[AMRadio] Suggestions for restoring a DX-100

Bill Smith billsmith at ispwest.com
Wed Apr 3 23:09:08 EST 2002 

You are very welcome.  Wish I had Internet when I was starting out.  :-)

Regarding leakage, Sprague published a formula for acceptable leakage which
was based on the capacity (and ambient temperature?  I forget) of the
electrolytic.   I have it here somewhere, but it is buried at the moment.
Generally, any more than 5-10 ma leakage is asking for trouble, with a new
capacitor's performance less than 1 ma.  You'll have to establish a feeling
for what is good and not so good.

A couple of additional thoughts.  First, make sure you discharge a capacitor
though a 1k resistor (value not critical) after reforming it.  Electrolytic
capacitors are like rechargeable batteries, and you can get a very nasty
shock from the terminals of a charged capacitor.  If the cap is good, its
charge can last days or even weeks.  You'll get a big, noisy arc if you
discharge using a screwdriver.

Second, (related to leakage) if the cap warms up to the touch, even
luke-warm, it is probably excessively leaky.  It might warm a little when
reforming (which indicates the reforming current is too high), but it should
not be warm when charged to its operating voltage.  A hot electrolytic (not
close to a tube) in equipment is a dead giveaway of a defective cap.  Tubes
radiate quite a bit of heat and can make an electrolytic feel 'too hot' just
from proximity.

Third, keep an eye on the reforming voltage.  If you place a resistor in
series with the diode, and measure the voltage output at the diode, and also
across the capacitor, recall that the voltmeter is reading average voltage
in both cases.

At the point after the rectifier, the meter will be averaging a pulse, and
will indicate a voltage lower than the peak.  At the capacitor, the meter
will be reading smoothed DC which, if the capacitor is any good will
eventually charge to a voltage which will be close to the peak voltage
delivered by the diode.

The point is you don't want to overcharge the capacitor beyond its rated
voltage.  If you leave a capacitor charging, and have set the variac based
upon the measured voltage at the output of the diode, remember that the
capacitor will eventually charge to a voltage well above this measurement.
Keep an eye on it so that the capacitor's working voltage is not exceeded.

By the way, the resistor value I calculated was for a current value assuming
a completely discharged capacitor (short circuit).  You will probably wish
to use a smaller value of resistance if you can limit the current with a
variac.

73 de Bill, AB6MT
billsmith at ispwest.com


----- Original Message -----
From: "john_walker_beasley" <jbeasley at earthlink.net>
To: <amradio at mailman.qth.net>
Sent: Wednesday, April 03, 2002 5:46 PM
Subject: Re: [AMRadio] Suggestions for restoring a DX-100


