[Hammarlund] Need suggestions of professional Hammarlund restoreres for SP-600-JX-1
Barry Hauser
[email protected]
Tue, 2 Dec 2003 13:52:10 -0500
I agree with Roy, though I sometimes employ the variac method, but in
combination with other pre-checks. This may serve to explain his advice
of "DON'T DO THAT, despite lots of advice from those who do."
1. If the radio has not been powered up for years, it's highly possible
that the filter caps have dried out, shorted and subject to instant
failure. At minimum, they're not ready to handle full power, and what is
ordinarily a momentary surge will be extended and too much too long. The
immediate risk is the power transformer among other potential collateral
damage targets, like those two large chokes in the SP-600. Bad paper
caps can short out coils, IF/RF transformers, etc. New caps are easy to
find, but the coils, chokes and transformers are readily unavailable from
Unobtanium Industries - unfortunately their phone number is unlisted and
there's no web site.
2. As a minimum -- caps should be checked with an ohmmeter. All tubes
should be tested, at least for shorts. If there is a resistence chart in
the service manual, you can go through that. Electrolytics can be tested
and re-formed with a cap checker that applies voltages at various levels.
They have to be disconnected from the circuit, and preferably removed
from the chassis. I usually put them in a sturdy metal box -- in the
event of explosion, even though I step the voltage up gradually and check
leakage at each level. I've had no explosions or thusfar, but YMMV. I
did have a couple of smokers, but that's when I took the word of the
seller that the unit was powered up recently when it might not have been
so and proceeded to practice unsafe radio. Also, if the rig has recently
been shipped, things can get out of place, tube elements can short, and
then there is the possibility of bad repairs, etc, so a close visual
inspection is generally in order.
3. There's a basic fallacy to bringing up a whole receiver slowly on a
variac. Tube rectifier(s) don't start to conduct until they reach a
risky voltage -- 90 or so. Hence, all that gradual-ness is illusory.
(That turn-on voltage is a generalization and may apply to the
transformer secondary, which can be a substantial multiple of the AC
primary voltage, so does not necessarily correspond to the variac
setting.) Some use the technique of subbing any tube rectifier with
solid state.
4. Part of the variac strategey is to gradually apply power (with solid
state rectifiers) and listen for crackling, watch for arcing, smoke or
odors. The theory is that if a wisp of smoke appears or an odor, you can
shut things down before there is major damage. However, these indicate
some damage has already occurred. One variant is to use a light bulb in
series with one side of the AC to limit current. That's usually done as
a quick check for the presence of a dead short. Built in fuses can
provide some protection, where the B-plus and filament lines are
separately fused in addition to the AC supply. The SP-600's generally
have a two-fuse setup. However, these are not fail safe either.
Some of us, under some conditions, would rather not first recap a piece
of gear before trying it out. So, we may choose to take a calculated
risk, partly reduced by various techniques. It's a question of -- as
Clint Eastwood said in the movie -- "Do you feel lucky?" Even if you
get past that critical powerup point and the radio works without damage,
critical caps should be replaced for low-risk long-term use and
performance. Other components should also be checked for shorts and out
of tolerance, etc. One haunting question -- why was the rig taken out of
service in the first place? Of course, there is also some risk when
doing a full-blown recapping job.
Bottom line though, simply powering up on a variac, without doing
anything else first, is not a good idea regardless of how lucky you feel.
Those who do this usually take some other precautionary steps, and are
willing to take the risk. Also not a bad idea to have a fire
extinguisher handy.
Barry
On Tue, 02 Dec 2003 12:16:23 -0500 Roy Morgan <[email protected]>
writes:
> At 09:21 AM 12/2/03 -0500, you wrote:
> ...recently received a generous gift of an
> >SP-600 which has not been plugged in for at many, many years.
> >
> >I haven't plugged it in myself, even on a variac.
>
>
> DON"T DO THAT, despite lots of advice from folks who do. It's an
> invitation
> for disaster.
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