Thanks for that feedback Dave.?
At this point in my struggle with this machine, nothing about it surprises me.? ?It could have been hit by lightning for all i know.? ? :-P?
One bit of good news, the modules do appear to be working.? I was able to view the scan and vertical outputs on my scope and it does look as i'd expect.??
Nice little mid week surprise for me there.??
I can't see what could destroy Q15, except perhaps C18 shorting.
Dave Wise
I don't have a HV probe currently, but its on my shopping list.? ? I'm going to go with 0v at the moment though, given the state of Q15.??
I guess I'm just wondering?if I should expect other damage to have occurred when this part failed.? ??
I will replace Q15 this week, get a probe and report back though!?
On Wed, Sep 18, 2024 at 10:59?AM Michael Bafaro via
<m.bafaro=
[email protected]> wrote:
Bill,
?
Q15 is the FET that is in the HV regulator circuit.? The gate to the other leads will read open if the ohm meter is connected to back bias the junction.? The question is what voltage is present at the control
grid of the CRT?? If you have a high voltage probe you can measure it directly at the green lead on the PCB that goes to the CRT socket.? I measured -2580.? What voltage do you see??
?
Mike
?
?
Checking things out last night in the LV side of the HV supply, I saw on the edge of Q15 a small delamination. Very tough to see in that tight space but just enough to warrant further?investigation.? ?I pulled Q15 and a sizable chunk
was blown out and barely hanging on, so it's physically OPEN between pins 1 and 2.
I've ordered a replacement.? Anything else to be concerned with in this area in such a situation?? Q13 and 14 don't seem to be damaged as far as I can tell, at this time.? ??
?
On Tue, Sep 17, 2024 at 9:27?AM Bill Berzinskas via <bberzinskas=[email protected]> wrote:
Probably not related strictly to the PSU, so happy to make a new thread if needed.? ?
I've gone over the A5 board at a high level.? ?Checked all the test points and all but #7 appear to be as expected on the scope.
One of the schematics has voltages at the wiring pins, and those also look ok.? ?I have not checked per-node voltage within the circuit yet.? ??
Is this "fairly conclusive evidence" that A5 is generally OK?? ?I don't want to dwell on it, I'd thought that maybe Blanking or Pulse were having issues?
but now i'm thinking not.? ??
More power supply related, I don't see any oscillation on the HV oscillator.? ?The labelled 7v is closer to 12v so I'm thinking the problem is around Q13, 14 or 15.? Q15 looks kind?of not good, seeing .1v breakdown with a DMM diode test in-circuit.? ?I know
thats?not the best test, but it does seem to work for most of the other transistors I've tested.??
?
The worst bugs make the instrument diverge from the schematic, often in ways that would look like nonsense if you drew it there.
"From over 40 years of fault finding and repair experience in a wide range of electronic products, not just instrumentation,
the weirdest symptoms often have a really simple root cause!"
This has been my experience as well! Makes it very easy to go down a rabbit hole. Which is why I always do a thorough visual
inspection first. Also checking mechanical components of switches, pots, etc. Giving them a good clean right off the bat and testing those first can save you a lot of headache.
