All good suggestions... but don't overlook the possibility that a leg of
the bridge rectifier has gone bad. Putting a scope on the output (as was
suggested in one post) will quickly verify excessive ripple. Ripple should
be under a percent or two of full DC voltage. Actually there should be
minimal ripple at the output of the bridge rectifier... if you see more of
a sawtooth waveform, the rectifier section could be the culprit.
I had this exact problem with a midi accordion my father used to play in his
band... When he went to some gigs, the AC voltage was marginal [that
happens when they string a couple hundred feet of extension cord from the
house out to the gazebo].
Of course the symptom he told me was, "damn thing has never worked right...
sometimes when I play it I get AC hum from the amplifier... it gets louder
with the more keys I play simultaneously." Naturally when he brought it
home where the AC was adequate, he couldn't duplicate the problem for me!
The source of the trouble was revealed when I finally brought my scope over
and hung it on the DC supply line. Sure enough, press no keys, little
ripple... the more keys pressed, the more the ripple. Hindsight is nearly
perfect! It became apparent that a lower AC main voltage, this symptom
would be exagerated. My father had trouble relating the circumstance to the
symptom.
I got a free spaghetti dinner out of that repair job! I can't tell you how
happy he was that I located the problem that had plaqued him for over 5
years. A few years ago, I acquired all his musical equipment and I found a
note in the books about this repair job! Something about tears of joy
coming to his eyes. Sorry... I'm getting off topic here.
Good luck with your troubleshooting.
73 de Bob - K0RC
----- Original Message -----
From: "Steve Matda" <smatda@yahoo.com>
To: <rtty@contesting.com>
Sent: Friday, August 23, 2002 12:03 PM
Subject: Re: [RTTY] Help!!!
>
> Seems to me that most power supply problems usually occur in four areas:
the pass transistors, the regulator (usually an LM723 type), the transistor
controlling the pass transistors (usually a TIP41C or similar) and the
protection circuits. Sometimes the protection circuits work too well, and
hide the real symptoms of the problem.
> I'd pull the pass transistors and check them first off with an ohmmeter.
If they are bad, I'd replace them as well as the ballast resistors. Do
you know if the 20 amp rating is peak or continuous? If it's peak, I'd be
immediately suspicious of the pass transistors...
> Working backwards, I would then check the current amp driving the
passes...and then last I'd check the operation of the regulator itself to
make sure it was driving the current amp correctly.
> That being said, the first thing I do with any supply I get is to
socketize the regulator IC if it isn't already...it's then easier to replace
when it dies (and trust me, it will).
> As for expense, it really is worth your while to keep a couple of sets of
pass transistors, regulators, and current amps handy. It's hard to believe,
but 4 30-amp pass transistors, a regulator, and a current amp runs a total
of about 12 dollars. (Which raises another point--why are power supplies so
darn expensive??)
> I don't consider myself an expert (and you shouldn't either)...sometimes
I'm lucky and fix the problem, and then sometimes I'm left scratching my
head amid a cloud of smoke. This is just the approach I would take.
> Hope this helps...from a person who has spent quite a few late nights
staring at a schematic and wondering "WHY IN THE #$*% DOESN'T THIS THING
WORK??!!!"
> Steve, KE4MOB
> Note: forwarded message attached.
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