[Amps] Harbach Heathkit SB-220 Soft Start Update

[Geert Jan de Groot]

> I have had some time to investigate my Harbach Soft Start failure and found some very unusual results. 

A couple things to ponder.

First, a SB220 that is not RF-keyed is just a HV power supply, a 110V 
power supply, and a hefty power supply to power the filaments of the 
tubes (5 volts at at 15 amps each).
(oh, there is some scale illumination and a fan, too).

Take the tubes out and the big load is gone. What is left is the HV 
power supply and capacitors charging, etc.

I have used this fact to operate the SB220 in "scale down" power supply, 
instead of 220 volts, just 2 volts on the input. Things like the HV 
power supply work, but with voltages and currents scaled down.
And while you still want to be careful with HV (don't build routine to 
touch it!), at least there is no HV during testing.

This is how I did a lot of my testing (after several updates) before I 
put tubes in and full input voltage on.



There is a second thing to ponder. A SB220, wired for 220 volts, has two 
110V windings in series to be configured for the 220V. This arrangement 
is for both transformers.

Unfortunately, the middle connection point of both transformers is 
joined, so the join of the HV transformer is is connected to the join of 
the filament transformer.

The HV transformer is not directly connected, there is a switch that 
switches the input windings of the HV transformer (to switch the HV 
voltage) for CW/SSB operation.

There is a failure mode where the SSB/SW front switch may have a bad 
contact, In that case, the filament transformer (whose voltage is not 
switched) only feeds one half of the HV transformer. When this happens, 
the 110V filament transformer gets much more than 110V, and I wonder if 
that is what caused your problems.

This problem is described in Heathkit service bulletins, specifically 
SB-220-19 and more specifically SB-220-27.
See https://puck.nether.net/pipermail/heath/1998-January/028165.html

I wonder if your SB220 is operating on 220 volts and if true, if the 
excessive high voltage on the 120v power supply is what caused the 
failure of the harbach slow start (and the 120 volts power supply).
Come to think of it, perhaps the Harbach failure has prevented you from 
a burnout of the filament transformer if the above scenario is true.

I worry that perhaps the filament transformer is still damaged by this.

And I would *not* operate any SB220 without SB-220-27 implemented since 
these transformers are now hard to get.

73,

Geert Jan PE1HZG