Hi there!
I am repairing a Rotel RA 870 (not the BX) amp, and the phono stage has some hum+noise problem. I tried to find the schematic for this early version with all transistor stages, but on the web I found only the IC phono and gain stage variant's service manual.
I wonder if anyone here has the schematic for these circuits (the phono stage has 30 transistors on its board!). Any help would be greatly appreciated.
Greetings: Andrew
I am repairing a Rotel RA 870 (not the BX) amp, and the phono stage has some hum+noise problem. I tried to find the schematic for this early version with all transistor stages, but on the web I found only the IC phono and gain stage variant's service manual.
I wonder if anyone here has the schematic for these circuits (the phono stage has 30 transistors on its board!). Any help would be greatly appreciated.
Greetings: Andrew
There is very little difference between the BX and non BX version.
Download the Rotel RA-870 manuals for free - Hifi Manuals
Download the Rotel RA-870 manuals for free - Hifi Manuals
Well, then I must do it the hard way, without schematic (that thing is a nightmare to work on, the way it is put together with all those wire wrap connectors between boards.... ) Anyway, thanks for your replies!
I tried, power stage almost identical (with lower p.s. voltage), but the rest is not even close... phono board is 12 transistors vs. 30 in the 870, it has tone controls, the 870 doesn't, etc. 🙁
Thanks for the idea anyway.
Thanks for the idea anyway.
Thanks, but that is the power amp only, my problem is in the phono section of the preamp stage of the RA 870...
So, here we go again..... I found another problem that initially skipped my attention. On one channel (power amp) everything is fine, DC offset 4.5 mV, bias adjustable, set to 30mA per output transistor.
But the other, seems to get me, despite some experience in repairs. Whatever I do, the output is stuck around -200mV, and the worse part, it does not respond to bias adjustment, stays steadily at around 100mA. I checked everything,(statically) transistors, diodes, electrolytics, resistors, trimpot,: OK.
Actually the amp works, the protection does not engage, but the "bad" channel is louder than the other by about 6dB.
It is a PITA to work on, at least to measure voltages, because the opening on the bottom is rather small, so I cannot access many parts to measure, to get the board out is a nightmare, every connection is wirewrapped on it.....
Does anyone have any idea what might cause this problem? (oh yes, checked it with scope too, no oscillation either.)
But the other, seems to get me, despite some experience in repairs. Whatever I do, the output is stuck around -200mV, and the worse part, it does not respond to bias adjustment, stays steadily at around 100mA. I checked everything,(statically) transistors, diodes, electrolytics, resistors, trimpot,: OK.
Actually the amp works, the protection does not engage, but the "bad" channel is louder than the other by about 6dB.
It is a PITA to work on, at least to measure voltages, because the opening on the bottom is rather small, so I cannot access many parts to measure, to get the board out is a nightmare, every connection is wirewrapped on it.....
Does anyone have any idea what might cause this problem? (oh yes, checked it with scope too, no oscillation either.)
Attachments
Does the Vbe multiplier work? If gain differs by 6db maybe one of the two feedback resistors is open circuit.
Hello, I can not send the PDF. I send you picture of the sector phono RA 870.
There is transistorized. Use IC's.
Regards.
There is transistorized. Use IC's.
Regards.
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