Re: Hello from newcomer Fabio Trevisan - My first Tek Scope 464 + DM44

 

On Tue, 27 Sep 2016 19:00:14 -0300, you wrote:

Hi Dave,
Thanks again for your feedback... I am suppressing my original post and
leave only your comments and my replies.
I hope this won't mangle the whole thing this time (Ì'm doing it from my
mail client).
Rgrds,
Fabio
I usually reply in line when posts get complicated enough.

I assembled and cleaned up a set of color schematics for the 464 which
Kurt has been nice enough to host at his site:

http://w140.com/tekwiki/wiki/464

2016-09-22 2:15 GMT-03:00 David @DWH [TekScopes] <
TekScopes@...>:

On 16 Sep 2016 17:07:00 -0700, you wrote:

...

My notes say it is 8.5 kilovolts. That is enough difference that I
would suspect failure of the high voltage multiplier is causing the
problem.
I concur with you that the 464 should be 8.5kV, But I have another theory!
I think the correct anode voltage is indeed in the 7kV ballpark for 2
reasons:
1. because this is exactly what is marked in the 464's HV's Cage Warning.
It says: CAUTION 7000V inside !!!
2. 7kV of PDA plus 1500V of cathode voltage makes up exactly for 8500V
acceleration !!!

Moreover, I`m considering that even the 7.22kV I`m measuring may be already
HIGH by 220V
and this can be exactly an indication of the inverter being driven harder
to compensate for some
deficiency on the cathode circuitry (either the winding or something else).
This rational is consistent with the other winding (600V) which also
measures around 620V
So, it really seems the problem is at the Cathode circuitry...
I will try to confirm that by measuring the HV doubler voltage or the 600V
while the scope
heats up towards to failure.
Ah, you are right. My 7000 notes break out PDA (post deflection
acceleration) and cathode acceleration voltages separately but my 465,
22xx, and 24xx notes do not. It is time to add a couple more columns.

The schematic and detailed theory section both say -1470 volts for the
cathode voltage and about 7kV for the PDA voltage. The theory section
says the cathode voltage is altered slightly between normal, variable
persistence, and fast storage modes. This will also affect the PDA
voltage.

The resistors could not cause the problem anyway.
Agree, but I replaced them anyway, by glass insulated (3kV class) metal
film resistors. (but no appreciable change in status)

On my suggestion of possible replacements of the HV diodes... you wrote...
The PDA voltage will decrease a little bit but the effect on
deflection calibration will be minor.
Agreed! So much that I decided to take the step and replace all 3 HV diodes
(the 2 at the HV doubler and at the cathode circuit).
When repairing these types of circuits, I prefer to replace all of the
high voltage resistors, capacitors, or diodes at the same time if they
are of a common type. It is just easier and safer this way.

Leakage through either of the diodes will cause problems. The
capacitors could also be causing the problem.
I have not ordered from them yet, but these guys look like a good
source for high voltage diodes and capacitors:
<http://hvstuff.com/>
About hvstuff.com... I have seen their website before, I think that I will
ultimately need to source from
them if I need to replace the capacitors, as the sources in Brazil for HV
capacitors are either
unreliable or only affordable for the industries (no hobby market).
We used to have Newark/Element14 in Brasil but they closed their operations
here.
You are not the first person in Brazil to mention having problems
finding parts.

As I mentioned above, I decided to take my chances with the ESJA53-12
diodes I found on a loca store...
After I replaced the HV diodes, I noticed a great improvement... but
unfortunately it wasn't the definitive fix...
Now the HV doesn't seem to fail anymore if I run the scope without the
outer casing (cooler).
While this is good news in some sense, it made things more difficult to
troubleshoot.
When I install the outer casing, the time to fail increased from 20~30
minutes to about 1 hour.
So it helped but did not fixed the problem.

I measured the leakage of the original cathode diode (CR1503) and under the
same voltage of about 3600V,
it's 25 times bigger than the 1.1nA that I measured in the new diodes,
but 28nA is still very small leakage and I think that it may just have
given the inverter a little bit more breath
so now it takes longer to fail.
But what is the leakage at higher operating temperature? Whatever
part is causing the problem is only doing so after it warms up.

I would change the high voltage capacitors before messing with the
transformer. Make sure the board around the high voltage multiplier
is clean when you finish working on it.
That's my next step... It will be difficult to find 6.8nF x 5kV around
here...
But at least for testing, I may resort to an association (parallel)
It's not HV friendly to have the uninsulated connections that would be
required
but there's plenty of space where the original capacitors are (they're
huge!)
Usually it is the voltage multiplier capacitors which have problems
and their value is not critical. The value for the capacitors which
filter the cathode supply affect the regulator's frequency
compensation which is partially controlled by the network built around
R1473; I think you can get away with increasing them to 10nF but I am
not positive about this.

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