Heathkit SB-401

[ButchB]

Hi All, I recently was gifted an sb-401 that does not function. I purchased another for in hopes I could get one of them working, but it had a shorted cap in the high voltage. Replaced the cap and the radio has the EXACT same problem. Putting the radio's in tune mode relay 1 does not fully engage and relay 2 does nothing. I put the gifted 401's relay driver tube into my dads sb-400 and the tube worked fine. Manually helping relay 1 to close caused relay 2 to close but there is no plate voltage or current. High voltage reads normal and ALC is mid scale in standby and CW open key. when helping relay 1 close High voltsge stays the same and ALC pegs to the 0 sidew of the scale. I traced the 250 V + dc to coil 1 and ground but only 10V+ crross the coil. When put in tune mode it reads about 50vdc across the coil. Im a bit confused but since i have 2 units with the exact same problem I think its probably pretty common. Any thoughts or sugestions would be greatly appreciated

73 KC7GRH

 

[BJ radionut]

https://www.w6ze.org/Heathkit/Heathkit_043_SB401.pdf

may help...

https://antiqueradios.com/forums/viewtopic.php?t=414547&start=20

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[nomadradio]

I haven't looked inside a SB401 in 40 years, so all I can offer is a best guess. Start with the power supply, where the B+ branches off. If you have a 'scope that would be an unfair advantage. I suspect you would see a lot of rectifier ripple where you should fairly-flat DC. This causes DC meter measurements to be lower than they should. Can't remember how many electrolytic caps are in that one, but they have all exceeded their use-by dates. If you can find a kit with the whole list of aluminum electrolytic caps that would be the next logical step, re-capping it. Might not fix the fault you're chasing, but it will prevent predictable failures.

73