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


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Richard Knoppow
Los Angeles, CA, USA