channel cutting out

[Jagman]

I was given a Marantz 2238B receiver that sat unused for some time. I am having problems with both the channels cutting out shortly after start up. This affects the left channel more than the right. Sometimes turning the power off and back on will bring back both channels, or just the right. I am a neophyte at this I should point out. I tried checking the transistors while in circuit, (not ideal i know) and they all measured the same. Could the problem simply be with the power switch or balance pot? Access to most components is not easy, and was reluctant to do much disassembly Not sure where to start. I have a digital multimeter,and that's it. Any help appreciated.

marantz_2238b_service.pdf

[Jazz P Bass]

Your first step should be to find out where the signal is failing (power amp or preamp)
On the back of the amp are 'U' shaped steel jumpers.
One for each channel.
The jacks are marked Pre Out & Main In.
This jumper is the preamp signal to the power amplifier circuit.
Get on the jumper (one at a time) with a meter or an o' scope & monitor it.
Red to jumper / Black lead to chassis ground.
When the unit is running a signal through the amplifier, you should see a low voltage AC signal on the jumper.
When the fail condition happens you need to know what is happening at this point.
If the signal fails at this point, you have a preamp problem.
If not (check both channels, then it is a power amp problem.

[Enzo]

Most likely a dirty control or switch. Clean the volume and balance controls, then try flipping the tape monitor switches off and on to see if they are iffy. Any source selector switch could be dirty, and so clean them and any tape monitor switch or tone bypass switch. ANy switch that is in the signal path can be dirty. Don;t forget any speaker selector switches. Is there a "link" switch or something like it on the rear? it would connect the preamp to the power amp. Some amps have a preamp out jack and power amp in jack, kinda like an FX loop, so you can use external EQ or something. Any of those can get dirty.

Transistors won;t be the cause of coming and going.

[Enzo]

Aha, Jazz reports there are jumper plugs rather than a link switch. They can be dirty too.

[52 Bill]

I was given a Marantz 2238B receiver that sat unused for some time.

Welcome to the place. Anytime that a piece of electronics gear sits for a long period of time, there is a great chance that any switch or interconnection contacts can become oxidized and dirty. If the unit was not stored under ideal conditions or was used in a smokey environment, then the problems with the contacts gets even worse.

I would suggest that as a starting point, you get a can of Deoxit spray and clean the selector switch, the tape monitor switch, the speaker switches, the power amp in/preamp out connectors, etc. This may be the only thing that you need to do to fix it.

Edit: I see that Enzo and Jazz have already responded.

[g1]

As you mentioned it may come back if you turn the power off and on again, I was thinking maybe an output relay if it has one. Oxidized relay contacts could cause such a problem.
As others have mentioned, after sitting in storage, general cleaning of the pots and switches is in order.

[Jagman]

Wow,thanks to all for such quick replies. Three questions
--I have some 99% isopropyl alcohol. Will that do, or is deoxit better.
--the switches on the front panel are behind a metal panel and covered by the tuner section from above. Any hints on the easiest way to get access, and where to spray the deoxit (or alcohol)?
--I don't see any U shaped jumpers, just the main in/pre out rca jacks. Not surw how to proceed. Do I measure from the center to the outside of the pre out jack? The outside is chassis ground is it not? If not there is a separate lug.

Sorry for the basic questions. Gotta start somewhere right?!

[Enzo]

Alcohol is not appropriate. Deoxit is specially formulated for exactly this purpose.

If you cannot get at the panel switches from above, take the bottom off and see if you can get them there. Sometimes you even have to remove the outer front panel. But in the pictures it sure looks like I can see the switches from the bottom view, and really it looks like there is a space between the front panel and the preamp board, which should allow access.

If music comes out the speakers, then your pre out and main in are connected somewhere. Those jacks in the back are EMPTY?

OK, according to the schematics and block drawings, the connection is inside, a cutout contact on the power amp jacks. SO those contacts can get dirty. If you take a regular stereo RCA patch cord and plug from pre out to power in, it bypasses this.

[Jazz P Bass]

O/k no pre/ pwr jumpers.
I would still use those jacks to nail down where the problem is.
If you start digging around & spraying you may just cause more problems.
If it turns out to be a control or switch problem then spray away.
Use the Pre Out jacks & do the meter/ o'scope measurement.
Look for a change in the measured signal when the problem occurs.
If there is no change then we can help delve into the power amp section.
The amp does have relays which could cause the problem.
But you need to divide the problem in half.
Preamp or Poweramp.

[Jagman]

Okay,I will get some deoxit and start with switches and contacts. Probably take me a few days. I shall report back.
Thanks again.