[Amps] SB-220 Flash/Bang

[Jim Kennedy]

Hi guys, I need some ideas. I have an SB-220 circa early 70’s and am the original builder. I converted the amp to 6m about 4 years ago at which time I installed RFP 3-500ZG’s. At that time I also installed all the Harbach mods except for the fan mod. I also have a 5 ohm 2W MOF in the in series with the B+ rail in hopes if a failure that it would act as a fuse. In retrospect I probably should have installed a real glitch resistor.  The amp has played flawlessly for 4 years on a daily basis. I work a lot of digital MS throughout the year along with the normal E’s season operation. At one time the Harbach diode bias string went bad and I replaced those diodes with 2Kv  2 amps. Long periods of FSK  441 MS operation is what I contributed that failure to. Now for the real story: Yesterday while I was working CW there was a bright flash/bang on the capacitor bank side of the amp. ( I operate the amp out of the cabinet and saw it out of the corner of my eye). The amp didn’t shut down. I shut the amp down and pulled it for inspection. I found the 5 ohm 2W resistor on the B+ rail intact but pieces missing off it. I measured the resistor and it was open.  The B+ lead  had come off the end of the resistor also. I inspected the amp and found no other indications of damage. I replaced the 5 ohm 2w MOF and tuned the amp back on. It didn’t come back on. Nothing. I traced the primary AC circuit and all was good up to the step start board where I found one of the step start resistors open. Replaced the resistor in hopes that was the problem. Again nothing and the same step start resistor opened again. After shorting the B+ rail it shows the cap bank charging with an ohm meter so I think the rectifier board is ok.   My next step is to isolate the secondary of both the filament and plate transformer so no load exists and see what that brings at turn on. Sorry for the bandwidth but maybe some input from you guys will prevent me from chasing my tail around the block. BTW the 220 circuit breakers never tripped. Thanks in advance


Jim W7OUU DN22