Sony De835 rear channel square waving, help.

[Joe]

I had a amplifier transistor pair I replaced on the rear channel
board, but the audio from it is distorted. I put a scope on it and the
audio has a square wave in it. Cn anyone help me identify the most
likely part to cause this square wave in the one chanel? I guess that
Sony experience isn't needed here, just audio circuit experience. The
other parts to go when the final amp went were the emiter resistors, I
think. They were 180 ohm resistors that burned when the transistor
went out.

Any help is appreciated. It has been a long time since I did any
repairs, maybe 20 years. I'm not that old, I just went into a
different line of work.

Joe massimj@NO_SPAM.hotmail.com

 

[Jerry G.]

If the bias current is too low on an output stage, it will start to square a
bit. Did trace back with the scope to see if the input of the output stage
is clean?

--

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Jerry Greenberg GLG Technologies GLG
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I had a amplifier transistor pair I replaced on the rear channel
board, but the audio from it is distorted. I put a scope on it and the
audio has a square wave in it. Cn anyone help me identify the most
likely part to cause this square wave in the one chanel? I guess that
Sony experience isn't needed here, just audio circuit experience. The
other parts to go when the final amp went were the emiter resistors, I
think. They were 180 ohm resistors that burned when the transistor
went out.

Any help is appreciated. It has been a long time since I did any
repairs, maybe 20 years. I'm not that old, I just went into a
different line of work.

Joe massimj@NO_SPAM.hotmail.com

 

[Joe]

Jerry, thanks for the reply. I do not have the schematic of the board
I am working on, but it is single sided. I may be able to draw a
schematic of it so I can see the path of the audio.
For now I can say that I tested various points in the circuit and I
see the square wave everywhere. There isn't anything to adjust here,
so I'm thinking that one other part failed whern the power transistor
went out. I replaced two burned resistors that were tied to legs on
the power transistors.

I guess my only choice is to compare the other channel with a constant
tone going through to see what the differences are, it might put me in
the correct area of the board.

My only problem is that this is the rear channel board, and the only
way to put any signal to it is with decoding of the inputs. Stereo
won't do it, it has to be prologic, or Dolby Digital. I think I have
one of those DVD test disks, it should let me activate each channel
with a tone for system adjustments. This may work.

Thanks, Joe

 

[Joe]

Jerry, what I did find was the diode module, for lack of a better
description, in the limiter circuit, to be bad. I checked this in the
diode mode of my fluke 87. The good part shows zero conduction in all
combinations. The suspect bad part shows 2.5 to 3 volts. It starts out
at approx. 2.5 volts and climbs. I suspect that C720 is charging
through the shorted parts of this module, and the one good diode left.
In any case, the part should read zero conduction because of the way
the diodes are situated, as the good part indicates.

Now for the hardest part of this do it yourself project. How do I get
this diode module, which must be a Sony specific part?

It appears to be two 2.2 volt zeners, and two of some other type of
diode. I could try to fake it with two 2.2 volt zers and try some
standard type glass diodes in palce of the unknowns.

The part is labled as D701 through D704 with part number 1SS133T shown
twice. The part is actually one large white plastic part, with three
metal posts coming from the top. The three posts are the three
junctions of the diode configuration. This makes it very easy to test.

Since this part is part of the limiter circuit, I imagine that this is
why I have clipping, or square waving in the output stage.

Any input on this is welcome and appreciated.

Joe massimj @ hot mail. com

 

[Mark D. Zacharias]

The part number for D701, 704, 705:

8-719-911-19 (diode 1SS133T-72)

part number for D702, 703 is

8-719-109-51 (diode MTZJ-T-72-2.2)


according to the service manual.


Mark Z.

 

[Joe]

I did get the help of somone that has been watching here. I received a
scan of the schematic and board, plus some real expert troubleshooting
guidence.

