Harman Kardon PM655 can't setup idle current

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XgN

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Joined 2014
#1
Hello,

got this one classic Harman Kardon PM655 integrated amplifier for repair.
I see it was repaired previously - most probably output stage transistors were blown as I see many parts replaced in that stage. Most of them are original with one or other analog component. Nevertheless, the amplifier turns on, even played (with very doubtful quality). Client said he had to switch something in order to sound appear.
Started from making adjustments according to the manual:
Screenshot by Lightshot

Could easily setup DC Offset to 0.
When it comes to Idle Current maximum what can be achieved is 4 mV to one channel, and 1.5 mV to another.

Triple-checked all the components, voltages. Seems to be repaired good previously. Maybe some voltages on the base of the output transistors are smaller (by 0.1 V not more) but the current should be higher either.
Screenshot by Lightshot
Screenshot by Lightshot

Adjustment pots are cleaned with special regenerator. Power transistors, driver transistors, power supply checked. Bad joints resoldered.
Maybe some ideas guys?

Schematic part:
Screenshot by Lightshot
 
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#2
Your probably need to take some voltage measurements to see
where it's failing. No response to VR adjust typically caused
by failed silicon. If bias does respond somewhat to VR adjust
then it may be a problem with the VR or even the diode chain
D409.

Start with the bias transistor Q429, measure b/c/e each to GND.
SM gives -0.5/+1.1/-1.1Vdc respectively. Adjusting VR403 should
affect these voltages

If Q429e is high (+0.5 ?) then you should look at Q427b/e
voltages, is it turned on?

Q428c/e voltages set emmiter voltages at Q433,5 and in turn Q441,3
Check these voltages.
 
#5
Q441 and Q443 transistors voltages are OK: Vb = 0.6V and -0.5V respectively. Voltage rails are OK too. Despite these facts, voltage across emitter resistors is max 4 mV for one channel, and 2mV for other. Checked those resistors, they are good...
 
#10
Its common for non original semiconductors (i.e. modern production) to have differing forward bias voltages that put adjustment out of the range of the Vbe multipliers adjustment. This typically needs just a single tweak of an appropriate resistor around the preset.
 
#11
Mooly said:
Its common for non original semiconductors (i.e. modern production) to have differing forward bias voltages that put adjustment out of the range of the Vbe multipliers adjustment. This typically needs just a single tweak of an appropriate resistor around the preset.
Click to expand...
So the plan is to replace R457(470ohms) with 680ohms? before power up set

VR to minimum, ideally use a dim bulb tester.
 
#12
mbz said:
So the plan is to replace R457(470ohms) with 680ohms? before power up set

VR to minimum, ideally use a dim bulb tester.
Click to expand...

Changed R457 from 470 Ohms to 200 Ohms now I get high idle current (voltage) values. Maybe too high and potentiometer turn is not effectively used, so I consume 330 Ohms value resistor should be perfect.
Going to try this trick on the other channel.
But as it seems this is the solution! Thanks all for help 🙂
 
#13
XgN said:
Changed R457 from 470 Ohms to 200 Ohms now I get high idle current (voltage) values. Maybe too high and potentiometer turn is not effectively used, so I consume 330 Ohms value resistor should be perfect.
Going to try this trick on the other channel.
But as it seems this is the solution! Thanks all for help 🙂
Click to expand...
I learn something new every day... thanks Mooly...
 
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