> Thanks everyone !
>
>     One more question.
>
>     What is an acceptable amount of leakage ?  Less than 1ma?
> What latency should I expect, for example, less than 1ma after
> 5 minutes under voltage ?
>
>     I know these questions are stupid to someone who is
> knowledgeable with the procedure.  I'd rather sound stupid than
> flame this rig !
>
> Thank You
> John, WAØSTX
>
> ----- Original Message -----
> From: "George Pritchard" <gpritchard at comtechpst.com>
> To: <amradio at mailman.qth.net>
> Sent: Wednesday, April 03, 2002 2:16 PM
> Subject: RE: [AMRadio] Suggestions for restoring a DX-100
>
>
> > Reforming caps, or verifying leakage on them is easy if you have one of
> > those older 0-400 VDC high voltage variable supplies (heathkits $10 at
> > fests). I used it to verify leakage on "New Old Stock" caps and noticed
> very
> > high leakage, >10 mA at only 100 Volts! I'd leave them at 100 volts
until
> > leakage dropped to < 1 mA... then go up slowly to 200 volts, and wait
> again.
> > Repeat the process untill you get to 400 volts on the 450 volt rated
caps
> > with less than 1 mA current. I did this for the replacement caps on my
> > DX-100. Guess what... They only lasted two weeks and shorted again
anyway!
> I
> > ended up with big oil types mounted where the HV 5U4s used to be before
I
> > solid-stated the supply. The oils will never fail. BTW... I used the
> > Heathkit supply to verify the shorted capacitors in the first place. It
> > saves fuses and possible the transformer!!! Good luck.
> > George AB2KC
> >
> > -----Original Message-----
> > From: amradio-admin at mailman.qth.net
> > [mailto:amradio-admin at mailman.qth.net]On Behalf Of Bill Smith
> > Sent: Wednesday, April 03, 2002 2:52 PM
> > To: amradio at mailman.qth.net
> > Subject: Re: [AMRadio] Suggestions for restoring a DX-100
> >
> >
> > Don't turn it on and expect the electrolytics to survive the shock.
Bring
> > up line voltage slowly with a variac, or light-bulbs in series with the
> > power line.
> >
> > Start with about 60 volts.  Just enough to light the filaments in the
> > rectifier and feed voltage into the power supply.  Ramp it up slowly
from
> > there, over the next several minutes.  It should begin to operate at
about
> > 90 volts.  Play it at 110 volts for a while (ten or so minutes) before
> > giving it the full gas, watching for trouble.
> >
> > If you go through this simple procedure, you may recover all the
> > electrolytics.  If you don't, you'll be sure to replace at least one
can.
> > Look at the circuit to see how B+ is switched, and see if there is some
> way
> > to reform any cans that are switched on only during transmit.
> >
> > Unfortunately, there is no recourse to tubular paper caps, they become
> leaky
> > over time and will likely need replacement.  They may seem "ok" when you
> > first use the equipment, but will quickly degrade with use.
> >
> > 73 de Bill, AB6MT
> > billsmith at ispwest.com
> >
> >
> > ----- Original Message -----
> > From: "russ dworakowski" <wb3fau at hotmail.com>
> > To: <amradio at mailman.qth.net>
> > Sent: Wednesday, April 03, 2002 11:35 AM
> > Subject: Re: [AMRadio] Suggestions for restoring a DX-100
> >
> >
> > > John,  what to do   with  your DX-100 has a lot to do with how  it
has
> > been
> > > stored.  Transformers  are  not a common  problem in the  DX 100 as
> they
> > > are the potted type  and  less subject  to moisture damage.
> > > Replacement of  power supply  electrolytiics  is  most  common
problem.
> > > These rigs sit around  not  in use  for  long  periods causes  the
> > > capacitors to go bad.  There are a  few in the   audio  section  to
> check
> > as
> > > well.  Good  luck and  have  fun!   Russ
> > >
> > >
> > > >From: "john_walker_beasley" <jbeasley at earthlink.net>
> > > >Reply-To: amradio at mailman.qth.net
> > > >To: "AM Radio Reflector" <amradio at mailman.qth.net>
> > > >Subject: [AMRadio] Suggestions for restoring a DX-100
> > > >Date: Sun, 24 Mar 2002 11:43:38 -0600
> > > >
> > > >Gentlemen:
> > > >
> > > >     I have a DX-100 and I am looking for any tips or advice
> > > >before I begin working on it.  It is in good cosmetic condition
> > > >and the quality of the soldering looks good.
> > > >
> > > >     I am familiar with the replacement of electrolytics, however,
> > > >my expertise with aging components pretty much stops here.
> > > >
> > > >     What I  am mostly concerned about is trying to avoid the
> > > >problems some people talk of such as losing mod transformers
> > > >from the failure of other aging components.
> > > >
> > > >     What precautions should I be taking? What other components
> > > >should be scrutinized. ?
> > > >
> > > >     Any suggestions or links that may be good reading on this
> > > >topic ?
> > > >
> > > >     Thank You
> > > >
> > > >     --... ...--
> > > >     John WAØSTX
> > > >
> > > >
> > > >
> > > >
> > > >
> > > >
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