Quad FM2 stereo tuner not very stereo

[Synchrodyne]

That looks like the schematic for the original outboard decoder that was intended for use with the original Quad FM.

Basically the same circuit was used for the inboard decoder in the FMII, except that Si PNP transistors were used in placeof the original Ge type, a stereo beacon was added, and the switching arrangements were somewhat different.

Cheers,

[Synchrodyne]

For the Quad FMII at interest, it might be worth checking that the switching system is working as intended. From the schematic, it would appear that in the quiescent state and when receiving a mono signal, switching diodes MR108 and MR109 are biased on, and so allow mono signal from TR100 emitter to pass to both the L and R outputs. Concomitantly MR106 and MR107 are biased off, thus blocking any signals from the bridge from reaching the L and R outputs.

When a stereo signal is received, pilot tone rectifier MR100 and MR101 produces a DC output (as well as a 38 kHz output for synchronizing the oscillator Tr2), and this DC is used to trigger the stereo beacon (via Tr103), to bias MR108 and MR109 off, thus blocking mono signal access to the L and R outputs, and to bias MR106 and MR107 on, thus connecting the bridge to the L and R outputs.

I imagine that any irregularities in this switching network could detract from the “stereocity”, as it were, of the output.

The network formed by RV100, R122 and R123 might be reasonably critical, as well. This compensates for the fact that a switching decoder is being used with an input signal that is not a true switching waveform, but one with truncated harmonics (this gets back to the Zenith side of the GE-Zenith equation). Significant unbalance between R122 and R123, and too much or too little signal from RV100 would affect stereocity.

Cheers,

[barretter]

That is most interesting but I am not sure that I am up to checking the operation of the board. I would probably have to remove the board from the case, in which I might as well buy the One Thing Audio board and install it. I have discovered that it is still available.

[trsomian]

If either of the circuit diagrams above are correct for your unit, then since they are both of the pilot tone doubling type, if any of the tuned circuits has drifted over the years you may improve the separation by carefully adjusting the coils. Obviously this is best done on a suitable test signal, but it may be possible on a standard trasmission. Carefully note how much you adjust things by, so that you can put it back if you need to.

[SteveCG]

Can I revisit the answer in post no.4?

Thanks to Dave kindly posting the circuit diagram for the FM2 in post no. 18, I can see the pin assignment on the DIN socket. Does barretter have a continuity meter to just double-check that the phono flying leads are indeed connected to pins 3 and 5 of the DIN plug?

My reason for asking is that I've come across quite a few DIN to phono leads that use pins 1 and 3 only. Using such a lead on the FM2's output would give the mono output on one channel with only one of the stereo channels on the other - so producing a greatly reduced 'stereo' image.

[barretter]

That was one of the first things I checked.

[SteveCG]

Fine, thank you barretter for the info.

[Kentode]

Also, since I had a look at the specs. the FM2 needs to see a load of at least 100K into the preamp. If this is different to your Troughline that may make a difference in sound quality.

I agree with Syncrodyne in post 22, and would add that all resistors and caps need checking.

You can borrow my Taylor 62a wobbulator and we should be able to crack this nut! Don't give up Dave.

[barretter]

Well, something strange happened. Last night I decided to "adjust" some of the coils basically just by twiddling randomly with a screwdriver to see if anything happened. Nothing seemed to happen so I switched off for the evening. I turned it on again this morning and did my comparison with the Troughline. Still no stereo. However, having left it on all day, I had another listen this evening and it was now stereo, quite nice stereo too, and louder than the Leak, which it hadn't been before. No idea why!

[Sideband]

Originally Posted by barretter

However, having left it on all day, I had another listen this evening and it was now stereo, quite nice stereo too, and louder than the Leak, which it hadn't been before. No idea why!

Classic case of duff electrolytics I would say. Possibly they have reformed somewhat during the day-long run. It would do no harm to change them all for reliability reasons.

[barretter]

I'm not clear how reforming of the electrolytics would turn mono into stereo.

[Kentode]

HI Dave, you have a pm.

As its gone quiet I wonder if the problem has been solved, or at least discovered? Perhaps moving VR100 shifted some oxidation or moving L100/L102 brought up the voltage?

[barretter]

Hi Ken,
I've no idea really. Private message received and replied to. Cheers, Dave