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Sony TA-4650 Restoration

I'm getting ready for my sixth Mouser order. There are so few parts that HAVEN'T been replaced at this point, I figure I should swap out whatever is left to try to root out the (final?) problem.

I can't find any of the transistors below. Can you suggest replacements?
Q308 - 2SA840 or 2SA1013
Q309 - 2SA639S or BF423
Q310 - 2SC1670 or 2SC2383

Will the 1N4148 serve as replacements for 1S1555 and 1T22A?

Finally, I've read about putting in a second diode in the PS, so 2 diodes in series, with the middle connected to the center tap line to the transformer. Has anyone done this and if so, how is it done?
 
The only reason I'd not swap in the .001uf caps for the small pf caps is that it means extra soldering that you shouldn't have to do. Order and install the correct parts and you're done swapping (even if it doesn't fix the issue).

And you need to figure out WTF is going on with that -75V supply. Your earlier scat markup says you have -110V at R413, but then show -49 at the power amp (R322 and R318). That is not possible unless you have a broken wire/bad connection/broken foil trace.

2SA840 - 140V, 500mA, 750mW, B=150, ft=45MHz
2SC1670 - 140V, 500mA, 750mW, B=50, ft=50MHz
2SC926A - 210V, 30mA, 320mW, B=100, ft=160MHz
2SA639S - 180V, 50mA, 250mW, B=50, ft=130MHz

Probably sub the KSA1220AY and KSC2690AY for the 2SA840 and 2SC1670. And I'd try the ZTX696B and ZTX796A for the 2SC926A and 2SA639S.

Dunno about the IT22A, but the 1N4148 wll do for the 1S1555. The rest of your post doesn't make any sense. Perhaps you have a link to something...?
 
This is great stuff. I'll look at this again when I'm not so tired. Yes, put in the new caps. And I'll link the dual-diode posts tomorrow.

Awesome. Thanks EW!
 
That is very interesting, although not the same dual diode problem I think of when I think of VFET amps.
 
Okay, clarification on the -75V power supply:
With just the PS, r413 reads -96v and verrry slowly ticks down (0.1v every 2 seconds).

With the amp connected, it starts at -85v (ish), ticks down fast and within a minute or so is down to -49v. The reading is the same at r413, r322, r372.

+75v line is similar. PS-only, it holds at +94. With the amp, It ticks up to over 100v.

So, any thoughts on what to look at that would make the +/- move away from each other? Do you think it's on the PS or the amp?
 
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I'll double check tonight but I am pretty sure that it is connected for both readings.

What will make voltage drift like that?
 
The pre amp is connected, but those voltages are solid.
I replaced the 8p and 10p caps. No change (didn't really expect them to be the culprit).
At this point I'm shooting in the dark. I have two options:

- replace transistors Q307-310. Don't know why they would drift the voltages apart like this, but that is where the voltages get weird.
- Put in the vfets and see if the feedback (don't really know what I'm saying) evens out the voltages. The gates don't go below -55 for a minute or so. I could measure to see if the voltages stabilize and shut it off if it doesn't. This - obviously - scares me. If I did this, what is the minimum voltage at the gates before the vfets fry?
 
Connect one more diode in series with D403 and the common point between these to ground.This will make the bias supply immune to load imbalance.

This design flaw cost lot of 4650's lives.Last week I repaired a 4650,imported from japan as junk, have same problem. VFETs failed, but lucky to have some spares.
 

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Been MIA for a few days with some repair issues of my own. I do think I remember Nasta saying something about the diode mod a while back, but I blew it off as I hadn't seen a 4650 in some years. When you mentioned it (post #101), I did not recall that conversation. Seeing joshvi's post brought it back to me. I've not done it, since I see these devices so infrequently, but it looks like a good idea. There's still an uneven draw on these supplies, but I think that's less of a problem than the lack of a proper ground reference causing the supplies to drift.

IIRC, Sony also does something like this on the 5650.

Nice first post there joshvi.
 
I have 2 functioning 4650s. I think I'll try the diode mod, if the group thinks it is safe to do. What diode is suitable?
 
I used the 1N4004 to replace the other rectifiers. It's 1A, 400v.

Thanks EW, for the vote of confidence. I'll be trying it today. Hope your repairs went smoothly.
 
<Cue music> I feel good!! I knew that I would...:banana:

I did the diode mod! Pics here and here.

I think I heard a sigh coming from the amp as I turned it on. Here are some numbers:

r330/380: +/- 66
r322/372, 319/369: +/-84
r319/369: +/- 69
vfet source: +/- 47

Most importantly, the numbers are solid--no drift!!
I measured the S and G of the vfet terminals according to joshvi ("Make sure that the voltage between gate and source of all vfets are between 8-18 volts.") and they were ~+/-16v and holding.

When I adjust the pots the gate changes. What number should they be? There are at 66v now.
The DC offset is 17mv left and 70mv right. I'll probably match transistors later, but that's safe for now, right?

Sooooo, am I ready to put in the vfets? I'm going to double-check all the voltages, check the continuity of the vfets once they're in. What else????

I'm SOOOOO excited!!!
 
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Hi Andrew Hess,

Yes ,Now you can put vfet back and adjust the bias to 65-75 mV across R333 or R334 with no input signal after 1-2 miniute warmup . Please refer the service manual page no.4 and 5.

Regards
joshvi
 
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You have to picture the other side of the camera--a tired, happy dude with a beer, trying to decide between Goldberg Variations, Beck, Shaun Colvin, Ella...

Okay, it's been a month and a half since I heard any music from this amp, but I really, really think the sound is much awesomer. I hear more air, more snap, texture...

Thank you ALL so much for your patience and thoughtful comments-especially Echowars, joshvi, DRecovery, Ilimzn - I'm sorry if I've forgotten anyone. I'd be a sad sack without your help.

Now, I think I'll wait a day or so before matching transistors to take care of that DC offset! :)
 
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