Carver CT-Seven

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Looks like the TC9152P has gone leaky. Between 6 and 14V with high source impedance present at the two output pins. As it's a bilateral switch, this also backfeeds into the sources and phono stage. The 'tuner' input does not couple signal through at all. I've got audio from the tuner board when signal tracing.

This DC leakage gets thru the coupling caps for opamps IC452/453, since they are the 'wrong' way around. With DC offset and their bias upset, the audio is rather distorted. Feeding a source into the tape inputs bypasses the TC9152 and the output signal is fine.

But wait, there's more! One or more of the opamps in the big bank for the 'Sonic Holography' may also be leaky. When engaging into the signal path, there is a pop in both channels. Immediately, right goes dead and left still has signal, but no effects can be heard whatsoever. This backs up my guess of bad opamps...

PDFs too big, so links are here:



-Paul

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Re: Carver CT-Seven

I'm going to use a bank of 5 DPDT signal relays and 5 PN2222s as coil drivers. Five 4148s will be used as back-EMF suppression. The owner may not want the Sonic Hologram fixed, but I've got something in mind for the opamps.

Replying to your own post- what a way to blow it.

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Re: Carver CT-Seven

Gonna try some JRC/NJR NJM4580s. Even though they're 8-pin, it won't be a problem.

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Re: Carver CT-Seven

All done!

I just now posted- this was completed a few days ago, after all the parts came.

It turned out that another analog switch was leaky, from the -15V supply to GND, so another relay replaced it.

The bank of 5 are the input switches and the one by itself is the 20dB 'mute' attenuator.

The caps in the power supply were replaced with 105 degree units. The ones on the input/preamp/hologram board were also replaced.

I also was asked to look over the M-500t power amp. I vacuumed out the dust and resoldered some connections on a few power resistors. As well as make sure heatsink/chassis screws/etc were tight. This is a 'rail-switching' amp- up until a certain output level, the rails run at +/-40V. When more voltage is required, a comparator drives driver transistors that drive those four big (commutating) ones right below the power trans. This instantaneously brings the rails up to ~+/-74V. Some fast diodes prevent the high voltage from backfeeding.

By way of C503 and C504 in the power amp, chassis 'ground' floats at ~60V, unless there's a true path to ground. My guess is that the power amp was left plugged in and connected to the CT-7 via the audio cable. Possibly a grounded source was unplugged from the CT-7. This would cause the following:

Grounded source connected, both shell and tip of RCA plug- all is well
Beginning to remove RCA plug, shell breaks but tip still made- BAD! Now the leakage, no longer 'bypassed' by the shell of the RCA, flows from ground into the ground pin of the 'input select' analog switch. It then backfeeds through the IC (kills it), out of the signal pin(s) and up the center conductor of the audio cable.

Only a guess though...


Enjoy the pix, and those TB-34 output transistors!
-Paul