denon pma-900v no bias/idling voltage

potatopie17

I bought this amplifier broken not to long ago and the right channel did not work. All of the emitter resistors .22 ohm were open and the 4.7 ohm resistors were out of whack so I suspected bad final transistors which it was. I also found burned open resistors r552, r560, r596, r424, r432, and shorted d402, tr412 and tr416. I replaced all of the right side amplifer transistors and parts except tr402 and tr404 which are fets. and matched up the hfe on the transistors. Turned on the amplifier and nothing seems to be shorted and the relay clicks on. I cannot adjust the idling current. it just reads 0 all the time and will not change. its supposed to idle at 8mv +/-3mv. The left after 15 minutes idles at 6.6 like it supposed to. So I went and started doing voltage check differences between the left and right channel. I found that d504 was zennoring at -8.8 and d502 zennoring at +8.65. So I replaced both of them and now they are both zennoring at the same voltage now. but the amplifier still will not idle it just stays at 0. I also replaced the op amps and it did not help. This amplifier is dc coupled with some kind of differential zero feedback circuit I think, well thats what I read when I googled. And I have never really learned much about op amps so I am sort of not sure what to replace or do. when I check the base at the finals im only getting around 300mv on + and - side when it first starts up and it just slowly drops to near zero. but the good left speaker side just stays solid around 1v i think. I am attaching the idling page and schematics from my service document.
I hope somebody can help.

megasat16

You are having issue with the right channel only? But for measuring voltage, you can reference to the left channels since both circuits are the same for all output and driver stage.

You need to reference all measured voltage to the left channel in the schema and take more voltage readings.

Zener d504 should be at -8.7V and the D502 at +8.7V. Did you replace with the same type Zener or use a general purpose zener?

At the base of TR508 should be 1.1V and TR510 should be -1.08V respectively. If they are not right that voltage, check the pre driver circuits in 2nd page starting from the FETs. Measure voltages with reference voltage to the left channel in the schema.

I wouldn't worry about op-amps till you've checked the pre-driver stage. Since you get the amp out of protection, I think there is something in the pre-driver circuit you need to look at.

potatopie17

the zenor diodes, I crossed the original part number with nte part numbers.
But all the transistors are originals.
I will check the voltages in the pre driver circuits.
thanks.

potatopie17

I have done some voltage checks. The voltages on the pre driver section seems to be ok. The voltages on the amp section right where the op amp is are reading strange. 0.7 volts off the output of the op amp when its supposed to be .145v. I can turn the trim pot one way or another and the voltage varies between .45v to .95v. I also tryed putting in random unmatched hfe transistors and it did not really seem to effect the problem. I also have all of the electrolytic caps replaced. I have taken out almost all of the ceramic caps and checked them and none of them were even out of tolerance. The parts I have replaced are all of the transistors except the 2 fets and all of the diodes except d508 & d510 which are 7v zenors. Ive replaced the op amps and whatever burned up resistors there where. And have checked like all the resistors at least 3 times. I have attached the scanned pictures of my voltage checks on the pre driver section and the amplifier section.

potatopie17

Thanks for the help guys. I built me an oscilloscope buffer for my soundcard from diy plans I found on internet and finally fixed the problem. I ran a 1000hz sine signal into the input, and the waveform was ok until I got to the amp section. I noticed the waveform was really big on the pnp transistors negative side of the amp. And the waveform was big all the way to the finals. But when I checked the the npn transistors positive side of the amp. They had that same big signal until I went on the other side of r552 a 100 ohm resistor. The signal was small? R552 was open. Its the resistor that connects tr504 and tr508 together. I replaced the resistor and now I have very beautiful sound. I have been been playing it for the last few weeks hooked up to a pair of kef model 102 speakers back from the 70's and they sound very open. They are 4 ohm 200w speakers. The pma-900v barely even gets warm.

ben62670

Do you have a link for the DIY oscilloscope?
Thanks
Ben