[Elecraft] F5UL: PA blow-up again

[Brian Broggie]

Sent to the reflector on behalf of Robert, F5UL in Senyod, France:

Hi the list
 
Some months ago, I got a PA blow-up and with the help of the list and 
particularly of Gary, it was solved for I thougth a long time. Well it 
happened again and the results are differents ! I replaced both 2sc1969 
and also Q13 who was shorted CE which was the cause of PA loss!

I end up with a power out of 12/13 w continus with an collector curent 
aroud 5 A, and after search an auto-oscillation around 2.5 Mhz was 
found. The power control have no action CCW or CW position it's the 
same. It's the 2SC1969 the culprits, or at least their presence, if I 
remove them the instability is not there, and the driver output is about 
5 volts PEP on each base ( 9/10 V PEP base to base). At this point the 
signal is clear in the control receiver CW or SSB.

I tried other transistor (2sc1969, 2sc1971, MRF476) to no avail, always 
this square wave signal with frequency variation from 2 Mhz to 3.5 Mhz, 
but always full +/-12V PEP.
I removed all components, up to Q5 circuits, put them back in after 
rewinding the cores with no results.

So I look for some help with this new problem. Can the DAC converter be 
involved?
The receiver is working OK, the PA bias is staying with 0.62 volt. With 
only the driver the curent is around 0.480 A but more than 5 a with the PA.
 
73/72 from F5UL