Re: HV Multiplier defective in 7603?


raymonddompfrank <r.domp.frank@...>
 

Thanks, Don, that's what I had in mind and I have already been looking for a suitable R until I just realized that there are several on this very same thick film device... Should be an easy thing if they're all ok.

Raymond

--- In TekScopes@..., Don Black <donald_black@...> wrote:

You can measure the 24.5 M resistor by putting a lower value resistor in
parallel and then measuring. If it's completely open you'll just get the
value of the parallel resistor but if it's OK the resistance will be
lower. Then you can calculate the value of the 24.5, there are little
programs on the net to do it for you. I'd suggest using a 10 Meg in
parallel if your meter can go that high, otherwise a 1 Meg.

Don Black.

On 25-Apr-13 6:30 AM, raymonddompfrank wrote:

My idea about loading the circuit came from the thought that the
multiplier input would see a peak-detected DC voltage that had to see
some load to discharge. Nonsense, of course. My excuse is that it was
at 7am this morning and I had been working all day and night (not on
this problem).
Just pulled Q1201. Cathode voltage goes down to far below 2KV so
Q1206, Q1214 and associated circuitry could well be ok. Looking at the
circuitry driving Q1201 now (R1245 etc). R1245B could be open but I
can't easily measure 24.5Mohm.

Next would be the Grid Bias circuitry.

Cathode filament is not open (2 Ohm across P1275 pins 1, 2).

--- In TekScopes@... <mailto:TekScopes%40yahoogroups.com>,
David <davidwhess@> wrote:

I will not say that is impossible but if I designed it, the cathode
voltage regulation would function correctly with or without the high
voltage multiplier attached. I think the load on the 6.3 volt AC
heater supply which also comes from T1225 would overwhelm any effect
from the high voltage multiplier load.

You might want to check that the CRT heater is not open. I think you
said earlier it is not shorted to the cathode.

The error amplifier design Tektronix used seems rather unwieldy to me.
I wish they had marked the typical operating voltages on the schematic
but maybe there was too much variation for that to be useful.

The 7613 and 7623 use the same basic design while the 7623A and 7633
use a single ended oscillator driving the transformer instead of a
push-pull oscillator. I looked over the schematics for all four of
those and none provided any insight into the typical operating values.

On Wed, 24 Apr 2013 17:11:45 -0000, "raymonddompfrank"
<r.domp.frank@> wrote:

David,
I thought that the multiplier was needed to allow regulation to
work, so -4KV would be reasonable without any load.
Of course, load by a healthy multiplier and CRT would be minimal,
so you maybe right, also re. destruction of the multiplier as a
consequence of the -4KV.
I will take a look at the feedback/regulation circuit.

Thanks for helping me on my way.

With regard to encapsulating the discrete HV components: What you
describe is what I had in mind once the circuit would work "supended
in the air", plus thanks for reminding me to get rid of air bubbles.

Raymond

--- In TekScopes@...
<mailto:TekScopes%40yahoogroups.com>, David <davidwhess@> wrote:

Concerning your point 4, with the presumably bad high voltage
multiplier disconnected, shouldn't the high voltage test point settle
at -2975 volts?

That makes me think something failed in the regulation circuit built
around Q1201, Q1206, and Q1214 including feedback divider R1245.

If the high voltage regulator ran away then maybe it damaged the high
voltage multiplier instead of the reverse.

As to building one with discrete parts, that is what I would do. The
esoteric thing that comes to mind other than standard high voltage
construction methods is the use of a vacuum to remove air from any
potting mixture. I would build the circuit supported in air by its
three external connections without using perforated board and
then pot
it.

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