Multiplex Board Issue

[jlovda]

My issue is with a KLH Eighteen but I am asking this in a generic fashion.

In this tuner the mono output comes out after a single gain stage. The audio is loud and clear. The stereo output is low in level and somewhat distorted but is equal in level on both sides. There are quite a few coils in the circuit. According to the schematic, some are series resonant notch filters and some are parallel resonant bandpass filters. I assume these are 19khz and 38khz. The stereo lamp seems to operate normally. What is more likely, bad transistor(s) or coils that have shifted in value? Quite frankly, I have no test equipment to do any alignment. It's a cute little unit but I don't know what to do with it.

 

[39cross]

You could add a bad e-cap to your list of suspects. If it was mine, I'd recap it first.

 

[txturbo]

Sounds like the MPX section is out of alignment...

When listening in Stereo do you have separation?

This is a trouble shooting tree from the Sencore SG-80 manual...

 

[rcs16]

The ckt basically uses a BP filter to separate out the 19KHz pilot and doubles it/filters to create the 38KHz switching frequency. Thus the filters need to be adjusted or peaked and phase aligned. You need a FM composite signal with a 19KHz pilot modulating it. You might be able to use a strong over the air stereo signal to make these adjustments. In some case you need to independently modulate L & R in order to adjust the L/R separation adjustment(s) if they exist.

 

[jlovda]

I should have posted this earlier. Here is the multiplex section. The mono output comes off the emitter of the second transistor. It isn't in the picture but it comes off the top transistor, Q704. The stereo output comes off R737 and R740. This tuner uses an NE-2 as the stereo lamp. It looks like the drive for this lamp is a resonant class C amplifier (?) to get the high voltage. I cannot determine the frequencies of the various LC filters because the inductance of the coils is not given.

 

[rcs16]

Old school double balanced modulator. peaking coils and transformers.
Hopefully it is an adjustment vs a bad component, it could be either or both.
R727 is the separation adjustment.
L706/7 are the sub carrier rejection filters.
Look at the adj procedure for an equivalent design of the era, early 70's ie. Pioneer TX-900

 

[chazix]

If both stereo channels are low compared to mono, it seems to me that C711 should be under heavy suspicion. It couples the baseband signal into the demultiplexing circuit.

In re the resonant circuits, the ones with L702 and 704 are 19KHz, the ones with L703 and 705 are 38KHz, and the one with L701 is probably 67KHz (but I'm not as sure, and it probably matters way less. Do many broadcasters still use old-school SCA subcarriers?)

Oh, yes, L706 and 707 - probably for a subcarrier (probably 67KHz) as rcs16 says, though I suppose they could also be meant as additional traps to get rid of any residual 19KHz in the demultiplexed outputs.

HTH,

chazix

 

[jlovda]

chazix:

Thanks for the suggestions. Since C711 will get replaced, I will change all the electrolytic caps on the board. I'll report back in a few days.

John