Marantz 2500 problem setting idle current

[mikey99]

Trying to set the bias....idle current on my Marantz 2500 and it wont go above .4 mv
on the right channel. Measuring on PX01 TP3/TP4 on the right channel. The left channel will
set correctly at 25 mv.
The DC offset is set to 0 on each channel. The receiver works great, both channels sound
great and the volume levels are the same between channels. I did notice that when I turn
the volume up, the voltage increases and will go as high as 25 mv, at high volume levels.

This 2500 had a blown right channel and had all eight of the right channel outputs
replaced with On Semi MJ21193G/MJ21194G awhile back.
At that time I also replaced the two TO-220 devices D610/B630 on the heatsink with MJE15032G/MJE15033G.
Also replaced two bad/open transistors on the P700 board, Q711/712 with MJE340G/MJE350G.
I did not replace either QM02 or QM01 (the sandwich) on the right heatsink. I did remove
these parts, cleaned and applied new thermal grease, and tested with an ohmmeter, comparing
to parts on the left channel.

I'm wondering if this inability to set bias could be due to different electrical characteristics
of the new parts I've installed. I'm thinking that eventually I may need to remove the amp tunnel
to troubleshoot...but would like to avoid that if possible.

Could either QM01/QM02 be bad and causing this type of problem ?

 

[mikey99]

Resurfacing to answer my question.....QM02 appears to be bad.

I pulled the amp tunnel over the weekend, checked the QM02 2SC1568S transistor, it
reads okay from B-C and B-E using an ohmmeter. However I'm also getting readings
about 10 ohms one direction 50 ohms the other direction measuring from collector
to emitter. So there appears to be some internal leakage in this transistor.
And of course this is the only transistor on the heat sink that I didn't replace when I repaired
this unit :-(

The service manual calls for either 2SC1568 R or S, I found 2SC15680R in stock on Digikey.
Not sure what the extra 0 is for but looking at the datasheet it appears to be the correct part.
I have a few BD135-16 already, which should work okay, but will probably go ahead and order
the correct parts from Digikey, along with some other parts I need

I'm also concerned about the QM01 STV-4H diode/varistor sandwiched on top of the 2SC1568S.
It reads high megohms in both directions, but the one on the other channel heat sink reads
similar.

 

[bktheking]

You need to test the diode stv4h in diode test mode, should get around 1.7v in one direction. They typically don’t go bad and if they do they blow your outputs. Check resistors in the bias circuit , one may have drifted high.

 

[mikey99]

Thanks for the info ! I tried the diode test on the STV-4H using my digital multi meter , it doesn't read any voltage on either STV-4H.
The test does work on a single 1N4148 (reads 500 mv) , but if I attach two in series it doesn't read anything. I suspect my meter
doesn't output enough voltage to test the multiple diodes. I may need to try the 9v battery and resistor test, looking into that.

 

[bktheking]

You may need to desolder it to get a reading, be careful with the leads.

 

[mikey99]

With the P700 board removed from the heat sink , and QM02 removed from the socket, the STV-4h is not connected to any other components.
So I think it should be okay to test it without removing/desoldering.

 

[bktheking]

Do it up!

 

[mikey99]

I did a lot more work on the 2500, replaced trimmers and a few other parts on the P700 amp boards.
Also, cleaning up the left channel output transistors, replaced with new OnSemi devices, new mica and
fresh grease. Replaced the bad bias transistor on the right channel, used the genuine 2SC1568 purchased
from Digikey. Hopefully this bad transistor was the issue with the bias setting.

Tested the STV-4H devices using a 9V battery and a 1.5K resistor, probably drawing slightly less current
than spec, but both diodes measure fine, about 2.2 V. Included pictures showing the test setup.
The STV-4H is attached directly to pins 5 and 6 on the circuit board, and is isolated from the
circuit when the P700 board is removed.

Need to double check everything ..... reassemble and test.