spam@... wrote:
My Tek model 570 vacuum tube curve tracer seems to have
lost the ability to keep the high voltage running. When
it is first powered on, I get a nice trace, but the HV
oscillator tube (6AQ5) looks like it's really unhappy
about it. In a minute or so, the high voltage goes
away with the usual dim expanding trace.
I've replaced the tubes, and don't see anywhere the
supply is overloaded, so I'm guessing a failed
capacitor or shorted turn in the transformer. Anyone
know where to get small quantities of the 1,2,3, and 4
kilovolt capacitors? I've found about half of them,
a couple apiece at five different vendors (I'd end up
spending 10 times more in shipping than in parts).
Are some capacitors more likely to fail than others?
I don't have a high voltage capacitor tester, though
I plan to go through it with the one I do have if I
can't just replace 'em all. And if I'm really unlucky,
anyone know where I can obtain a replacement HV transformer?
Failing all the above, can anyone recommend a place to have
classic Tek gear repaired?
-- Thanks,
John Rehwinkel
spam@...
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Hi John,
You can use just about any scope as a high voltage capacitor checker. You
just set the vertical input to DC coupling and some fairly sensitive range
like 0.1 volts/division. You then (carefully) connect some low ripple DC
(like the +500 volts from an older Tek scope) to one end of the cap and
touch the other end to the scope input. A very good cap will not leak DC
and the scope trace will return to the screen when the cap is charged. A
leaky cap will cause the trace to remain off screen. You can actually
measure the amount of leakage by realizing the scope is putting 1 megohm on
the end of the cap to ground and forming a DC divider with it. By measuring
the amount of DC deflection on the scope and knowing how much DC you are
applying to the cap, you can actually make a pretty good calculation of the
internal DC leakage resistance of the cap. It should be very very high in a
good cap . . . To get familiar with this testing technique, try some known
good mylar caps (very low leakage) and then try some older "black beauty"
type caps (usually quite high leakage). Make sure you don't apply more DC
than the cap is rated for. Less is OK but won't give you as good a test as
testing it near its rated voltage. The 500 volt supply in an older Tek
scope is DEADLY. BE CAREFUL.
I know of no source of good 570 High Voltage transformers although I know
one guy who rebuilds certain old Tek HV transformers. I am not sure he does
570 transformers, though. He is: Bill Schell, 10102 Winder Trail, Orlando,
FL 32817 Phone: (407) 282-4289.
I had a HV transformer failure in my early 570 and found that I could
substitute a HV transformer from an RM15. It provides a slightly different
CRT cathode voltage than the original but it was close and I think I can
make it work OK if I adjust it to keep the total CRT accellerating voltage
the same as it was originally. I have only partially restored my 570 so I
don't really know if this is a good solution or not.
Stan
w7ni@...
