if you lift one leg of C22 (towards V15A), it will isolate the variable
oscillator, and then you can troubleshoot it without involving other parts
of the circuit.
Same with C17 - the amplifier for variable oscillator - where you can
disconnect it from mixer and check its input and output.
Check V4 heater voltage, and I hope C402 was checked. I cant recollect
precisely, but I had issues around these components as well.
if you want I can check the DC bias voltages for V4 and update you over
weekend.
On Tue, Nov 5, 2019 at 1:48 PM Bob Albert via Groups.Io <bob91343=
[email protected]> wrote:
Thanks but that's not it. The zero adjust controls work okay and the
problem is simply that the variable oscillator doesn't oscillate. I have a
tiny signal from it whose frequency can be varied a bit with the tuning
slug. But it's nowhere near enough to drive the mixer.
Bob
On Tuesday, November 5, 2019, 11:54:04 AM PST, Rajesh VS <
rajeshvs@...> wrote:
Bob,in case it helps..my LC130 was misbehaving when I acquired it, and
the cause was a broken solder joint around the Zero adjust controls (front
Panel) . Don't recollect if it was in the fine or coarse control.
Location is top left in the pic -
On Tue, Nov 5, 2019 at 11:00 AM Bob Albert via Groups.Io <bob91343=
[email protected]> wrote:
Yes I am aware of many of the failings of various capacitor types.
However, I am not convinced that the problem in this circuit is caused
by a
poorly behaving capacitor. I will once again attempt to measure voltages
and even a few components to see where something isn't right.
For now, though, my attention is directed elsewhere. I have other means
of making measurements that the 130 can do, so my interest is only to
have
some fun with the unit. If I never get it working, it still is a vehicle
for education. The lowest range, 0 - 3 pF, intrigues me but I can make
those measurements with the Q meter if need be. I have a few 2 pF, I
think, capacitors that I want to characterize. They are the ceramic
dogbone style.
I also have a box of variable capacitors but those are easy. It pains me
to have these, as I have no projected use for them and they are almost
works of art, with the lovely bearings and shafts and other moving parts.
Back in the day, one might build an amplifier or tuner, but I don't need
either. So I will stuff them into the closet and let my heirs dispose of
it all, as they will be obliged to do with all my precious possessions.
It
might be prudent for some local enthusiast to make friends with me and my
family in anticipation of this event.
Bob
On Tuesday, November 5, 2019, 10:26:50 AM PST, Richard Knoppow <
dickburk@...> wrote:
There are two kinds of mica caps, silvered mica and plain
mica. Silver mica of a certain period suffer from something
called silver mica desease, probably due to moisture penetration
through the Bakelite casings. More modern epoxy dipped caps do
not seem to have this problem and it is only occasionally found
on the Bakelite type. It results in the cap becoming unstable,
sometimes jumping in value and sometimes having "scintillation"
or rapid variations. One should be aware of it but mica caps do
not have the predictable degradation with time of paper caps. The
other type of mica caps, usually found a HV transmitting caps,
are made of stacks of mica dielectric and foil pressed together
and held under pressure. They seem to be nearly perfectly
reliable but silver mica have better HF characteristics.
Ceramic caps depend on the type of ceramic. Those made with
high K material do age and change value. They also have
considerable voltage coefficient of capacitance. The ones made
with low K such as zero TC caps, are extremely stable. Some of
these caps fail, particularly the disc type, because the leads
have been stressed where they enter the body and have cracked it
allowing some air and moisture to enter. Disc caps can fail so
should never be left out of trouble shooting routine but neither
mica or ceramic need to be shot gunned as do paper or
electrolytic caps.
On 11/5/2019 12:10 AM, Craig Sawyers wrote:
Mica caps are a known weak point in the Racal RA17 receiver. They are
not a replace-on-sight because
there are lots of them, but worth bearing in mind that they can
occasionally be problematic.
Craig
-----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of
Ed
Breya via Groups.Io
Sent: 05 November 2019 00:04
To: [email protected]
Subject: Re: [TekScopes] 130 LC meter
Ceramic caps and even micas can get leaky after many years - not
often,
but I've seen it. Ed
--
Richard Knoppow
dickburk@...
WB6KBL
--
/Rajesh
--
/Rajesh
