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

Fabio Trevisan
 

Hi Dave,
Thanks for all your comments / input.
See my comments next to yours...Again, I`m keeping only the last
conversation (to easy up to parse)
Rgrds,
Fabio

2016-09-27 23:20 GMT-03:00 David @DWH [TekScopes] <
TekScopes@...>:

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
Thanks for that, it was useful already.
I do have a scan of Service Manual that has the diagrams in colors, like
yours, but each diagram is split in 2 or 3 A4/Letter Pages (that I was
still in need to put together).
Yours came in handy, as I printed them in A3 paper (Tabloid) and it's great.

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.
Now that you mention, I feel silly for having a "theory" when careful
reading
of the schematics should have been enough! It's right there in the
schematics!
And indeed, the storage mode in effect change voltage divider of the
inverter's
regulator feedback.
By my rough calculations, if "Normal mode" the cathode voltage is -1470 V,
in "Variable persistence" mode it should "rise" to about -1450, and in
"Fast" mode,
it should rise just a bit more to "-1445".

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.
I agree 100% with you... would this kind of HV components easier to
source or find in Brazil... But since it isn't, I sometime need to resort to
more conservative methods,
As I write this, I have dismantled most of what's around the cathode supply,
which includes the Focus divider, for proper clean up (which was due anyway)
and I will take the opportunity to test all critical components under
controlled situation.
I will connect each diode and capacitor to High Voltage (that I will borrow
from my Kenwood
scope) and look for leakages, while heating them up with a heat gun...
If the problem is in those components, I will find.

You are not the first person in Brazil to mention having problems
finding parts.
Yep... It gives us some headaches...
If at least it would be easy to import in Brazil... it wouldn't be a
problem...
But our customs are very picky and gives to the shippers all sort of
problems so, a lot of them even refuse to ship to Brazil.

So it helped but did not fixed the problem.
But what is the leakage at higher operating temperature? Whatever
part is causing the problem is only doing so after it warms up.
That's the difficult part... even with the original diodes,
which is the only thing I replaced so far, the problem takes quite
some time to happen with the outer case removed.
I measured the voltage drop at R1504, and it's ~1.4V
@ Intensity MIN (140uA), and ~2.0V @ Intensity MAX (200uA).
And I meausred the voltage dropt at R1522, and it's basically 0V
@ Intensity MIN (0 uA) and 0.5V @ Intensity MAX (50uA)
In both cases, I`m having the Intensity control limited (by R1406 trimpot)
to the point
where the CRT presents its 1st peak of intensity (double peaking... you
know)
But all those measurements were taken while it's working normally.
I didn't have chance to measure it after the HV collapses.

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.
Since I already replaced the diodes and the capacitors, I will follow
your wisdom and replace the 3 HV multiplier caps.
Those are not so difficult to find here.

I have not quite decided what to make of this but I have some ideas.
And they're welcome!

If the diodes were the problem, then I would have expected changing
them to have completely fixed it instead of making it a little better.
I agree... I think that they're just helping to keep the whole thing cooler
and therefore, farther from heating up to the point of collapsing.
After doing a good HV test on them (as I mentioned above), I will put them
back... If they are not the culprits, it's nice to keep the scope original.

What I might try is looking for some other part which is getting hot
causing enough leakage to pull the high voltage supply voltages down.
If you do not have an infrared thermometer to scan with, a cotton swab
dipping in rubbing alcohol could be used to cool suspected parts after
the HV supply starts to collapse to see if it has any effect. Q1486
and Q1484 would be especially good targets for this test.
Hmmm... That is a good idea... I`m only afraid that a bare cotton swab
will not cool down Q1486 so easily!

Freeze spray is what would normally be used for this but I would be a
little leery about using it around high voltages where condensation
might cause problems.
I would too (be leery)... Freeze spray is ruled out for now!

Thanks again for your time!



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