[AMPS] Blown TL922A... What to do?

[measures]

.......
>As the subject line indicates, I've got a little problem with my Kenwood
>TL-922A.
>
>The background is  that this amp has been in storage for a long time, almost
>a decade.  We have moved a few times since then and the tubes were out
>of the amp in their own foam-lined boxes. The tubes were re-installed when
>we moved
> here about six years ago.
>
>I decided to bring the amp out of the closet and see if it still works.
>
>I have read here about soft-starting with a variac, but not having one,
>I plugged it into 240 vac and hit the switch. 

A suitable means of limiting current is to remove the two fuses and 
temporarily attach two,  60w to 100w, 120v lamps across the fuse holders. 
 The mode switch should be set to CW.  

> The fuses blew, and there
>was a faint burning smell, you know what I mean...
>
€  downtown pukesville. 

>I took the top and bottom covers off, and seeing no apparent damage, decided
>to try again. This time I put the inside covers on the HV and RF
>compartments and triggered the HV with the inside  safety switch. With power 
on and HV
>off, the filaments lit on both tubes. With HV triggered, there was an arc in 
the
>RF compartment. In the process of locating the arc, I blew up a small 
resistor attached
>across an RF choke on one of the pins 

€  (one of the grid pins)

> at the bottom of one of the  tubes.
>I had also noted the  grid current pinned the meter, 

€  This is a fairly reliable indication of a parasitic oscillation c. 
120MHz.  D-14 prevents destruction of the meter movement during the surge 
in grid current during the intermittent parasitic.  

>but this occurred
>during the first attempt. I did not  see a pin number on the socket, 

€  The pin that is by itself is #3.  Count clockwise.  Pins #1 and 5, the 
fil. pins,  are directly opposite #3. 

>but the
>choke/resistor
>assembly appear to connect directly to ground.
>
>So... Now I know where the arc was, 

€  Not a sure bet.  

>but don't know what caused it. I'm
>looking for advice. Is the amp worth repairing? 

€  yes.  The 922 has a tank circuit that uses enough copper, the tank has 
forced air cooling, and the 922 is instant on.  In my opinion, the person 
who designed the cooling system was definitely very cool.  

>I assume it is, although probably
>not by me. 

€  My guess is that, if you can solder and read English, you can do it to 
it.  The good news is that if do this project you will not be just 
another schmuckoid appliance operator.  

> The second question would be where to repair it and what would the
>cost be?

€ If the tubes are not shorted, the cost should be minimal.  The trouble 
with a Kenwood-san repair is that they must restore the amp. to factory 
stock config., which may result in an instant replay/big-bang.  .   //  
Shipping a 922 requires a Kenwood shipping carton.  The trogs at UPS 
Ground are virtually certain to drop it.  //  On my Web site, there is an 
article about Circuit Improvements for the TL-922.  This is the article 
from which the article about parasitics in the Oct. 1988 *QST* came from. 
("Improved Anode Circuit Parasitic Suppression for Modern Amplifier 
Tubes".)  

>This amp is fairly low hours and in good shape otherwise.. HI!
>
€  what's funny?

>I'm guessing a problem with the one tube, and wondering if I re-tubed would
>I simply be setting myself up for another bang? Replacing the resistor is 
simple
>enough,
>but I'm wondering if my tubes are bad due to the long period of non-use?

€  non-use is no problem unless there is a seal leak. 

>IN any event how could I tell?
>
€ To test the tubes, you need a 8kV high-pot.  Anode-grid leakage should 
be under 10uA @ 8kV.  Grid-fil. leakage should be under 2uA @ 5kV.  

good luck, Gary

-  Rich..., 805.386.3734, www.vcnet.com/measures.  
end


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