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Sony TA-4650 Restoration

Well, that's why we're here--we don't want their relics going to heaven increasingly rapidly.

I'm hooking it all up today--measurements willing. Question: can I measure DC offset without the preamp? I'm only using the amp section of the Sony, using a Logitech Transported (modified w/ tubes) for the preamp.
 
Help please.
I've got the amp in (no vfets) and nothing is smoking. :banana:

I started taking some voltage measurements and some worry me. I've marked them on the schematic here.

The left side of the schematic has reasonable values, I think. But around Q307-310 the numbers are either too low (-25) or too high (112). The vfets source looks okay at around +/- 47.
Are the weird values because the vfets aren't in? Should I be worried? What should I check.

I REALLY don't want to put in the vfets until these numbers are worked out.
 
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Sorry. Try it now.
Also, the voltages to 2SK60's gate is -26. The voltage to 2SJ18's gate is 112. Both channels.

And I put in new DC bias pots but haven't adjusted them yet.

And when I first turn it on, the two voltages seem fairly balanced, around 60-70, but then each starts moving apart until I have the numbers above.
 
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You should be getting a much higher voltage from the negative unregulated supply. R413 may be burnt. Measure the voltage on each side of it.
 
R413 is a new resistor (I replaced them all). It measuresd about 83v on one side and ~4v lower on the other, but kept lowering voltage over the course of a minute. After about two minutes, it was 60/57v on either side and still dropping. (hmm, I may have missed the minus sign on these numbers.)

update: I just went back to it after about 5 mins--it was ~ minus 50V.
 
While I was measuring it it went from about -94 and slooowly ticked down to about -92, but I think it kept going. It seemed to measure the same on both sides.
 
If it measures the same on both sides, then there's no current flowing. Confirm that by clipping the leads on either side of the resistor and measuring the voltage across it. 0V = no current flow.
 
With leads on either side of the resistor = -0.1 mv
That's pretty close to no voltage, yes?
I removed the resistor and it tested at 220 Ohms.
C413 and C414 both measure 52uF.
Next step?
 
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You know you can test the whole amp without the V-Fets ??
I serviced mine, when I was finished, I put a headphone in the socket and gave a source on the tuner input.
If all OK, you should be able to hear the input etc.
 
Good to know. But I'm still going to have to figure out this voltage issue. My v fets are lonely sitting alone on my bookshelf.
 
If you have no current flow, but the voltage is low and still falling, you have bad caps or leaky rectifiers or both.

See if the voltage on the two 4700uf 50V caps is stable. If so, I'd replace D410, D411, C413, 414, and 415.
 
Ugh!
Okay, I put in the old 4700 caps and put in new caps for C413, 414, 415.
Things seems essentially unchanged:
I get minus 0.4mv across r413--which I think means basically no current. Measuring one lead of r413 with the other to ground measures -82.5, but it ticks down about 0.1v every few seconds.

So, new diodes? Can I replace D410 and 11 with the 1N4148 diodes I used for the death diodes? (I'm thinking not.) If not, what diodes should I get? 512-1n4004?
 
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Well, with the amp boards removed you won't really see any current flow, and that's OK. BUT, whatever you see at R413 you should also see at R401. Voltages should be pretty equal.

Positive your meter isn't acting up?

Yes, you need a real rectifier for D403, 405, 410, 411. The 1N4004 is fine.
 
Still waiting on the parts...
Recapped the preamp section, just to keep my iron warm.
Just found this review of the TA-4650. Brought me back. Did I really pay $400 when I was 16???

Interesting comment in it about how the two speakers are run in series so operating both doesn't get you below 4Ohms (I think that's the point.) I've always avoided running both A and B, but maybe I don't have to.
 
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