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marantz 2238b power main board transistor

EnzoFerenzo

EnzoFerenzo

New Member
I am looking for a suitable replacement for the thermally coupled transistors, in particular Q708, part number listed in the manual is:HT315672BO. I've looked online, and it seems like an old, obsolete part. Any help getting a substitute is appreciated. I've included below part of another thread explaining how I got here.

If anyone has info on a better diagnosis or if should I replace all the thermally coupled transistors, please advise.

Originally got the receiver with bad distortion on the right channel, lots of intermittent static. Cleaned all pots etc. New speaker switch.


" I am using my phone and an adapter to RCA plugs. Magically the all outputs were quite clean, aux, tape monitor, and FM. Turned the amp off, and later it had some intermittent static again, which cleared up some after leaving the amp on for a while.

I thought I may as well finish the recap on the main power board, as I only recapped the ones I could easily reach at first. Now I did them all except for a few under the speaker switch rail that were blocked.

Now when I turned it on, the relay didn't click on, later after checking my work for pins shorted, polarity issues etc.(I checked at least 10 times after I recapped and again now.) I found no errors.
The next day the relay did click on, and I got only some static. Now I'm getting some signal through, but very low volume and crunchy.
The DC voltage at J722 reads 35 V, checked the emitter voltage at Q708 was really wonky, reading anywhere from 130 to 180 DVC and bouncing all over the place. Should I take it out and measure it then? All the other transistor seem ok, testing .5v from emitter to base."
 
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Hi,

The answer is fairly simple: The replacement for the Q708 and Q707, which is a 2SC1567, is the KSC2690AY.
In case you want to replace its complementary type: Q709 and Q710, which is a 2SA794, is the KSA1220AY.

You may want to check this out:

https://audiokarma.org/forums/index.php?threads/marantz-transistor-diode-trimmer-sub-guide.926805/

It was made by @Steven Tate and others who shared their information here on the forum.

Of course, you want to double check the old ones when replacing a transistor by a newer type. Not just solder them in head over heels.
What i'm trying to say is that if the emitter voltage is so high, you might as well want to check your output transistors. If the voltage on the base of Q712 is that high too, it could be blown. Its voltage is only limited between 100 and 150 volts.
 
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I just rechecked the voltage, not plugged in, also I have this voltage at two of the output transistors. Q713 and Q714 of 32 VDC but no voltage on the other two Q711 and Q715.

Any help isolating the fault is much appreciated.

Thanks all in advance
 
EnzoFerenzo said:
I just rechecked the voltage, not plugged in, also I have this voltage at two of the output transistors. Q713 and Q714 of 32 VDC but no voltage on the other two Q711 and Q715.

Any help isolating the fault is much appreciated.

Thanks all in advance
Click to expand...
I would start by checking the B+ voltage (J707) and B- voltage (J708) coming into the board to make sure I have both. Should be around +/- 36V
 
Steven Tate said:
I would start by checking the B+ voltage (J707) and B- voltage (J708) coming into the board to make sure I have both. Should be around +/- 36V
Click to expand...


Thanks for your response, yes, I checked those voltages and I have +37v and -37v respectively.
I also put the input directly into the main and the output is perfect. I have no radio at all now, and the left channel is gone except for some static.
 
So, you get good sound out of both amps if you put your signal into Main In? Also, check for 14V out of J724. That powers many of the other boards. Also check for 35V at J722.
 
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I have 35v at J722, I have nothing at J724!
Getting somewhere, what next? Problems at Q753? Q753 has no voltage at E to ground, E to C I have -49.3V, C to B I have +49.3V. There is continuity from C to ground.
 
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What is the voltage at the emitter of Q751? That would check the zener voltage of Q752. Otherwise, Q753 looks like the suspect. Pull it and test it.
 
Yeah, I've got 14.75 volts at both Q751 and Q752. Looks like Q753 is the culprit, it also has a collector to emitter short.
Does anyone know the part number for a common replacement?
 
The odd thing about this quandary was the 14V problem at Q753, it was fine before I did the recap, then it appeared. I desoldered the pads on that Q753 transistor and I found some severe corrosion (see picture), and a missing pad basically. I tested the transistor, and it was fine, so I reinstalled it and did what I could to get a solder joint on those rusty pads. Voilà, I once again had my 14V back and FM is working.
So there was barely a connection at those traces and pads, and it finally gave up after the recap.

I still had that gnawing annoying static and intermittent cut out of the left channel, though. I remembered seeing another trace that was wonky, this time on the tone board, so I went over that exposing some copper and fluxing and soldering and like magic, all the static was gone.

Can anyone suggest a good product to use to inhibit the further corrosion on the board? You can see from the picture that the other pads are corroding as well, the amp must have been in a damp place stored for a while, I am thinking.

Holding my breath that all is ok, I let it burn in for a few hours and whew, I think I'm good. This one really had me thinking hard, quite a learning curve, I loved it! NEXT!
 

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