It appears that the 1000pF red ceramic capacitor was replaced with the
flat maroon capacitor in yours. Someone left the 1000 pF stock capacitor
in the chassis but disconnected it. Maybe it was suspected to be
failing. A HV test with a hi potter would determine that. Meanwhile, if
the amplifier was working on all bands before, I would not reconnect
that red capacitor unless you can test it first with HV. If the 5800 pF
is of high enough voltage rating (4-5 kV recommended) and is of low
enough inductance that it's first series resonance (internally) is above
30 MHz, then you are probably good to go. The capacitor will have to
support some RF current being bypassed to ground, as the red choke would
have the lowest inductive reactance (Xl) at the bottom of the 80 meter
band. This would result in some RF current going through choke to this
capacitor where it is bypassed to ground. This is why it should be a
ceramic made for such service.
The green choke to the shorting switch and feedthru is a secondary 8.5
uH and on the other side of that wall is another 1000 pF but it is a
large ceramic disk to ground for additional bypassing. It keeps RF out
of the power supply/metering area.
A more scientific approach would be to find a grid dip meter and measure
around that area, coupling to the green choke. Then disconnect one cap
and connect the other and see if the first resonance (suck out on the
meter) moves.
73
John
K5PRO
Date: Sat, 15 Oct 2016 15:41:28 +0000 (UTC)
From: Catherine James <catherine.james@att.net>
To: <amps@contesting.com>
Subject: [Amps] SB-220 question
Message-ID: <1403430264.272807.1476546088267@mail.yahoo.com>
Content-Type: text/plain; charset=UTF-8
I've just bought a Heathkit SB-200. Superficial inspections looks good; no
sign of burned contacts, corrosion, etc. Transformers appear fine.
There do not appear to be any Harbach board modifications made. I will need to
install the softkey before I can use the amp, and I am planning to put in the
soft-start at the same time.
There is one connection that I cannot reconcile with the manual, but I am no
expert on this amp. See the photo below:
https://s3.amazonaws.com/files.qrz.com/r/n5wvr/SB_200_plate_wiring.jpg
The plate connections of the two tubes run through a capacitor/inductor circuit
to a green RF choke. There is a short uninsulated wire that runs from this
choke to a lug, and it appears that it once connected to a screw terminal on
the capacitor bank wall. However, it is hanging loose.
There is also a capacitor of about 5800 pf running from the end of the green RF
choke to a grounded lug on the sheet metal panel above the capacitor bank. I
am not seeing this in the manual at all.
Is there a known SB-220 mod where this direct connection is replaced by a
capacitor? What purpose does it serve?
73,
Cathy
N5WVR
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