2238 Power Supply

[ocdecio]

As per the Service Manual, I should be getting 46V at the collector of Q803. However I am getting 47.6V which I believe is responsible for the regulator becoming too hot (around 160F). The voltage before R801 is 50V.

I am pondering if I should up the R801 resistor a bit to get it to the 46V prescribed and hopefully tame the regulator temperature but wanted to ask if there could be issues doing this. I have a 27ohm 2W resistor I was planning on starting with.

 

[Steven Tate]

I wouldn't think 1.6V would make much of a difference at that spot. It sounds more like a current problem than a voltage problem to me. Have you tested R801 to make sure it is OK? If it is, I would be checking Q803 and all of those resistors around it. Something is letting too much current through R801 if you think 160 degrees is too hot. That is 71 degrees Celsius, which I think is not uncommon in these power supplies.

 

[rBuckner]

That voltage varies with the line voltage and the drop across R801 seems about right. I don't think anything is wrong. I would replace Q803 with the higher rated MJE15030. That old guy had had a long hot lifetime of work!

 

[Leestereo]

...I am pondering if I should up the R801 resistor a bit to get it to the 46V prescribed and hopefully tame the regulator temperature but wanted to ask if there could be issues doing this. I have a 27ohm 2W resistor I was planning on starting with.

Increasing the R801 resistor to 27ohm from 22ohm is OK, it will help lower the heat dissipated by Q803 a bit. In addition to the MJE15030 recommended above, KSD526 is a good replacement for Q803 (2SD330).

 

[petes'picks]

Hook up receiver to a variac set to 117v then take the reading again.

 

[ocdecio]

Steven was right on the money actually, thank you everyone for your help. It was a somewhat complicated issue - one of the outputs was not entirely failed, but just enough to pull more current than usual on left channel. I kept seeing the drivers on that side get warmer than the other but since it is close to the power supply I didn't think much of it. Until of course R757 10 ohm at one of the drivers started to smoke. It took down that driver with it.

So what was happening is that channel was drawing murh more current than normal, which made the power supply sag with MJE15032. I replaced it with TIP31 and it kept it stable but in doing so it took down the driver (which was being subject to higher currents due to a failing output)

Please let me know if my analysis makes sense. It took me longer than it should have for me to just decide to refresh the outputs because I have a tendency of keeping the OEM whenever possible. Maybe that makes me change my mind.