Runaway plate current

[WB4AQL]

A buddy was transmitting using a Johnson Viking Challenger. He said plate current began rising, he smelled smoke and shut it down. It is on my bench now.

I`m suspecting parasitic oscillation and have swapped tubes- 6DQ6A driving a pair in the PA. I`ve replaced some bypass capacitors, out of spec resistors, coupling caps, parasitic chokes on the 6DQA`s, checked choke and pwr transformer resistances, tube pin resistances, but not voltages-I`ve been reluctant to do this because plate current begins rising as soon as plate voltage comes up.

I`m no electrical genius, but can usually bonehead my way through most problems. Anyone had this problem with the Challenger? Thanks!

[N4LTA]

If it was me - I would stop changing out parts until I found out what the real problem is. The last thing you need to do is compound the problem . It is unlikely that the amp just started having parasitic problems out of the blue.

I'd look for a biasing problem. I am not familiar with this rig - but find the problem that caused the runaway current before you do anything else.

Disconnect the plate and screen from the output tubes and measure some voltages.

I am sure others on here can guide you through the process.

Pat
N4LTA

[KM1H]

That rig was aptly named
As Pat said check the bias, its probably time for the filter caps to crap out.

Carl
KM1H

[WQ9E]

Was he using phone or CW when it went up in smoke?  The final uses the clamp tube as a modulator and the keying system on CW relies upon the clamp tube to prevent high plate current when the key is up.  The Challenger is a low cost unit with no bias supply (or any other expensive parts).  Take a close look at the clamper circuit and it could just be that tube died.

I had to bend the plates of the loading capacitor in mine to prevent shorting since simply putting the case together (and the case and panel are not bent or damaged) would stress the loading capacitor enough that it would short when the transmitter was in the case.  It worked fine while out of its case.

[WB4AQL]

He was using CW at the time, but the plate current will slowly rise in the AM mode as well. I`ve substituted all the tubes including the clamp tube. I`m not sure how this rig is biased, but I will find out and voltage checks will be next.

I appreciate everyone`s help.

Mike WB4AQL

[sndtubes]

a gassy tube could be the problem

[WA1GFZ]

bias going away, drive going away. Too many variables give more information.
What is the grid current doing. get a meter and measure the bias.

[WB4AQL]

I put the VTVM on it. No bias voltage. Now I just have to determine why no bias...

Michael  WB4AQL

[Gito]

HI

Looking at the schematic ,This challenger has an automatic bias, that means no fix bias,so the bias depends on the drive of the driver ,
Try to switch to tune position ,tune the oscillator and driver stage for correct final grid current,or measured the min voltage between C 16 to ground ,it's the min bias developed across R11in series with R12 for the final,
Min bias =( R 11+R12) X grid current

Check it if the drive (grid Current) stay the same for a 10 to 15 minutes .Does it change?,
Because when the drive (grid current) change ,it also change the bias of the RF tube and also changed the plate current.

if it stay the same than the problem  can be in the final RF tube circuit.
Try to check it step by step,


Gito

[N3DRB The Derb]

johnson yellenger makes a excellent big linear driver. simple speech amp changes and you got it all. switch it to fone, power goes down to 15 watts, on cw 100.

[WB4AQL]

Yes. If I can get this thing going, I think he will let me borrow it for AM use.

However, the plot sickens! In the process of troubleshooting, one thing I did first was to measure the resistance of the power transformer leads. All OK. After several sessions with the Variac, today I was working with it in the CW position, key up. The 5U4 rectifier fizzed and a line fuse blew. This was followed by the stink of a transformer gone away. Now when I check transformer lead resistances they are way out of spec.

Since my friend saw smoke when this all started, I wonder if this has been a transformer problem all along, or have I fried the transformer during the course of my efforts. I`ve been ever so careful with the Variac...really!

Michael

[N3DRB The Derb]

those transformers aint much. very budget tx. but the one I had rocked for driving amps.

