[Drake] News about TR7 PA problem

[ik2bcp - Guido Tedeschi]

The problem is in the driver; fortunately, the final transistors semms to be 
healty...
I removed the PA, desoldered the wires and discovered that it was repaired 
very badly with all (yes, all...) the transistors and the 7805 susbstituted.
The driver pair was changed with one MRF475 and one 2SC2098 (matched pair! 
HI HI) and the pins are very long...
I connected my signal generator to the input, a dummy load to the output and 
powered the PA with 12V (2A limited) and I traced with the scope that it is 
the driver that has a gain difference of about 12dB between 18MHz an 1.8MHz 
and this explain the 150W -> 10W difference between 1.8MHz and 18MHz
I'm not sure if the long pins and the different transistors can give 12dB of 
gain difference, maybe it can be one of the transformers or other 
components: I think that I will resolder all the "looking bad" components 
and then, at last, the transistors will be changed with a matched pair of 
2SC1969 (more cheap and easy to find in Italy than MRF475).
Any suggestion and or comment before ordering the new transistors?
73 and thanks
Ciao
Guido, ik2bcp