TA-4650: Bringing it back to life

ahess

Member
2016-04-26 11:17 am
I'm hoping you can help me bring my beloved TA-4650 vfet back to life (I bought it in 1977 when I was 16!).

Background
I’ve been using just the amp section, connected to a tube-modified Logitech Transporter as a preamp. It stopped working and I heard a pop. I tested the vfets and they were good. I saw burned resistors and decided to do a full upgrade--new resistors, new caps, new pots, fix the death diodes. Vfets are out.

All was great until I connected the PS to the power amp and misaligned the pins, sending + and - where they didn't belong. Caps bulged. Transistors popped and smoked. Very stupid, and very sad. :headbash:

Since then there have been many layers of troubleshooting.

Now

After many changes, a new zener diode was got the PS to pretty good numbers. Click here for the schematic and voltages of 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
+B4

In the power amp, the last step was to put in Wima film caps to replace the mylar caps. After that the whole left side of the amp (in the schematic) settled into good numbers. The numbers had been drifted up and down. Now the numbers are steady. I'm Getting excited.

Now: Click here for the schematic and numbers of 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.

Thoughts about what is next?

<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.

Also, Ilimzn wrote about replacing one of the PS diodes with two and connecting them in series to the center tap. Would this help with the voltage drift? Also, can someone provide some details on this--which diode to expand to two? D403? D410?

Thank ahead of time, everyone, for any help you can offer to get this part of the family working again!
 
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