I took my Nakamichi DR1 out of hibernation of 20 years (bought new in 1994). I replaced the drive belts but one of the channels has no output. When I switched between Type I, II or IV positions there was an intermittent sound on the dead channel, but it would cut out again within a split second. I hooked the output to a scope and found that the dead channel has a large DC offset that can go from -4V (Type I position) to +3 to +4.5V (Type II or IV position). When the DC offset switched between negative to positive while switching between Type I or II position the sound would come on for a short time until the DC offset gets high again. It is like the DC offset saturated the amp. When I keep it playing for some time, the DC offset would sometimes come down low enough and the channel would play (but I can still see 1-2V DC offset).
I found the schematic from the internet, and was able to isolate the problem to be in the playback amp PCB. I replaced all 6 electrolytic caps and in fact found a bad smelly one from the dead channel (C206 in schematic). I thought that would fix it but the dead channel is still dead. The DC offset is still there. I measured every point in the schematic and compared the measurements on both channels using a scope (while playing music), so I was able to see the signal plus DC offset. Both channels measured very closely except the output from the bad channel has a DC offset after the capacitors on the output side. The output is AC coupled so I couldn't figure out where the DC offset came from. Everything to the right of the dotted line in the schematic matched very closely between the channels but the left side has a DC offset in the bad channel.
Anyone has an idea what is going on?
Thanks.
I found the schematic from the internet, and was able to isolate the problem to be in the playback amp PCB. I replaced all 6 electrolytic caps and in fact found a bad smelly one from the dead channel (C206 in schematic). I thought that would fix it but the dead channel is still dead. The DC offset is still there. I measured every point in the schematic and compared the measurements on both channels using a scope (while playing music), so I was able to see the signal plus DC offset. Both channels measured very closely except the output from the bad channel has a DC offset after the capacitors on the output side. The output is AC coupled so I couldn't figure out where the DC offset came from. Everything to the right of the dotted line in the schematic matched very closely between the channels but the left side has a DC offset in the bad channel.
Anyone has an idea what is going on?
Thanks.
Attachments
Update: I gave the PCB a good clean (not that it was dirty to begin with) and the DC offset was reduced substantially, from a few volts to a few 100 mV. Over the course of a day the offset was down to a few 10 mV now. But it still wouldn't completely go away. I suspect there was a leakage between the traces in the PCB but it was hard to find. The circuitry downstream from the playback amp can deal with a few 100 mV DC offset so the deck seems to play flawlessly now. Man, the DR1 sounds so nice that I had long forgotten about.