SB-401

[Kenneth G. Gordon]

On 17 Aug 2010 at 18:43, Jim Schnurr wrote:

> I have an SB-401 that has been working perfectly. It recently decided
> that after about an hour it would suddenly key itself up.

As I remember it, something very similar occurs in the SB/HW transceivers 
when the final amp tubes get gassy or exhibit grid emission (or something 
like that). This is caused in THAT case by something weird in the screen 
circuitry. It can be fixed by adding one wire to the switching relay which 
grounds the final amp screens when in the receive mode.

I know this sounds strange, but it is mentioned in a Heathkit service bulletin.

> I replaced
> the 6D10 , 6EA8, 6AV11 and 6J11 with new stock I had on hand. It still
> does the same thing after about an hour warmup. I wonder if anyone
> else has experienced the same problem. It is probably a component that
> is getting hot in the vox-keying circuit.

That may be, all right, but I would first either remove the final amp tubes 
(6146s), or replace them, and see if the problem no longer occurs. If it 
doesn't, then there may be a service bulletin that mentions the fix.

Ken Gordon W7EKB

-----------------------------------------------------------
Products bought, sold or traded here is the responsibility of the
parties involved.  This list and the City of Tempe are not responsible
for losses or misrepresentations of any kind.  Buyer beware!

-----------------------------------------------------------
This list is a public service of the City of Tempe, Arizona
-----------------------------------------------------------

Subscription control - http://www.tempe.gov/lists/control.aspx?list=HEATH
To post - HEATH at LISTS.TEMPE.GOV
Archives - http://lists.tempe.gov/archives/HEATH.html