July 30, 2024 Day 86
From Mike:
Approx 7-29-24.
I jigged the chassis to my 21FBP22 today, connected to a variac for first power up. When I got to full voltage, nothing was alive in the power supply. The D.C. voltages should be 385v,,,, 275v… and 140v. (roughly). All filaments were lit to normal brightness except for the 5U4 Recifier tubes, which were both blown. NO continuity through the filaments. This usually means that they died from an overload somewhere in the power supplies. I checked for shorts to ground, but nothing obvious. I then checked the fuse in the 385 volt line only to find that it was OK. BUT, somebody thought that it was an OK thing to install a 3.5 amp fuse into a place where it there should be a 2 amp fuse. Some technicians seem to think that it is an intelligent thing to do that.
Upon replacing the two 5U4 tubes from my “good used” pile, and replacing the fuse with the proper value, I tried again only this time I monitored the D.C. amps inline with the fuse upon power up. It got to just under 2 amps when the fuse blew. The D.C. 385 volt supply never got over 50 volts with the variac at about 75% which would be around 80 VAC line volts. Still no obvious shorts using an ohm meter, which means that the short is only there with voltage applied. It is time to start changing all the electrolytic capacitors in all sections of the power supply. There are 4 “cans” on the chassis, and all of them are in the front where the crt insulator boot can interfere with the 2 in the front, if the cans are too tall. I learned this one the hard way a number of years ago when I restored my set. The best way to fix this problem for maximum reliability is to re-locate the caps with individual units under the chassis, and leave the cans in place for cosmetic purposes. Fortunately, there is a lot of room to work with under the chassis to accomplish this task. I cut out the studs on the cans one at a time for further clues with the ohm meter. What I found was that several of them were leaking voltages from section to section, rather than the normal failure mode of stud to ground. This did offer some explanation to the overcurrent condition with voltage applied. I spent a lot of time over the next few days mounting tie points for the new capacitors.
Pictures 1 through 4 are the newly mounted electrolytic capacitors and some others that I replaced since I was already working there where the new ones were mounted.
Upon the next power up, I had 325 volts of D.C. on the main output of the rectifiers. Some success! It will likely be much more like 385 later when I install NEW 5u4 rectifiers, but this voltage is fine for now during more troubleshooting. I now have weak audio but NO high voltage at all. A quick check of the Horizontal output and damper tube showed that they are probably OK. So, a quick check for a Horizontal Oscillator drive signal on the control grid of the output tube showed NO signal there. The oscillator tube checks ok but I tried a new one anyway. No change. I then checked for B+ at the osc. stage and that looked OK. Then I got lucky when I did a careful physical exam on the osc. board. I was prompted to do this since I noticed that some tech ahead of me mounted new capacitors on top of the board. That is not a good thing to do because it often times weakens the solder connection on the bottom side. That is when I found the capacitor that is across the oscillator coil had a cold solder joint. These oscillators work much better when the coil and capacitor are actually connected to the circuit. (sarcasm) .
Picture 5 The red pen shows the point of cold solder joint.
Picture 6 The previous techs “handy work” on top of the board. (sarcasm)
Picture 7 Signal now present at the grid of the H. Output tube.
But, still no high voltage even after I changed the 3A3 HV rectifier and the 6BK4 Regulator. Further troubleshooting continues.
Cheers, Mike
Authors note: Spoiler, we do have HV! Description to follow.
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