My purpose in adding a resistor was to limit the inrush current. I believe my problem was a bad HV transformer. Based on symptoms, I think there was a partial short in the output. This could have the effect of changing the input-output ratio. So the regulator circuit would have to drive the transformer harder to get the output voltage up to spec. However this level of regulation would be outside the regulator design.
Without the current limit ( snubber?) resistor, the regular circuit would first overdrive than over correct and under-drive resulting in a ringing waveform instead of a sine wave. Much like a new car driver learning to steer. By limiting the input a bit, the regulator circuit was just able to work within its design. I finally got a working HV output and trace. But after about an hour it started to go unstable and then slowly got worse. I finally turned it off figuring to replace the transformer before it failed completely and took out more components. This of course is just a guess on my part. It's still on the shelf as I have gone on to other things.
Also it seemed to me that the whole HV box heated up a lot faster than other 7603s I have worked on. This is subjective, but would fit as the HV circuit would be driving more energy that normal.
Bob
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--- In TekScopes@..., "gshashte@..." <gshashte@...> wrote:
Bob,
My Q1216/Q1218 are overheating.
Looking at the Diagram from your website, the 15.7 ohm resistor was added to the Q1216/Q1218 loop. I am in "first grade" electronics, lacking some elementary knowledge but comparing my circuit with yours(I have a much older 7603), mine does not have resistor R1217 (47 Ohms). Seems to me that in later versions they added R1217 to limit the current opening Q1216/Q1218 and consequently their current. Did you try limiting the Q1216/Q1218 by using a higher value of R1217.
I am not totally sure how the circuit works but it seems to me that there are 2 ways that the current on Q1216/Q1218 can increase:
1) Decreased resistance on the gate (Q1214 is failing, the driver is putting higher current or a resistor has lower value).
2) Q1216/Q1218 are deteriorating and allowing higher current or a short on the transformer.
In the end, were you able to completely solve the problem?
--- In TekScopes@..., "Robert" <go_boating_fast@> wrote:
This might help. It is some extensive tests on a 7603 HV I did a few
years ago.
;
=a096bf125eb899358c4d218d16fd078d
<;
D=a096bf125eb899358c4d218d16fd078d>
Bob
--- In TekScopes@..., "gshashte@" wrote:
I bought a broken 7603, the fan was working but nothing else.
I opened the power supply and started looking but did not really find
anything.
I did not find on the power supply board the TO transistors that
Stefan mentioned (referenced below). After moving to the LV Regulator
Board, they were there - 6 of them. Re soldered all those connections
and now I have graticule lights and power led on.
After I reconnected the power supply to go to the LV Regulator I
noticed something was smelling. Turned the power off and decided to
change the position of the cable that goes to the Z-Axis Amplifier board
and supplies 15V unregulated. P870 on the PS has 3 pins but the cable
only two wires. The smell got stronger and I noticed that the big
transistors connected to the HV board were getting hot. So that was a
bad move I made.
After accessing the High voltage board I noticed that it was just
floating, the one metallic post and the 3 plastic ones were not secured.
It seemed to me that the contacts of the transformer could have been
making contact with the metallic enclosure.
I also noticed that the big HV disk cap had some melting on both
sides.
I secured the board with the metallic post (don't have plastic screws
for the other 3)and made sure that there was a good separation between
the big HV disk cap and adjacent caps.
At this moment nothing smelling burned so I suspect the transformer
was making contact with the metallic enclosure.
However, Q1152 on the Z-Axis board which seems to supply 130v to the
HV board on line B overheats. When I remove the cable that supplies 15V
unregulated the problem goes away.
At this point I am not sure how to continue. I only have a Multimeter
to check continuity. I may get a cheap old Radio Shack Transistor
tester and a cheap Chinese ESR tester like this one:
;
PN-PNP-MOSFET-RESISTOR-METER-/160943358553?pt=LH_DefaultDomain_0&hash=it\
em2578f8c259
or this one:
;
984.m1438.l2649
How do I go about testing the transformer on the HV board?
Stefan Trethan wrote:
The 7603, unlike the 7623A, has a linear power supply.
There are big TO-3 series pass transistors on a heatsink, the
problem
is that they are in sockets that combine an unfortunate choice of
plating with an unfortunate mounting arrangement. Thermal expansion
and contraction stresses the solder joint between socket pin and
PCB,
which eventually causes even the leaded solder to fail. The failure
line is very thin and difficult to see even if you are used to
looking
for this. Usually the driver transistor has failed from carrying the
load current, but the TO-3s are still good once you reestablish
contact.
Another common failure is big Sprague capacitors with internally
corroded terminations (reading no capacitance / excessive ripple on
the supply).
As you can see I know more about the 7603 than the 7623A, my 7633
has
been working fine so I was never in there.
ST
