Valiant: Another 866 Failure Mode and Microphone wiring

[ka4koe]

I found out why my 866's blew last night; luckily nothing else went with them. Be SURE to check the octal plug in the back. In my case, the male plug-in has a loose connection that I did not discover until afterwards. Most of the filaments in the radio were not on when HV was switched on. It had sat for about 5 minutes when I switched to MANUAL. So I essentially killed the rectifiers by unintentionally switching on HV to cold mercury rectifiers.

KABANG! Works every time. Fortunately my hand was on the switch when this happened so the thing didn't cook for minutes on end and take out the HV transformer. I got lucky. Everything worked fine when I swapped out the bad tubes for Taylor 866AS-1 solid state replacements. I was holding off doing this as I was researching the drawbacks to swapping (minimal), and things actually worked better when I did, ie easier to load up, more power output, etc.

Also hooked up the EV638, keyed up the transmitter in Am mode, and I think one or both of the heavy power resistors near the bias tube V21 started getting hot. Again, killed power and checked and no apparent damage. I need to check and make sure there is not something awry with the microphone wiring, although checking the schematic not sure why this would happen, except perhaps the line cord plug is reversed and I have AC hot on the chassis. Any thoughts?

I am learning this transmitter, but given my luck, I hope I don't blow it up first. I've already read the manual.

Philip
KA4KOE

[N8ETQ]

Hey Philip,

  Please change "R25" to at least 4.7Meg (6.8M better yet)
while your in there..  Also be aware that PTT voltage on
a Valiant is pretty high, 200 or so comes to mind but
really don't remember.

/Dan

[WA2OLZ]

Philip,

I'm cheating 

I decided to not apply any of those volty electron thingies to my Valiant until you clear all the landmines from my path. Seriously, I'm glad you got away with this one and hope that is the end of your saga!

73
Jack

[KM1H]

Now you can use the 866's for the only thing they are good for...target practice.

Of course there is a bit of a mercury issue too

[ka4koe]

Someone suggested that my microphone issue may have been due in part to the dummy load only being a foot or so from the microphone, and that RF conduction caused the audio stages to oscillate.

The overheating issue was not noted when the microphone was unplugged from the transmitter.

Philip