Hi all,
Thanks for the help...
I hooked the monitor up to the isolation transformer and variac, brought up
the voltage slowly while watching the VCC with the scope hooked to the (-)
of the large filter cap and the lead hooked to VCC on the 3842. Voltage
climbs to +17vdc and sits. If the chip was working, I would see some sort
of drop when the oscillator started running. Nothing on any of the output
lines either. All diodes around the IC test ok. All diodes on secondary
test ok.
Removed the 3842, put in strip sockets, installed new 3842 and the montor
fired up. Used the light bulb in place of fuse and as the monitor would
cycle up, the light bulb would get bright as the filter cap charged and then
settle down when the monitor came up.
The original was a KA3842A and I replaced it with a UC3842B. They both
cross to the same NTE and from the little I can garner from the specs, the B
is a higher tolerance version of the A.
Kirk S.
"tekman" wrote in message
oups.com...
wrote:
Since the equipment under test is being powered via an isolation
transformer you should ground the scope probe ground to the negative
side in the hot chassis in order to obtain proper waveform
measurements.... no problem, no sparks, not an issue.
Obviously voltage and current are still present and standard personal
safety proceedures are still required, but because the scope is safeyy
grounded there are not hot chassis issues since they are floating
thanks to the isolation transformer.
electricitym
All correct. An isolation transformer is always a good part of your
personal safety.
The UC384x type supplies I hadto repair have had these usual sympthoms:
1. Dead UC 3842. Change.
2. Freid resistor at chip output (in the connection to the gate of the
FET). Replace.
3. Dead FET. Replace.
4. Capacitor about Ucc of UC384x: Leaky or lost capacity. This is often
the "twit..twit...twit" sound of the startup.
google for the article of Keith Lofstrom (or similar , may be my memory
is not correctly remebering): "fixin scitching poer supplies".
hth,
Andreas