The part that I thought was a diode module was actually the duel
emitter resistor. This had opened on the bad channel. I ordered a
replacement from Moteva.com. Apparently, this part often fails when
the output transistors short. The part number for this part is R726
and is a 2 watt .22 ohm duel emitter resistor. The part I located was
a 3 watt part, but it should fit OK.

Mark, I see that you changed your ISP. I did write directly to your
old address and got a bounce. Sorry, I won't write directly to you
per your request. Thanks for checking in here and responding to my
earlier misguided attempts to troubleshoot this.

So, in this repair, I have identified these parts that failed.

Q706 MN2488, Q707 MP1620, R726 (duel emitter .22 ohm), R724 (180 ohm)
, and R725 (180 ohm) I obtained all of them from www.moteva.com
Moteva didn't care any heat sink compound, so I got some Dow Corning
340 off of ebay last night. I ended up with enough to last me 450
years, so, I got that going for me

The left rear channel had been putting out bad audio ever since I
replaced the power transistors. I didn't hear it because I sit on the
right side of the system. Once I relocated the receiver and
approached each channel to verify output level, I noticed the
distortion.

I'm still not sure how it happened, but I am going to be very careful
about how I load-up each channel, and check that none of the wires are
in danger of shorting.

Thanks to all who replied.. Joe

 

[Mark D. Zacharias]

If the emitter resistor has opened up, you could have one or more 2SA988
protection "gate" transistors bad, even on other channels. They've changed
the resistor rating to 5 watts for this reason. Also watch out for small
resistors bad, like 180 ohms and 10 ohms at 1/8 watt.

Mark Z.

 

[Joe]

Mark, I put new a 2SA988 in on the bad channel, and replaced the
emmiter resistor R726. All else, I thought was good until I turned on
the rear channel and got the protection again. The only thing I can
test that comes up odd is the reverse voltage on D701 and D704 in the
bad channel. These both read proper (.5) forward, but 2.2 volts in
reverse. Teh good channel shows zero in reverse for these two diodes.
Everything else I check looks good.
What am I missing here. I'm ready to pull my hair out..

I could only get 3w parts for the R726, but I replaced both channels
since I had the board out and spare parts that were 1 watt greater.
The 5w parts were out of stock.

Thanks for the help.. I do have the schematic in front of me and can
see that the other two diodes that look good , happen to be 2.2 volt
zeners. ????

Joe

 

[Mark D. Zacharias]

Try unplugging CNP713. It's an 8-pin cable coming off the rear amp board. It
will disable the relay for the rear channels, but it should also disable the
protection circuit there as well, allowing you to troubleshoot.

Mark Z.

 

[Joe]

I did the tests again this morning with a clear head. The onlyy part
that has suspect readings in D704 . I am getting 1.6volts in reverse
when it should read zero. The other channel's same part reads zero.

Can anyone crose reference this part to a common number that i may
find in Radio Shack or in my parts box? It appears to be a regular
diode, and it backs up to a zener.

Could this indicate tha tthe part has failed, and could this cause
my protection circuit to trigger again?

Thanks, Joe

 

[Mark D. Zacharias]

D704 should check like a regular diode. It should be OK to remove the diode
for test purposes - it's there to help limit excessive signal to the
outputs..

Mark Z.

 

[Mike ZOD]

Mark Hi,

I have similair problem but frount left channel
checked all parts as best I could still blows left output pair.
I can post Sony's PDF of DE835 right off thier CD if you need it.
I could use as much help as possable to resolve this problem.
Seems we are not the only ones haveing this happening. I read a review
on this model in witch the owners repair center had help from Sony
support will ill results. File is 8.4 meg and is the actual service
manual, but there are no trouble shooting methods contained, only
schematics,board layouts, parts lists and the like.
Let me know where you might want me to upload??

Mike ZOD

 

[Mark D. Zacharias]

Hi,

I wasn't the original poster. I have the manual.

Thanks.
mz

 

[Mark D. Zacharias]

Sorry, didn't read your post correctly. We'll just need to start from the
beginning. Parts you have replaced, other parts to check, etc.

Mark Z.