Guido, Since the normal base to emitter voltage on the PA transistors self limits at about 0.63 volts, the best way to check the bias voltage is to remove the PA transistors and measure it. With the PA transistors out, if the base voltage during a TUNE is between 0.6 and 0.63 volts, then Q11 and Q13 are doing their job and you have faulty PA transistors.
For proper heatsinking, yes the PA transistors must fit flat against the heatsink. Before soldering the PA transistors in place, I position them on their mounting hardware and secure them temporarily with a nut (no lockwasher, no thermal pad). That places the tab parallel with the RF board. Solder the transistors and remove the nuts - add the thermal pads and mount the heatsink. 73, Don W3FPR Guido Tedeschi wrote: > Tom W8JI wrote: > >> This is almost certainly thermal drift from heat. >> >> Poor heatsinking of transistors will cause this, so make sure he used >> the CORRECT very light amount of any heatsink compound and that the >> transistors are clean underneath and bolted to correct tightness. >> >> Also a very small upward drift in bias voltage will cause this. This >> can be because of leaky series pass bias transistors or bias pass >> transistor heating from inadequate heatsinking. >> >> > Many thanks to Tom and the others: I've noticed that PA transistors are > not installed perfectly horizontal and it is possible that the > mechanical contact with the cover is not so good. > This poor heatsinking can explain the drift but, probably, not the 1.2A > with only 0.5W... Maybe, also the two transistors used for bias voltage > are not so healty... > > ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
