK in that regard. It just looks like poorly done work with rosin core solder. Missing is any evidence of corrosion on the connections. Now, poor tinning, poor wetting and flow- that is another matter.

No progress yesterday, as I got involved with another project. My new (to me) tube tester showed up so I played with it instead. Back at the T-60 tomorrow. :wink:

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Last edited by Clutter on Dec Wed 14, 2011 7:46 pm, edited 1 time in total.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Wed 14, 2011 2:12 pm 
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A 200KHz oscillation is about right if you have a poor screen bypass capacitor, or a drifted-high grid resistor.

I would stop fooling around with the original caps and change ALL the bypass caps to new disk ceramics with short leads to ground. And I'd get a BIG soldering gun and solder the ground lugs to the chassis (if it's steel).

Long ago I had a cheap CB set with similar issues-- never did figure out exactly what was wrong.


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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Wed 14, 2011 2:14 pm 
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Oh, it's also possible that somebody ran the xmitter with the plate cap off the output tube-- that will burn out the screen grid in about 2 seconds, causing all kinds of problems like the one you're seeing. Try a new output tube. You should keep a spare around anyway.


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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Wed 14, 2011 4:06 pm 
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That hystersis you're seeing in the driver tuning is a sure indication of the set going
into self oscillation.
I'd also go along with a missing, bad or improperly installed bypass cap.
Does the parasitic dissappear or appear when you adjust the driver tuning?

Pete

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Wed 14, 2011 6:59 pm 
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Thanks, All-

I'm getting some great suggestions here. I had this small insight today...

Considering that this is a basically working transmitter provided one carefully detunes the Drive Tune just a little, and it shows 30 watts output on the Heath HM-2140 wattmeter even detuned...

I noted with great interest that when I put my DMM on V1 pins 3 and 9, which are the oscillator and multiplier plates, I saw 520V both places. That's right, it's the same as the 6DQ6B final plate voltage, for crying out loud! Yikes! Perhaps this is normal, however- this was measured key up. Need to get a better sense of whether the electrode voltages make sense in those stages. Looks like I'm dealing with series dropping resistors and not dividers here.

(Sigh...) I'm embarrassed I had overlooked this most basic of tests up until now. :oops: But still these may be normal readings. The work ahead is to restore the correct B+ distribution in any case. It stands to reason that a number of caps should be replaced on general principles. It is looking like one of the decoupling caps somewhere on the B+ might be the culprit. BTW there are no tubular type caps at all in the affected circuitry. Everything is disks there.

Peter Bertini wrote:
That hystersis you're seeing in the driver tuning is a sure indication of the set going
into self oscillation.
I'd also go along with a missing, bad or improperly installed bypass cap.
Does the parasitic dissappear or appear when you adjust the driver tuning?

Pete

Yep, that's it exactly. Looks like at the exact peak on the Drive Tune for 40m it breaks into oscillation. Detune to either side and the oscillation dies. Today, the sinusoid I was left with was pretty much 30W on the meter and looked reasonably clean. For some reason, changing out the cathode bypass C11 on the PA made a big difference. It was a huge 600V mylar. Now it's a tiny 250V box polyester with way less lead inductance. Remember, the parts list callout was for a .05 uF disk.

Gentlemen, this old T-60 will live again, and I'll put it on 40CW for its inaugural QSO at some point.

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Last edited by Clutter on Dec Wed 14, 2011 7:50 pm, edited 1 time in total.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Wed 14, 2011 7:37 pm 
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The oscillator and multiplier voltages proved hard to pin down with the DMM, even on the supply side of the RF chokes- RF effects on the meter. I'll try later with a 2:1 resistive divider to bring the voltages within my scopes 50V/div so I can look at them DC coupled. My impression so far is that they are probably close to normal.

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Last edited by Clutter on Dec Wed 14, 2011 8:02 pm, edited 2 times in total.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Wed 14, 2011 7:40 pm 
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Ancient_Hacker wrote:
A 200KHz oscillation is about right if you have a poor screen bypass capacitor, or a drifted-high grid resistor.

I would stop fooling around with the original caps and change ALL the bypass caps to new disk ceramics with short leads to ground. And I'd get a BIG soldering gun and solder the ground lugs to the chassis (if it's steel).

Long ago I had a cheap CB set with similar issues-- never did figure out exactly what was wrong.


I'll try replacing C12, the PA screen bypass. Ultimately, I think a number of bypass caps ought to be replaced as well. You're right, it's also time to start checking some grid (and other) resistor values.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Wed 14, 2011 7:45 pm 
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Ancient_Hacker wrote:
Oh, it's also possible that somebody ran the xmitter with the plate cap off the output tube-- that will burn out the screen grid in about 2 seconds, causing all kinds of problems like the one you're seeing. Try a new output tube. You should keep a spare around anyway.


