Broken Carver Pre-Amp -- need help repairing

[sundownz]

Hey guys, I've been an occasiona lurker on here for a while but this time I actually need some help! So here goes... A friend of mine works as a stereo installer and he brought me two broken pieces of equipment; an Adcom stereo amplifier (circa 1987) and a new-ish Carver Pre-Amp.

I've already repaired the Adcom amp (I'm still a newbie compared to most of you guys on this forum, though). It had a bad ground... a faulty repair job (wrong resistor installed, burnt eyelet pads and traces from poor soldering -- ran some jumper wires to bypass)... two blown fuses... and a faulty RCA jack. So it was a pretty easy job. It's 100% functional again, I was proud of myself.

The Carver unit is a bit out of my league... it's most ICs... and well, I know alot less about circuits with ICs than I do discrete circuits. Again, I'm not a huge expert like some of you guys, but I know quite a few things about electronics.

Now that I've kind of introduced myself and what I'm doing on to the question. This pre-amp looks like it just uses a bunch of Op-Amps to power everything, nothing discrete involved here. Well... the customer said it had NO output so I figured I'd have an easy "find the burnt part" repair.

Not so... it has SOME output, almost inaudible, you have to put your ear up to the speaker to hear it. My setup is rigged through a shelf CD player and the Adcom amp I mentioned above -- that CD player straight to the Adcom is actually quite loud -- so I seem to have a good bit of signal to achieve ANYTHING from the Pre-Amp. And the volume know of the pre-amp must be ALL the way up to hear it from more than 6" away. I'm talking less than whisper level output.

Since I'm not afriad to admit I don't know alot about IC circuits I'm wondering if you guys can point me to the first things I should check for. I'm trying to get ahold of an O-Scope, so if one is needed don't be scared to mention it. I appreciate ANY help and I'm hoping I can learn something from this experience. Thanks all!

[Netlist]

If both channels of that preamp are down I would start looking in the power supply.
Usually, opamps need +/-15V.
Also, a mute circuit could be active but then you'll probably need a schematic.
Here's a link to them: http://www.carveraudio.com/default2.htm
Good luck.

/Hugo

[sundownz]

Hugo,

Thanks for the suggestion. I just checked the Op-Amp power supply voltage and all of them are between 13.5 and 15 volts. So that seems to check out. You may be on to something with the muting circuit. Do you think it would be possible that they enabled it with the remote and then lost the remote? The chassis doesn't have a mute button on the front panel. Thanks again.

[Netlist]

Do you measure minus 15V as well?
I don't know the preamp but that problem with the remote could be an option. One never knows with those modern processor chips.

/Hugo

[Netlist]

What's the type # of the preamp?

/Hugo

[sundownz]

The pre-amp itself is a Carver CT-24 and most of the Op-Amp ICs inside are "JRC 45580 6208G" models.

To answer your other question, I went back and measure the + and - power input. ONE of the ICs, number 11, has a positive that is the proper 15v but the other side is about 24 mV. Would there be any reason for this to be so?

[Netlist]

Pin4 should read -15V and pin8 +15V on all 4558's.
You could trace back the pcb to see if perhaps some safety resistor is broken.
Here's the datasheet:
http://www.tkhifi.com/datablade/ae04052.pdf

/Hugo

[sundownz]

Hugo,

Thanks alot! I'll go back and check for nearby resistors when I get a chance. If that isn't it I'll just keep looking until I find out why that one IC doesn't have proper voltage.