[ARC5] SCR-522 Project: Finding Bad Bypasses in 20 Minutes.

[David Stinson]

The BC-624AM receiver from SCR-522.  As mentioned
before, this is how I find bad bypasses quickly:
Tube filaments are not powered.  A trace of the
schematic shows two B+ busses; one from chassis
connector pin 3 and one from pin 5 (powered for
RX, off for TX), shows that with tubes unpowered,
there should be no legitimate paths to ground for
B+ in the -AM version of the receiver except
through the squelch control circuit, which should
draw about 3mA at 300V.  
Connected a variable supply to buss 3 and slowly
brought the voltage to 100V.  This displayed a
current drain of 40mA- obviously leaky.
https://photos.app.goo.gl/tGark6fCSkzMHXrWA

40 mills at 100V represents 4 Watts of heat going
somewhere.  Four Watts doesn't sound like much,
but it builds-up over time.  Left the 100V on for
about 10 mins.  After putting my left hand in my
pocket, started feeling each resistor and
capacitor for heat and discovered 8 of them (yes;
I left the 100 Volts on.  A finger tip might get
"bit" but if it's only across your fingertip, 100V
ain't gonna kill ya):
 https://photos.app.goo.gl/ifjSWHv8aRvSg3Au7

Some of them were easy to spot without even
touching them.  Look at the lead going into the
cap on the left:
https://photos.app.goo.gl/vjATviQypZ5XUEMF9

After marking the "bad guys" with red, snipped the
Ground end of each and watched the current drain
drop with each unit it reached just a couple of
mills.  Testing the pin 5 buss did not reveal any
more bad guys, but did take some time to "reform"
some oil-filleds.
https://photos.app.goo.gl/YmxyNUruXbt9vtWG7

Next, connected both busses together and boosted
to the specified 300V.  If the busses were clean,
the total drain should be 3-5mA.  
Both busses are drawing 10 mA, so there's still
5mA of leakage in there somewhere:
https://photos.app.goo.gl/r3mkSpG2in3nsXxG9

At 300V, 5mA is 1.5W of heat being generated in
the leakage.  
Do NOT NOT NOT "feel around" for heat with 300V in
there.  Switch off before poking around.  Next
work cycle, I'll leave it connected at 300V
for 15-20 mins, which will allow enough heat to
build to make the bad guy/s evident.  At only 5mA
of leakage, the set could be fully powered to
check for basic operation, but I'll find the
remaining bad parts first. Last, will power both
busses at 300V and leave it for a couple of hours
to "smoke out" any marginal leaks.  Once all the
leaks are fixed, I'll power-up the set and see
if/how it plays, then repair stage-by-stage.

GL OM ES 73 DE Dave AB5S