I think I do have a used 6DQ6B around here. I'll try substituting it. Just yesterday my tube tester arrived. Once I get more modern tube chart data, I can test the final and also the 6HF8.

There's another question to the group- how would I go about getting tube tester settings updates for my new (to me) Hickok 538? It has the 6DQ6 on the chart, but not the B version.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Thu 15, 2011 5:23 pm 
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Today I tried replacing the PA screen bypass cap C12. No Help. Also replaced the PA plate bypass cap C13. No help. I found the ground end of the original C13 was either not connected or very poorly soldered. Gripped it gently with needle nose pliers and it either was free or very easily broke free.

I built up a simple 2:1 resistive divider to look at various plate and screen voltages using my scope with 10x probe at 50V/div. Just two 100K/2W carbon comps with clip leads either end. Everything looked reasonable, both key up and key down. I noticed that the key down voltages differed when the Drive Tune was set for LF oscillations. Osc and mult plates tended to read higher key down with the LF present, PA screen read lower.

I'll keep at it. Kind of ironic- I have a transmitter that is basically working right now, and the cathode keying note sounds nice in my receiver. Looking at the PA output waveform, I seem to have a pretty clean looking waveform. But tune the drive carelessly and it breaks into LF oscillations. FWIW I suppose I could use it on the air right now if I watched the waveform.

I need to finish my 40m VFO project. I have a working oscillator right now, but would need to get it in a shield box and add a buffer to bring the level up enough to drive the 6HF8. This is a Vackar circuit and it's nice and stable. Still have not found my other FT-243 crystals. I'm stuck with one rock at ~7010. Maybe I'll try a little DX one evening. :wink:
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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Thu 15, 2011 6:40 pm 
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If it is low frequency oscillations, then you might try adding a few electrolytic bypass
caps on the B+ supply for the driver section. 10 uF should be pretty effective.
I can't imagine why you are having this problem, but see if that fixes it. you could
also add some 2.5 mH chokes on the power supply feed to the driver plate and
screen... again, if the original design didn't need them they should be needed now.
But, it would be interesting to see if improving the bypassing and decoupling
improves the situation.

One other to watch for is if someone used a common ground point terminal
for bypass caps associated with the plate and grid tuned circuits. A poor
high resistance ground, or unexpected ground loop currents, can cause
regenerative feedback.

Pete

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Fri 16, 2011 2:27 am 
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Peter Bertini wrote:
If it is low frequency oscillations, then you might try adding a few electrolytic bypass
caps on the B+ supply for the driver section. 10 uF should be pretty effective.
I can't imagine why you are having this problem, but see if that fixes it. you could
also add some 2.5 mH chokes on the power supply feed to the driver plate and
screen... again, if the original design didn't need them they should be needed now.
But, it would be interesting to see if improving the bypassing and decoupling
improves the situation.

One other to watch for is if someone used a common ground point terminal
for bypass caps associated with the plate and grid tuned circuits. A poor
high resistance ground, or unexpected ground loop currents, can cause
regenerative feedback.

Pete


Good ideas- I'll try the additional electrolytic(s) for decoupling near the driver. For the PA, at least, I think those ground points are pretty well separated. Will take a relook at that. I'll also try an additional L + C section to the B+ feed to the driver, if the electro alone doesn't help.

Oh, one thing I should mention- the original RF chokes were 3 identical 5mH chokes. They are iron core, 3-pie types. Someone replaced the PA RF choke with one of those old, longer choke types, the ones with the ceramic body and massive metal end caps with radial leads. It is obvious from the puddle of wax beneath this choke that it got pretty hot at times. I figure this has to be a 2.5mH choke, as there is no powdered iron core. I may remove that non-standard choke and swap in one 5mH from below chassis. I have plenty of spare chokes, and do have some 5mH to replace what I borrow. I'd like to get that PA area back to being as original as possible. FWIW, as I worked on the radio, every little change I have made in the PA area has seemed to translate into less of a feeling of instability and less ugly waveforms, both in and out of the LF oscillation. The 'snap' noted when turning the Drive Tune has now noticeably diminished (but is still perceptibly there). My gut is still pointing me mainly towards that PA stage for the source of my problems. I'm not sure how easy it would be for an LF oscillation to originate in the driver and pass through the shuntC/ series L network feeding the PA grid. Perhaps not all that difficult. I guess that network must be basically a low pass?

I do think grounds are to be viewed with some suspicion in this radio. Again, too many mods by persons who apparently weren't thinking in terms of good RF design and layout.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Wed 21, 2011 3:32 pm 
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I suggest before you go any further that you put all wiring and components that may have been moved to other locations back to where they originally were in the transmitter. That alone might solve your problem.


