I figured that I'd pass this one along today, as I wind up fixing a T-4XB that had a number of problems.? It got sent to me because the owner has having trouble aligning it and got down a rabbit-hole.? It happens.?
It's a nice-looking example; cosmetically about an 8/10 with all controls having decent feel, etc.? I inspected it for charred components because the owner mentioned that he smelled smoke before boxing it up.
I couldn't find any burnt parts.? I begin testing and quickly find that I can get good output on 80 and 15, lower output on 10 and nada on 40 & 20.? I deceided that starting an alignment was the best start, as the owner said he had tried making adjustments.? Yep -- 40 & 20 had trimmers that were way off.? The best way to find these is to see if you can get the wattmeter to budge even a tiny bit.? That allowed me to get 40 & 20 back into the ballpark.?
A full alignment followed.? I could see that the finals had been replaced at least once because the Phillips screw was reinstalled on the back of the cage cover.? Checking neutralisation showed that it was off.? I did the normal tune for a dip, remove screen voltage and adjust the cap for minimum.? In this case, this gave me no joy.? I had to end up setting the neut cap by "feel".? This doesn't happen often but it happens.
Happy with plenty of power out on 160-10, I checked with a mic and had output.? Checked on CW with no key in the jack and got output.? Not thinking, I figured -- Oh, OK, CW has power (this would be okay with early T-4X's but not the B!).? Put it into my B Line station and CW isn't working right!
I find that the can cap is open.? This was the 60 uF part, as the 20 uF part was still reading 22 uF.? Replaced both with discrete caps underneath.
Go back on CW and... Still acting wonky!??
As it turned out, D9 and D12 were leaky.? In the diode test position of my Fluke 77, they tested okay on forward drop and something like 980 and 1100 on reverse,? I unsoldered one side of each and confirmed that they were leaking when reverse biased.??
It's best to partially remove that board to gain access to these diodes, which are close to the underside of the chassis.? Two lead connections to the tube and it was nicely accessible.? Replaced the diodes and everything now works.
There were two issues causing trouble with the VOX (and thus, CW).? One was the open cap and the other the two bad diodes.? Some had also replaced R94 with a 220k (should be 680k).? I will assume that someone played around and got the rig to work for a while before the parts got really bad.
Moral of the story is to try and find all problems.? If you replace a part and it still isn't working right, look for more problems!
73,
Steve Wedge, W1ES
Time flies like an arrow.? Fruit flies like a banana.
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