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Sony TA-4650 Restoration

ahess said:
Just found this review of the TA-4650.
Click to expand...

That is a classic Popular Electronics issue from 10/1975. Note on page 7 the ad for the Altair Computer. They didn't call it a PC back then but it was essentially the first PC. There was NO software for it so electronic geeks like Bill Gates and Paul Allen were forced to program it in machine code to get it to do anything. They created their own programming language for the Altair which they called Altair Basic and they started selling it to other Altair owners under their new company Micro-Soft. And as they say, the rest is history.
 
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Update!

I replaced D403, 404, 405, 410, 411 and C403, 403, 405. I had already replaced C413, 414, 415.

NOW the readings are -114 at r413, +76 at r401, +19 at +B3, and +3.5 at +B4. The readings are solid and are not ticking down like they were.

Some of the other readings seem okay. Connector 14 pin 1 is 48v. Pin 2 is 78v.

The readings make me think that this is a step in the right direction, but these values are still not right.

Advice on what to check next?
 
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Here are more extensive voltage measurements (updated link to correct schematic). The highlighted components are the ones I've replaced.
 
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I installed the amp boards, but the numbers are still off. The good news is that + and - voltages are + and - (it helps to plug things in the way they are supposed to). Numbers are off, though, and the ticking down is still there. The Q360 emitor starts at around -55v (where I think it is supposed to be), but falls off and is around -40 in a minute and in another minute around -24.

The PS voltages are so weird! Some are fine, but the A and D board outputs are way off as is the output around r413. The right side of the schematic looks good, but the area around r402 and the transistors is way off. The transistors seem to check out okay, and most of the caps and diodes are replaced. Any chance D406,407 are involved? Could I replace those with some of the 1N4004 diodes?

I just realized that I'm coming up on the one month anniversary of working on this amp. I think I'm starting to lose it. Why can't I get decent voltages?

Later: I replaced C407. Also Q402 and Q403 tested okay out of circuit.
 
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More later: saw online that N1004 can replace the 10D-6 diode. Saw a slight bump in the low voltage readings (1v at Q401 became 4, +B4 output went from 3.5 to 5.7v). Still really low. Plan now to replace the Zener D406. It's reading 3.5k both directions.

Two options:

This or this ?

I hate paying the shipping for Mouser. Anyone know of a local shop in Brooklyn that sells this stuff?
 
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Both Q310 and 360 start at -55ish then tick down. After a minute or two it's down to -35ish. They seem balanced.

Q308 and 358 start at around 69 and tick up. After a minute or so it was up to around 80.

So, the PS has very unbalanced outputs (-110 and 19) but is steady. Connected to the power amp the voltages seem to start right but then walk away from each other (tech jargon, eh?).
 
I'll look this over, but give me some time.

What meter are you using? Beginning to wonder about it.

Swapping out those resistors was a mistake...too many places you may have screwed up.
 
This one.

Sorry, couldn't resist. It's an Extech 330. I bought it for this project.

Resistors? Maybe. Frankly, the real mistake was plugging in the power to the wrong pins on the power amp board. I'd be listening to music right now otherwise.

Thanks, EW. I'll really need your insight into this. This has really gotten into fuzzy territory for me. I loved when I started and was able to identify burnt resistors and caps that were out of spec.
 
D406 is a 7.5V zener. I dunno if it's supposed to be a 1/2W or a 1W. A 1W ought to be pretty beefy with thick leads, and the 1/2W would be smaller with thin leads. D407 is more likely a small signal diode, such as a 1N4148 or a 1N916. A picture would help.

But you are correct to start there at that zener. Too many voltages on that board depend on the voltage at the emitter of Q401.
 
9.1 + the signal diode (another 600mV or so) is going to put you way close to 10V. The emitter of Q401 is only sitting at 8V, so were talking about a 7.5V zener + a diode.

I'd use a 1/2W zener and a 1N4004. That should get the voltage on the Q401 emitter very close to 8V.

One thing that bothers me is the huge voltage drop on R402 (140V to 33V). Check that resistor. And R405 as well.
 
Does this look okay?
Also, since I'm paying shipping (I could probably buy a whole amp worth of components for the cost of shipping!), should I get any other components just in case? Are there any parts that are next in line for troubleshooting, off hand?
 
Zener looks fine.

I'd have a replacement on hand for every semiconductor on that regulator board.

Both Mouser and Digikey will ship USPS. Reasonably cheap.
 
The +/- 75VDC supplies are referenced to the high current +/- 43VDC supplies, what are the 43VDC supplies doing at this time?

Craig
 
I think the 75VDC supplies usually act goofy without the VFETs installed, may be chasing your own tail on this problem.

Craig
 
Checked the zener and r402, r405. The zener was open both ways. R402 and r405 were fine. That 140V to 33V drop seems weird to me too (but a lot of this seems weird to me).

The +/- 43 supplies are solid at around 46.
 
Great news!

Step 1: I put in the new zener (why does that sound funny?) and the PS numbers are great!
Click here for schematic and voltages for the PS (the ones in red). +B3 and r413 are +/- 95, but that doesn't seem too far from 78 and seems like what others have found. I think the PS is DONE! (edit: not so fast--see below).

Step 2: I put in Wima film caps to replace the mylar caps and the whole left side of the amp settled into ballpark numbers and the drift went away--the numbers are steady. Getting excited.

Now: Click here for schematic and numbers on the amp. The other side is similar. The up and down arrows means that the number is ticking up/down. Everything to the left of Q307/309 looks good to me. I think the 107v and rising at the top of the page, and the -49v at the bottom are suspicious.

More info about the PS:
Disconnected from the other boards:
+B1 48 and steady
-B1 -48 and steady
+B2 +97 and ticking down by 0.1v every 1-4 seconds
-B2 -97 and ticking down by 0.1v every 1-4 seconds
(actually, I just noticed that the ticking down only happens w/ the meter on them, for both +B2 and -B2. If I come back a few minutes later, the number starts at 97 again.)
+B3 97 and steady
+B4 20 and steady

When connected to the power amp:
+B1 same
-B1 same
+B2 113 and ticking up by 0.1v per second
-B2 -68 and ticking down by 0.2v per second
(When I checked them after a few minutes, they were at -140 and +240.)
+B3 same
+B4 same

Help? I think I'm almost there!

<crazy thought?> Can I swap in some 0.001u caps (just 'cause I've got them here) for the 8p and 10p caps to see if they are causing problems? I read on a post that the little green ceramic caps (8p, 10p) can become erratic.
 
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