Hi,
Thanks for everyone's input! The problem is now resolved, a note from my
Weblog follows!!
Regards
Nicolas
www.m1hog.com
10th July - Main Board - I investigate further; after a good few hours of
wandering through various tests and a few trial and errors, I find the PA
Bias is grounded, instead of showing 2.7k resistance! Which is not helping
matters.. The Pre Driver Bias is OK though. I read through the emails, and
chat with Sean. Further digging and checking, but no real progress. Then off
for a swim, and a chance to properly consider the various email suggestions;
DC voltage checking, PA shorts, removing components, failed components..
Clearly, this problem requires a hot iron and a cold beer. Looking again at
the circuit diagram something is dragging the PA bases to ground, now there
aren't that many components that are, or could be connected to ground, I
start removing them and checking the PA base ground as I proceed. After
various capacitors and the 2 PA transistors! have been removed, the ground
still remains.. I look across to the LPF connection and remove Z1
(Effectively a link to part of T4). The ground to the PA base disappears..
Progress..
Taking another breather, I consider T4 (As well you might) something from
the LPF side of T4 is grounding across T4? Puzzled by this; I remove one of
the T4 output wires(3), and replace Z1, the base ground disappears. After
some wonderment I resolder the wire and the short does not reappear. I fail
to quite see the logic here, but clearly there was something odd happening
with T4.
A considerable time is spent cleaning up and resoldering all the removed
parts, checking along the way. 4039 is once again connected to its dummy
load, and tested. This time we have RF!!
Looking again at the PA circuit diagram also reveals that T4 should have 3
turns on the output not 2, this could explain the slightly high current
being drawn (2 turns are recommended for optimum 5 watt operation. The next
task then is to take the heatsink apart again and rewind T4!!
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