[Boatanchors] Sx-111 Problem?

[Glen Zook]

Try another 6C4.  That particular tube type is infamous for having problems in oscillator circuits.  They will check fine in a tube tester but just will not work as an oscillator.  Some 6C4 tubes work fine for a while and then start causing problems.  Other 6C4 tubes will work up to a certain frequency and then won't work.
 
Glen, K9STH


Website:  http://k9sth.com



On Friday, February 28, 2014 5:49 PM, K5MYJ <macklinbob at gmail.com> wrote:
 
I have a SX-111 that has developed a problem with the HF LO. It does not run anymore.

When I last used it several months ago it worked on all bands but 40M. 

Today it does not work on any band. It does not appear to be a band switch problem. I'm suspecting one of the common capacitors but I am not sure.

I have checked continuity to all the coils. No apparent problem there.

I removed the 6C4 and connected a signal generator to the cathode pin. I connected my scope to the grid pin. For all bandswitch positions I got a signal back on the grid.

I doubt the cathode RF choke is bad. But getting in to remove and replace the two capacitors (cathode to coil tap and the capacitor back to the grid) are really difficult to get to.

Any suggestions before I tear this thing up? I'm thinking of replacing the coupling capacitors.

Bob Macklin
K5MYJ
Seattle, Wa.
"Real Radios Glow In The Dark"

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