**Denon amp oscillating, need help**

[silversweden]

**Im working on a Denon PMA 1060.
Want to have some tips from people who are good
at amplifyer repair and troubleshooting.

Have a nasty oscillation, read on.

Schematics here: www.itxpress.se/1060.pdf

The previous owner said that it had been struck by lightning, (maybe not a direct strike i hope

And after this it did not work. I replaced the fuse on the AC line wich had blown.

When i turned on the reciver the amp jumped off the table and smoke came out of it stupid of me to think that it was only
the fuse!


Outputs shorted and drivers in half.

I have replaced all the components i found broken.
But still the drivers go up in smoke after about 1 seconds.

I have a oscillation, i think its a oscillation.
A sine wave.. www.itxpress.se/2443.JPG <---- this is thesignal i get. Settings on scope: 1V/Div 20uS/Div

I have this signal on the 1and 3 legof the drivers.
And on the first and third leg of the outputs.

And it changes in amplitude if i try to set the bias.


What can cause this...must have help in wich direction
i should search because ive checked all the transistors
and semiconductors in the "power-amp" part of the
amplifyer and i cant find any defect componets here.

I have music signal on the input fets pre drivers drivers
and outputs.


Please take a look at the schematics and tell me what
you think. In desperate help after a week of troubleshooting
and still no solution.

[jaycee]

Check the small compensation capacitors. Also check the capacitor thats across the bias stage (C535).

Is there any sign of the oscillation in the first and second stages ? Check on the bases of TR505, 507 and the bases of TR509, 511

[silversweden]

Hi jaycee!
I have replaced both elektrolyts over the bias compensation
transistors.

Wich is the other compensating capacitors?
Are you talking about the smaller electrolyts in general?

I will check the TR505 507 509 511 when i come home from work!

I still think its strange that ill have this signal on the 3 leg of each
output transistor, it should not be any signal att all here when
i dont even have te outputs in place! or am i wrong?

Anyway, im going to check te transistor you told me about.

[jaycee]

Compensation caps would be C505, C509, C527, C529, C531 and C533. You may also want to check the Zobel network - C541 and R573. These are the left channel's parts numbers - check the equivalent parts on the right channel too. if the outputs went up in smoke, definitely check the zobel network parts as they are probably zapped.

You'd be best limiting the current some how rather than just powering on - since as Denon have foolishly omitted fuses on the secondary side of the supply! Wire up a common light bulb (not an energy saving one) into the Live wire of the supply so it is in series (ie, wall socket live -> light bulb -> amplifier live)

[silversweden]

there is a 4A fuse on the primary side of the power trans.
i use a 100w bulb in series with AC line

Dont help the drivers will fry in 10 seconds anyway.
Can i troubleshoot this with the drivers and outputs
out of the circuit?

Just to se if the oscillation dissapears before i put
new drivers in there. Or can the "missing" drivers
in fact make the oscillation appear?


I will check those parts you told me about on both
channels!

Stupid question maybe but what is a "Zobel network"
Can you explain that breif for me =)

Thanks!

[silversweden]

By the way,what do you mean when you say check the caps?
I dont have a capacitance-meter. I use to check caps with the
diod function on my DMM.

I also have a scope...

[jaycee]

Zobel parts are C541 and R573 in the left channel, and the equivalents in the right channel.

You could probably check capacitors with a 'scope some how but im not sure how. It might be worth just getting a cheapo meter thats got a capacitance check on it.

[silversweden]

okay, ill have the oscillation on the following caps.
c510, c528, c536, c534, c542

i also noticed that if i place a 47uF elyt over the
c534 the oscillation disapears..maybe it just smooths
out the oscillation, dont know.

this was a very very hard nut to crack

its so hard to track where it comes from, there is
so many point that have this oscillation

[GregC]

Hi silversweden,
I use an Equivalent Series Resistance (ESR) meter to test electrolytic caps.
It is very useful to identify bad caps especially in old equipment or equipment that has suffered "stress" i.e. high temperature or over voltage (like lightning)!

All electrolytic caps deteriorate over time & their life is shortened by high temperature as in power supplies. Most caps are rated at 85 degrees C as you may see on the side of the cap.

The advantage of the ESR meter is that it can usually be used with the capacitors in place to enable a quick diagnosis.

I bought my meter from EVB in Portugal, check the following link:

http://clientes.netvisao.pt/greenpal/evb1.htm

I have no connection with them, just a satisfied customer!

Best regards,
Greg.

[silversweden]

Hi Greg!
Can you check ceramic caps with that meter?
Or is it just for Elyt?