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Early ‘70s Sony TA-1140/ST-1150

Japanese numbers are a real pain as they are never reused by a second source, like the US and European numbers.

I try to suggest parts in RS. Cricklewood are also trustworthy, but never buy from eBay.
I have not found a datasheet for the 2SC632

Please examine you actual PCB very carefully and give us the actual part numbers, there are variations between closely related models.
The 2SC632 and 2SC632A are not exactly the same. Swapping parts with better noise or higher voltage is fine. Too high or low gain can be an issue and too fast can turn a very poor PCB layout into an oscillator.
 
davidsrsb said:
Please examine you actual PCB very carefully and give us the actual part numbers, there are variations between closely related models.
Click to expand...

Thank you. Probably one of those times where an image is worth more than words:

53338332297_d8c5ab4b82_b.jpg


According to the schematic…

Q501/601 and Q503/603 are 2SC632A
Q502/602 are 2SA705
Q504/604 and Q508/608 are 2SA706
Q505/605 are 2SC633A
Q506/606 and Q507/607 are 2SC1124

One thing I notice from the image is Q505/605, which are listed as a 2SC633A appear similar to the C1364 transistors in the 2SC632A positions. That said I don’t know how to read this stuff; I’m assuming, maybe wrongly, that the C1364 is the ‘thing’ and the three numbers below is just a batch or date code? I only know how to read valves!
 
Tony L said:
I’m assuming, maybe wrongly, that the C1364 is the ‘thing’ and the three numbers below is just a batch or date code? I only know how to read valves!
Click to expand...
Correct and they also omit the leading and pointless "2S"
Be very careful with the 2SA706, there is no modern medium power part with base middle pin
 
davidsrsb said:
Be very careful with the 2SA706, there is no modern medium power part with base middle pin
Click to expand...

As long as I understand the pin out and the leads are long enough there I’ve no issue sticking them in the holes out of physical alignment. I’m sure I could twist things without risk of shorting, but they may end up a little higher from the board to allow for lead-bends, maybe even some heatshrink.

PS I had a go at cleaning the ones I pulled with a pretty decent level of success just using an eraser:

53339342061_b03cde6d9f_b.jpg


The corrosion is very much on the surface and I’m confident I could get a very solid solder joint. My issue is I’d never trust the amp as I have no idea what the situation is like inside the package and I can’t explain what happened yesterday morning. I want to restore this amp to the point I’d be confident to use it to drive my Tannoys, a pair of speakers I’d not sell if someone stuck £6k in cash on the carpet in front of them! That is the point I’ll consider this restoration done.

PPS I’d cleaned the 706 before reading your post telling me to be really careful with it! It still tests as a transistor!
 
It was the 632 that had a bit of a reputation for failing.
You could do with one of those speaker protection kits before connecting non-capacitor output amplifier to high value speakers. Peace of mind
 
If you can use your component tester to select higher gain ones for the 632.
I presume that these were originally chosen for this
 
I’ll measure the one that was stuck in Q605 later and see how different it looks.

Given the nature of the failure are there any clues where the problem I described actually lies on the schematic? I’m half tempted to stick the transistors all back so I can see where things are now after hoofing all the carbon comp resistors out and cleaning the transistor legs (I’d reflow that channel too). I’ll not do it immediately as I’ve ordered a fibreglass pen which should be more effective than my old eraser, though that did seem to do a surprisingly good job.

I feel like I’m totally in the dark here; I don’t understand the substitutions at all, and I don’t understand where the failure point is. As an ex-IT manager I do know that is a terrible place to be from a fault-finding/logic perspective!
 
The composition resistors are far more likely to be at fault than the transistors. The chances of both channels having bad transistors with the same crackle fault are very low.
 
davidsrsb said:
The composition resistors are far more likely to be at fault than the transistors. The chances of both channels having bad transistors with the same crackle fault are very low.
Click to expand...

Could that explain the massive DC offset after yesterday’s failure? At this point I’m not worried about the crackle (the resistor change may well have fixed that), but the fact the right channel DC offset slammed up to 20V (Volts, not mV) soon after power on the last time the amp was in one piece for me to measure. That is what prompted me to pull the transistors out of the board. I assumed at least one had catastrophically failed and was swinging the output stage of that channel hard to one side.

I still haven’t checked the output transistors.
 
