BOOOM!!!! Valiant Goes Tits Up In Spectacular Fashion

[KA2DZT]

Philip,

BTW, I forgot to mention in my previous post,  no need to apologize for the screw-up.  We'll let you slide this time. BUT, make that mistake again and we'll be forced to come to your QTH in Ga and pull all the tubes out of your set.  In some extreme cases even tear up your license

Just thought I'd mention it.

Fred

[ka4koe]

Guess I'd better keep the Mauser Karabiner handy in case I see angry hams coming up the street with torches, yelling for my red, spurting blood.

[W2VW]

Flex should have an option where parts break and the owner gets to troubleshoot.

They could also simulate lousy audio of different rigs and make the operator substitute circuitry.

Maybe even smoke pills like the old train sets to simulate blown up stuff.

[WBear2GCR]

Stu,

It seems to me that the problem would come only IF the voltage differential across one of the caps exceeded its rating.

The time to charge to peak is somewhat irrelevant, since both caps are charging simultaneously, a relatively small differential at the center point will not blow the caps.

Regardless, I said to use the balancing caps anyhow.

                      _-_-

[ka4koe]

Got caps replaced tonight with pair of Xicon, 450V, 100 uF. Checked bleeders across each one and reading 20K as per spec. Performed smoke test.....no smoke. Reset HV to -70V and set clamper to 10 mA per the manual. Bias voltage looks nice and clean on an oscilloscope. Need to get Flex hooked back up so I can compare the output with both filter caps working.

The caps are held together with hot glue, applied sparingly. Both are mounted to the chassis with adhesive velcro. I know some will wince at my method of execution. Yes, its a double entendre.

Loads up to around 170W CW carrier into 50 ohm dummy. So, I guess we're good to go until it breaks again. I am under no allusions that it will stay fixed.

[K5UJ]

I've never owned a valiant so I'm curious about the filter capacitance.  I'm assuming these (to me) high value electrolytics are in the h.v. supply.  I'm surprised EFJ used so much capacitance.  Is there no filter choke, or is the supply filtering capacitance only? I'd expect in a rig of that vintage, that there'd be a 10H choke with a couple of 8 to 12 uF 1 KV oil caps in there.  I'd consider attempting a filter scheme such as that so this electrolytic blowouts happen no more.

what's the B+ on the final of that rig?  I see what I think is a rectifier socket and if I'm right it is phenolic.  I'd look for ceramic sockets.

[ka4koe]

600V B+. 866AS rectifiers (2) in a full wave bridge with center tap transformer. Large choke on the output on the full wave bridge.The new caps are rated 450V, not 600V as indicated on the sketch.

The original network spec's 450V @ 80 uF. I don't recall the choke in-duck-dance.

The first replacement cap blew as I had it in backwards. The resistive divider also supplies voltage to the finals.

In other news, I put the Beast on 7290 last night for the AM net. It did not melt into a pool of bubbling iron, glass, copper, plastic, and human flesh.

Philip

[K5IIA]

600V B+. 866AS rectifiers (2) in a full wave bridge with center tap transformer. Large choke on the output on the full wave bridge.The new caps are rated 450V, not 600V as indicated on the sketch.

The original network spec's 450V @ 80 uF. I don't recall the choke in-duck-dance.

The first replacement cap blew as I had it in backwards. The resistive divider also supplies voltage to the finals.

In other news, I put the Beast on 7290 last night for the AM net. It did not melt into a pool of bubbling iron, glass, copper, plastic, and human flesh.

Philip

Should just be a full wave. Not a bridge rectifier. Koe rect?

[W8VG]

Phillip:  Good luck with the Valiant.  I started with a real basket case in 1998 and learned many things--some of them the hard way:).  At least you haven't blown the modulator driver tranney while "experimenting" with positive (hmmmmm wasn't that supposed to be negative?) feedback.  But it was then that I made the fateful decision to entirely redo the speech amp.     

Eventually you will get that beast running where you want it and you'll look back and realize how much knowledge and experience you've gained.  And you get a pretty good sounding/reliable transmitter to boot. 

But it doesn't stop there--you'll start drilling and blasting on the next boatanchor or on a homebrew rig.  That's the fun of it all!  Of course if you're a real glutton, you just go out and find another Valiant  .  That will be time for a visit to the doc!

Best 73's Geo

[ka4koe]

Yep, misspoke.

600V B+. 866AS rectifiers (2) in a full wave bridge with center tap transformer. Large choke on the output on the full wave bridge.The new caps are rated 450V, not 600V as indicated on the sketch.

The original network spec's 450V @ 80 uF. I don't recall the choke in-duck-dance.

The first replacement cap blew as I had it in backwards. The resistive divider also supplies voltage to the finals.

In other news, I put the Beast on 7290 last night for the AM net. It did not melt into a pool of bubbling iron, glass, copper, plastic, and human flesh.

Philip

Should just be a full wave. Not a bridge rectifier. Koe rect?

[ka4koe]

But it doesn't stop there--you'll start drilling and blasting on the next boatanchor or on a homebrew rig.  That's the fun of it all!  Of course if you're a real glutton, you just go out and find another Valiant  .  That will be time for a visit to the doc!


-Geo

I may be insane, but I ain't crazy.

[K5UJ]

600V B+. 866AS rectifiers (2) in a full wave bridge with center tap transformer. Large choke on the output on the full wave bridge.The new caps are rated 450V, not 600V as indicated on the sketch.

The original network spec's 450V @ 80 uF. I don't recall the choke in-duck-dance.

The first replacement cap blew as I had it in backwards. The resistive divider also supplies voltage to the finals.

I see what's probably going on.  The filter choke I bet is pretty low inductance, maybe 3 or 4 H and low resistance.  the stock filter C is 40 uF total.  EFJ probably wanted a fairly stiff B+ on CW with the leading edge of the keying envelope. 

if it were me I'd probably change the filtering eventually if I didn't run the rig on CW but maybe it doesn't matter that much.  I like to use oil caps as much as possible.

[kf5qeo]

Just FYI, I've had similiar and some of them scared me so much, I thought I was never going to touch a soldering iron again!