The AM Forum

Took the patient over to Dennis, W0JX, who performed a thorough exam and narrowed the problem to the tank circuit, We had a direct short to ground but could not figure it out, Finally, Dennis found the culprit!!! The load cap was shorted to ground. There is a spade lug connection on the back of the load cap and when I reconnected it after replacing the switch wafer I managed to short the cap-thus explaining why it had no effect on tuning. TNX to all who posted ideas as we checked off all of them in our troubleshooting. Once again proving what a fresh pair of eyes and a website can do to solve a problem. Once I get all the kinks worked out( have to fix tune cap that arced when all this started) I'll do a write up of how I did the replacement (being sure to mention possible pitfalls like shorting load caps!) without tearing the whole amp down and put it on my website.

Dan W8CAR

Without going into great detail here is the deal. My 220 had a bad output bandswitch. I replaced it and thought everything went fine. If I drive it with 25 watts-I get ZERO output. I have checked that I installed the wafer in the correct position (simple shorting BSwitch). I have looked until my eyes water-checked relay, continuity, caps. Everything is nominal but the tune and load controls have no effect on tuning and zero output. I have grid and bias current. Full voltage in both CW and SSB position. Has anyone seen this? Tubes appear fine no apparent shorts and they were good when I took them out of the amp. Next stop is a friend's place to replace tubes (even though I think they are fine!!!!) It is like the drive power disappears into the void in the amp-I know it must be something simple but I'm stumped and need some fresh ideas.

TNX Dan W8CAR

addenda: Plate choke is good, power through relay with just driver, internal relay is switching with outboard switch with continuity from relay to output,bypassed dowkey directly to antenna, IP about 500 mils in transmit, Have checked the bandswitch install and it is installed correctly, trip to friend's shack today to troubleshoot-more later.

Check for a cracked DC blocking cap, Dan.

You say, checked relay but I see no mention of checking that the internal SB-220 relay is being keyed by the exciter. Even if functional, are the relay contacts doing their job. With the SB-220 shut off, can you pass exciter RF into the SB-220, through the relay, out the SB-220 and into antenna, dummy load, etc.? Have you tried it on more then one band?

Could the plate HV choke be open? Are you getting grid current from the drive?

You did not mention if there is plate current!

All you stated was tune and load made no difference. 

You got Ip ?

If so, you put the shorting switch segment in backwards!

I bet that's what I did to the gonset.  :-X

Thanks for posting the solution....enquiring minds want to know!