Marantz 2238B Distorted Left Channel

tonefoto

AK Subscriber
Subscriber
Picked up a 2238B in decent cosmetic shape with a cut power cord. After powering it up on a DBT, relay clicked. I noticed left channel distortion and loud popping, right channel is good, no distortion and good sound.

After a dioxit to all switches bias and dc offset was attempted, NO bias on left channel but DC was adjustable, right channel could be adjusted properly for bias and dc offset.

I suspected the left channel differential pair, but both left and right voltages on Q701/703 and Q702/704 seem to read ok: C= -36V, E= 0.6V, B= 0V. Found a driver PNP Q709 read open and replaced the pair with an NTE equivalent, Q701, Q703 tested ok with a diode test when I pulled them. R759 150Ω read open and was replaced.

I pulled the outputs and they all tested good with the exception of the left channel PNP 2SB541 which I replaced with a supposedly equivalent sub (ECG285) that came out of my Marantz 2245. Have an NEC 2SB541 coming.

It now has a strange pulsing/soft popping sound on left channel, a little hum present, but no loud popping still distorted. Right channel still sounds good, doesn't go into protection.

Where do I go from here? The differential pairs are still suspect.
 
Last edited:
I saw the three letter words that we never speak of in my shop...NTE and ECG. Especially the first one. That may be the cause of your continuing troubles.
 
I saw the three letter words that we never speak of in my shop...NTE and ECG. Especially the first one. That may be the cause of your continuing troubles.
Picked up a 2238B in decent cosmetic shape with a cut power cord. After powering it up on a DBT, relay clicked. I noticed left channel distortion and loud popping, right channel is good, no distortion and good sound. I suspected the left channel differential pair, but both left and right voltages on Q701/703 and Q702/704 seem to read ok: C= -36V, E= 0.6V, B= 0V. Found a driver PNP Q709 read open and replaced the pair with an NTE equivalent, Q701, Q703 tested ok with a diode test when I pulled them. R759 150Ω read open and was replaced.

I pulled the outputs and they all tested good with the exception of the left channel PNP 2SB541 which I replaced with a supposedly equivalent sub (ECG285) that came out of my Marantz 2245. Have an NEC 2SB541 coming.

It now has a strange pulsing/soft popping sound on left channel, a little hum present, but no loud popping still distorted. Right channel still sounds good, doesn't go into protection.

Where do I go from here? The differential pairs are still suspect.
NTE is not kosher, I got that.
I checked the preamp section by connecting an amp through the pre out jacks and both sides worked fine, so I am working in the correct area.

Helpful advice is appreciated.
O
 
While waiting for the marantz gurus...

The preamp test indicates it's a power amp issue.

Yeah, could be any transistor, solder joint etc... Maybe even pitted contacts on the relay.
Suggest use RCA cables and cross the pre-out/main in ie, PRE-OUT(Left) to MAIN-IN(Right) etc...
Relay problems will still appear on the left channel
 
Check that the left channel reponds to a bias adjustment. Should really work through the adjustments in
chapter 6&7. This will not find the fault but it's a good health check that the amp is behaving as expected.

Assume you have no oscilloscope. An audio probe may be useful.
 
While waiting for the marantz gurus...

The preamp test indicates it's a power amp issue.

Yeah, could be any transistor, solder joint etc... Maybe even pitted contacts on the relay.
Suggest use RCA cables and cross the pre-out/main in ie, PRE-OUT(Left) to MAIN-IN(Right) etc...
Relay problems will still appear on the left channel
Will cross the precut main in to see if it stays on left channel
 
Check that the left channel reponds to a bias adjustment. Should really work through the adjustments in
chapter 6&7. This will not find the fault but it's a good health check that the amp is behaving as expected.

Assume you have no oscilloscope. An audio probe may be useful.
I should have mentioned in original post that there is NO bias on left channel. I do have a scope but am not too familiar with tracing signals
 
Check that the left channel reponds to a bias adjustment. Should really work through the adjustments in
chapter 6&7. This will not find the fault but it's a good health check that the amp is behaving as expected.

Assume you have no oscilloscope. An audio probe may be useful.
I crossed the pre out left to main in right as mentioned and distortion stays on left.
 
While waiting for the marantz gurus...

The preamp test indicates it's a power amp issue.

Yeah, could be any transistor, solder joint etc... Maybe even pitted contacts on the relay.
Suggest use RCA cables and cross the pre-out/main in ie, PRE-OUT(Left) to MAIN-IN(Right) etc...
Relay problems will still appear on the left channel
Cleaned relay contacts and still has distortion on left side
 
Check that the left channel reponds to a bias adjustment. Should really work through the adjustments in
chapter 6&7. This will not find the fault but it's a good health check that the amp is behaving as expected.

Assume you have no oscilloscope. An audio probe may be useful.
I do have a scope, just not too sharp with it and reading schematics yet
 
Could be speaker selector, maybe try spkrs on set B. Also try through headphones, any distortion?
 
Could be speaker selector, maybe try spkrs on set B. Also try through headphones, any distortion?
Close to the speaker switches is R776 (parts list shows 390Ω) which has been replaced with 100Ω, I can't locate R776 on schematic, only on parts list. Google photos of this amp shows a Diode at R776 position, weird.
 
Close to the speaker switches is R776 (parts list shows 390Ω) which has been replaced with 100Ω, I can't locate R776 on schematic, only on parts list. Google photos of this amp shows a Diode at R776 position, weird.
Can't find it, maybe post a pic. Try headphones and connecting to the other spkr set (System 2?)
 
Can't find it, maybe post a pic. Try headphones and connecting to the other spkr set (System 2?)
I tried speaker B and still get a pulsing clicking sound at zero volume on left channel and distortion at volume.

Tested with headphones and get no pulsing or clicking and no distortion on left channel, slight ground level hum,

Posted a photo of R776. Looks like a diode attached to one leg of resistor. I found another example online of this. Some have the diode alone while some have a resistor, found one with a diode attached to a resistor.image1.jpeg
 
I don't see the diode/R776 as an issue, it just provided a better/more controlled volt drop maybe, R776 looks a bit stressed, could
also just be a lighting issue.

Headphones present a higher impedance load, lower load current, so amp not working as hard. Could be protection relay but more probable
speaker switches. I would be inclined to use the oscilloscope. What make/model? An oscilloscope, basically, is just a very fast volt meter. It
includes the normal voltage scale along with an additional time scale. Will get some test points.
 
Probably easiest to probe at the large emitter resistors. For left
channel need to probe at R763, R765, point B as per pic. Right channel
is R764, R766, point C.

upload_2018-11-17_7-18-26.png
 
Posted a photo of R776.
That shot looks pretty cruddy. You might consider blowing out the insides with some compressed air, afterwards using an adequate non-residue cleaner and brush to clean the PCBs. It'll make it easier when inspecting for damaged traces and cracked solder joints.
 
Probably easiest to probe at the large emitter resistors. For left
channel need to probe at R763, R765, point B as per pic. Right channel
is R764, R766, point C.

View attachment 1332176
Scope is Hitachi V-222. I put one of the probes on point B, sort of hard to probe point C as its insulated wire.d. what am I looking for at these points? point b shows the pulsing in the waveform
 
That shot looks pretty cruddy. You might consider blowing out the insides with some compressed air, afterwards using an adequate non-residue cleaner and brush to clean the PCBs. It'll make it easier when inspecting for damaged traces and cracked solder joints.
I shot the diode resistor before I cleaned the board, its been cleaned thoroughly
 
Back
Top Bottom