Did you figure out where the yellow electrolytic in the photo was
connected? Power supply filter or meter integrator?

It's connected across the meter.


Find an oscilloscope that can tolerate some high voltage (i.e. 100x
probe). Look at the power supply line. Got pulses? If so, get rid
of them with a filter cap.

Already tried that.... or are you talking about the AC line feeding my
transformer/rectifier/cap power supply?


Old carbon composition resistors tend to change value with age (and
moisture). With the power off, you might walk though the pile and see
if there are any that are radically off value. I don't think this is
causing the problem, but it might be contributory.

Checked them all, and all are well within their specified tolerances.


I wonder if a row of 10 9-volt batteries would work?

Probably, but rather expensive. It should work with your external
power supplies, but due to the apparent lack of power supply
filtering, it might be oscillating.

If all else fails, trace out the schematic. It doesn't look all that
messy and you can probably use the DG-9 schematic as a guide. The
author of the web site where we found the DG-9 schematic is the
founders son. Send him email asking if he has a DG-7 schematic or if
he knowns anyone that can help.

I might do that. Can't think of anything else it might be, as it's really a
rather basic circuit. BTW that small tube with the 2 leads I mentioned is a
5841 (voltage regulator), and I don't have a sub for it.