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Marantz Model 1200 Left Channel Issue

lo-d dad

lo-d dad

Active Member
Hello AK, I recapped my Model 1200 in 2011 thanks to all the help from various members. I still love the sound and look of the model, but for a while I have been noticing the left channel is quieter than the right channel, especially right after I power on. If I keep using it, the left channel is back to about the same level as the right channel eventually. When the left channel is quieter, the sound is also distorted a little as well. If you have any suggestions what I should check, please let me know. Or, if there are any similar discussions somewhere, please also let me know. I still don't have much experiences working on old electronics products, but I surely enjoy it. Thank you in advance,
 
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MarantzRefrb said:
If you switch your Tone Control knob, on the top far right, to Out, does the volume or distortion change at all?
Click to expand...

I have never tried to switch Tone Control knob when this occurs, but I will do that next time. If the Tone control changes the volume/distortion, would this be the one I really need to clean? By the way, I can hear pop sound when I switch "High Filter". Does this mean that I should clean this one too?
 
I'm currently working on a 1200 with a Left channel problem also. The Tone Control, Out position, will bypass the 3300 Pre-amp section, this will make it easier to see if your problem exists in the 3300 pre-amp section, or the 250 power amp section. When I switch to Out on Tone Control, my Left channel problem goes away, and it sounds correct.

I don't believe your High Filter setting should pop, but a good De-oxit cleaning on everything except the Power Switch, should help. The Balance pot on a 1200 seems to be the most common problem. Does your Balance, or Volume pot, feel sticky or gummy at all?
 
MarantzRefrb said:
I'm currently working on a 1200 with a Left channel problem also. The Tone Control, Out position, will bypass the 3300 Pre-amp section, this will make it easier to see if your problem exists in the 3300 pre-amp section, or the 250 power amp section. When I switch to Out on Tone Control, my Left channel problem goes away, and it sounds correct.

I don't believe your High Filter setting should pop, but a good De-oxit cleaning on everything except the Power Switch, should help. The Balance pot on a 1200 seems to be the most common problem. Does your Balance, or Volume pot, feel sticky or gummy at all?
Click to expand...

OK. I just tried all the tone positions. It doesn't make differences. In my case the issue only occurs sometimes. When I turn on the power in the morning (after I haven't use it overnight), the issue is more noticeable.

The volume control feels sticky for sure, but the balance is not.
 
If you have Freeze Spray, you could try spraying your Left 250 power amp's transistors, to see if that brings the problem back.

EDIT: Measuring DC offset at the speaker terminals, when it's having the problem, and when it's not, would be good to know.
 
MarantzRefrb said:
If you have Freeze Spray, you could try spraying your Left 250 power amp's transistors, to see if that brings the problem back.

EDIT: Measuring DC offset at the speaker terminals, when it's having the problem, and when it's not, would be good to know.
Click to expand...

I don't have Freeze Spray, but I can get it. So, if the problem brings back by spraying left output transistors, could those be faulty?

I just measured DC offset when it's having the problem. The right channel is about 0, but the left channel is 11~12mV. I will try to measure them again when the problem goes away.

Thank you for your help.
 
Your DC offset sounds good, well done.

Since you only seem to experience a problem when your 1200 is cold, The Freeze Spray can help quickly replicate that condition. The spray can expose a transistor that has thermal issues, or it can expose a cold solder joint (expanding and contracting with heat).
 
MarantzRefrb said:
If you have Freeze Spray, you could try spraying your Left 250 power amp's transistors, to see if that brings the problem back.

EDIT: Measuring DC offset at the speaker terminals, when it's having the problem, and when it's not, would be good to know.
Click to expand...

When the problem goes away, the right channel is 4mV and the left is 22mV.
 
According to the 1200 SM, 50mV is the correct voltage per side:
84SvJYI.jpg


You're well within spec on the DC offset.

Bias measurement:
JRfCPTz.jpg
 
Bias and DC offset have nothing to do with intermittent operation. There is no magic knob in one of these, or any other amp or receiver that someone is gonna twiddle and fix their problem. But there are a couple that you can twiddle and cause a whole host of new problems.
 
MarantzRefrb said:
Your DC offset sounds good, well done.

Since you only seem to experience a problem when your 1200 is cold, The Freeze Spray can help quickly replicate that condition. The spray can expose a transistor that has thermal issues, or it can expose a cold solder joint (expanding and contracting with heat).
Click to expand...

OK. So, I can identify which transistor has the issue with spray and replace it. This question sounds silly, but I have to ask you since honestly I forgot it. If you face the front of the amp, the left power amp board is on my left and next to two 20K caps, not the transformer side. Is this correct?
 
lo-d dad said:
OK. So, I can identify which transistor has the issue with spray and replace it. This question sounds silly, but I have to ask you since honestly I forgot it. If you face the front of the amp, the left power amp board is on my left and next to two 20K caps, not the transformer side. Is this correct?
Click to expand...

In my 1200, the Left channel is called A, and B is on the Right, but yes, facing the front of the unit, my amp on the Left, is the Left channel.
 
MarantzRefrb said:
They can indicate a problem, no?
Click to expand...
To someone who has experience and knows what they are looking at. Even then, it's the last thing I look at when signal is cutting out. Telling laymen who are not experienced techs to fiddle with the bias and DC settings is irresponsible and not helpful, IMCO. There are more than a few vintage amp designs that will flat out go up in smoke in an instant when the bias string has an intermittent open. Like the kind caused when someone moves a 40+ year old bias trimpot that hasn't been touched since it left the factory.
 
In my experience, calling people laymen without giving them the benefit to experience, or learn something, will keep them in that spot you call, 'laymen'.
 
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