[Amps] SB-220 question

[John Lyles]

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 at att.net>
> To: <amps at contesting.com>
> Subject: [Amps] SB-220 question
> Message-ID: <1403430264.272807.1476546088267 at 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