Audio Research D70 mark1 + schematic

I agree with Johnson. If you only have 1 volt drop across that regulator, it could be shorted.

Why the regulator is shorted might be the bigger problem. They normally go short from too much current draw in the load, which means you might fix the regulator only to reveal the load problem... the root cause.

When the current draw gets REALLY high, they first short out, then blow open, so it is not likely an extreme overload.

They can also go short because they have exceed thermal capacity, they got too hot. Older gear can have age related problems with heat sinking that causes thermal death.

Or in some cases the design was just pushed a bit outside of the envelope of the parts capabilities. Sort of an unintentional time-bomb. If that is the case, be prepared to replace it again in 2047. =-}

Well, This is a pretty old thread now, but maybe I'll get lucky.
I have a D-70 I'm troubleshooting for a drop in the bias after about 4-5 min of warm up.
I've have it current limited on a variac and was initially elated when I was able to bring the screen voltage up to spec by adding some capacitance across a 530 uFd cap. 
This seemed to cure a slow cycling splutter sound coming through the outputs and a day later, I tried adjusting the bias.
I found at the higher voltage AR recommends for setting the bias, that, although the spluttering was gone, it began drawing current after about 4-5 min.
I found this was due to a drop in bias voltage, which I've been trying to find the cause of since.
I tried monitoring the pre-regulated bias supply while separately monitoring the post bias supply.
I found the pre-reg stable at -54.6 VDC (Spec is -57) and initially the regulated was 30.5, but began dropping after 4-5 min.
The zener in the circuit measured neither short, nor open.
I've heard the circuit board is a bit of a nightmare to work on, as it's very thick and the tracings lift easily, so I'm trying my best to determine a cause before just tearing into it.
Any ideas?
Thanks

P.S. I realize the post regulated bias spec of -30.5 is low, spec being -37, but it's the instability that really hurts.
Thanks again