[Tuber]

I just had the same thing on a DX-100 (runaway plate current and fuse blows) and found the 2.5 mH RF choke that supplies B+ to the plate of the Driver (5763) had the bottom lead broken off (corrosion)............

I spotted it by poking around (unplugged), but by pulling voltage checks around the LV side it would've shown up pretty soon.  That may not be my only problem so I'll keep checking it out to be sure.............

Good luck,
Steve

[WB4AQL]

That fits with Gito`s train of thought. I`m thinking more about the driver or PA circuit. Thing now though, I gotta find a power transformer.

Michael

[N3DRB The Derb]

if you smelling stinky transformer yep thats it. The good news is that rig is cheap as dirt. look for a beat up looking one on ebay and you get all the other parts too.

[WQ9E]

Steve,

Check your clamp tube circuit in the DX-100.  It should clamp the screen voltage of the final to a low level when drive is lost (like what happened when the driver stage lost B+) preventing excessive plate current flow.  It may just need adjusting or you may have a bad clamp tube or associated component.

I just had the same thing on a DX-100 (runaway plate current and fuse blows) and found the 2.5 mH RF choke that supplies B+ to the plate of the Driver (5763) had the bottom lead broken off (corrosion)............

I spotted it by poking around (unplugged), but by pulling voltage checks around the LV side it would've shown up pretty soon.  That may not be my only problem so I'll keep checking it out to be sure.............

Good luck,
Steve

[WB4AQL]

Thanks all.

I`m looking for a "parts" Challenger now. I priced a transformer from Peter Dahl (used to be) and a new one is $180. Not ready to go that rich. I`ve got a Ranger I`m trying to resurrect as well!

Michael WB4AQL

[The Slab Bacon]

Also check / replace the postage stamp mica cap that couples the driver plate to the final grid. I had one short in my Valiant (buffer to driver). It put the LV B+ right on the grid of the driver and launched a 5763 to the happy hunting ground.

In a transmitter like the Yellinger, you could probably easily get away with an old TV power transformer. Why spend the big bux!!

[Tuber]

WQ9E:  Thanks -- Symptom:  I was watching the plate current as I slowly loaded the DX-100 when the current started very slowly rising at first,  then it shot up to peg the meter and the fuse blew.  Not believing what I just saw, I replaced the fuse, checked the grid drive (it was an OK 5ma at that point) and dipped the plate to 100ma and stopped there.  Watching..... about 5 seconds later the current starts to very slowly rise.....slowly rise to accelerate and shoot past 300ma where I shut off the plate HV at that point.  (somewhere in this sequence, the RF choke lead broke off, I guess, but I had grid drive at the beginning of the last powerup--I have repaired the old choke for testing, but a new one is on order)

The 6AQ5 clamp tube was new and checks good.........but maybe when it heats up/under load? it fails??  I have put in another new clamp tube but haven't powered up yet.  Still checking resistances and visuals.  6AL5 bias tube checks good, too.  The plan is to power up without the rectifiers and check the power transformers to see if we damaged one as in the case above.  Then power up with the rectifiers back in and check LV side= bias, B+, pinouts.  With dummy load, if OK so far, power up HV side and see what I have...........

Thanks for the info.  This is a new one on me........will let you know what I find........... GREAT info and help in this Forum!
73s, Steve

[WB4AQL]

Thanks Bacon. I checked that mica for a short and it was OK. I bridged it to check for open. Seems to be OK although I`m learning those old micas don`t live forever.

Tuber that is exactly what I was experiencing with the Challenger before the tranny went. I could peak the grid just fine in the tune position. Switch to CW or Phone and the plate current would begin climbing.

[WB4AQL]

Another thing about the Challenger. It has a clamp circuit, but I don`t think it has means of adjustment like on the DX-100 or DX-60

Michael WB4AQL

[KM1H]

Have Gary rewind it better than the original for a lot less.
http://members.tripod.com/tubes_tubes_tubes/transformerrewindingservice/

The guys on the Johnson forum dont call it a Challenger for nothing, its probably Johnsons most troublesome TX to keep running according to several owners.

Carl
KM1H