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

Fabio Trevisan
 

Hi Dave, thanks again for your comments...
I have made some progress and I think I found the culprit for the HV
inverter collapsing under heating.
See my comments next to yours below,
Rgrds,
Fabio

2016-10-02 5:07 GMT-03:00 David @DWH [TekScopes] <
TekScopes@...>:

That could work. I usually end up hacking together a quick high
voltage supply as needed. Periodically I consider designing a
variable high voltage low current bench supply for doing high voltage
leakage tests up to 3 kV or so.
I removed and measured all critical components for leakage under approx.
3kV, heating them up with the heat gun.
I controlled the heating between 70 and 90 deg celsius (which is already
way hotter than they should get).
The 6.8nF 5kV ceramic capacitors measure around 1uA cold and less than 5uA
hot
The 12nF 4kV film capacitors measure < 1uA cold, and < 2uA hot
The 1nF 10kV ceramic capacitors (HV doubler) measure under 1uA cold or hot.
Maybe 3kV is still little for
these 10kV beasts.

The HV transformer... between the HV winding (Cathode & PDA supply) and the
600V bias winding...
Cold, it already measures 10~15uA under 3kV (A WARNING SIGN)
Hot...enter the Drum Roll... 400uA!!! and my test HV supply sags to 1500V!
It didn't take all the heating above to present the increased leakage...
Leakage skyrocketed after about 40 deg celsius.

So... I think I (we) have a culprit!!!
At 400uA, this leakage alone is taking up 200% of the current actually
drawn by the HV circuit under normal operation.

P.S. I didn't have to resort to my Kenwood's HV supply...
I found a broken mosquito zapper (HV racket) hanging around (the racket was
bent and shorting).
It delivers about 3200V from 2 x AA batteries and it's safe (not connected
to mains and low power)

So I think now I`ll have to order one transformer from Sphere.ca.

I am still puzzled as to why changing the diodes improved the
situation without fixing it. I have difficulty believing that the
diodes are leaking just enough to cause such a marginal problem.
Looking at the orders of magnitude of the diode leakages that I measured
(less than 0.1uA),
the order of magnitude of the capacitor leakages and now at the HV
transformer leakages...
I`m also not sure if the replacement of the diodes really made a
difference, or if it was just
a mistaken conclusion... Probably just a "wrong detour".
Maybe the HV transformer leakage is erratic and better in some days and
worse in others.

I usually do not care about keeping the stock condition if I can make
some improvements.
Well... at this point I want first to fix it, whatever it takes...
So, I agree with you.
But given the unreliable source of the components here, I can never tell if
it's really what's labeled so,
ff the original diodes are not definitely bad (even if my superficial tests
tell me that the new diodes seem
better), I rather prefer to stick with the originals (because they may be
more reliable / better quality).

So use a few of them at the same time. The heat of vaporization for
most liquids is really high which is why evaporative cooling works so
well. Water is especially good but common rubbing alcohol has quite a
bit of water in it.
I`ll keep the tip, but I think there's no need anymore to go down that road!
The HV transformer looks pretty guilty at this point.

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