1976 Toshiba Music Centre, looking for help

[marty2be2000]

The system having trouble is my 1976 Toshiba SM-3100, when I got it the system was non functional mechanically (Both the record deck and cassette deck) and its electronic state was unknown.

I've sorted all the mechanical issues, the record player works again as does the tape deck.

I am however left with the following issues.

On FM Stereo, I only get audio from the left channel. The right channel has audio but its pretty quiet. FM Mono has sound from both speakers, and from what I can tell is both channels merged together. The service manual I think shows both using different electronics.

Sound balance from the onboard sources (Tuner, Turntable and cassette deck) is lacking bass and balance from the left channel. You can compensate for the lack of volume by shifting the balance knob to one side, but bass on the other channel is still stronger. However when using the Tape monitor (Aux) input the sound is perfect on both channels. I suspect the pre-amp but don't know where to start looking.

I have the service manual (B&W photocopy) in electronic form, electronic diagnosis to component level is not really my thing. I did electronics at college and understand the basics, circuit diagrams I struggle with. It looks like all the components are original, none of the capacitors show signs of leakage but that of course does not mean they are not faulty.

Looking for advice really on what to look at next. I can upload the manual as a PDF if the forum will allow.

[Nuvistor]

Check the audio coupling capacitors in the preamp, they are probably electrolytic and one may be low capacity in the left hand channel. You should also do voltage checks on the preamp stages.
I cannot see a manual in the service manual section of the forum, can you post a circuit of the preamp and the stereo decoder or the full circuit if possible.

[marty2be2000]

I have uploaded the service manual to my google drive, the link is below.

https://drive.google.com/file/d/1aXm...ew?usp=sharing

I tried to upload to the forum but it is just over the size limit.

[Nuvistor]

For the lack of bass on the Tuner, turntable and cassette check the circuit around TR 303 and 304, one is left channel, one right, not sure which, also make sure the switches in that circuit are clean and working correctly.

The aux channel is fed into the player after these transistors.

[Nuvistor]

Is there a stereo mono switch for FM or is it automatic? I cannot see one but that’s probably me.

You could feed a test signal in to IF Block outputs for FM, F is right channel, G is Left and check if both perform the same. Fix the pre amp first though. Again confirm the switches are clean for this area.

[marty2be2000]

Thanks for the pointers so far, the system has separate function selectors for the Tuner.

There are selector buttons, identified as LW, MW, FM and FM Stereo, then the Phone, Tape and Aux input.

When you are in FM mode, it's mono but the Stereo light comes on.

I have a video on youtube which I uploaded a few weeks ago, where I am up to.

https://www.youtube.com/watch?v=4o6IcHcPmQI

[Nuvistor]

When tuned into a weak stereo station does switching to FM mono reduce the background noise or just put the audio through both channels, I am just trying to sort out the switching?

We sold these units, must be 45 years ago, they didn’t give problems then so I have no known faults to fall back on.

[marty2be2000]

Thanks again Frank

This morning I've once again removed the tape deck and turntable and started to look around, tracing components back from the main amp which does identify the left and right channels.

I've established that the left channel has these capacitors, maybe more in the pre-amp. I may have missed some from the circuit diagram.
C309, C311, C314, C315, C317, C321


I've uploaded to my google drive the service manual for the SM-3150 which is basically the same unit but with a different turntable and cassette deck. This scan has a much easier to read main PCB diagram.
https://drive.google.com/file/d/1CO7...ew?usp=sharing

C314 has leaked, why I did not spot that before or what it does as I cannot see one for the right channel.

Apart from sorting C314, if you had to take a stab which would be the most likely culprit.


Another thing I am not sure of, is how i am going to deal with replacing these, the board looks impossible to remove without a lot of work and restringing of the analogue tuning dial. I was thinking of just cutting the exposed legs and soldering the replacement caps to the remains of the old legs. I'm not worried about cosmetics, just function.

As far as FM goes you get hiss on both stereo and mono.

There is a diagram of the radio circuit which shows to IC's.

