Hi all,
Been working on this receiver for a bit and have reached a blockage point. I'm hopeful for some insight on what's going on - thank you much in advance for your input!
This was a "not turning on" unit when I got it, that I tracked down to the MPX unit shorting the Vcc rail to ground (I could have fried some eggs on the regulator heatsink prior to fixing it). Upon recapping it (and the rest of the lythics - some were leaked), and tinkering a bit more with the connections to the board (such as reflowing the solder, cleaning up the grounds, etc.) it's now:
- working as far as preamp/amp stages go (which I adjusted and they're working very well);
- the tuning meter now deflects as I dial the tuning up and down (which wasn't happening before - neither AM, nor FM, as they were fed from the same clamped rail);
It seems the lingering issue is now still with the MPX module. I don't get audio out, and some voltage readings are enclosed (on top: stereo, bottom: mono, where different). Some obvious trouble seems to inflict Q401, Q402, Q403, though I don't have a good understanding of what's going on there.
I suspected Q401 has some issue (short?) between gate and source, and replaced the 2SK23 with a PN4393 selected for a Vgsoff of 1.17V and Idss of 8.45mA (which I feel is a suitable replacement, but welcome thoughts) but that returns returns the exact same values (and it seems to amplify the input signal to a decent waveform). I believe I also disconnected the input to the MPX and got the same elevated gate voltage, so the issue doesn't seem to be originating upstream. I am increasingly becoming weary that the coils there (MU401, T401) may be damaged. All transistors seem to pass the diode test out of circuit, though that's obviously not always positively relevant (so it very well could be that a certain transistor - such as Q402?... Q403? - is failed). But I just can't see how some failure of Q402/403 can trigger the Q401 gate issue, therefore my conundrum.
I am not sure the stereo bulb is conducting, but I assume R411 is there to prevent a burnt bulb from stopping stereo operation, so I didn't worry too much about it.
Any input much appreciated!
Been working on this receiver for a bit and have reached a blockage point. I'm hopeful for some insight on what's going on - thank you much in advance for your input!
This was a "not turning on" unit when I got it, that I tracked down to the MPX unit shorting the Vcc rail to ground (I could have fried some eggs on the regulator heatsink prior to fixing it). Upon recapping it (and the rest of the lythics - some were leaked), and tinkering a bit more with the connections to the board (such as reflowing the solder, cleaning up the grounds, etc.) it's now:
- working as far as preamp/amp stages go (which I adjusted and they're working very well);
- the tuning meter now deflects as I dial the tuning up and down (which wasn't happening before - neither AM, nor FM, as they were fed from the same clamped rail);
It seems the lingering issue is now still with the MPX module. I don't get audio out, and some voltage readings are enclosed (on top: stereo, bottom: mono, where different). Some obvious trouble seems to inflict Q401, Q402, Q403, though I don't have a good understanding of what's going on there.
I suspected Q401 has some issue (short?) between gate and source, and replaced the 2SK23 with a PN4393 selected for a Vgsoff of 1.17V and Idss of 8.45mA (which I feel is a suitable replacement, but welcome thoughts) but that returns returns the exact same values (and it seems to amplify the input signal to a decent waveform). I believe I also disconnected the input to the MPX and got the same elevated gate voltage, so the issue doesn't seem to be originating upstream. I am increasingly becoming weary that the coils there (MU401, T401) may be damaged. All transistors seem to pass the diode test out of circuit, though that's obviously not always positively relevant (so it very well could be that a certain transistor - such as Q402?... Q403? - is failed). But I just can't see how some failure of Q402/403 can trigger the Q401 gate issue, therefore my conundrum.
I am not sure the stereo bulb is conducting, but I assume R411 is there to prevent a burnt bulb from stopping stereo operation, so I didn't worry too much about it.
Any input much appreciated!