A sudden shift like that could be a blown transistor, which your tester would see, or a bad solder joint.
The circuit is unusual, not the usual long tailed pair input
 
I’ve just tested the right channel output transistors:

53341913563_8a38f62378_b.jpg


This is the top one.

53341693531_c3d13acd68_b.jpg


This is the bottom. Both look like transistors to me, but I know nothing.

The heatsink design is really bad with no protection against shorts at all aside from nudge alignment. I got so frustrated fitting the top transistor back, which shorted every time I tightened the bolts, that I applied some heatshrink to the legs to act as spacers.

53342020309_109ab842a9_b.jpg


I’ve only worked on a Quad 303 before, which has much larger holes and non-conductive plastic inserts to prevent this. Anyway no shorts now.

I‘m waiting for a fibreglass pencil to drop through the door, when that arrives I’ll clean the right-channel small transistors and fully reassemble the amp for a quick multimeter test. I clearly haven’t found a fault here and I so far out of my depth it makes sense to just put it back together again and see where it is. The carbon comp resistors could possibly have been an issue, so it is worth re-testing.

PS If I get it working again I’d be tempted to drill the heatsinks out to allow for plastic inserts, or even just leave more space so there is absolutely no prospect of shorting. I hate that design. Just so little clearance as-is.
 
The plastic inserts are good. I'm wondering if some plastic or ceramic inserts have got lost over the years. Sony engineers wouldn't have depended on luck.
 
davidsrsb said:
The plastic inserts are good. I'm wondering if some plastic or ceramic inserts have got lost over the years. Sony engineers wouldn't have depended on luck.
Click to expand...

I’ve seen no evidence to suggest I’m not the first person in here. Even the mystery 15k resistor on the back looked identical in every way to the one on the front of the board. There was no signs of rework, nothing different left to right that I can see.

PS Do you think my logic of fully reassembling and re-testing is sound?
 
A bit extreme, but as all transistors look to measure OK in affected channel with 20v DC offset, why not swap them all with the other channel and re-assemble (maybe need to be careful with that biasing “badge” resistor that seems to have been added at factory). If fault moves to other channel you know one is breaking down under load, and you can selectively swap pairs to identify. If not you know to look elsewhere.
 
I’ve reassembled it and it seems to be working again. I’ve not connected anything other than a multimeter and headphones to it and everything seems back to pre-failure point. I only left it on for ten minutes or so, but the DC offset seemed right and the bias was where I’d expect it cold (I set it after it had warmed up for at least an hour as it gradually rises, so it is 50mV when warm, less cold).

I did’t have headphones plugged in when turning it on as I didn’t want to risk blowing them up (a lovely mint pair of HD-414s), but after a minute or two when it hadn’t done the DC thing I plugged them in and did notice a little crackling. Mostly, maybe entirely on the left channel, i.e. the one I haven’t reworked, so I guess it could be dirty transistors. Both channels have the new resistors. I’ll let it cool down fully then turn it back on and listen to what it does through the headphones.

This has obviously left me totally confused. I would like to explain exactly what happened that led me to read a potentially speaker-destroying 20V on the right channel. I don’t understand what would do that yet not show up as a broken component. I’m also not assuming the crackle and the 20V voltage swing I witnessed are connected, other than things got real loud at that point (loud popping too). I want to understand it as I’ll never be able to trust the amp until I do.

I’d still prefer to replace all the small transistors regardless, certainly the ones with horrible reputations such as the 2SC632A. I don’t really understand how to read the circuit, but given they are up next to the resistor that seems to trim the DC offset could one of them glitching out somehow swing the whole output stage hard the other way?

PS I plan to strip the left channel transistors out, clean, measure, and reassemble regardless. Just cleaning the legs, reworking the joints etc can do no harm. I also ordered some fresh mica spacers and plastic inserts so I’ll completely redo the output transistor/heatsink interface. I just don’t like the way they are!
 
Dowser said:
A bit extreme, but as all transistors look to measure OK in affected channel with 20v DC offset, why not swap them all with the other channel and re-assemble (maybe need to be careful with that biasing “badge” resistor that seems to have been added at factory). If fault moves to other channel you know one is breaking down under load, and you can selectively swap pairs to identify. If not you know to look elsewhere.
Click to expand...

The confusing thing is it failed cold. I‘d just turned it on first thing in the morning and it went. I also wanted to keep the one working channel untouched as a reference. I’d really prefer to understand the logic and what could cause what happened.
 
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