transistors Marantz 1122 dc

[Depaj]

Yes indeed, there is a possibility to couple it or to bypass the coupling cap but there shouldn't be any dc coming out of the preamp when the only signal I feed in is a 1kHz sine wave. Right after the preamp stage there are some 50v 4,7µF and that's where I measure the dc (it's a coupling cap between the preamp and the tone amp). The dc goes up whenever I turn the volume button and then settles again at 0v but that's enough to open the relay each time. When I switch to AC coupling between pre and power amp there is no problem.

 

[Ketje]

If the DC changes when you turn the volume there is DC on the vol-pot, that's not normal.
Mona

 

[Depaj]

I checked and have no dc on the volume pot, not even when I turn it. Here's the schematic for the preamp, I put the reading before the tone amp because nothing changes when I bypass it so I assume that's not where the fault lays.

 

[Depaj]

I took a risk and turned the amp on without the current limiter and the dc problem is gone, does that make sense, is it possible that with the voltage drop caused by the bulb the circuit couldn't follow ? I do have 46mV of bias now and that seems way too high. As the manual doesn't say anywhere what the bias should be it's hard to figure out if this is how marantz intended it or not (with pioneer sa-7800 the bias setting is at 66mV so could 46mV be ok here ?)

 

[Depaj]

I found that one of the bias transistors was open base to emitter so I replaced all four of them by MJE340/350. For a while the amp seemed to work fine although DC offset was a bit jumpy (that seems to be a common problem with this amp). The bias adjusted fine and I could listen to some music with headphones for a while. Also the little heat sinks were a lot less hot than before. I was getting quite excited but than the horrible whistle appeared on the left channel (same as before). I measured with the oscilloscope and it turns out to be a 5MHz sine wave (strange because I can hear it) . Something appears to be oscillating but I don't know where to look.

 

[Depaj]

Finallyyyyyyy !! As I did a few times along the way I left the amp sitting for a few months and decided today to give it another chance, guess what, I found the problem right away. A little polystyrene cap between the collectors of the differential input was cracked open on the bottom, touching the cap would either get rid of the problem or make it worse. I replaced it with a WIMA for now and no more oscillation. It's a 1600pF polystyrene, hard to find value, would I be ok with a 1600pF polypropylene or should I stick with polystyrene and get a 1500pF or 1800pF ? (It's C704 on the schematic BTW).