It's a complicated circuit. Q401 and MU401 generate a 38 kHz signal out which drives Q402. It's a resonant circuit that picks up the 19 kHz pilot tone and doubles it. MU401 is adjusted to give the maximum amplitude of 38k signal at C405. Q402 should be switching on and off at a 38 kHz rate in stereo mode. There should be a buffered version of the audio present on the output center tap of T401. I am assuming you have a scope to look at this stuff.
You are showing .25V/0V at collector of Q402. It should be a higher voltage and it should be switching on and off if you look at it on a scope.
You are showing .25V/0V at collector of Q402. It should be a higher voltage and it should be switching on and off if you look at it on a scope.
msubully82 - thank you very much for thinking on this with me. I've currently parked the project for a while - there's only so much room on my bench, and so many projects!... - but will get back to it and hopefully will add more data and measurements on here to get this up and running.
Hopefully soon! Thanks again for your thoughts.
Hopefully soon! Thanks again for your thoughts.
It's been a while, but just got some time now to catch up with this project.
msubully82 - (see note below on levels in) there's distinctive switching stereo/mono, though not so much with Q402, much more measurable with Q401. I enclose a new voltage chart, newest readings (I believe the main difference is stronger signal into the tuner from FM generator) in red.
A few hopefully useful notes:
msubully82 - (see note below on levels in) there's distinctive switching stereo/mono, though not so much with Q402, much more measurable with Q401. I enclose a new voltage chart, newest readings (I believe the main difference is stronger signal into the tuner from FM generator) in red.
A few hopefully useful notes:
- I believe I successfully replaced Q401 (2SK23) with a PN4393 selected for a Vgsoff of 1.17V and Idss of 8.45mA. PN4393 I have a small supply of, but I think it can still be relatively easily sourced, while obviously the 2SK23 is not. This was suspect by some measurements, and taking it out to further check it triggered its loss of a leg. But I guess for the common good and for science, knowing the above replacement should do OK is a good thing.
- it's a very sensitive stage to incoming signal level. Though may be in good part misalignment, I could only get the MPX to switch stereo/mono with levels as high as 700mV in. I haven't mapped all the possible levels and exactly where the threshold is, but earlier tests at lower levels didn't trigger stereo/mono switching and threw me off for a while.
- Ultimately, it seems (due to no collector DC biasing of Q402), that all signs were concurring to T401 having open windings. Once taken out and measured with an ohmmeter, only one section presented resistance (3.4ohm over bottom/left as you look at the schematic), all others were open. How's this possible, I am as puzzled as any one. I may have to try to open the can and see how could all those windings be open, and maybe give a shot at reconnecting them. I read multiple testimonies of similar occurrences, and the only explanation that stands to (my) reason is maybe a mechanical manufacturing (maybe a particular run?) issue/defect. Something consistently triggered way down the line by repeated mechanical action such as vibrations, shipping, etc.
Attachments
Alright, everyone, good news here. It's fixed! 🙂
Further notes:
Further notes:
- I was able to find a replacement module and transplanted T401 (which, btw, had continuity on all windings). Before plugging it in, I made extra sure everything else is working. Further notes on this below.
- KSC1845 is a perfect replacement for 2SC634 (Q402). It didn't need replacing, but although I got reasonable measurements from the 2SC634 out of circuit, I wanted to make sure another, known good transistor would eliminate the possibility of the 2SC634 failing in some way under load.
- I didn't realize this, but the board is "littered" with tantalum caps (C410, C424, C425, etc.). They should be closely examined - tantalum caps have funny ways of failing, some particularly spectacular, others completely inconspicuous - not much in the middle, but obviously the latter case is more troublesome as it may slip examination. I replaced the ones just mentioned.
- Replacing T401 restored full operation.
- For those wanting a very original look for this, I've found a 4V/40mA "grain of wheat" lamp with wire leads that works great.
On to alignment now!
jkvw - it's been a while, but I believe the ones I used are these: https://www.elliottelectronicsupply.com/51490.html. Took a minute to find them back then, but hopefully this helps you and others down the line.