I'm looking for the TUNE variable capacitor for a Heathkit
SB-220/SB-221 Linear Amplifier. The one that's in the one I just
picked up has been arc'ed so badly that even after carefully filing
away the burnt/melted parts, it still arcs occasionally. The cap is a
250 pf. variable cap, and the Heathkit part number was 26-131. I
haven't looked at a Heath SB-200/SB-201 manual to see if those amps
also use the same Tuning cap.

If anyone has a junked/parts unit that has this variable cap
available, please let me know.

Larry

Hi Larry
It doesn't mean that the cap is bad because it arcs, Try installing a
better parasitic suppressor on the tubes first. Your amp has this problem
because of the use
of high VHF-Q copper inductors between the TUNE capacitor and the anode
connections or plate caps on the 3-500z's. What you see is the 2600V of
arcing caused by the intermittent 110MHz parasitic oscillation. This
voltage takes the path of least resistance and can take out other
components very quickly.
The factory suppressors are made out of copper wire. This keeps the You
can easily fix this by making LOW VHF-Q paracitic suppressors using VHF
lossy nickel-chromium-iron alloy (NICHROME-60) wire. Replacing the
original copper braide between the DC Blocking cap and the HV RF Choke
with a pair of unequal length of nichrome-60 wire. This will dampen the
effect of the VHF Q and give you the VHF stability so it won't it won't
arc anymore.
If you use the amp as is it will eventually burn your bandswitch contacts
.. As for your Tuning Cap, I bet it's fine for another 40 years.
73 Chris VE3NGW/W4 Florida


I'm looking for the TUNE variable capacitor for a Heathkit
SB-220/SB-221 Linear Amplifier. The one that's in the one I just
picked up has been arc'ed so badly that even after carefully filing
away the burnt/melted parts, it still arcs occasionally. The cap is a
250 pf. variable cap, and the Heathkit part number was 26-131. I
haven't looked at a Heath SB-200/SB-201 manual to see if those amps
also use the same Tuning cap.

If anyone has a junked/parts unit that has this variable cap
available, please let me know.

Larry

He is right about the VHF parasiics.
A new capacitor will not stop it from arcing.

Look here for answers and parts to fix it.

http://www.somis.org/

I have installed his Low VHF-Q parasitic
suppressor retrofit-kit in my own SB220
and it has never arced.

Chris wrote:
Hi Larry
It doesn't mean that the cap is bad because it arcs, Try installing a
better parasitic suppressor on the tubes first.

I'm looking for the TUNE variable capacitor for a Heathkit
SB-220/SB-221 Linear Amplifier. The one that's in the one I just
picked up has been arc'ed so badly that even after carefully filing
away the burnt/melted parts, it still arcs occasionally. The cap is a
250 pf. variable cap, and the Heathkit part number was 26-131. I
haven't looked at a Heath SB-200/SB-201 manual to see if those amps
also use the same Tuning cap.

If anyone has a junked/parts unit that has this variable cap
available, please let me know.

Larry

Unfortunately, depending on how much arcing has occurred, the arc marks or
rough edges left make it even easier to arc again, even with the suppressors.
Suppressors are cheap enough and easy enough to install to give it a try. But,
you may find out that the arcing still occurs. Some people have successfully
deburred the rough edges created from the arcing to get a nice smooth surface
edge again, and the arcing may stop.
Good luck. Bennett ko2ok

Thanks to all the responses, but yes, I have installed the updated
suppressors (Harbach modifactions with new suppressors, parts
underneath the chassis, etc.) Unfortunately, this TUNE variable
capacitor is so badly melted and mangled that it really does need to
be replaced. It no longer arcs on 10M now, but now arcs on 20M, etc.
I've cleaned up the cap as much as possible with some small files, but
it's still in bad shape. It even arcs down at the base of the rotary
plates now, not so much at the outer edges that were originally melted
down.

I'm still looking for a replacement 250 pf. cap from a Heath
SB-220/SB-221.

Larry





(Bpn518) wrote in message ...
Unfortunately, depending on how much arcing has occurred, the arc marks or
rough edges left make it even easier to arc again, even with the suppressors.
Suppressors are cheap enough and easy enough to install to give it a try. But,
you may find out that the arcing still occurs. Some people have successfully
deburred the rough edges created from the arcing to get a nice smooth surface
edge again, and the arcing may stop.
Good luck. Bennett ko2ok