Stereo to Mono Output Problem

After using my perfectly rehabbed unit for a few months, I noticed that sometimes the stereo signal was off and the output was in mono.  I would turn off the stereo, turn it back on and most times it would start working in stereo again.

However, after a few more weeks, this was no longer working to get the output to be in stereo.  Now, when the tubes are warming up, I can hear it trying to put out a stereo signal (the stereo light is lit), but after about 10 seconds, the stereo light goes dark and a mono output comes through loud and clear.

I'm looking for some directions on what could be wrong before I take everything apart in a few weeks when I have the stereo moved back home.

Thanks in advance for any guidance!

craig.

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So I finally took the radio home to do some testing.  I checked a number of the tubes, including the main ECLL800, with my voltmeter and they were performing quite well.  When the stereo auto decides to output the mono sound, it sounds great.  But when the stereo sound is played at the beginning, it's crackly and faded before switching to the mono sound.  

Is it the decoder that is powering the decision to play mono or stereo?  I have recapped the entire radio and am at a loss for what the problem could be.

Any help is appreciated!

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...Yes, the decoder is what makes the 'decision' to play FM in mono or stereo, and that decision is based on the strength of the signal entering the decoder.  A number of things can affect this:

--the location of the receiver (what kind/construction of building, etc.)

--the antenna you're using (you may get better results with a better antenna than the built-in dipole)

--overall alignment of the FM section

...and even possible misalignment or a component problem in the decoder itself.  The stereo mode sounds the way it does because the decoder is having difficulty processing the stereo signal for whatever reason. If you have good strong FM reception (sounds like you do if you have clear mono), I'd say we can eliminate the first three things and say something is not quite right with the decoder.  Given the relatively late age of the set, it's probably a transistorized decoder; perhaps one of the transistors is on its way out or perhaps there are capacitors that need attention (especially electrolytics)...

...oops. I stand corrected.  The stereo decoder is a separate unit ('Stereo-Automatic-Decoder 6', link included in the model page), not included in the unit schematic, and it is not transistorized.  It is a small metal box with a cable that plugs into a socket on the main board.  It has two tubes of its own (ECC81 and EC92).  I'd start by checking the ECC81 (aka 12AT7) tube; it may be getting weak (the EC92 is only used for controlling a relay that operates the stereo indicator and switches the output of the decoder from stereo to mono; you probably hear it clicking when the stereo indicator goes on or off, so it seems to be working OK).  This decoder has only one electrolytic capacitor and no paper capacitors, so recapping it probably won't help...  

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Hi Todd,

Thanks for the great write up on what the potential problems are.  You were able to correctly diagnose that the ECC81 tube was bad.  My guess is that since I had not changed out that (C12) particular electrolytic cap, since it wasn't on the standard schematics, it might have caused the tube to fail.  

I changed out the electrolytic first, retaining the existing tubes.  Same problem.  I then bought replacement tubes for both ECC81 and EC92 and now the stereo quickly comes up with a great sound, but doesn't seem to really even try to go with the stereo mode.  I pushed the button to try both stereo and mono and there is no difference in the sound.  

Seems that there may be a different problem, but the sound even in mono is really good.  Any other thoughts?

cheers,

craig.

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I'm still experiencing the stereo output problem.  The latest is that when I turn it on with the stereo button pressed in, the light for the stereo indicator goes on for about 4 seconds, then turns off as the stereo warms up.  I still get great sound in mono but am stumped if the issue is still with the decoder or not.

Any help is appreciated!

craig.

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So I've been working with a friend who has a EE degree and he's been helping me to narrow down the problem on this.  It's definitely the decoder 6u that has the problem since the mono sound is fantastic.  

Last night, we checked all the diodes, resistors and capacitors.  We then hard wired the output to be in stereo, no matter what the EC92 tube decided.

The sound was crackly with no music until I turned up the volume nearly to the top end of the dial.  Then I could hear the music faintly in the background and it came out of both speakers so my thoughts are that the signal splitting into stereo is working, but there's no amplification of the sound.  It's even in terms of the amplification doesn't work on either.

So my question is twofold:

1. Would you agree with my assumptions on what is wrong?

2. If true, I assume that the ampification transformer on the 6u decoder is bad.  How does one test whether or not the transformer is working properly?

