Re: HV Multiplier defective in 7603?


 

Both the high voltage multiplier and the cathode supply work off of
the peak voltage from T1225 but since the regulator only uses the
rectified and filtered cathode supply for feedback, it does not care
what the high voltage multiplier is doing unless it draws too much
power.

The high voltage and cathode voltage outputs are actually designed to
track pretty closely so the regulator indirectly controls the
acceleration voltage by regulating the cathode voltage because the
former needs to be stable for proper horizontal and vertical
calibration. Thinking about it now, I wonder if CR1244 is there to
roughly temperature compensate the diode voltage drops through the
high voltage multiplier. As the temperature goes up, the cathode
voltage will drop by roughly 120mV/Cdegrees (59.5 x 2 mV/Cdegrees)
which would be about 30mV/Cdegrees for each diode in the multiplier. I
am not sure that the post deflection accelerator voltage needs that
much accuracy but it would explain CR1244.

I would be really suspicious of Q1201. It only operates with
microamps of collector current so if it was suffering from high
leakage, that would explain the high voltage.

Another easy check would be the anode of CR1244 which should be +50.6
volts.

I just noticed that there is no cathode voltage adjustment. The 4
slot 7000 mainframes have a trimmer to adjust the cathode voltage.

On Wed, 24 Apr 2013 20:30:48 -0000, "raymonddompfrank"
<r.domp.frank@...> 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@..., 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@..., 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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