1976 Toshiba Music Centre, looking for help

[Craig Sawyers]

Unless something is physically in the way, you can check the voltages on the ends of the resistors that feed the transistors without taking the board out. The preamp transistors seem to be on the RF block, page 15.

[marty2be2000]

Today I stripped the carcass, so I had access to TR303 and TR304 on the main PCB and measured the voltages for base, emitter and collector on each.

Both TR303 and TR304 measured the same voltage, and they are pretty much identical to the voltages posted by Craig with the emitter and collector being slightly higher.

Vb = 2.2V
Ve = 1.6V
Vc = 15.8V

Does that mean the transistor is working and the fault lies somewhere else in the pre-amp.

I'll leave the top off the unit for now, she is looking rather sad.

This photo shows the preamp area with the wooden chassis removed. I've also included the main PCB

[Craig Sawyers]

Yes - the transistors are OK. Which means that the associated resistors are also OK:

R319, R317, R323, R321, R320, R318, R323, R324.

But I'll tell you what I have just spotted - the screened cables are wire wrapped to pins soldered into the (single-sided) circuit board. The screen of the wire must be solder pigtailed to single core wire, because that is wire wrapped too. Even the cores must be pigtailed - solid core screened cable would be an odd thing indeed in mid 70's Japanese kit.

So possibilities: a ground wire or signal wire has come adrift in the left hand pigtails. Or there is enough time passed that the left hand channel wire wrapping has got flaky. Or there is a break in the board solder joint to the wire wrap pin.

So next thing to check is continuity of the wiring - the red and white screened wires. Try to check from where they connect to the actual component to the bit of the switch bank they go to.

The screen is only connected at the board end, and not at the switch.

Does any of that make sense?

Craig

[marty2be2000]

Ok, so I've unplugged the unit and stuck the DMM onto ohms, and using a common ground point I have gone over all the pins marked E on the board and confirmed continuity to ground, I have followed the wires from those pins to the other side and the result is the same. The grounds for the left and right channel appear to be connected together on the same pin.

I have also confirmed before unplugging that the voltages on TR301 and TR302 are the same, they are also on the preamp circuit but seem to be only active when FM Stereo is selected.

FM Stereo does not work correctly anyway, there is no audio from the right channel. However there is a hum from the right channel if I touch the right channel input into the preamp.

Not worrying about the FM Stereo at the moment, need to see if I can get the preamp going.

[marty2be2000]

These are I think the only caps in the left channel pre-amp for my reference or any comment.

I have measured in circuit the values of C309 (left channel) and C310 (right channel) and the values are the same. Since they are in circuit I cannot get the actual values. I used the capacitance setting on the DMM and ensured the test leads were the correct way round.

I cannot get access to the legs of the others as there is not enough clearance between the capacitor and the PCB.

C309 0.47 mfd 50WV

C311 ceramic 10 PF

C315 1mfd 50WV

C317 1 mfd 50WV

C321 Ceramic 500PF, K

[marty2be2000]

I don't know if this is relevant.

If I plug headphones in, I get a hum on the left channel that is not present on the speakers.

The hum is present regardless of the balance control or volume setting and only present on the left channel, plus maybe its my ears or the headphones but the bass on the left channel appears better with the headphones but the balance control still needs to be well off centre as per the speakers.

[Nuvistor]

My advice would be fix the preamp first.

[Craig Sawyers]

Well yes - I agree with that. But the OP has measured the BCE voltages on the transistors (one per channel) and they are correct. So the two transistors and eight resistors are A-OK. And if the resistors are OK, the gain has to be OK since it is the ratio of collector to emitter resistor (8x)

The only other components in there are 1uF electrolytics on input and output of each transistor. And the wiring.

Craig

[Craig Sawyers]

Originally Posted by marty2be2000

C309 0.47 mfd 50WV

C311 ceramic 10 PF

C315 1mfd 50WV

C317 1 mfd 50WV

C321 Ceramic 500PF, K

C309 should be 1uF 50V, not 0.47u

And you don't need to get access to the leads - measure between components to which they are connected (resistors, wire wrap pins etc)

[Nuvistor]

The coupling caps were suggested in post #2, have they been checked?

[marty2be2000]

I've got replacements on the way for C315 and C317, I am just going to replace them with new Nichicon of the same rating. They should be here tomorrow (RS next day delivery and showing despatched), and I'll have a few spare as the same caps are used elsewhere as well.

Depending on the time the caps get delivered, I hope to replace them tomorrow. The work will not be pretty, but it should function fine and they are not visible once the turntable is reinstalled.

I tested C309 (Left, faulty channel) and C310 (Right, good channel) in circuit with the capacitance setting and they both read the same on the multimeter.

I am going for the electrolytic's first as from what I understand ceramics are ok generally.

I also have a replacement new old stock for the transistor just in case.

[Nuvistor]

Hope it fixes it for you.

[marty2be2000]

is it me or does it look like C317 was the wrong way round. I have confirmed that the transistor emitter is connected to the plus symbol

[marty2be2000]

Well C315 was the one, changed C317 and it made no difference so went onto C315 and normal service resumed, I think.

[Craig Sawyers]

Awesome! Well done.

And good timing - we're off to Heathrow in the next couple of hours to head off to Australia.

Craig

[Nuvistor]

Sit back and enjoy the unit.

[marty2be2000]

Now the unit is all back together, and the capacitor replacement is not nice to look at it all seems to be working fine in the amplifier department.

The FM Stereo problem is not really an issue for me, its primary roll is going to be vinyl and tapes with occasional Radio 4 which is ok in mono anyway.

There is still a small deviation from centre on the balance control, but I'll put that down to mismatched components or even the balance control itself.

It's good the unit is back on song, currently sitting here listening to the Rag N Bone man, Human LP on Vinyl.

[marty2be2000]

And my thoughts of the balance control are sort of confirmed I think. The lineout on the back which is basically the pre-amp output is showing a nicely balanced signal when recording to an external deck with the record level controls equal.

[marty2be2000]

And further thoughts regarding having to use the balance slightly to one side, have discovered when listening with headphones that from zero volume audio, is heard from the right channel a fraction before the left, so maybe a volume control problem. This can stay as it is.

[Nuvistor]

Originally Posted by marty2be2000

This can stay as it is.

That is what I would do, it’s possible that there are other components drifted slightly out but if it is small I would leave it as it is.