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475 CRT High voltage problems


lmaccone
 

My 475 (which I had fixed thanks to this list, thank you!) has lost
all CRT signal. Apart from that, it seems ok. All voltages are more or
less ok, except for the 110V that is around 86. I checked the power
supply stage and it looks ok (that was what had failed
previously). The 110V drives the high voltage section, and one of the
transistors there (Q1354 if you have the schematics) is heating up a
lot... I don't have the tools to work on HV and I'm ready to ditch the
unit (it's older than me!), but I'd love to fix it if possible. Any
ideas?

Thanks,
Lorenzo


ehsjr
 

lmaccone wrote:
My 475 (which I had fixed thanks to this list, thank you!) has lost
all CRT signal. Apart from that, it seems ok. All voltages are more or
less ok, except for the 110V that is around 86. I checked the power
supply stage and it looks ok (that was what had failed
previously). The 110V drives the high voltage section, and one of the
transistors there (Q1354 if you have the schematics) is heating up a
lot... I don't have the tools to work on HV and I'm ready to ditch the
unit (it's older than me!), but I'd love to fix it if possible. Any
ideas?
Thanks,
Lorenzo
I had a HV mica cap go bad in my 475. Lucky for me it was easily
found using the ohmmeter. The bad cap measured 5000 ohms. So
you could try measuring the resistance of the caps in the HV
section - they all should read as open on the ohmmeter. As I
recall, I measured the resistance in circuit (obviously with
power _off_) and found it that way without the need to unsolder
each cap. However, I don't know if that is a "bulletproof"
technique.

So, before you give up, at least try measuring the caps with
your ohmmeter.

Ed


KB6NAX
 

Not all faulty capacitors can be caught with the usual DVM or DMM ohmeter that measures only up to 10 or 20 megohms. Connect a 100 volt DC supply to the capacitor with the DVM in volts mode to measure the actual leakage resistance of the cap. The DVM's input resistance, usually 10 megohms, is the sense resistor for making the current measurement. The capacitor's leakage resistance is 100 volts - the volt meter reading / calculated current through the DVM with is the voltage reading / DVM input resistance. If your cap isn't up in the thousands of megohms replace it (this doesn't apply to electrolytic capacitors).

Arden


I had a HV mica cap go bad in my 475. Lucky for me it was easily
found using the ohmmeter. The bad cap measured 5000 ohms. So
you could try measuring the resistance of the caps in the HV
section - they all should read as open on the ohmmeter. As I
recall, I measured the resistance in circuit (obviously with
power _off_) and found it that way without the need to unsolder
each cap. However, I don't know if that is a "bulletproof"
technique.

So, before you give up, at least try measuring the caps with
your ohmmeter.

Ed