Having solved the original AM problem with this set from my earlier thread, another question has arisen...It is a stereo FM set, and I receive stereo stations OK, but there is a switch on the unit that you push for stereo reception, or leave it off for mono. What happens is--When I switch to stereo mode from mono, the volume drops to half of the mono volume on all stations. I'm not good at reading the more complicated German switching logic in the schematic, and am wondering if someone could help determine what is happening when I switch to stereo in the circuit. It is almost like the power doubles in mono...or maybe that is normal for the set...I have tried to post the schematic Edd used in my earlier thread---If it doesn't work, please refer to the schematic in my previous thread.

http://img5.imageshack.us/img5/7697/mk40.png

I own a copy of this set and out of curiousity, turned it on this morning and fiddled with the mono/stereo pushkey to see how it behaved. In my set, activating FM Stereo (key down) causes the sound level to INCREASE ever so slightly. I repeated this several times, with the same result. The difference is noticeable but not substantial, as it apparently is in your case (in which the effect is reversed between mono and stereo).

It appears to me that the two decoded audio channels are fed to a slide switch that either wires in the Balance pot from each side signal to ground, or shorts the two decoder output channels together (effectively mixing them) before they go to their first stages of audio amplification. That seems to be the only difference. So, main candidates for investigation are the stereo/mono slide switch (dirty contacts will increase resistance and decrease volume) and the Balance pot (was it replaced with a value lower than its original 1 Megohm? If so, it may be excessively loading the decoded audio- and this would only show up in Stereo mode, since it is completely bypassed in Mono mode).

OK CV, glad you have same set to help with this...I pulled the chassis and the 1M balance pot is correct, and working. The signal strength on the eye tube is the same in either stereo or mono button positions. I notice if I engage the Stereo button (down), and I put the unit on AM, the signal (stations) doesn't get thru (AM), no sound..When I release the stereo button (up) I get the AM back...Does yours do that? Please try that on yours when you can. Also---If I touch the volume pot CT, I get a strong hum signal with the Stereo button up or down, showing the problem is before the volume..Hope these 'clues' help.

Yes, I just checked: in any AM mode pressing the Stereo key down will kill the sound on my set, too.

Aside from the lowered volume, when in Stereo mode do both channels come through with equal volume when the Balance control is centered, and have you got good channel separation?

Yep...both channels are equal in volume (reduced) in Stereo mode, and the separation is very good.
I'm wondering if there is something sucking the output in the stereo demod section (ECF80 or ECC808?) With both channels weak, that is a clue somehow. I may try to read some voltages on those tubes with the stereo in and out of circuit.

I think that your problem is AFTER the ECF80 decoder stage and BEFORE the first audio amp (ECC808). Thats where the source (AM/FM/tape/phono) and mono/stereo switching takes place. If the problem isn't the Balance pot it must be in the switch.

You might try spraying some Deoxit into the Stereo linear switch and activating it a few dozen times.

I give the switch cleaning a try. I am trying to verify the components on the multiplex board, but it is difficult seeing the values on the caps.I really could use the schematic info that show parts location and value on the boards, but the only site that has that is Radio Museum, and you have to jump through hoops to join them. Anyway, there is 10mf electrolytic cap shown on Edd's schematic on the cathode of the 'simple' side of the ECF80 that I can't find on the board, which is part of the reason for needing the pics of the layout of the MPX board...