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Test Equipment / Re: I'm new, what better way to start with a faulty Marconi 2018 RF generator
« on: July 27, 2017, 02:10:55 AM »
Well, there it is, all done.
It turned out to be a flaky TR1 (BFR90). I think when I had my scope probe in there measuring, when I touched that junction the impedance of the probe must have pulled the bias of that particular part of the circuit enough to kick start that transistor.
I then reflowed all the solder joints around the junctions and surrounding components because I couldn't find anything out of the ordinary.
Then fired it up and all was well, I was displaying the output on my scope, I thought, why not dial in some modulation, as soon as I thought this, we had some intermittent activity on the output, cutting out, noise, spikes, the works, so I got to with the probe whilst watching the chaos going on and I probed at the oscillator, that was good, then as soon as I touched the probe on the Base of this TR1 (which is the input), it stopped, I could bring it back to life by putting my thumb on it, so I thought ah ha, I hit it with the freeze spray, it cut out, hit it with the heat gun, it started up again....
So there it was, dodgy transistor!!
BFR90 is now obsolete, so just the other day I thought to myself, there is a chance it could have been a transistor, so I found out the modern sub, which is a SOT23 package, so ordered up a bunch, never dealt with something that small before, but it was okay!! So anyway, I got the old TO50 transistor out, and managed to get the BFR92A in there, I have enough magnification to do it so there it was, installed.
The Marconi is now running on the bench, been running at 90mhz for the last 2 hrs, I think its fixed!!
I took a pic so you can see the retro fit....the hole in the board was the size of the other device as I am sure you're all aware, but yeah new to me....Lucky I happened to get some tweezers the other day in anticipation of having to do SMD...
It turned out to be a flaky TR1 (BFR90). I think when I had my scope probe in there measuring, when I touched that junction the impedance of the probe must have pulled the bias of that particular part of the circuit enough to kick start that transistor.
I then reflowed all the solder joints around the junctions and surrounding components because I couldn't find anything out of the ordinary.
Then fired it up and all was well, I was displaying the output on my scope, I thought, why not dial in some modulation, as soon as I thought this, we had some intermittent activity on the output, cutting out, noise, spikes, the works, so I got to with the probe whilst watching the chaos going on and I probed at the oscillator, that was good, then as soon as I touched the probe on the Base of this TR1 (which is the input), it stopped, I could bring it back to life by putting my thumb on it, so I thought ah ha, I hit it with the freeze spray, it cut out, hit it with the heat gun, it started up again....
So there it was, dodgy transistor!!
BFR90 is now obsolete, so just the other day I thought to myself, there is a chance it could have been a transistor, so I found out the modern sub, which is a SOT23 package, so ordered up a bunch, never dealt with something that small before, but it was okay!! So anyway, I got the old TO50 transistor out, and managed to get the BFR92A in there, I have enough magnification to do it so there it was, installed.
The Marconi is now running on the bench, been running at 90mhz for the last 2 hrs, I think its fixed!!
I took a pic so you can see the retro fit....the hole in the board was the size of the other device as I am sure you're all aware, but yeah new to me....Lucky I happened to get some tweezers the other day in anticipation of having to do SMD...