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[N3AJB]

My Delcon t-210 had a major failure. The amp is a traditional grid driven using a 50 ohm non inductive input circuit  with 3 4cx250s.  I was in the tune position (no screen voltage)  with no drive.  I turned on the HV switch and immediately the plate current meter (1 amp) pegged so violently that it bent the needle and remained in the far right end of the scale.  It's kaput.  I estimate 3 (just a guess) amps at 2500v surged through the amp.  I instantly turned off the HV but of course far to slowly to prevent damage. No flashes or explosions occurred.

Both fuses in the primaries (240 input) are good.  I checked the plate choke - no burned sections, full continuity.  I checked the blocking capacitors and no shorts. I checked the plates of the tubes for shorts to ground, filaments and screens  but found no shorts.  I checked the tank circuit plate and load capacitors (both are vacuum variables) and found no shorts to ground.  If it's not listed here I haven't checked it.

I realize that without a schematic it's difficult to pinpoint possible problem areas.  However, I need suggestions as to how to proceed with trouble shooting.  Any help would be appreciated.

JON N3AJB



 

[WW1SS]

Possibly a shorted tube. Remove them and test again with only one in the socket.

[K1KP]

If it was mine I'd pull the tubes and slowly bring the amp up on a variac. You need a way to monitor plate current so if you don't/can't repair the plate meter you'll have to use a separate DVM to monitor plate current (or watch the current on the variac). Given the symptoms you report, I'd suspect the grid bias circuit failed, allowing the grids to go positive and pull tons of plate current. Another possibility is screen supply failure.

But before that, I'd attempt to trace out the circuit. Plate current in tube amps is usually measured in the return side of the HV, so you'll need to figure that out. Also, you will  need to figure out the grid bias and screen supply.

Always follow HV safety protocols! One hand in your pocket, never go poking with the AC on, discharge caps with a chicken stick, etc.

-Tony, K1KP

[W1VT]

Tracing out the circuit is a great idea!
You may find differences between the schematic and the actual circuit.
You may come across the fault while doing the close inspection of the circuit.

Repairs are rarely figured out by engineering talent analyzing circuits.

[K5LXP]

I have my T210 still on the bench.  The grid supply is bad and I need to order some parts but if there's something I can measure/verify here just let me know what.  I have a couple electronic copies of the manual and so far they appear accurate.  I've tried contacting you about your power supply cable, maybe we can help each other here or offline, mycall at arrl.

FWIW collins panel meters are the same form factor, maybe even the same OEM.  That might be a path to replacing the one you have.

Mark K5LXP
Albuquerque, NM

[N3AJB]

Update:

 I found 2 blown resistors, which I replaced, in the metering circuit.  I removed the plate meter, opened it up, and freed the stuck needle.  Surprisingly, the movement seems to be ok but I'll check it out further. No shorts were found on any of the tubes from the plates to any of the elements. So far I haven't found anything that would cause the meltdown.  Perhaps a tube flashed over and destroyed it's internals.

 However, there is no plate, screen, or bias voltage.  Fuses are all good.

Fingers crossed that the plate transformer isn't blown.  I'll check out the power supply shortly.

JON

Mark, I contact you shortly.

[N3AJB]

 The Delcon has an outboard power supply (65 lbs).  The plate and filament transformers, as well as the choke, are all good - whew.  I can operate the power supply independent of the rf deck but will do some more "cold" trouble shooting before I turn on the power supply.  I never hold probes when working on a live HV

K1KP: I have full schematics for the rf deck and power supply.  The screen voltage is obtained from the HV through a dropping resistor to 3 VR tubes.  There's a separate bias supply.  I'll check their functionality when I turn on the power supply.

[KH6AQ]

Update:

 I found 2 blown resistors, which I replaced, in the metering circuit.  I removed the plate meter, opened it up, and freed the stuck needle.  Surprisingly, the movement seems to be ok but I'll check it out further. No shorts were found on any of the tubes from the plates to any of the elements. So far I haven't found anything that would cause the meltdown.  Perhaps a tube flashed over and destroyed it's internals.

 However, there is no plate, screen, or bias voltage.  Fuses are all good.

Fingers crossed that the plate transformer isn't blown.  I'll check out the power supply shortly.

JON

Mark, I contact you shortly.

I think you tested the tubes for shorts using an ohmmeter. That is probably not going to find the tube that might have arced over.

[W1VT]

https://www.w8ji.com/fault_protection.htm
Tom talks about protecting amplifiers with a glitch resistor.
Perhaps you could install a glitch resistor that is intentionally small in wattage and  high valued in resistance to act as a sacrificial fuse in case there is something waiting to blow up that you haven't found yet.  Alternatively, folks have tested tubes with current limited  high voltage.  But the temporary glitch resistor may be more practical for someone who only intends to fix one amplifier.  It could also be replaced with something intended to be a permanent form of protection.

[N3AJB]

With additional trouble shooting, I found the zener diode (91v, 10W) in the bias circuit had shorted to ground (plate to grid short??).  I have a spare and will install it.  Then fingers crossed

The plate meter had pegged but once I took the movement out of the case, the needle moved freely and responded to the ohm meter test.  It's a 1 ma movement so I'll check it out to see if it still is accurate.

The amp has a current limiting relay supposedly set to 900 ma.  Clearly, it didn't do it job.  A glitch resistor is on the list of upgrades.