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Marantz 1152dc problem

K

khelvan

New Member
Hello, I recently purchased a vintage 1152dc which appeared to be in pristine condition, and sounded amazing for about two weeks. Then suddenly the left channel cut out almost completely. When I isolate the left channel and turn up the volume I can hear a tiny bit of low and mid frequency sound, no high frequency. I've tested both the 1 and 2 speaker system inputs and different audio inputs, plus different speakers at different impedance, and the problem remains, so it's somewhere internal in the amp across all inputs.

Any ideas on what I can do to address this, other than finding a professional to assist me? (Which I would love to do, but the only vintage place that I have found recommended in the SF Bay Area doesn't seem to be answering its phone during pandemic times)
 
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With the unit off, I would plug an RCA cable in and out of the Preamp, and Main amp jacks several times. If you have some contact cleaner (DeOxit or similar) spray a little in the holes first. The unit has switches in the Pre/Main jacks that frequently go bad, and cause issues of weak, distorted, or no sound on one or both channels.
 
Thank you, sadly the RCA cable trick has not worked. I have ordered some DeOxit, we'll see if that helps.

If not, what steps would you then suggest?

On further testing (playing with every switch and setting on the amp), turning the volume up high when the balance is set to anything but right-channel only, I get a very loud low frequency hum that builds and builds in the left channel.
 
You can use a pair of RCA patch cords to cross connect the Pre and Main connections (left to right and right to left), to determine if the issue is in the preamp or main amp. If the issue moves to the other speaker, the defect is in the preamp, but if it stays in the same speaker it is in the main amp.
 
Ok, upon sending my pre-amp out reversed to my main in the defect did in fact switch channels. Do you have advice on how to proceed from here?
 
Well that indicates the issue is in the preamp circuit. From this point, you will need some instrumentation to do any further troubleshooting. At least the preamp circuit is safer to troubleshoot and repair (less chance of catastrophic failure).
 
AE Bob said:
Well that indicates the issue is in the preamp circuit. From this point, you will need some instrumentation to do any further troubleshooting. At least the preamp circuit is safer to troubleshoot and repair (less chance of catastrophic failure).
Click to expand...
Is this something I can reasonably do on my own? I have a background of putting computers together, but nothing like analyzing broken audio equipment. I have a multimeter, that's about as advanced a circuit tool as I have.
 
khelvan said:
Is this something I can reasonably do on my own? I have a background of putting computers together, but nothing like analyzing broken audio equipment. I have a multimeter, that's about as advanced a circuit tool as I have.
Click to expand...

If preamp section, you want to clean all switches and pots first to eliminate that. You may want to invest in a signal tracer to help with signal tracing. You should download the service manual and at least try to understand the circuit too - the signal path.
 
I've done a full cleaning of every switch & pot with Deoxit, nothing made a difference. I'll download a service manual. I'll look into a signal tracer, can you tell me a bit more about that?

Edit: I've had my eye on a used HP 8922M Multi-Electronic Test Set in my area on craigslist, will this fit the bill as a signal tracer?

I also have my hands on a service manual, but it's been three decades since I took circuit courses in college so this is pretty intimidating!
 
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khelvan said:
I've done a full cleaning of every switch & pot with Deoxit, nothing made a difference. I'll download a service manual. I'll look into a signal tracer, can you tell me a bit more about that?
Click to expand...

If you can follow the signal path, start from the RCA jacks, and at each juncture at the next component, you should still hear the music or whatever your source is playing. Since you have one good channel, follow it all the way to the pre-out. Then do the same for the missing channel, you would be getting full signal at the input, and eventually you will lose it somewhere along the path. That's where the problem would be. All of this is in theory of course. You want to be really careful in isolating the probe so only the tip is exposed and careful not to accidentally short something...
 
slowpat said:
If you can follow the signal path, start from the RCA jacks, and at each juncture at the next component, you should still hear the music or whatever your source is playing. Since you have one good channel, follow it all the way to the pre-out. Then do the same for the missing channel, you would be getting full signal at the input, and eventually you will lose it somewhere along the path. That's where the problem would be. All of this is in theory of course. You want to be really careful in isolating the probe so only the tip is exposed and careful not to accidentally short something...
Click to expand...
Does this mean doing something like wrapping the lead up to the tip in electrical tape?
 
Or strip a piece of insulation off of a piece of wire and slip that over the tip. There is no place in audio electronics for elec-chicken's tape.
 
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