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Denon PMA-777

 
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horias2000



Joined: 03 Dec 2010
Posts: 4

PostPosted: Fri Dec 03, 2010 9:09 pm    Post subject: Denon PMA-777 Reply with quote

Hello to all,

I'm working on repairing a Denon PMA-777 amplifier.
One channel is working but the other one not.

There were a few transistors burned, including final ones, drivers and pre drivers. I changed all of them but still no success. The thing is that I get a DC voltage at the output of the first stage (TR213 and TR215). The DC is around 100mV.



Does anybody here have some experience with Denon's?

Please help.

Thanks,
Horia

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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Sat Dec 04, 2010 2:30 am    Post subject: Reply with quote

I used to deal with Denon products frequently. In general, when/if you are faced with shorted txistor(s), you MUST check any low value resistors(fusistors) all the way back to the front end.--(marked as FR & NB) Also, leaky diff pairs or diodes can easily upset dc bias. As long as you have a known good channel, you can do a quick compare of resistor readings at several random points thru the offending circuit. OR if you have a curve tracer/tracker, you can do the same thing and compare any discrepancies in the resulting signature patterns.
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horias2000



Joined: 03 Dec 2010
Posts: 4

PostPosted: Sat Dec 04, 2010 8:06 pm    Post subject: Reply with quote

Hey vtech, thanks for the reply.
I replaced a couple of seriously burned resistors. I will look closely at the ones you specified. I think that the root of the problem is that 100mV on the output of the first stage. After this stage, IC301's output is totally different than the one on the schematic.

I will get back with news, hopefuly god ones Smile

Thanks again
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vtech



Joined: 08 May 2006
Posts: 1264
Location: USA

PostPosted: Sat Dec 04, 2010 10:11 pm    Post subject: Reply with quote

Backup all the way to the FET's and work your way forward. Remember, it is critical to have a balance of plus/minus readings at each stage-- not so much as the value indicated on the schematic. Of course the 100mv is too high. Also make sure D301,D303 are clamping correctly at plus/minus 15V and if in doubt for a reading, use the working channel as reference.

...& watch out for the slipping probe!
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horias2000



Joined: 03 Dec 2010
Posts: 4

PostPosted: Wed Dec 08, 2010 9:51 am    Post subject: Reply with quote

I'm working on it now. I started mapping all the voltages and currents of the good channel. I will then start measuring the bad one and compare it to the good one. I hope to discover a bad resistor as all the semiconductors are new on that channel, I changed them all Smile. I will also look at all the diodes to make sure all are ok.

There is still one thing, D309 (MV-1YH), was changed with a 1N4148 in the bad channel. As far as I can tell, MV-1YH is a temperature sensible diode. The 1N4148 was "glued" with thermal paste to one of the pre-finals. I do not like this setup at all, I bet that 1N4148 does not vary with temperature as MV-1YH, but I can't find this diode. I found NTE-600 and it seems to be similar to MV-1YH. What do you think?

Thanks,
Horia
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horias2000



Joined: 03 Dec 2010
Posts: 4

PostPosted: Fri Dec 10, 2010 7:47 pm    Post subject: Reply with quote

Well I measured the good cannel and then the bad one and I can't find what is wrong with it. It all seems ok until TR207 where the colector voltage is around 8V and in less than 10 seconds it rises to 10 and it keeps going up. It reached 15V and it was still rising. The thing is that the base voltage of TR207 is the same. I do not think this is the reason of the 120mV at the exit of TR213/215 as the 120mV is there from power on. The voltage on the colector of TR207 migh be a secondary effect of the 120mV. I'm very close to giving up...maybe I'll try looking at it in the morning, who know. If you have any thoughts on this, please share Smile

Horia
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