Re: Hello from newcomer Fabio Trevisan - My first Tek Scope 464 + DM44
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)
2016-09-27 23:20 GMT-03:00 David @DWH [TekScopes] <
I assembled and cleaned up a set of color schematics for the 464 whichThanks 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
The schematic and detailed theory section both say -1470 volts for theNow that you mention, I feel silly for having a "theory" when careful
of the schematics should have been enough! It's right there in the
And indeed, the storage mode in effect change voltage divider of the
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
it should rise just a bit more to "-1445".
When repairing these types of circuits, I prefer to replace all of theI 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
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 problemsYep... It gives us some headaches...
If at least it would be easy to import in Brazil... it wouldn't be a
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.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
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 problemsSince 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 changingI 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 hotHmmm... 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 aI would too (be leery)... Freeze spray is ruled out for now!
Thanks again for your time!