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Denon PMA-777
Denon PMA-777
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Old 12th October 2010, 07:42 AM   #1
horias2000 is offline horias2000  United Kingdom
diyAudio Member
Join Date: Oct 2010
Default Denon PMA-777

Hey guys,

I'm working on repairing a Denon PMA-777 and I got stuck.
Looking at the attached schematic, TR311, 315, 313 and 317 (output stage) were all destroyed. Going further, TR307 and 309 were also smoked and also TR303 and TR305. After replacing them, and some resistors and diodes, I still do not have good voltages, as shown on the schematic. I should have +/- 1.7V on the bases of TR303 and 305 but I have - 0.8V and + 3.5V. I removed IC301 and the voltages are now +/- 2V...at least they are symmetrical but still not OK. Going further towards the input, everything seems OK. As far as I can tell, the input stage is a class A amplifier and the output is Class B.
I have a few questions. First, what is the role of IC301? The output is linked to the +/- 1.7V rails. I looks like a voltage stabilizer but I'm not sure. I will try to replace is with OPA2134 as I have this IC at home.
Can anybody give me a few suggestions? I would really like to repair this amp but I got stuck. One channel is OK but the second one isn't. The amp got smoked because of overheating. It was repaired before and D309 (MV1YH) was changed with a 1N4148. I know that this is not ok but I couldn't find any replacements available in Romania. I was looking of replacing it with NTE601.

Thanks, any help is greatly appreciated.

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