An Afternoon with the Tandberg 3001 Supply

Rather than try a bunch of things and have then be wrong, I ordered lots of spare transistors of the one's I could order, and I spent most of the afternoon today looking for the bad one in the 25V supply.

After replacing a host of little transistors I finally found the one that was causing the problems. The supply is 95% done. The critical 15V and 25V supplies are working perfectly. It wasn't a complete waste of time though, a lot of the transistors were being upgraded to 65V parts, which will make this unit much more stable should I ever have to remove it and modify it on the bench again.

Unfortunately, I'm still having serious problems with the two 5V supplies though. Let me share with you my current (haha!) problems. Take a look at the relevant part of the schematic:



To the left of the 33 Ohm/4W resistor is 15 Volts, as it should be. The problem I'm having is that the two transistors you see here should be outputting 5 and 5.2 V respectively. However, there is too much of a voltage drop through R822, leaving me with something like 2.8 V instead of the 5.9 I should expect. The problem seems to be the current draw through Q814(Q813 on the board). The way I see it, in order for the power supply to behave as documented, there should be about 4.1 mA through R827, and about 1.4mA through Q814. Instead, I'm getting 4.8 mA through Q814 alone. What I don't understand is WHY. The parts all seem to be working correctly. If I remove Q813, and replace Q814 with a brand new part, I still have the same problem. I can't get the voltage at the base of Q813 up to 5.9V. And from what I can see, besides actually changing the value of R827 I really don't have a chance. The input voltage remains a rock solid 15.04 volts throughout though.

I've removed the electrolytic, and measured all of the resistors. Everything is measuring what it should.

Suggestions? Is the schematic bogus to start with?