C33 Preamp Problem - very very low sound from all outputs and headphones?

slayboy3

New Member
Just received an older (80's) C33 Preamp. Has not been powered in decades. Sound is extremely low from all outputs including headphones, all other controls/lights appear to work. Any thoughts??
 
Muting circuit, loudness circuit, power supply. Could probably think of a few more.....How do you know the controls work if all the outputs are basically dead? I had one recently with a similar issue, can't remember what it was. Probably was the complicated muting circuit. Should keep your local tech busy for a few hours diagnosing the issue common to both channels.
 
How do you know the controls work if all the outputs are basically dead? What little sound can be heard, does change when you adjust any of the controls.
 
Is there a experienced Mac tech near to you? Mac preamps like the C32 and 33 are not for the faint of heart repair tech, the schematics are 3 double sided 22X36 sheets with 13 different sections.
 
Is it the same for all inputs? The same for both channels? Are the tape outputs also low?
 
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The Analog Store in NC is a couple of hours away. CORRECTION, Tape outputs: 1, 2, and 3 work. Tape output 3 (headphone jack on front) works. The two processor outputs on the back work. Another strange thing, when the Rec EQ button is pressed the front five EQ knobs only affect the left chanell. The main outputs and 1 & 2 - all very very low sound/signal - barely audible.
 
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What does affect all inputs and all outputs ? Power supply seems to be the first area to control .

And as Chris said : the schematics are 3 double sided 22X36 sheets with 13 different sections ! yes ! not easy for someone who don't do that every day !

I don't remember if muting circuit is working also on tape output .....
 
I would not be surprised with a 5 hour labor estimate and some parts needed that have been discontinued for years but subs are available except for one.

That one part Patrice inspired me with his exceptional made up substitute.

I would suggest having the unit fully restored once the problem is solved.

These preamps are exceptional and when restored will stand up against many new units. I have one client that keeps asking me why it sounds better than his restored MX110 as well as his other tube preamps. Well worth the dollars to get done right but like I said not for the faint of heart repair tech or one looking to get it quickly off their bench.
 
I had a similar issue with my C32 where only one channel played when I engaged the EQ circuit. It turned out to be the OpAmps on the EQ board. The C33 OpAmps may be a bit different but I'll bet you they are the source of your problem there. They're cheap to replace so you won't loose much if that is not your issue. I posted some pictures on this thread:

http://audiokarma.org/forums/index....q-out-1-ch-would-go-out.725566/#post-10312826

Posts 6, 8, and 9.

Mike
 
Took the unit to a repair guy south of Raleigh, used to work for McIntosh. Bad transistors on the muting circuit. Only cost $360. Picking up the unit this week.
 
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