[Heathkit] SB-101 Driver Preselector Problem...answer # 1...

[Kenneth G. Gordon]

> My SB-101 Peaks differently on Recieve and Transmit - If I peak my reciever at
> 7.2 mhz and the preselector is about the 12 O'clock position, my transmit peaks
> at around the 1 to 2 O'clock position - This occurs on all bands - I'm sure
> there is a simple fix for this - Any help would be appreciated

There is NOT a simple fix for this. The problem is caused by what is called 
"Miller Effect". Since I cannot remember exactly what this is, I won't try to 
describe it, but any really good book on vacuum tubes, like Terman's, will 
cover it in detail.

Basically, the fact that you are using the exact same tuned circuits with two 
sets of completely different tubes, causes the problem. Those tuned circuits 
peak the receiver RF amp and Mixer stages, and the final amp driver and 
last transmitter mixer.

Heathkit had a fix which while not really trivial, is not really all that hard to 
implement either. It involves adding two (or four) small adjustable padders in 
parallel with the tuning capacitors which are switched in for receive, and out 
for transmit, or vice versa, probably via small switching diodes.  I will look in 
all my Heath info to try to find that article and a link to a URL which contains 
it.

BTW, this fix was FIRST published in either Hints and Kinks, or Ham Radio 
magazine about the time the SB-101 first came out. Heathkit then took that 
(or those) articles and slightly re-engineered their results to make it easy for 
owners of an SB-100/101/102 to install. If I'm not mistaken, later models of 
the SB-102 included that mod as a normal installation item.

There is also an RIT mod for those rigs.

Ken W7EKB