SB-220 Troubleshooting

[Kenneth G. Gordon]

On 1 Aug 2010 at 18:00, Goss, Ed wrote:

> Hi Folks,
>      A friend just bought an old SB-220; fairly good physical
>      condition but it's been stored for a number of years.  He brought
>      the unit up on a variac (120 v input); no problems so he switched
>      to 240 v power.  He sent me the following:
> 
> I installed a 240 VAC
> > receptical yesterday to power the amp. All ok in receive mode, but
> > the instant I activated the Antenna Relay Tx mode, a very loud pop
> > sound came from the amp. No RF input was applied yet. I immediately
> > switched off the Tx mode and main power. When turned back on, the
> > everything seems normal in receive standby mode.
> > 
> > From what I have read about the amp, this could be anything from
> > antenna relay problems, a shorted tube (grid to cathode), or
> > parasitic oscillations. I wouldn't think the latter is likely since
> > no RF input was applied yet.

The existence of parasitics does NOT depend on an input signal! The very 
word "parasitic" means that the tube is oscillating on its own. No "input 
signal" required.

> Any ideas of what to check first?
>
>  --Ed--

If it were my amp, the first thing I would do is to make absolutely certain 
there is no HV left anywhere after unplugging the amp and letting it sit for a 
while.

Then, I would spend considerable time, after removing all covers and 
shields, looking for an obvious arc or burnt component. 

If I found such, then I would spend more time looking for or trying to figure 
out a cause.

The problem may be as simple as a HV arc having been caused by a build-
up of dust, dirt, bugs, spider-web, animal leavings, etc., in or near a HV 
point, and when the HV was applied it simply arced over to ground.

In fact, that would be what I would suspect first. Something simple.

Secondly, although I am not familiar with the exact causes, 3-500Zs DO 
have a history of various problems, many of which appear, and disappear, 
after prolonged storage. Research on the web might find these.

Lastly, I DO know that the grids in those tubes are VERY close to the 
cathodes, and can short out to the cathodes fairly easily. 

However, your description does NOT sound like this sort of problem. In fact, 
it sounds more like there was a momentary short from some HV point to a 
nearby ground that cleared itself when it arced.

These are usually found most easily visually.

Ken Gordon W7EKB

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