1976 Toshiba Music Centre, looking for help

[marty2be2000]

Thinking about the FM Stereo problem, I did notice that when listening to the FM Stereo input through headphones that if I use the balance control and move it almost all the way to one to the right side to almost mute the left channel there is a very low volume output of the right channel.

it is clearly the right channel as you can here the stereo effect between left/right, its just you need to have the volume almost on maximum.

I have confirmed that there is a good connection between the selector switch and the pre-amp, as if I press my fingers on the selector either the input our output pins for FM Stereo then I can induce a loud hum from the right dead channel.

So the issue with FM Stereo right channel is somewhere before the selector switch. it is almost like there is no gain into the switch on that channel.

[Nuvistor]

Well you have had one duff electrolytic, check C214 is the right channel, C215 left, may get lucky.

[stevehertz]

As has been said, clean those switches - all of them - using contact cleaner. Then with the set off, operate the switches a few dozen times each, or more. This may seem like a somewhat un-scientific approach, but believe me, hifi equipment from the 70s can be crammed with faults as a result of oxidised switches. At least, if you do that, you can more or less eliminate them from the problem. My current main interest is hifi receivers from the 70s, and without any exaggeration, each and every one I get has 'faulty' switches due to oxidisation, and that can lead to all manner of effects.

[marty2be2000]

Originally Posted by Nuvistor

Well you have had one duff electrolytic, check C214 is the right channel, C215 left, may get lucky.

Thank you for the pointer, got around to looking at it today and changed C214 and it fixed the problem.

[Nuvistor]

Good, hopefully all sorted out for a while.

[marty2be2000]

Perhaps I should say almost fixed the problem.

I now have the following observations with the tuner in FM Stereo mode.

1) FM Stereo light comes on when tuned to stereo station and audio can be heard from both left and right channel. But, its difficult to get the station to lock in and when you do speech is very sibilant. As you manually tune, the left channel kind of just appears but balance is good between left and right channel (Stereo station)

2) When tuned to a mono station (in FM Stereo mode), or static the right channel is louder than the left.

This is where I may have been a stupid person a while back, I may have tweaked something in the alignment as an attempt to fix the previous now fixed fault. They would have been only small tweaks, and when nothing changed the tweaks were reversed (but not to the original positions it seems). Back then I did not understand what alignment was.

I am guessing the FM tuner is slightly out of alignment, it tunes into stations fine so can I do anything without the correct tools to improve things. Does not have to be accurate, just better.

[Nuvistor]

I cannot check the circuit until next week, hope fully someone will have some ideas. It is still possible that you still have faults that require fixing.
Alignment is a possibility especially if you have moved the cores.

One of the first things “drilled” into me at 15 years of age starting in the trade was don’t touch the alignment unless you know it’s wrong. Wise advise.

[stevehertz]

Originally Posted by Nuvistor

One of the first things “drilled” into me at 15 years of age starting in the trade was don’t touch the alignment unless you know it’s wrong. Wise advise.

I largely agree, but that's assuming it was performed right in the first place - not always the case if you're looking for absolute optimum alignment. For those with the tools and the knowledge, very often an improvement to the alignment can be achieved, especially given the age of equipment we are talking about, where on top of factory setup inaccuracies, there's also component value drift to contend with. But in general, leave well alone if you don't have the tools or the knowledge.

[marty2be2000]

Originally Posted by Nuvistor

I cannot check the circuit until next week, hope fully someone will have some ideas. It is still possible that you still have faults that require fixing.
Alignment is a possibility especially if you have moved the cores.

Remembering back in time the control I messed with, its down in the service manual as being SVR201 which is the separation adjustment.

It was the right channel that was previously almost dead in the stereo tuner mode, and the service manual says.

1000HZ modulation frequency
Stereo R with Pilot for modulation mode
90% (67.5 kHz), 10% Pilot (7.5kHz) for modulation level
Output indication is an oscilloscope across the Left channel speaker terminals
Adjustment is a 5k ohm resistor semi fixed resistor (SVR201) which is adjusted for minimum.

So SVR201 may not be set correctly, I wonder what the symptoms of having the separation set incorrectly are. When I get some spare time, I may see what happens if I tweak that control slightly as it may only be a tiny bit out.

[marty2be2000]

I had a look at the FM Stereo quality problem over the bank holiday.

Using headphones, some stations did not pop on the stereo indicator and those that did had terrible sound and there was a weird noise on the right channel.

Anyway, I had a go at sorting this using nothing more than my ears.

Tuned into Classic FM, (one of the stations that did not pop on the stereo light). I then tweaked the 19khz control on the tuner, and bingo the stereo light came on. I continued tweaking until the sound on the left channel was ok. I then moved onto the 38khz control and managed to get the right channel sounding pretty good.

The sibilant speech problem is gone, and the weird sound on the right channel is also gone and tuning seems to be improved.

I am sure it could be better, but its way better than it was.

[stevehertz]

Well done. A tip when adjusting those pots without monitoring values or waveforms is to adjust the pot and leave it in the centre of the range in which the set functions as expected. However! this 'centering solution' is not always correct as in some cases the pot is best set to the point at which the desired effect is achieved. Sorry I can't be more specific, but hopefully that helps for future reference.