Audio Research D70 mark1 + schematic

Now this D70 has got new filtercaps ! One thing I notice is that if the wipers on the biaspots should lose their contact
completely or partial there will be a currentrush in the powertubes . Judging from the position that these wipers have now with 65 mA through the new 6550:s ( Tungsol reissue ) it
is possible to add a resistor to each biaspot .... between the wiper and the side of the pot that have -47 VDC . I think this
resistor can have 120K or a bit less resistance. Kevin O´Connor describes this security - resistor in one of his books
about guitaramps . If the wiper should lose contact full bias-voltage will turn down ( choke ) the 65 mA current to a lower value ...... Wouldn´t it be possible to add this "mod " to a D70 ???? Thanks for some comment. Stewen

I've never seen a bias pot fail in a D70 or D115. They can get noisy however. It's 65mV as you are measuring across the resistor.

Hallo again ! I have been working with other items lately but today returned to the nice D70 . Measured the voltages at the
TIP 41 regulator circuit as the former advice pointed out as important . The correct values shall be 335VDC and 327VDC so
the difference is about 8VDC . I have 338 and 337 VDC !!!
The only component I´ve checked so far is the 1N914 diode which was OK. Can I check the zeners (2)? What component is
probably failed ? The IC ? The transistors is in a Darlington coupling . Think I can check them in the circuit,
right ? I mean , I don´t have to desolder to measure them ??
Well , thanks for any help , advice . Regards Stewen

TIP41 likely shorted out.
Collector should be about 340 and the output to the B+ for the 6922 should be 304V ( in the D115)

The D70 and D115 are hard to troubleshoot.

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. =-}