[Nuvistor]

C314 is the voltage supply decoupler for both channels in the preamp, you will need to clean up the leakage. That leakage could have caused damage
The easiest way to fault find is either to signal trace with an oscilloscope or inject a signal at various points in the signal chain. That way you can determine where the signal is lost. You can also check the voltages on the preamp. No point guessing, a simple logical approach using either technique will find the faulty stage. Remember to not overload the stage when injecting a signal.

The Stereo FM, glad you said the hiss is there in with the stereo switch in either position, that makes sense.
Once you have the preamp sorted out inject a signal into points F and G on the IF block, these are the stereo outputs of the decoder, if you get the same level from each channel the fault is in the IF block, if one channel is down, check the components from F or G to the switch, hopefully find the fault there. It’s possible that the IF block could pull the signal down and confuse but worry about that later, see what the tests show.

If you break it down it’s a simple circuit, just looks a lot, logical signal tracing and voltage checks should reveal the problem.

Access, yes they were not really designed for easy servicing, as I said they worked for many years and a long service life but perhaps not 40 odd years.

[marty2be2000]

Hi again, been doing lots of probing about with an input signal.

If I connect the probe to one side of R329 then the sound is quieter than if I connect the probe to the other side. There is a small ceramic capacitor in parallel with the resistor, should I be looking at that capacitor?

R329 is a 47K resistor.
C321 is a ceramic 500PF, K capacitor.

On the circuit diagram they are the last two components before the tape monitor switch.

I got bad sound from both sides of C315 before.

I've attached a picture of the area that I've been working on.

[Nuvistor]

Use the good channel as a reference, it should be easy to tell if the same point in each channel gives the same result.
You can expect a reduction in level through R329/C321 it depends on how much, a bad C321 will give loss of treble not bass.

[marty2be2000]

I am getting completely puzzled.

The sound at R329 sounds perfectly fine, if I connect to one side its quieter than the other. I've got the other channel plugged into aux connection and I am switching between them using a mono audio source.

On the + side of C315 the sound is quiet, on the - side its louder.

if I connect my audio source to pin 29 on the board that has the volume controls the audio is fantastic.

I Have the input selector on the onboard cassette module.

I am stopping for this evening, poor light is now not helping.

I'll return to this in two weeks when I am next here and able to look again.

[marty2be2000]

Sorry for not posting much, I have still not got around to fixing the SM-3100.

However I have been using it on and off over the last few months, but between the last few uses the sound from the left channel has taken a turn for the worse. The balance control used to only need to be slightly left to maintain level sound across both speakers. Now however it's 3/4 of the way round, so the caps or something else in the pre-amp are having problems.

The issue is still only present when using the onboard tuner, turntable or cassette deck. With an external source connected to aux-in the balance control can be returned to centre and you get perfect sound thus ruling out the main amplifier.

I need to get the pre-amp recapped, I was going to do it myself, but the work involved and the risk to mess it up have put me off. So does anyone know any good repair places in either Hertfordshire or Dorset who would have the knowhow to tackle the job. The reason for not wanting to do this is the tuning capacitor is on the same board, and the stringing and some link wires are my big concern.

[Nuvistor]

You can post for help in the repair wanted section, don’t ask for capacitor replacement, ask for it to be repaired.
Capacitors are a very possible reason for the fault but proper fault finding would confirm what is wrong.
https://www.vintage-radio.net/forum/...play.php?f=145

[ben]

Re: post 8- That does not look like leakage to me. It is the old brown glue manufacturers used to use. Unless it is across the print side of the pcb it is unlikely to affect anything.

[Studio263]

Step 1: leave the capacitors alone, it won't be those in Japanese equipment of this era. The new ones you can buy are unlikely to be as good as the ones which are already in there.

Step 2: simple things first, have you checked the selector switches themselves? A dose of switch cleaner would be a good start. When I've had this sort of trouble with Toshiba equipment this has always been the cause. After cleaning, it would be wise to check the contacts in the suspect area with a meter to make sure.

Step 3: The real enemy in Japanese sets of this period is the small transistors, they get noisy or suffer with low gain. An easy way to check is to swap each one with it's counterpart in the other channel if you don't have spares on hand. For the pre-amplifier stages, BC549B replaces all the 2SC / 2SD... types, BC559 replaces 2SA and 2SB... Be careful though as the pinning is different; looking with the numbers towards you the Japanese ones go:

B-C-E

Where as the BC types mentioned go:

C-B-E

Be methodical and it should be easy.

