Re: Got the V2000 HV blues...

I'd like to thank everyone for their ideas and recommendations. I'm
going to combine several responses into one loooong reply:

John Robertson wrote:
> If your monitor uses the (801 - Electrohome) HV doubler they do tend to
> fail...or if not, the HV diode can become defective and start
> overheating - drawing too much current leading to fuse failure. If you
> have a remote temperature probe monitor the HV parts to see if any
> start to heat up, or if the rubber end caps on the HV diode are
> black/melted then the diode may be bad (or just old rubber from previous
> bad diode).

It's a 15V2000, so it has a normal HV diode. After initial rebuild, I
was hearing an intermittent HV "hiss". I cleaned the diode holders and
installed a new diode with fresh dielectric grease. The "hiss"
persisted, and it would still blow F102. Since the springs in the
holders didn't look very good, I cut apart the holders, soldered in a
new diode directly, and reused the holders gooped with silicon to
insulate it. The "hiss" persisted, and it would still blow F102.

So, the HVT or CRT were then suspect in my mind. When I tested the CRT
with my Sencore CR-70, both cutoff and emission tested bad. After a
restoration, emission tested ok, but cutoff was still bad. Since I had
one, I went ahead and installed a brand new crt (15ST4730R). I also
swapped in a known working HVT that I pulled from another HV board.
Now, the HV "hiss" was gone, but it would still blow F102.

John Robertson wrote:
> Then the problem could be a defective picture tube that has an
> intermittent shorting screen or similar flaw. You could test this
> hypothesis by simply unplugging the picture tube neck plug, leaving the
> HV anode and the aquadag grounds in place and again wait to see if the
> fuse fails. If it does go to the next tests below.

I didn't try this test, but presumably got around it by replacing the
crt. I should check the neck connector, though.

John Robertson wrote:
> You might try turning out the room lights and see if there is any glow
> around the transformer or on the underside of the PCB indicating a HV
> leakage path...

Did that, and couldn't see anything.

Rodger Boots wrote:
> Did anyone else notice that the manual (in a note) says that the 1 A
> fuse used to be a 1.5 A fuse on early monitors?

I did notice that note on the schematic. This monitor has a build date
of December 1980. It also has R100 & R101 on the deflection board.
Those both suggest to me that it's an earlier build as opposed to a
later one. Unfortunately, when I started the monitor rebuild, I threw
away F102 without checking what value it was first.

By now, this monitor had really annoyed me, so I just spent an hour and
shotgunned the entire HV board. Every resistor, every capacitor, and
(almost) every semiconductor were replaced. I replaced all the zeners
for a second time (even though they tested fine) and all the
electrolytic caps for a second time (even though they also tested fine).
  The only things not replaced were the coils (L900 & L901), Q900
(didn't have a new BU409 on hand), Q903 (couldn't figure out a sub for
2SD525R), and the big focus pot. I did resolder those.

After all that, with the monitor outside the cabinet, it still blew F102
after 30-45 minutes.

Rodger Boots wrote:
> Possible causes: bad fuse holder causing extra heating of fuse, leaky
> electrolytic (runs warm, goes into thermal runaway as it gets warmer,
> blows fuse), open electrolytic (causing ripple current back into fuse),
> flyback being driven at wrong frequency, misadjusted voltage regulator
> in HV unit (is HV level correct), problem with regulator (does HV change
> as unit warms?).

HV has been set to 14.5kv using my Tenma HV probe. It appears as stable
as my other monitors. From inital power-on over about 15 minutes of
warm-up, it'll slowly increase by ~0.05kv. I haven't watched it for a
longer period to make sure it stabilizes.

John Robertson wrote:
> If you have a couple of voltmeters and some time you could put an
> ammeter in series with the fuse and a second meter monitoring the screen
> output (400VDC) of the HV cage.

I removed F102, clipped a test lead onto each pcb-mount fuse clip, and
connected a DMM in series with a 1.5A SB in a separate fuse holder. I
then watched and noted the current draw. It started at 0.56A,
stabilized at 0.62A in less than 20 minutes, and stayed at 0.62A for
another 100 minutes until I turned it off.

Since the thing had actually worked for a whole 2 hours, I removed the
meter, but left F102 (still a 1.5A fuse) in the separate fuse holder
connected by test leads to the deflection pcb, then threw the monitor in
the cabinet, closed it up, and turned it on. It then worked fine for
more than 7 hours until I turned it off.

So, two possibilities come to mind. First, I may have some funky 1 amp
slo-blo fuses. Second, as Rodger suggested, I may have a bad fuse
holder. I did resolder it, and the clips look fine and have good
tension. I will go ahead and replace the fuse clips (as soon as the new
ones arrive).

Mark suggested monitoring the level and stability of the B+. I haven't
done that yet. I can dig out an old laptop, connect it to a DMM, and
use that to continuously record things. I did check the voltages at
P900. They all seemed fine, but I didn't watch them over time.

He's right, though, I've done a lot of "dart throwing" with this
particular monitor. As a test engineer, I should know better - although
in my defense, my area of expertise is aerodynamics, not electronics.

Alan

-- 
Total Impulse - Just Having Fun!
Alan Estenson, totalimpulse@comcast.net
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