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Old 29th March 2011, 08:43 PM   #1
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Default Denon POA 2200 - TO LOW IDLE CURRENT

Hello every.

Hoping for some advice. I have the right channel distorting when playing music. After a quit measurement i found out that i only have about 0,37V on the base of the outputs with the idle trimpot adjustet to "max".

I guess i have whats called cross-over distortion. I have been trying to find out why i cant get the voltage is low.

Can someone look at the schematic and maybe give me som advice?

http://vintageshifi.com/denonpoa2200.pdf

I am going to change all the elyt.caps on the amp board and i am hoping that this will solve the voltage problem. Or maybe not?
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Old 29th March 2011, 11:09 PM   #2
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Quote:
Originally Posted by silversweden View Post
Hello every.

Hoping for some advice. I have the right channel distorting when playing music. After a quit measurement i found out that i only have about 0,37V on the base of the outputs with the idle trimpot adjustet to "max".

I guess i have whats called cross-over distortion. I have been trying to find out why i cant get the voltage is low.

Can someone look at the schematic and maybe give me som advice?

http://vintageshifi.com/denonpoa2200.pdf

I am going to change all the elyt.caps on the amp board and i am hoping that this will solve the voltage problem. Or maybe not?
it should read 8 mV on VR504. I had the same problem on my poa 1500. On mine, there was a transistor (1815BL) on the back of the right channel board that was not soldered correctly. Start with that.
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Old 30th March 2011, 04:53 AM   #3
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You mean when adjusting VR504 i should have about 8mV
on T.P for bias adjustment?

I will start to check the solderwork on TR522

Strange thing is that according to schematic i should have
+49mV where L502 is placed, this is going in to the IC502 OP.
I only have 3,5mV!!
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Old 30th March 2011, 05:51 PM   #4
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Hello again everyone

I found a pair of bad solders on the board and i also changed all
the electrolytics now. Frustrating that i have exactly the same
clipping sound as before.

I took some pictures when measuring with my scope.
This is a 100Hz test tone from cd-player.

With speaker relays off: (right channel)
Click the image to open in full size.

With speaker relays on: (right channel)
Click the image to open in full size.

With some music it looks like this:

With speaker relays off: (right channel)
Click the image to open in full size.

With speaker relays on: (right channel)
Click the image to open in full size.

I get distortion as soon as i hit the relay switch.
I have a test speaker connected.
All measurements is taken from both the output
transistors and the coil before the relay.
Get the same result.

When i release the speaker relay the distortion is
gone again. Why?
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Old 30th March 2011, 07:37 PM   #5
CBS240 is offline CBS240  United States
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perhaps one of the driver transistors has become disconnected somehow. This might explain why the signal is only corrupted under load. Check the voltages on the driver stage in comparison with the other channel, you said you found some cold solder joints elsewhere.
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Old 30th March 2011, 10:27 PM   #6
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Looks like something i've seen a while back - now's the time to use that cheap hfe tester on your multimeter that you thought had no use at all. Lift all the small (usually to-92) transistors in the input stage and measure them. Chances are one of them will have much reduced beta.
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Old 31st March 2011, 04:59 AM   #7
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Thank you foor the advice! I will try this tonight
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Old 1st April 2011, 02:03 AM   #8
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Quote:
Originally Posted by silversweden View Post
Thank you foor the advice! I will try this tonight

any luck?
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Old 1st April 2011, 05:51 AM   #9
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Yes i actually repaired and destroyed it in the same time! ):

There was 2 more cold solders on TR523 and 525, they had a small
almost invisible crack, had to push the transistors a bit to identify this.

I resoldered them and cleaned the board a bit, i then powered the amp
up (without the heatsink mounted).

I could now set the bias correctly.
Strange thing is that left channel sounds OK but the bias is way to low for
having VR in max position, i think there is some cold solders in L channel as
well.

Okay now the sad part, after discovering that i could set the bias i
thaught that i should just test with my 100hz sinewave and measure
the signal all the way.

I plugged the signal into the wrong terminal i think, it all happend so sudden =)
The amp has a CD-input with volume knob and a "normal" with full volume all the time.

Anyway after about 3 seconds with the sinewave testing i heard a strange sound
and shortly after there was a (LOUD!) sound, like a gun going off =)

C587 exploded! Desolderd all the outputs and yes all 4 of them are shorted.
Dont know if i should cry or laugh. Right now im laughing because i dont want
to be sad =)

My guess is that there was nothing wrong with the amp but several mistakes i did
made the output transistors overheat and fail.

1. I set the bias according to service manual, waited for 30-40 seconds and
readjusted because the bias was going up all the time.

I should have mounted the PCB to the heatsink allowing it to thermal compensate correctly.
I should also have watched the bias a couple of more minutes before testing anything.

2. I did not mount the heatsink because it was easier to work without it.
Maybe the transistors would have survived correctly mounted.

3. I did not use my "bulb in series with the AC cord" as i use to! Maybe i lost
respect because the problem was never overheating until i fixed the bad solders.

Sad but true

I still wonder if it was just the combination of not waiting for the bias to
stabilise, not having the transistors clamped to the heatsink, and driving it
to hard this way with my cd 100hz sinewave or if it was something else
that killed the transistors.

There was no speaker load connected at the time so it could not draw any
current trough the output-transistors!

This leaves it to the bias current killed my amp because of not waiting for
bias to stabilise and not having it clamped to the heatsink i guess.

Any reflections would be welcome have ordered some new (expensive) parts........

Last edited by silversweden; 1st April 2011 at 06:04 AM.
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Old 1st April 2011, 06:04 AM   #10
GoranB is offline GoranB  Poland
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Silversweden your mistake - HEATSINKS!!!!!!
Check all before you solder new parts. Maybe more transistors are damaged, not just outputs. And dont forget heatsink.
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