"jakdedert" wrote in message
. ..
Richard Knoppow wrote:
"jakdedert" wrote in message
.. .
Scott Dorsey wrote:
wrote:
hi again, tnx for the replies..i did clean all wafer
sws & pots,
didn't clean main tuning cap..have bama prints for
rx..will try
voltage & resistance checks..full alignment sounds
like a good idea..i
have sig gen..1st & 2nd ifs would be difficult because
sig gen calib
not great..i dont think if alignment is my main
prob..but will do all
hf aligns..i'm glad to get back into tubes..its like
restoring a
classic car, but alot cheaper...hi hi. tnx agn..if u
have any more
ideas, pse email me.
Before you touch the alignment, look for leaky paper
caps.
--scott
That's kind of like looking for when Bush is
lying...it's when his lips are moving. How do you know
if they're leaking? They're the paper ones--they all
leak...or will.

jak

You need a capacitance bridge to measure the
dissipation factor. The by-pass and decoupling caps in
the HQ-180 appear to be mostly disc ceramic caps. These
have relatively low dissipation factor and are considered
quite reliable but can go bad. Electrolytic caps, mostly
used for power supply filter caps, at least in vacuum
tube equipment, have relatively high dissipation factors
when new but mainly fail when not used for some time. One
plate of the capacitor is s chemical film which depends
on the presense of a voltage to form so if not used for
some time the thing simply stops being a capacitor.

Good info which explains 'why' old electro's should simply
be replaced.

Paper caps can actually be pretty good. If
reasonably well sealed they have long lives. The
notorious Black Beauty caps, as used in Hammarlund
SP-600-JX receivers, had a manufacturing problem which
caused them to be short lived. Actually, they were
intended and sold to be long lived high performance caps
and are found in all sorts of very high quality
equipment. I suspect that moisture getting into the caps
is the main cause of failures but they started getting a
bad reputation not long after they began to be used so it
might be something else.

I've read that the paper was not all that high
quality...contains acids which will inevitably break it
down. Moisture is the main culprit, but only leads to
breakdown sooner than later. Wax as a sealer was never
going to have as long life as the other parts of the
electronics in which these were installed.

Good paper type caps should have dissipation
factors (at 1 khz) on the order of 0.01 or less and very
low leakage types like ceramic, silver mica, and plastic
(polyethylene, etc.) of perhaps 1/10th of this.
Electrolytic caps will have dissipation factors much
larger, however, they really should be measured with DC
on them and some capacitor checkers allow for this.

Conventional wisdom is that they'll leak sooner or later
anyway. Better to shotgun them and be done with it. The
wax will likely melt sooner or later, anyway.

OTOH, Chuck says there are no paper caps in the 180, so
I'd defer to that.

My bet about the HQ-180 is that the power supply
filter caps are sick.

...and should be replaced. If they're not sick now, they
could fail in spectacular (or not) fashion down the line.
It's about the easiest thing to do...replace the filters.
If they're not bad now, it's just one less thing to worry
about.

I operate some vintage tube gear in which I've not
replaced the filters; but I realize I'm sitting on a
potential time bomb. First sign of a hum, I'm turning
power off...which might be too late. Just because I don't
follow my own advice (all the time) doesn't make it bad
advice.

jak

Electrolytics can have a very long life. Generally,
they go bad slowly with an attendant drop in supply voltage
and increase in hum. They rarely have catastrophic failures
except after a long period of disuse when they become
non-capacitors. Sometimes they can expode resulting in a bad
smelling mess. This is pretty unusual. The power supply caps
also act to by-pass some signals, especially audio, to
ground. So one symptom of bad filter caps may be audio when
the volume control is all the way down or noise or
distortion.
I would not replace good caps simply beause they are
old. As long as they are being used the film which
constitutes one plate, will stay formed. Occasionally
something will cause a short but, again, thats not too
common.
Paper caps are all over the place. The old oil-filled
ones were quite reliable and of high quality but were large.
Most are high-voltage types. Unfortunately the dielectric
oil used for many years turned out to be a form of PCB which
is a cancer risk. These were sold with names like Pyranol,
Dykanol, etc. Again, if you have some and they are not
leaking, leave them alone. The main virtue of these oils was
that they are not inflamable which was not true of the
mineral and vegetable oils used previously.
Some tubular caps used a wax impregnant as the
dielectric. Some were simply wax covered. Keep in mind that
the latest of these guys is probably getting on toward half
a century so its not surprizing that there are failures.
The Black Beauty caps found in a lot of 1950s equipment
were intended to be deluxe, low leakage, long life types.
For some reason they began to fail after only a few years. I
think an improved version was made because some later
equipment has caps that look the same but don't seem to fail
early. Another type of capacitor was made by the same
company under the name Orange Drop. These shared the same
plastic impregnated paper dielectric but were equipped with
dipped epoxy cases rather than the molded Bakelite cases of
the Black Beauty types. Orange Drop caps seem to be quite
reliable which leads me to think the problem wtih the BBs
was the casing rather than some problem with the impregnant
or paper. Of course, at this late date, I doubt if we will
ever know.
Hammarlund got burned badly with the BB's which had to
be replaced in thousands of military receivers so I think
they avoided them afterward.


--
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Richard Knoppow
Los Angeles, CA, USA