SB221 Relay Failure?

[KN4SK]

I just replaced a failed filament transformer in an SB221. The filaments are now getting 5V and running at proper current (and lighting up). I'm also getting ~110VDC rectified to the relay bias circuit. So it appears that the transformer is wired in and operating correctly.

When I ground the ANT Relay line the relay is making a quiet buzzing sound but never makes the switch from Rx to Tx. I'm measuring 110VDC on the relay coil when the ANT Relay line is open, it drops to 45VDC when I have the ANT Relay line shorted to ground. I'm  measuring ~10mA through that relay coil during the shorted condition.

I believe the relay happened to fail at the same time the transformer failed (maybe the xformer failed in such a way as to over-voltage the relay coil).  The relay is original from the 1960s and this linear has seen a ton of use.

Is there something else I might be overlooking here?

Thanks all,
KN4SK

[w4bfs]

I believe I would check the bias circuit even tho You see -110V ... sounds like the filter cap has dried out ... a scope would tell all ....its a one-eyed snitch !

[KN4SK]

I did replace that filter cap since it's a very old electrolytic and being leaky could possibly have caused the filament transformer to go bad in the first place.

I did throw a scope on it to see if anything looking funny and the relay started working. A few minutes of digging and I found a jumper wire from that cap's negative terminal to ground had become intermittent. Problem solved.

The specific suggestion might not have exactly been it but it made me kick things around and find it.

Thanks!
Ray

[w4bfs]

nice going om ...73

just a minute .... did you say negative terminal .... I believe that would be positive terminal for a bias supply or did you use a nonpolarized cap ?

[KN4SK]

In this build the negative terminal of that cap is soldered to a terminal block and there is a jumper from that point to ground. That jumper was intermittent. I've got a polarized electrolytic (I replaced the old one with a new one with a higher voltage rating).

[w4bfs]

I see .... dug up a sb220 schizmatic .... multipurpose + lv supply .... alc offset & bias & relay supply

as Gilda Radner's SNL character said;   "never mind"

[KN4SK]

No sweat, thanks for checking the post. I keep thinking I'll just fumble my way through each fix of my dad's linear without really studying the circuit to understand it. It's keeling me in this perpetual state of feeling like I halfway know what I'm doing.

Now, one tube developed a grid filament short last night after we started tuning on 20m. I just replaced the things Jan 2015. Coincidence? Unlikely. Maybe it had an intermittent filament grid short that overloaded the transformer and took it out?