Having restored receive functionality to this old set, and having then established that the transmit function was u/s, I set about figuring out a strategy to find out what the problem, or more likely problems were which prevented me from transmitting.
You always need a strategy of some sort, and I wanted to come up with one. The most obvious thing I noticed at this point was that the ALC reading on the metering was "pinned" when I put the set to transmit.
This could be cause, or a symptom. Anyway, some place to start, a little piece of evidence to build on.
Whilst all this was going on in my head, the radio was powered up and receiving lots of signals. I began to notice an unusual tone to the received signals, and upon investigating this I realised that the frequency had developed a nasty "wobble", jumping about by small amounts. I also noticed that the radio was pulling around 100mA more current from my power supply than it used to, so something was clearly wrong.
I established fairly quickly that the 8V regulated rail from the AVR unit had increased to nearly 9V and was fluctuating. It was possible to trim this back to 8V but the fluctuations continued. This is when I noticed Q6 on this board running VERY hot.
Further investigation of voltages around the circuit established that the Q6/Q7 pair (the shunt voltage regulator error amplifier) was "up the creek" (Remember Q5 in this circuit was one of the first faults I found on this set).
Q6 was found to be short circuit from collector to base, and when Q7 was removed, I found ANOTHER case of "rotten transistor leg syndrome".
I am very suspicious now that at some point in the past this transceiver had some unpleasant, corrosive liquid spilt into it, or something of that nature way back in it's past. That might explain a thing or two ...
Replacing both of these transistors has restored everything to normal - 2N3704s used to replace the 2SC945s - the base and collector terminals just need to be transposed.
Right, where were we? . . .
You may remember way back when you first undertook the restoration of this rig that you found some spillage and i commented that you will no doubt find some more.
ReplyDeleteCan i say i told you so.
I just had to say that mate sorry.
73
Ken G3SDW
You can say what you like, Ken, and no need to apologise!
ReplyDeleteYou are of course correct.
I don't think this is a conventional case of contamination, as there isn't a great deal of evidence to see which is obvious.
However, something has been troubling me for a while about this radio, and that is why I have found _so_ many broken bits in it.
One would normally expect the set to be put to one side and / or repaired after the first of these faults occurred - to find so many (and I clearly haven't got to the end of them yet) defies simple logic.
Consequently I have been wondering whether some of these have been caused during the "storage phase" of this radio's life so some kind of contamination whilst stored has to be a possibility.
To be continued ...