Subject: Re: [drakelist] AC-4 Cap values
From: k5vkl{at}juno.com


k5vkl{at}juno.com made an utterence to the drakelist gang
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Carl,

Now might be a good time to review some slow-start feature design
reviews,
looking forward to incorporating one into the turn-on function of the
power
supply.  When you have a "thump" from turning on a power supply, it 
generally is indicative of too great of a surge current going into the
filter 
capacitors and stressing them unnecessarily.   It doesn't help the
rectifiers,
either.  Also, I'm not sure that it is a "leaky" capacitor that causes
the
"thump" you heard at power supply turn-on....I've always been led to 
believe that the "thump" was from the large flow of current into an
uncharged capacitor, although I've never had it proven to me that this
was the actual cause.
You could, however, benefit the power supply considerably, by inserting a
resistor of the proper resistance and wattage, into the 650 volt output, 
between D2 and the connection to R1.  This would also serve to bring your

650 volt HV (now approx. 700 volts, as you indicated) back down nearer to

the correct 650 volt value.
Your could also do the similar action with a resistor added between D5
and
the connection to C3, and also reduce the 290 volts down to the original
design value of 250 volts.   
Possibly, the original design of this power was slighted somewhat, by not

including a choke in each of the two points I noted above.   That would 
have produced a significant reduction in the capacitor surge current at 
turn-on.   Chokes would also have been more effective in reducing the 
ripple you originally saw on the outputs.   
I'm naturally assuming, of course, that you do intend to reduce power 
supply outputs down to their specified voltage levels. 
I haven't intended to sound critical of your modification efforts here,
Carl.
Moreso, I am critical that Drake didn't put choke filtered supplies in
this
equipment.   I would sure have liked my T-4XB/R-4B's AC-4 to have been
choke filtered.   (Guess they figured the cost of the choke, a place to
put it,
and also a slight change in the secondary output winding in the power
transformer secondaries would cost too much to add into the equipment
price.

Anyhow, and Meanwhile, back at the Ranch (as "they" say, somewhere)
keep up the good work, and keep us posted.

Bill Flowers   K5VKL   
    
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On Wed, 02 Mar 2005 20:37:21 -0800 Carl Strode 
writes:
> 
> Carl Strode  made an utterence to the 
> drakelist gang
> ----------------------------------------------------------------------
> I have completed re-capping the AC-4.
> 
> After looking at the actual caps installed, and comparing it to my 
> supply of modern electrolytics, I have replaced the 40 mfd caps 
> (shown  as 40 mfd on most schematics) with 33 mfd.
> 
> The HV filter caps were replaced with 150 mfd 450 volt units.  ( I 
> had a  bunch of these laying around).
> 
> Since my original late model AC-4 had a 100 mfd cap installed at C3 
> and  C4a, and an 80 mfd cap installed at  C4b,  I decided to  keep 
> everything  simple and replace these with 150 mfd caps as well.  I'll
save all 
> of my  82 and 100 mfd caps for some other project.
> 
> The 150 mfd caps I have are circuit board mount types, with both 
> leads  on one end.  This made installation above decks a snap.  I
removed 
> the  old caps, and then reamed out one of the holes used for the old
cap 
> ____________                   __________________                 
________________
 When I was finished, there was essentially no ripple at all!  only 
> about a tenth of a volt on the 650 and the 250 volt, and about 0.2volts

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