Re: SC503 HV, long message

Tom Miller <tmiller@...>

It looks like the measurements you are seeing are good. Without the 33 volts, the inverter will certainly not work.
The negative to counteract the two plus supplies (and the HV adj from <10>) comes from the neg 1900 HV through the 25 Meg resistor.
I would check the two inverter transistors and all the transformer windings to ground for a shorted condition. The blown track means someone tried a larger fuse or jumper to force a condition.
Check all the output caps and diodes.
Good luck,

----- Original Message -----
Sent: Tuesday, December 22, 2009 10:32 AM
Subject: [TekScopes] SC503 HV, long message


An SC503 I recently bought did not power up and the +33V fuse was blown. After replacing 2 transistors and a shorted tantalum in the +5V regulator, the LVPS is good and powers up. I replaced Q7031 and Q7032 in the HV oscillator circuit and cleared a short on the +33V load. I installed a 1amp fast blow fuse and found that with the power switch in the STBY position I had no voltage on the collector of Q7031. I quickly found that continuity was intermittent from +33V to Q7031 collector, I found a damaged pad where T7041 is mounted, repaired with a solid jumper and found good voltage at Q7031-C. Turned power switch to CH1 and found 1amp fuse blown. No voltage at Q7031-C, 0.65V at Q7031-B, U6021-6 at -10.5V, .4885V at U6021-2 and .0003V at U6021-3. The negative output turns Q6031 on, causing the .65V to turn on Q7031. I grounded U6021-2 to see if pin 6 would go positive. I found that grounding U6021-2 does not cause U6021-6 to change. Is grounding pin 2 enough? The book says pin 2 should be near zero during normal operation. This causes me to wonder where the negative comes from to counter the +12V and +110V. I checked the HV ADJ line and found it zero. What should the HV ADJ line normally be?

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