[Amps] AL-1200 debug at a distance: we might have a winner(W5GHZ)

[TexasRF at aol.com]

With say 1.3A cathode current (plate + grid) and 5.8v, that will be 7.54W  
dissipation. So, the 10W rating seems ok but not a lot of headroom.
 
Considering the diode is using the chassis as a heatsink, it is unlikely it 
 will ever see 100C and even 50C seems unlikely if the insulating hardware 
is the  type that helps transfer heat. But, is it that type? Does it use 
heat conducting  grease?
 
73,
Gerald K5GW
 
 
 
 
In a message dated 10/19/2010 2:14:51 P.M. Central Daylight Time,  
knormoyle at surfnetusa.com writes:

Boye  posted from a search
"Zeners was replaced too to a 50W type and 2A  fuse
insert to cathode DC connection"


This is interesting  because I was also looking at the
50W zeners and wondering if the 10W might  be undersized.

The 10W diodes are only rated to 10W up to 50C.  at
100C, they derate to 6-7W.


Couple that with poor thermal  mounting and they might
not take much at the high internal temps of the  amp?

The 50W versions need a larger mounting hole (1/4")
which is  doable, but I'd also need the 1/4"
insulating hw.

I replaced a  cooked final transistor in an icom 756 a
while back with a NTE part and it  seemed to work well.

So I thought maybe I'd try NTE parts here. I  ordered
two NTE5182A (7.5 volt 10W) and am going to use them.
(while  ameritron and rfparts sell the 1N2971A, it
seems not widely  available)


The non-switched diode was definitely open. I  don't
think I had any excessive grid current events...well,
I guess none  that would be visible on the meter.

I'm not sure about the suggestion  of evidence of
multiple issues. No evidence of that yet? 
The no plate  or grid current is consistent with an
open on the non-switched diode,  right? 

Now if the non-switched diode was shorted, that would
be a  different story.

Thanks for the info on the other diodes to  check.
Will check them.

Thanks for all the comments everyone. It  really helps  me.

-kevin
AD6Z



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