On Fri, 23 Apr 2010 10:38:23 -0700, "Chris F."
wrote:

Schematic of the DG-9 model:
http://www.curtcass.com/detectron/dg9-sch.jpg
Probably quite similar to the DG-7.

I came across that already, but it's quite different and therefore not much
help.

Oh, I was hoping they were similar. Sorry(tm).

Replace it with what? Is silicon, the B+ is probably too high.

I tried putting a decade resistor in series with the diode, but even at
100k-ohms it still didn't make a difference. It still oscillates even with
the diode disconnected.

I don't know exactly what series resistor is going to work. 100K is
certainly too large. When I was doing the same thing on much higher
power ancient Motorola radios, about 10 ohms 25 watts was about right.
I'll probably get yelled at for this suggestion, but use an ohmmeter
to mearsure the selenium recifiers forward resistance and use a
resistor with a slightly lower value. I dug around with Google and
found recommendations varying from 30 to 90 ohms for low power
devices.

http://wiki.answers.com/Q/Can_you_replace_a_selenium_rectifier_with_a_silico n_rectifier

Yes, I checked these with a tube tester, and even trying subbing them with
new-old-stock tubes.

The tubes are probably fine since it does go click on your vasalene
marbles.

Here's a photo of the circuit:
http://s282.photobucket.com/albums/k...=detectron.jpg

Yech. Note the big paper electrolytic cazapitor. I can't tell if
it's a power supply filter cap or across the meter to smooth the
output. If across the power supply, replace it as judging by the age
and type, it's almost certainly dead. If you have an ESR tester, use
it. (If you don't have one, buy or build one).

I'll stand on my comments that your clip lead and external power
supply kludge is probably causing problems. Find some suitable
electrolytics and place them across where the battery would normally
connect. The idea is to reduce the impedance that the counter sees in
the direction of the power supplies. That should reduce the
motorboating (oscillation).


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