Vic,
Glad you found the problem. ive seen alot of components break down under load,or in other terms under circuit operations. Before i was in engineering i worked as a depot repair tech. working on everything and anything. Most of the "dog" repairs as we techs call them,were either semiconductor failure under load or capacitor failure under load.
the failures were from the junction break downs, either BE, BC,or sometimes CE. these even tested good on the good transistor testers and also on a scope.Only when applying the required voltage and allowing current to be detected,in the circuit, would you see the breakdown of the signal from input to output.,and when said components were replaced, it solved the problem in a majority of cases.When current,or the pressure of the electron flow gets to a big enough pressure,you have punch thru on any or all junctions that were damaged from heavy static and or lightning.Sometimes you can pick this out with the component heating up,but alot of times the component will act like all the others off the shelf until put to the test.
73 David ac9xh
On Thursday, June 16, 2022 at 04:58:34 PM EDT, Vic WA4THR via groups.io <vhklein@...> wrote:
David, you win the prize! After confirming continuity right up the the leads of Q911/912, I pulled them from the circuit. Alas, they tested OK on the little transistor checker, so I confirmed that all the resistance values and circuit paths were OK and then installed 2 new 2N3904's after matching hfe values on the little tester (not really needed, but couldn't hurt). Surprise, output power has returned! The event that caused this had clearly taken Q90, and it did test bad after removal, but these 2 must be breaking down under load or something as you suggest. Power is only slightly less across the bands than before, but that could be that I am running slightly lower voltage to the finals today. While things were spread out across the bench I did remove the regulator circuit that was providing the lower power to the main board, but only after the transmitter repair was successful.
Evan, that RF probe you suggested has the identical circuit, including the component choices and values, to the one I cobbled up on the bench that helped trace the problem. Still, it is a much neater package than my perf board and clip leads, and quite inexpensive, so I have ordered one.
Thanks to you as well, Raj, for your suggestions. I was rocking T8 around and even bridging the capacitors with parallel? caps trying to get that signal to the next stage. The support from the community here is fantastic and I may have just chucked this thing without all the ideas, but it has been one of my favorite rigs and has traveled with me a lot, so I am glad to have it back and functional.
By the way, I also have a BitX40 and it blew a driver stage some years back because of being near a 500w amp that was transmitting, so the back-to-back diodes mod was made as it clearly had a weakness there. I was never sure the uBitX had the issue, but I had installed the diodes as many had suggested before this event. At one point in the diagnosis of the problem I pulled them, suspecting they might have shorted in the event, but that made no difference. I haven't bothered putting them back in at this point since they clearly didn't help in this case.
=Vic=
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