Help repairing Akai AM-M939

tallman5353

New Member
Hi there,

Recently picked up a nice matching akai system from a garage sale, the amp being the AM-M939
Unfortunately upon firing it up the right channel appeared to be barely working, crackling some music at times.

I took the cover off and did some diagnostic, no outstanding dry joints, all caps and transistors don't appear burnt out. I've cleaned all of the pots and switches for balance, volume, tone etc with deoxit.

I did swap the output transistors from each channel to the other, the problem persists still on the R.
I also tested the idle current, the left channel measures correct at 20mv, however the right is a flat 0v. Adjusting the VR pot does nothing.

Anyone have advice? I had a peek at the service manual schematic, but it is a bit of a maze to me.
Thanks in advance.
 
Welcome to AK.. On the bad channel. Verify that the originally installed outputs are not shorted. Then verify that the emitter resistor block R39 is still serviceable. Check that both resistors within the block are not "open"..
 
Hi there,

Recently picked up a nice matching akai system from a garage sale, the amp being the AM-M939
Unfortunately upon firing it up the right channel appeared to be barely working, crackling some music at times.

I took the cover off and did some diagnostic, no outstanding dry joints, all caps and transistors don't appear burnt out. I've cleaned all of the pots and switches for balance, volume, tone etc with deoxit.

I did swap the output transistors from each channel to the other, the problem persists still on the R.
I also tested the idle current, the left channel measures correct at 20mv, however the right is a flat 0v. Adjusting the VR pot does nothing.

Anyone have advice? I had a peek at the service manual schematic, but it is a bit of a maze to me.
Thanks in advance.
Speaker Relay ?
 
Welcome to AK.. On the bad channel. Verify that the originally installed outputs are not shorted. Then verify that the emitter resistor block R39 is still serviceable. Check that both resistors within the block are not "open"..

Thanks avionic, checked both resistors. Both measure the same at 5ohms attached to the board. Any other ideas?
 
Would be nice to know what the bias transisrtor is seeing.
If possible measure TR901b b/c/e voltages to GND/chassis.
This can be done at the jumpers as per pix, ie, inturn,
measure each jumper to chassis, expect about +/-2Vdc

upload_2018-9-15_17-6-54.png
 
Ok, have pulled them out, readings are shown in pictures attached. Readings are the same for both resistors
 

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OK, those look good. Your test leads are probably adding .4 or .5 Ω to the measurement.
 
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Ok, measurements taken. Definitely a discrepancy between the two tr901's
These are measurements taken from 901b
 

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And these from 901
 

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Nice photos...Which is the emitter? Base? collector?

Just list the voltages. photos are not necessary.
TR901 ---- 901b
E----------------- ?
C------------------?
B------------------?
 
Next voltage measurements TR1 and 1b. TR1 is a JFET(junction field effect transistor)
Expected readings...Drain,Source & Gate

D-- 0v
S-- 10.5v
G-- 0v
 
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Ok, i think i have the ecb the right way around.

TR7b
E 47v
C -0.6v
B 48v

TR6b
E -47v
C -0.6v
B -48v

TR7
E 48v
C 1.25v
B 47v

TR6
E -48v
C -1.25v
B -47v

Could these transistors (6b & 7b) be the culprit, or is it further up the circuit?
 
I'd try replacing TR901b and zener diode D3b.
 
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