Amplifier Denon-PMA-680 rele Speaker protection out of service

C

Chilerwater

New Member
Dear Moderators, pls locate this thread were applicable. Thank you.

Hello,

This time I´ve obtained Denon PMA-680R from a well known auction. The amp is apparently dead. The The amplifier turns on perfectly but the speakers are not heard.
I have reviewed the electronics and the problem is because the protection relay does not reach tension. I put in the post plane of the circuit.

I changed the capacitors and checked the transistors and resistors not detecting any fault. What I notice is that I cannot see 8,9 volts after the R415 and that it goes to the base of the transistor T405.

any clue in order to continue?

regards,
 

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Last edited:
Suggest start by checking for excessive dc voltage on the left and right
speaker line.

Connect black multimeter probe to chassis or any black speaker post
Connect the red probe to the left (+) test point near R349 (see pic)
Expect less than 100mV. More than 0.5Vdc would be a concern.

Repeat for other channel. Connect red probe to right (+) test point
near R350. Again, less than 100mV expected
DenonTPs.JPG
 
Thank you very much
In this moment I am in my job but when I will ariive at my home I will check it.
Tomorrow I will post the test.
 
Dear.

I have cheking dc voltage on the left and right speaker line.

And I have 13 Vdc in left and 13 Vdc in right. I have checked the TR-319-320-321-322 and it's ok all.

any clue in order to continue?

Thanks
 
Yeah, looks like the 13Vdc is triggering protection.
Next step is to identify the faulty channel. I'm assuming
the previous measurement was +13Vdc.

Probably need clips for this, measure the dc voltage
between left (+) test point near R349 (RED probe) and
the right (+) test point near R350 (BLACK probe). Probably
need mV scale. A positive result (+xxmV) indicates faulty
left channel while -xxmV points to right channel.

The next step would be to measure voltages at b,c,e at
TR316,8, 320, 322 (assuming faulty left).

With amp powered off/unplugged can you measure the resistance
of R323 and R324, 68ohm fusible, try measuring in circuit.
 
Dear,

I'm assuming the previous measurement was +13Vdc. It's correct..

Measure the dc voltage between left (+) test point near R349 (RED probe) and the right (+) test point near R350 (BLACK probe). I have -6,0 mV. I understand that the problem is right channel.

With amp powered off/unplugged can you measure the resistance of R323 and R324, 68ohm fusible, try measuring in circuit. The resistor are good, they haver 68 ohm.

The next step would be to measure voltages at b,c,e at TR316,8, 320, 322 (assuming faulty left). For me is right channel, but I have checed all transistor and all resistor.

upload_2018-4-20_8-36-35.png

The tension are reference at (-) that power supply.

The resistor R-345-333-347-335-346-334-348-336 are good but the mesure in circuit, not mesure aut the circuit.

Thanks.


 
Little busy at the moment, will need some time to digest measurements.
Initial analysis suggest both channels have the same problem.

The b/c/e measurements don't align with the service manual, appears to be missing
about negative 15 volts, the voltages at TR317,8, 321,2 base should be
negative but measure positive.

Problem might be failed TR325 affecting both TR311 and 312.
Suggest measure dc voltage at TR325 and 311,2 b/c/e
 
Not sure what you mean by this (it's late). Where was the black probe connected. Will check results.

The black probe connected as:
upload_2018-4-20_12-5-4.png
 
I have checked en TR 325.
b____ -50 Vdc
e____ -50 Vdc
c____ 13 Vdc

I go at remove the TR325 por to check and I will put the results.
 
indeed the TR 325 was broken.

I have changed it for the BF 423 and the relay is already activated.

I have tried it and it has been working for 1 minute when the source direct is pressed. The relay has been deactivated again and it smells like burning.

Next Monday I'll post news

Thank you very much.
 
Chilerwater said:
What it's correct?
Click to expand...
Both are correct, one view is from the bottom.

You probably have a short in the power amp that's causing TR325 to fail/burn.
You probably need to build a dim bulb tester, it's a current limiting device, many plans on OK.
Need to solve the short then replace TR325. The short is the root cause of TR325 failure.
 
This fault it's diferent. The TR-325 is good. I have -50 v en b,c,e,
Learning from you, I checked the right and left channels.
And I have 50 v in (+) test point near R349 and 0 v in (+) test point
near R350.
So I have problem in cannel left.
I have checked the R 323 and this have 90 ohm.

It's too late, tomorrow I'll continue
 
Sorry, I'm wrong is the cannel right.
The TR318 is short.
I think it is possible that it was broken because it had the collector solder TR322 bad (loose).

The TR322 is good

Tomorrow I'll buy one.
 
Dear.
Today I have changed the TR318, but the problem continues.

I have the I have the following voltajes:
View attachment 1171377

And the dc voltage between left (+) test point near R349 (RED probe) I have 0 dc.
And the dc voltage between right (+) test point near R350 (BLACK probe) I have -48,7 dc.

If I have -48 vdc near R350 I understand that the problem is in channel right.
 
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