Looking for someone very knowledgeable on the Henry 2KD Classic

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KU8L:
I have a 2KD Classic here for service.  Someone has been in it before me and there is a mod and some bad wiring.  Powers up but when received complaint was grid current ok and gts Ip but zero RF out.  Went thru all the output stuff--using a VNA.  Brand new 3-500s from RF Parts.  I found the main bias diode open.  Ordered an new one from Henry ($80.00 !!).  Put it in and it failed faster than I could see anything glitch.  Open.  It is a stack of silicon diodes in an epoxy rail.   THe fuse that is in series with it did not fail.

I'm wondering if there is a possible error in polarity of the other two diodes referencing B- to ground and causing a problem.  I see no evidence of grounds in the filament bypass caps or shorts in the transformers, etc.

I have a handful of the 50W zeners to replace the stacked original (13.00 each) but wondering if anyone has seen this before.  BTW, with the biase diode failed open, I got the expected full Ip current at idle.

The schematics are some of the worst I have seen in 55+ years of service work.

THanks for looking.  Off list is fine.

Curt
KU8L@flash.net

KF7WWW:
"BTW, with the biase diode failed open, I got the expected full Ip current at idle."

Sounds like grid to filament short somewhere to me.. All depends on how bad its been monkeyed with.. Have you checked the bypass caps? Visual inspection is not a real check.  ITs uncommon but a short or near short there will cause problems. 
I would also lower the fuse value for further testing..



" I'm wondering if there is a possible error in polarity of the other two diodes referencing B- to ground and causing a problem. "
Any raise in voltage from reference will cause problems ( that is more then the expected for metering )

Then again, Im sure you know this if the unit was brought to you for service. If all else checks ok. Try another set of tubes, just to be sure.

WBear2GCR:

It's difficult to see why the bias "diode" stack or zener should be $80... nothing magic there.
Was it RF bypassed? Needs to be, afaik.

Failed instantly?
Hmmm... iirc, it's in the cathode??

Could be a parasitic, possibly.

The bypass caps on the socket, and if it still uses resistors on the socket, they should be
checked too...

Most of the Henry amps have the B+ iron and the filament windings on the same core... makes it difficult to fire it up under lower B+.
But perhaps there is a way to fire it up with lower B+...?
There's something on the W8JI pages about a "glitch resistor" - might be worth looking into. Even if it is a temporary
installation...

KD6VXI:
Any amp I get here for repair gets a resistor in the b+.  If it's legal limit sized, an 8 ohm 10 watt.  Tubes with handles, or huge C in power supplies will get a 100 watt.  Why 8 ohms?  Got a deal.  If the tube shorts, etc. I have a two cent resistor go, not a higher priced part.

Pull the tubes.  Henry does the ass backwards design of running their filaments in series.  Insulate the hv connectors and turn the amp on. No problems?  Test the tubes with an ohm meter, filament pins to ground. If one shows a short, problem found.

And ditch the 80 dollar :zener:  A string of 6A10 or 10A10 diodes will do just as good.  For dynamic regulation stuff a few hundred mf cap across the cathode line to ground.

A 1.5 amp quick blow fuse in the cathode lead will be cheap insurance, too.  I out a cathode resistor in my amps now as well.  Overdrive the amp, replace the fuse.  Have a glitch?  The cathode fuse in concert with the glitch resistor and fuse will hopefully save transformers and zener.


--Shane
KD6VXI

DMOD:
1. Hopefully you have a HV probe such as 10kV probe to plug into your DMM. Without any RF drive, what is the B+ at the bottom end of the RF choke L1 when Keyed?

2. What is the bias voltage when keyed? A 10 or 50W 10-15V zener will work if the stud is grounded, such as http://www.nteinc.com/specs/5200to5299/pdf/nte5240a.pdf see for example NTE5254A or NTE5257A. Eimac actually specifies a 15V zener for over 3kV V plate.

3. When Keyed, does the relay function (pull-in)?

4. With no RF drive, what IP current are you seeing when keyed?

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