3. If I can identify the bad transformer, can it be repaired or replaced?  How to identify a suitable replacement?

Thank you for any guidance!

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...Do you have the schematic for the decoder?  It will help you follow the following description.

...First of all, I was mistaken way back in my first reply.  Looking at the schematic, I see there are at least six paper capacitors in that decoder.  They should almost automatically be considered suspect, and a couple of them are used in critical signal coupling applications and could be causing inadequate signal levels in a couple of areas.

...This decoder has four tunable coils/transformers.  Like the other tunable coils and transformers in a radio, they need to be correctly tuned (aligned) in order for the decoder to work properly.  They don't amplify (the tubes do that), but their adjustment is critical.  I've added a little precautionary information about them below.

...How the decoder works:  The composite multiplex FM stereo signal consists of several subsignals: an L+R (mono) signal at +0-18kHz, a low-amplitude 'pilot' signal at +19kHz, and a pair of L-R signals (also 18kHz wide) at either side of +38kHz.  (Plus an SCA signal at +67kHz, but that is not relevant, and there is usually a circuit in the decoder to filter it out.)  In this decoder, if the 19kHz pilot signal (actually the 38kHz carrier that is derived from it; explained below) isn't present, the L+R signal gets passed through to both channels as FM mono (since L+R IS mono) because the mono/stereo switching relay, controlled by the EC92, is not energized.  However, if the pilot signal IS present, it gets amplified by one section of the ECC81 and fed to the primary winding of one transformer (9235-501).  The secondary winding is center-tapped and fed to a pair of diodes, creating a frequency doubler with a 38kHz output.  This forms a 38kHz carrier that allows the recovery of the two L-R signals.  This is amplified by the other section of the ECC81 and fed to another transformer (9235-502) where the carrier is tuned in the primary winding and the L+R and L-R signals are mixed in the secondary.  They are added and subtracted by the matrix of four AA110 diodes that follows the transformer so the following results:

(L+R)+(L-R)=2L (left channel)

(L+R)-(L-R)=(L+R)+(-L+R)=2R (right channel)

...thus the two stereo channels are recovered.

...A portion of this internally-derived 38kHz carrier is also rectified and filtered to create a turn-on voltage for the EC92 relay driver, which energizes the relay, switching the signal from mono to stereo and lighting the stereo indicator.  The Mono/Stereo switch on the radio front panel grounds this voltage, disabling the EC92, when placed in Mono.

...When you 'hardwired' the relay for 'always in stereo', you're not getting any sound because the process I described above is not taking place, so you don't have your 2L and 2R signals and you've disabled the mono bypass path.  You need a signal generator (an audio oscillator will work, set to 19kHz) and an oscilloscope to troubleshoot and/or adjust the decoder: 

...IMPORTANT: Any adjustments you make to those four tunable coils/transformers you should make carefully.  One thing German radios were notorious for was tuning slugs that crack or break easily.  Also, these coils are 'unobtainium' now if they get damaged.  Rewinding them would be difficult, since they operate at low frequencies and would require a lot of turns.  You can (should!) check them for open windings before you start.

...Connect the generator to the composite input (pins 3 and 4 of the 10-pin plug or the junction of R11 and R12.  It would be better if you could actually disconnect pin 3 temporarily.)  Use a low signal level (but still sufficient to see it on the scope) and probe pins 2 and 1 of the ECC81.  You should see a stronger signal on pin 1.  Then probe the anode of either of the AA110 diodes and see how much signal you have, and try adjusting the transformer for maximum signal there.  Now, when you go to the junction of the two AA110 cathodes, you should now see twice the frequency there (without making any changes to the oscilloscope settings, you should see a lot more peaks a lot closer together).  Now, probe pin 7 and pin 6 of the ECC81.  You should again see a stronger signal on pin 6 than on pin 7.  Also, if you see a significant reduction of signal from the AA110 cathodes to pin 7, suspect a bad C14 (10nF=0.01µF); the schematic shows that being a paper capacitor which should probably be replaced anyway.  If that all checks out, go to pin 6 of the EC92 with a voltmeter and measure it, then see if adjusting the 9235-502 transformer makes a difference in the voltage (adjust for maximum).  If you have all that peaked, try reconnecting pin 3 of the 10-pin plug and see if you now have better sound; also try adjusting the 9235-504 transformer and see if that makes an improvement. 

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