[Collins] re: newbie question

[Dr. Gerald N. Johnson]

On Sun, 2008-06-15 at 11:34 -0500, Rodger wrote:
> If it is actually dropping almost 200 volts across choke L-123 you 
> either have a problem in the choke or a significant drop in your 
> resistance to ground downstream of the choke.  Before going further, 
> disconnect L-123 to preserve it if it hasn't already been damaged and 
> check your resistance to ground at pin 5 of C-217, it will take a few 
> seconds to get a valid reading as C-217 charges up.  Hopefully you just 
> have a capacitor with severe leakage and nothing else.  If the 
> resistance to ground is normal at the capacitor, check the resistance 
> across the choke and also from the choke terminals to ground (with the 
> both leads to the choke disconnected) in case the choke has developed 
> internal leakage.

The 2/3 amp it takes to drop 200 volts across a 300 ohm choke resistance
should have the chokes, the power transformer, and the rectifier tube
very hot in a short time. And the line fuse should be blown unless it
has a much larger current rating. If the 1.5 amp fuse is holding the
excess voltage drop has to come from an excessive resistance in the
choke.
> 
> Does the R-388 have the proper fuse in it?  With the current draw it 
> takes to drop this much voltage across a good choke I would expect a 
> properly sized fuse to open.

And the voltage drop in the first choke, the power transformer, and the
rectifier would not allow the high voltage tap to be normal voltage, so
the problem must be a high resistance in L123. Its resistance should be
300 ohms according to my Collins manual. That can only be checked
accurately with one lead disconnected from the circuit.
> 
> Rodger WQ9E
> 
> T
> 
>  wli wrote:
> > 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 initially 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
> >   
> >
-- 
73, Jerry, K0CQ, Technical Advisor to the CRA
All content copyright Dr. Gerald N. Johnson, electrical engineer