Tennessee Walker blows ac fuse instantly on key down

[MudDuck442]

All I can say is wow! I worked on a few of these throughout the years and almost forgot what a deathtrap they are. The HV section looks like I remember although the one I was working on had 4 total 4cx250b tubes in it. Very poor design but at least there was a chassis there. The lack of grommets and the overuse of tub and tile sealant on the one I saw to keep air from leaking out of the "pressurized" cabinet areas were enough for me. Just be careful working on it make sure everything is discharged first and then work on it.

I really agree with just about everything shockwave said. The zener does usually go and the uaual fix is to replace and add a choke. I have also seen other quirks as well depending on what happened when it went "pop" but the good news is, while it is a little morecomplex than a triode amp... it is one of the most simple designs I have seen.

 

[Truck]

Replace the zener diode shown inside the red circle. Add a 10 uh RF choke in series with the wire that has the red arrow pointing towards it where it connects between the two resistors. Then the RF drive will just be feeding the tubes instead of the bias circuit at the same time. This assumes you've checked the zener and found it to be shorted.

What ma choke is most commonused and what is the best place to source them out thx

 

[Shockwave]

Anything above 250 ma should work fine. If you can't find one locally, try Allied Electronics online.

 

[Crusher]

If you have some 43 material laying around, you can wrap about 8-10 turns around a half iinch core and that should be plenty.

 

[Truck]

I checked the zener and the other two diodes here with my fluke appear to be ok. That's disheartening...

 

[Shockwave]

Well, that's because yours is one of the very few that I've seen with a 22 uh choke already installed. The close-up pic reveals what looked like a resistor is actually a very small green choke that looks to be well under 500 ma. The excessive drive can easily burn this size choke open. Check to see if that green choke has continuity through it.

 

[Shockwave]

At this point you really need to confirm there is bias voltage with a meter. Unfortunately this requires testing it while powered up. I'm hesitant to provide instruction here because this can be dangerous. The safest and simplest way I can think of is to connected the negative wire of your volt meter to the wire I pointed out with the red arrow. Connect the positive wire of your volt meter to chassis ground. Make these connections with alligator clips so that your hands will not be anywhere near high voltage when you power it up.

Do not let the meter leads or your hands touch anything around the 8 electrolytic caps or the board with all the HV diodes on it. Place the meter where you can see it without holding it. You don't need a radio connected. The amp should be on and unplugged so that you will turn it on by plugging it in. When you plug it in you should expect your volt meter to show about 65 volts. If you see zero volts you know the boas it the problem. This amps keeps the bias voltage steady and switches the screen on and off.

When you're done with the test, unplug the amp and wait at least 10 minuets to be sure the bleeder resistors have removed the voltage stored in the HV caps before touching anything.

 

[Truck]

Will do this afternoon. I will report my findings. Thank you

 

[Truck]

1 volt AC measured..... Also checked the choke. It reads 1.4 ohms

 

[Shockwave]

You should be looking for DC volts. Change the scale on the meter to DC and let me know what you see?

 

[Truck]

 

[Truck]

I thought about that after posting . it continued to rise above 60v slowly

 

[Shockwave]

What do you mean by "rise slowly"? Did it get close to 60 volts quick and then slowly rise a few more volts or was it slow going all the way from nothing? This is starting to sound like at least one defective tube that may be arcing under load. If you have another you can swap out one at a time to see if one is shorted, that would be nice.

 

[Shockwave]

If you don't have extra tubes, I'd consider testing it with no tubes. Make sure the plate connectors for the tops of the tubes are not touching ground. Turn it on and key it with your radio for a short test. Obviously it won't make any power and your radio will see a fairly crappy SWR since there will be little load on the drive power. The goal is to see if all power supplies can fire up without blowing the fuse.

 

[Truck]

It got up to about 58v and then climbed slowly fro. I there. It was still climbing slowly when I took the picture. I ha e already confirmed that it will not blow the fuse keying it with no tubes.What next?