[Heathkit] SB1000 problem

[Kenneth G. Gordon]

On 15 Feb 2021 at 16:27, Bruce Hudler WK3A via Heathkit wrote:

> Hello Group,
> I'm working on a SB1000.
> Originally it had a homebrew Soft Start board (it is wired for 120V) in it, mounted on top of the power transformer. That had to come off and changed back to original configuration so that I could install the SBA2001-1 QSK board (used one), which lives on top of the power transformer now. 
> I've got everything working now after replacing one of the open PIN diode D101 on the QSK Board. 
> Tunes up OK and on the air tests seem to be fully functional 160m - 10m
> Now the real question - most often the damn thing trips the mains breaker instantly (15A 120V) in the house when powered up. After quick reset, the amp works normally and I'm puzzled by this. I powered off everything on the circuit, and tried a different breaker circuit.  No difference.
> I don't know if this is why the original HB SS was installed or something to do with the QSK board. 
> The amp is new to me so I don't have any past history. 
>  Thanks
> Bruce WK3A

Hello, Bruce:

I suspect that the original owner's SoftStart board was necessary to forestall the tripping of 
the breaker.

IMHO, this is often caused when a good, low-resistance, transformer exhibits close to a 
dead-short momentarily until the flux density rises enough. I think it has nothing to do with 
whether or not the capacitors are "full" yet.

I have a very heavy duty Variac here which will trip the breaker almost every time I turn it on. I 
had to add a SoftStart circuit to that in order to prevent the tripping. I had an old time-delay 
relay which I used to add a resistor in the input line for a few seconds. That fixed it.

It didn't ALWAYS trip the breaker. I suspect it only did it on the peak, or near the peak, of the 
60 Hz primary voltage.

Ken W7EKB