Quad FM + multiplex decoder not working

[willyvanwin]

Hello,
have refurbished a complete set: 2 * Quad II, a 22 preamp, FM and AM tuner.
all is working perfect. I installed a seperately buyed multiplex decoder for the FM tuner and placed the MPX switch to red in the FM tuner. Stereo decoder is not working: no output at all. On mono all is ok. DC voltages are ok.
Buyed another decoder which also did not working. I changed to silicon transistor and some other modifications from Keith Snook ( http://www.keith-snook.info/quad-fm1...o-decoder.html ). But even this is not working.
I have no stations at all in stereo ...
Have anybody some ideas

[Nuvistor]

When switched to MPX, is there any output from the tuner, check with an oscilloscope?

[willyvanwin]

When using FM tuner without decoder, there is output in both positions of the swith.
On red the signal is a litlle bit louder and distorted, which is normal.
But i will check later this evening with the decoder connected and a scope.

[Nuvistor]

Appears that there is an output, it was just the first thing that came to mind, if there was no MPX output stereo wouldn’t work.
The decoder could be stopping the signal but you have tried two different ones.

[brightsparkey]

Is it pulling down the HT or something like that - check the supplies when the decoder is in. I guess its possible that a connector is incorrectly wired and that no-one ever noticed since it never had a decoder in before...

Kevin.

[TonyDuell]

Check that you have the yellow and blue 4-pin power plugs from the decoder correctly connected to the preamp (yellow in the top socket, blue in the bottom, the middle socket should be empty). Then check (on stereo) that you have HT from both of the plugs. From the circuit diagram of the decoder, it appears that the yellow (left hand/mono channel) plug powers the tuner and the decoder, HT from the blue plug switches it into stereo mode.

[willyvanwin]

Indeed, thats why i tried the second decoder and even modified it to the adapted schematic from Keith Snook. I first thought maybe somebodey had messed up the alignment in the decoder, but the same problem in the two decoders ? Maybe signal strength...
Have to look on the spec's. The valves in the FM are ok in my AVO.

[willyvanwin]

Blue is connected to the blue connector on the 22 and yellow to the yellow. The middle socket is connected to the AM tuner. If the FM is connected directly to the 22, all is working well ( mono ). If the FM is connected to the decoder and decoder to the 22 I have no output from the 22, even in mono ( both switches in ). I think with the deoder connected even mono signals must be working. There is 330V on yellow in the decoder and when switching from mono to stereo on the 22 i have also 330V on blue. Both resistors after 330V are ok.

[TonyDuell]

With the decoder connected are you getting no stereo output (i.e. you still hear something, but it's always in mono) or no output at all?

If no output at all, I'd start by unplugging the blue power plug (used essentially as a stereo swtich signal). Then check (signal tracer or 'scope) you have audio from the tuner going into the decoder. Check it's on the emitter of TR1 (which I think acts, in part, as an emitter follower to buffer the mono/sum signal). Then check where it gets to through the switching diodes

[willyvanwin]

Will do the check this evening.
Also wondered if the modified schematics have a fault...
looks like C22 is connected wrong.
see second schematic in attachment

[Nuvistor]

C22 looks ok to me.

[ms660]

Looks like wrong polarity to me.

Lawrence.

[Nuvistor]

I thought it had +20v on it when in stereo, but admit I could be wrong. I am full of a head cold so perhaps not thinking straight, I give the thread a miss tonight.