Re: Type 503 Oscilloscope Issues

Dave Casey

The fact that the new caps took significantly longer to charge until the
meter went out of range is a good sign that the old caps don't have a lot
of capacitance anymore; the comparison assumes that you still left the
multiple sections of the cans tied together for your tests.

If any of them are particularly leaky once they get up to working voltage,
then you may not see improved performance by the power supply until you
remove the offending part from the circuit. That is to say that temporarily
tacking on the new caps will not necessarily show signs of improvement if
the old caps have high leakage current and are still connected. Temporarily
connecting the new caps with the old caps disconnected before you go
through all the trouble of hollowing out the cans and making the
installation more permanent is a good test. I suspect you will see marked
improvement, but I've been wrong before.

Dave Casey

On Thu, May 18, 2017 at 1:29 AM, enchanter464@... [TekScopes] <
TekScopes@...> wrote:

Hello all,

From the past week, I only had the chance today to look at the scope (but
the new capacitors came in yesterday), so just reporting in:

Doing the simple resistance test on C682 and C684, with the leads
disconnected, both seemed to show a similar trend. C684, on the 20k
setting, gave only a very brief reading before going out of range, which
was the same on the 200k setting. C682, however, showed the out of range
instantly on both settings. As such, I tested both on the 2M setting, and
this time both showed a reading in range, and then rapidly went out of
range (although I know it was mentioned that this setting would complicate
the readings' accuracy from the caps' faults). I performed the tests both
on the analog meter and on a DMM to confirm.

With each test, I switched both probes to confirm the test worked in the
reverse direction, and again the same rapid increase to out of range
occurred. However, the difference between the two caps was in the voltage
measurements. C684 took on a voltage ~3.5 VDC when attached for 1 min,
while C682 only went up to 0.4 VDC (even when connected for 5 mins). I am
not sure if this is significant (since the caps are different ratings), and
I do not have a capacitance/ESR meter to check the findings, but compared
to doing the same tests on the new capacitors, the new ones have a much
more gradual increase in resistance over time from 0 ohm to out-of-range
(over the course of many seconds at 20k, to a minute or two on 200k).

I intend to replace the caps anyway since I have the new caps in, but I
won't be able to do that until possibly the weekend, as I forgot that my
dremel bit for metal cutting had broken (and I want to put the caps in the
original containers). Both caps are can-negative, but that should be okay
since I will not be using the leads from the original caps (just threading
through the new ones in the modded can).

If I have time tomorrow, I'll just temporarily connect the new caps in
place and see what effect that has on the scope's performance and the -100V
bus reading.

- Evan

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