[Collins] re: newbie question

[Chris Kepus]

I had a similar problem with a different rig...undoubtedly simpler circuitry
but nonetheless, very, very similar symptoms.  

The problem was a failed disk ceramic bypass capacitor that had not
completely shorted.  I know, these things are not supposed to fail but in my
case, one did.  Here's the whole story if you care to read.

I was checking out the HV power supply with all the tubes out of the set.  I
had already checked the power xfmr with no connections to the filter system
and found it to be healthy
 At 117VAC input, it was cranking out 736 VAC
across the entire secondary.  When I plugged in my “diode 5U4” (which
allowed me to bring up the B+ gradually in the remaining B+ distribution
circuit minus tubes) and started bringing the AC voltage, my meter on the B+
DC line wasn’t showing the VDC advancing as it should.  I knew I had
problems.  I expected a minimum DC voltage of .9 Vin.  So at 100 VAC in, I
expected *at least* 100 VDC into the cap input filter (C-L-C) (possibly a
lot more since there was no load other than a 25K bleeder).  The actual VDC
reading at the output side of the choke was around 27 VDC.  So I stopped the
voltage tests and went into the troubleshooting mode.  I quickly eliminated
the choke and the filter caps as suspects.  Then what?

The B+ distribution circuit showed only 560 ohms to ground resistance
(rectifier pulled).  Not good.  When all the tubes were pulled and the
bleeder lifted off ground, there was no direct connection from the B+ line
to ground.  At worst, the resistance should have been 1 meg which was
provided grid bias and it checked OK at 1 meg+.   The only "suspect"
components left after triple checking all solder joints and the
aforementioned resistor were five disc capacitors that were between the B+
line and ground points.  But disc caps never fail, right?   Wrong.  The
second cap I disconnected to test was a .001 bypass on the modulation tranny
secondary
and VOILA!  "Infinite" DC resistance was restored between my PSU
and the remainder of the B+ distribution circuit.  I measured the resistance
across the cap after removing it and it was
..yep
.560 ohms.   I put a new
.001 cap in and fired the rig up again.  This time all was well.

Good luck, YMMV.
73,
Chris
W7JPG

-----Original Message-----
From: collins-bounces at mailman.qth.net
[mailto:collins-bounces at mailman.qth.net] On Behalf Of wli
Sent: Sunday, June 15, 2008 9:00 AM
To: collins at mailman.qth.net
Subject: [Collins] re: newbie question

Recently, I picked up a cosmetically clean R-388 Order 25067-PH54-55
Had all the tubes and both covers
Not able to test before purchase at a busy local hamfest (lame excuse)
It was almost deaf on all bands when I tested it initally at home.

I replaced the power cord with a new 3-wire cord and replaced weak/wrong
tubes. Both 47µF electrolytics seemed OK (both leak <0.1µA at 250v). Slowly
brought it up on a Variac, passed the smoke test. No internal components
seemed discoloured or broken. 

Funny thing: B+ at pin5 of C217B was only 50V, whereas it was 240v at pin3
of C217A. Next move could be to break the B+ line at C217B and measure the
mA draw...

Does this make sense?
What other things should I do before that?
What are the weak areas to look at in a R-388?

Thanks

W. Li
Mercer Island, WA
  




      
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