[marty2be2000]

Coming back to my unit, and the fact it's now easter with some time to look at this again.

I've been playing around a little, and have re cleaned the main controls and sprayed generous amounts of contact cleaner in the selector switches and the results remain unchanged.

Thinking about the previous post, maybe transistors are the problem.

The problem is definitely in the pre-amp before the tape loop on the back. I've got a second cassette deck on the tape loop, and when listening to this deck you place the balance back on centre and all is good.

Current symptoms.

1) When using the onboard tape deck, turntable and the tuner in FM Mono the left channel is lacking in bass, and has less output than the right channel. With the balance control 45 degrees to one side the sound output is level but not much bass on the left. When attempting to record to cassette, either onboard or the tape loop then the record balance is way off, the left channel is as per the symptoms above. The cassette deck are therefore recording the output from the pre-amp.

The main audio controls Bass, Treble, Balance and volume act on the main amplifier and work as expected.

2) When using FM Stereo one channel has almost zero audio.

I've worked a temporary solution for number 1, I fired up a vintage Realistic 10 band (5 per channel) equaliser and attached that to the tape loop and used that to increase the bass frequencies on the left channel and drop them slightly on the right. I now have audio that at least sounds ok'ish, albeit with the balance control at 45 degrees.

[Craig Sawyers]

Looking at the schematic, I think it has already been discussed that the "pre-amp" is just a single common emitter amplifier on each channel - TR303 and TR304.

Now it *could* be a transistor problem, but that would show up as the DC conditions being screwy. Now the voltage test values on the schematic are impossible to see, and could be worked out from the resistor values - but the main thing to test is that the base, emitter and collector voltages for both transistors are the same. They should be within around 10% of each other.

Having said that, Toshiba transistors have historically been known for reliability, they are in an unstressed part of the circuit, and I would guess that they are probably OK (but worth confirming).

Ah - just spotted you can make out the base voltage is 2.33V. That would make the emitter voltage 1.68V. That would make the emitter current almost exactly 2mA (which makes a lot of sense) and hence the collector voltage. Working back from the base resistors the voltage rail looks like 24V, which would make the collector voltage 11.7V. I *think* I can just about make out 11.4V - so everything is consistent.

So check both channels on the preamp transistors - Vb = 2.33V, Ve = 1.68V, Vc = 11.7V. All within about 10%. You should be able to measure those voltages on the resistors without having to mess with the transistor legs.

If they are indeed similar, then the only other components that can cause the symptoms you describe* are the input and output caps - all 1u 50V. If one of those has lost capacitance or gone electrically leaky that would explain everything.

Craig

* symptoms - left channel very low level and lacking bass for every internal source. Using external tape source, sound is perfectly OK.

[Craig Sawyers]

OK the voltage rail is determined by the zener in the power supply D003, which is 30V. Subtract the base emitter of TR001 to get a rail voltage of 29.3V. The schematic says 31V, which does not really make sense - but let's swing with 31V, since that is what Tosh says.

Some of that is dumped in R328, an 820 ohm resistor which with C314, 220u makes a filter that cuts at 1Hz - so reduces any power supply noise remaining after the series regulator TR001. The final rail voltage for the pre-amp is 31 - 4(mA)x820 = about 28V.

Plugging those numbers into spice (because I'm too lazy to do the detailed sums) gives

Vb = 2.2V
Ve = 1.5V
Vc = 15.5V

See what I mean by voltages needing to have a wide tolerance of acceptability? And the operating point it is very dependent on the current gain of the transistor, which for the 2SC1327 is very wide indeed (180 to 1040).

Regardless of precise operating point, the voltage gain is closely approximated by the collector resistor over the emitter resistor, or broadly speaking 8x.

Craig

[marty2be2000]

If only access was not such a problem, I would swap everything in the left channel preamp.

I've got a DMM, and will check the voltages on the three pins of TR303 which will require removal of the whole casing to access due its location.