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In Reply to: RE: fisher wx multiplex chassis posted by BFitz on June 11, 2018 at 18:25:43
Hi BFitz, thank you for great idea. I have one of those little modulators and will apply it to this job. I did replace that 1 uf capacitor but it had no effect on stereo functionality. Which is to say lack of same. I will replace those mylar caps and the diodes first and see what happens. Does anyone know a modern equivalent for those diodes? Can a generic 1n4004 be used or something like that? I don't mean the bridges, but the ones on the first tube and the one on the output signal to the magic eye at second tube input. --TH
Follow Ups:
You want "signal" diodes. 1N34(A) is still avail if you look. 1N914 is more modern Si repl. Another good repl is 1N5711, a schottky with similar characteristics to 1N34 wrt forward voltage drop.
FWIW, from your descriptions, I'm pretty sure your problems lie in the front end of the decoder where 19KHz is "converted" to 38KHz, not later audio matrix.
Stuff has happened. I put in several capacitors in the first stage and second. Both the diodes are now 1n34a. Nothing past the middle tube has been replaced. The eye now closes all the way but the -6V we are looking for at bottom of doubling diode (that goes to eye) is more like -2.2V. The stereo sound is distorted and reduced in level. If there is any separation, it is poor. The 38k coil is quite tight and resists any and all attempts to free it much less adjust it.
The schematic shows several "TSP" points which I assume are waveforms? I put my scope on these and there is all kinds of wave action. We do have one mono talk station and when I go there, all waveforms from the early part of the MPX board disappear so I take that as a good sign. But does anyone have a link to images of these waves and their desired amplitude? I am at the point I need to know what I am looking at and what might be wrong and where. Thank you.
Did you get "real" 1N34A diodes? A lot od sellers are passing fakes on EBay and elsewhere. You can tell from the voltage drop AT low current (1-2 ma). Most multimeter have a test, but all seem to do it at different currents.
You want TRUE germanium diodes, not anything else, and they should be matched for Voltage drop. There is one other new one that is OK, it's an HP mixer diode (NOS), but it may be sold out by now.
We had a good discussion at FMTuners a while ago on this subject,mostly centered on their use in detectors, but there were also used in tube MPX decoders.
Be very careful with a stuck ferrite core, if it is already broken, it's too late. Most stuck cores ARE. If broken you need a new ferrite core, or new transformer from another unit. Some folks have gotten the pieces out and re-glued them, but that never worked for me. It helps to remove the aluminum outer case to diagnose what is going on.
BFitz this is helpful. The diodes came from a local OTC brick n mortar. They are the old style I recall seeing in pocket radios from the 1960 era. They are mostly clear with orange bands at each end plus the dark band. On diode test mode of my DVM, they showed lower numeric value on the forward conduct path than the original ones. Something like 350 for these new ones vs 1200 on the old ones. Other direction was infinite on both. The cores on all xfms do turn. The 19k ones I have peaked but the volts to the magic eye are only -4, not -6 as desired though it does deflect. One thing I do not understand is where the 17V is supposed to come from on the cathodes of the last 12at7. Right now it is more like 29V. I do understand that twisting coil slugs is not to be done lightly. But this thing has some kind of component failure I am certain. With all original parts, the eye did not close and sound was pure mono equal in amplitude to direct mono path. Now after changing diodes and some caps in front part, the eye closes and the audio has changed A LOT. So it had some bad parts, but still has more bad parts is my conclusion. Rebuilding the entire thing is not a thing this client has money for; nor do I have the stomach for it.
It's possibly an alignment issue.
Found this good read that goes on for many pages regarding troubleshooting Fidher tube MPX sections. The modular ones in stereo tuners are all very similar, mostly the same circuit with minor differences.
On page 2 they show many scope traces of test signals at known points.
http://audiokarma.org/forums/index.php?threads/fisher-mpx-decoder-troubleshoot.516094/
Wow this is sort of what I have been seeking: people who have gone deeply into this thing and come out the other side. It will take a while to sift through all this. Thank you.
...generally no pics but descriptions of what to expect at each TP and signals that cause them. Some will be DC. Not what you want to hear but.....you're at a point where specific instruments / alignment procedures are necessary to get to the bottom of your issues. At minimum you need something that will provide composite output with L+R, L only, R only and switchable 19KHz pilot signal in proper proportion to audio signal. Alignment by ear using off air or FM modulator signal is crap shoot at best. Good luck.
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