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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Wed 21, 2011 8:24 pm 
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Tube Radio wrote:
I suggest before you go any further that you put all wiring and components that may have been moved to other locations back to where they originally were in the transmitter. That alone might solve your problem.


I am seriously considering doing just that. The thing is, the mods were done mainly in the B+ distribution, not quite so much in the RF lineup. My initial approach was to look only for any changes to that two tube 6HF8/6DQ6B RF strip. I found a few things, but not a dreadful amount of meddling, fortunately.

The other area that really stands out is that some previous owner had not only removed and kept the 12AX7 and 6DR7, but had also completely stripped out all the components in the audio section. As it came to me, it had only two bare 9-pin miniature tube sockets and a front panel gain pot. That's all that's left of the modulator. Yikes! But then ultimately all I care about is getting it working on CW on a couple of bands.

It's not as if this radio has any resale value, as it is. Someone either dropped it or threw it at some point- at least once, but evidence suggests twice. The case and chassis show obvious impact damage, which was sufficient to deform the little steel chassis in the transformer area. That end is shaped like a shallow downward "V". The front panel was bent by the impact, and clear drop marks are seen in one place on the transformer, in an upper corner. In the front panel upper left corner, someone straightened it with (looks like) a pair of duckbill pliers. They got it straighter, all right, but this in-artful repair left permanent tool marks in the panel. Ugh! I removed the panel and hammered it out best I could against a flat, hard surface. I got it maybe 80-90 percent better, and managed to not damage any of the silk screening. It's at least fairly flat again. It cleaned up 'OK', but has some sort of clear coat that shows filminess in that corner from the fracture it underwent. The top of the case has rust streaks which may speak to outdoor storage, perhaps in a storage shed, for an extended period. Fortunately, the chassis has not developed rust, only the usual tarnish one would expect in a radio this age. Someone removed the neon panel lamp and took a file to the mounting hole. They must have had a push button mounted there at some point- hand lettered in the area was the legend "Panic Button". Someone really disrespected the little radio.

I plan to neutralize the rust on the case and repaint it, also to clean it up enough to get it running without instability. Seeing the sad condition of this radio broke my heart. That mitigates against my really giving it an all out effort with the rewiring. The little radio will play again, but will always show its war wounds. I'm going to give it a Purple Heart and put it back to work on 40 meters.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Wed 28, 2011 4:13 am 
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It is kind of late here now and I may have missed a fix, but you say that the 5mh choke in the final plate was removed and replaced with a 2.5mh choke.
The low frequency oscillation you see is probably a TPTG (tuned plate-tuned grid) oscillation with the chokes.
The old books said that the plate circuit must be tuned higher than the grid circuit to oscillate. (I am 71, and the books in the library were old when I was in high school).
Drop the Q on the driver choke with a parallel resistor for a test and see if the problem goes away.
If so, then you need a new choke in the plate.

There are people who make new panels. Or you can find someone that can make you a decal to put on a replacement panel.
The modulator may have been removed when someone told mama that junior was outside the novice band on AM.
Happy New Year,
Pat W5THT

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Thu 29, 2011 2:22 am 
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Could it be possible that having the modulator removed and the B+ mods is causing the problem? My thinking is the modulator more than likely drew B+ current all the time and the power supply mods might mean the tubes are no longer getting their proper voltages or for that matter proper separation between the B+ going to the tubes which could cause low frequency oscillations I suppose.


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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Thu 29, 2011 8:23 am 
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Thanks, guys-

A couple of good suggestions. My work has suddenly gotten real busy, and when you add in the holidays, work on the T-60 has slowed way down for now.

Pat, I think you may be onto something. Two issues- I know for a fact that there was a factory mod where the builder was supposed to add a 27K/2W carbon comp resistor in parallel with that driver plate RFC. I know also that my radio presently does not have the mod. So between that and the PA plate choke being changed, I think you may be onto something. Those two points are pretty easy to check, so I'll try to do that in the next day or two. (Q-killing resistor first) I will then swap in one of the original 5mH chokes for the PA plate.

BTW I'm thinking that they could have just pulled those two tubes and hidden them until junior got his General class- LOL!

Tube Radio, I really haven't thought through what difference it would make not having the load of the modulator and mic preamp. I'm going to think about that a little. What I do know so far is that an incorrect value dropping resistor was seen as I went through the radio. As I remember, the value was less than the schematic calls for. I need to hover over the radio with the schematic in hand to work through this- but certainly it would help to restore the correct value resistor.

Happy New Year's, all.

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Last edited by Clutter on Jan Thu 12, 2012 12:11 pm, edited 1 time in total.

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 Post subject: Re: Help Troubleshooting Old Transmitter (T-60)
PostPosted: Dec Thu 29, 2011 4:24 pm 
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Yes restoring the correct value resistor will help and might cure most of your problem.


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