Re: Type 503 Oscilloscope Issues

Dave Casey
 

I've mentioned the caps as a possibility, but I haven't yet heard anything
back on them being checked. I agree that it's looking more and more like
they are to blame.

Dave Casey

On Mon, May 8, 2017 at 7:52 AM, Trevor Marshall trevorjmarshall@...
[TekScopes] <TekScopes@...> wrote:



Measurements point towards open or very leaky -100 volt rail filter
capacitors C682 and/or C684. Electrolytics can fail such as to become
leaky, or fail such that they have greatly reduced capacitance, many times
essentially zero, when they fail open circuit. Disconnect one lead of each
of these caps to facilitate leakage measurement with your DVM.
Then, if you lack a capacitance meter, another more crude capacitance test
is as follows: Put your DVM in the resistance mode on the 20k scale and
probe the two cap terminals, and watch the meter reading. If the capacitor
is functioning properly, the reading will start low and increase over the
course of a few seconds or so, as the meter charges to cap, and then read
"ol" (or whatever indication the meter gives for off scale readings.) You
can repeat the test, but will have to first discharge the cap by shorting
it.Ideally, the correct polarity should be employed, meaning that the DVM
positive lead should connect to the capacitor positive lead. In practice,
at the <2 volts applied, polarity should not matter. If you put the meter
on the 200k scale, the charging process slows down by a factor of ten, so
will be even more easily observed. Going to an even higher scale, 2M, the
capacitor's leakage resistance will begin to interfere with this method.
You can get a rough idea of the value of the capacitor by comparing the
results to another known good capacitor. So if you have an electrolytic cap
that is 20 to 80 microfarads, experiment with it.

Before connecting your DVM or a capacitance meter to any capacitors,
assure that the cap. is discharged. If the scope has been off for some
time, charges tend to decay toward zero. But to be safe. since some types
of capacitors can hold charge for weeks, manually shorting capacitors
before measurement is a good practice.
My apologies to Evan and the mentors if this is redundant advice, it's a
long thread and I did not thoroughly scour it.
Trevor


On Sunday, May 7, 2017 1:38 PM, "'John Snyder' Kochcal@...
[TekScopes]" <TekScopes@...> wrote:


I look like your making progress, some things you try don't reveal
things.

Dave's point of Turning all the front panel adjustments is a good idea to
see if it changes the loading on the -100V, if one has a big change it can
indicate a part of the circuit that is the problem. If the overload is
through one of the tubes the associated adjustment should change the tube
operating point and current that may release the -100V a bit.

But it might not.

I would try other things before changing any other "inside" calibration
adjustments other then R641. The existing calibration can have some value
as
it can take time and equipment to restore.

Other things to look at:

I don't recall if you have already measured the resistance of the -100V bus
to ground (be sure to turn power off, wait, ground -100V bus for a while
remove ground, before testing to remove any charge on the capacitor) that
would be a good starting point.

Have you done a AC voltage reading on the -100V to see if the problem is a
low DC or more of a AC wave on top of the DC resulting in a lower average?

The -100V should have a AC voltage that is very low compared to a volt.

And then the same question as to the +500V on C612

The 500V could have 1's to few 10's of volts of AC but much more then it
would be a concern and need some calulating.

I don't recall or missed if you checked the input DC power caps C611 or
C612.

If you have can you re-report the check?

If you have not, one way to help check those would be to clip on a pair of
350V to 450V caps in parallel with each one. Those generally have a very
large range of value like +20%,-80% so it might only take 30uF's or so to
function. And that would be a good way to help eliminate that possibility
that that part of the power supply that is cutting out 120 times per
second.

The scope is functioning enough that you might be able to use it self to
probe some circuits.

Do you have any probes and if so can you tell us, there nature, model
numbers or ratings? To see if they can be used for the voltages needed for
some tests you can do to help.

John

For later:

You can see on the schematics that there are typical voltages at nodes for
parts of the circuit. (best made if all the scope knobs are set as stated
in
the manual for testing) Measuring those and reporting them, might lead to
clue as to the location of the problem, but with the -100V low most will
readings will be offset in some way, and it will be a bit more challenging
to use them to locate the area of over current. But we might be able to
find
something.

I don't recall if you have done any tests to check D682, C682 C684, a leak
there would be generally independent of the tubes being pulled.

If you are running the scope for a while you can check for the temperature
of D682, C682 C684 if it's different from the other diodes, But be sure to
measure all the voltages 250V 100V -100V to be sure they are discharged be
for sticking a finger in there. Not a great test (see scope probe Questions
below)

You still have a visual check of everything connected to the -100V

And measuring the voltage of across each resistor that is connected to the
-100V

-----Original Message-----
From: TekScopes@... [mailto:TekScopes@...]
Sent: Saturday, May 06, 2017 10:39 PM
To: TekScopes@...
Subject: [TekScopes] Re: Type 503 Oscilloscope Issues

Yes, from the calibrator, the scope is out of spec (at 420 mV and 4.1 mV),
but the calibrator image is not stable. Here is a link to what I mean:
https://goo.gl/photos/BZzZ41sma3vz5R717
https://goo.gl/photos/BZzZ41sma3vz5R717.

From the images, the square wave's magnitude is ~0.8 cm (compared to the
expected full 1 cm). However, as seen in the video, the issue of the waves
"warping" is even more present here, aside from a general instability of
the
trace.


I still have not looked at turning each adjustment knob to see the effect,
but I will try to get that done tomorrow.


- Evan



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Posted by: enchanter464@...
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