Burned Grid Choked

[Robalo]

TL-922A has this problem:
L7(470UH/4.7K) is burned out and I'd like to know what is causing this?

Also, I'd like to know where I can get those?

 

[Robb]

TL-922 mods

Lot of info there for rebuilding. Not specific to your problem though . . .

 

[RatsoW8]

What were you doing when the choke burned out? If you bought it burned out then ask the previous owner.


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[Robalo]

Thanks Rob for the info, this is my friend's amp who got it on ebay. Let me see what I can do to fix it. So far this is what i found:

Grid chokes sometimes burn out because of instability of the valves.
That happens more to valves which haven't been used for a long time.
The short circuit goes via relay R2-1, the zener diode and the frame, the meter also will get a big knock.
Connecting fuses in series with grid chokes L7, L8, can make a protection. I didn't do this in my TL922, but as a preventive against grid to cathode (filament) short.

tl-922eng

This is the way that it needs to be done.

 

[Shockwave]

The amp was severely over driven at one time. Take a look at the bifilar choke on the cathode RF input. The center is very discolored. It takes a lot of drive to turn that part dark like you see. Surely that pounded the crap out of the grid current and took those chokes out too. You may have even had an oscillation there if the grid bypass caps have dropped in value.

 

[ElectronTubesRule]

Prob. a problem of a parasitic. If some one over drove it I would wounder why and how. If anything they prob. did not tune it up well. Not a lot of armature radios that go above 100 watts output that where commonly used with this amp from it's time frame. It has only been what the last 10 years that we have routinely seen 200watt HF broad band transceivers in wide use. This is why I suspect improper tune and load waltz over just being over driven.

If someone failed to tune it up properly you would expect to see damage in the grid in caps,resistors and chokes. So it could be an either or situation. Even caps that are not blown should be checked and any resistors too. The last thing you need is to fix the choke and then have other parts not obviously damaged taking out more parts. I would imagine anything that could burn and destroy that choke would damage other parts leading to it.

You might want to get rid of the Zener diode that is in it now plenty has been written about this unit and mods to make it more durable and efficient as well as their parasitic problems with some tubes. It seems that some of them had issues with the AMperex brand 3-500Z tube due to it's greater gain then the Emiac 3-500Z. I would review all the stuff that has been written about this model and see if you think any of it is relevant to your situation.

Because of the high voltage and the cost of new tubes I would prob. check the circuits from input all the way through the unit up to the final output.

Do let us know what else you find.Have you already checked the tubes? I do not think tube balance is much of an issue because OEM's do not match tubes during production the Head guy at AMeritron has publicly said this as have other's.