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Title: Sb 220 on 75 meters Post by: Detroit47 on December 03, 2018, 08:59:57 PM My SB220 is my SSB amp. It is giving me a run for my money. I hadn't been using it for a couple of years so I took the home brew 4-1000 out of line. With plans of running the SB220. Unfortunately it wouldn't tune up on 75. I immediately suspected the band switch. So I pulled it out of the case and started checking. I discovered the bypass caps on the grid had exploded. The chokes where still intact at least no HV arcs. So I decided to take the grids straight to ground with copper strap. So I fired it up and it wouldn't tune right on 75. I got full power on every other band, but only 300 on 75. It was obviously out of resonance. I decided to check it out with a dip meter. I got a nice indication of resonance on all bands except 75. So I started digging I pulled it apart, I took the band switch out nothing looked amiss the 500pf pader checked out. The tank coil assembly is original. I put an extra 500 pf in parallel with the pader for laughs no change. The plate block is the stock 1000pf door knob. It checks ok with my fluke so I stuck it on my old Cornell Dubilier bridge. It said it is ok. I'm like WTF the tank coil didn't change. So I'm all ears anybody got any ideas.
John N8QPC Title: Re: Sb 220 on 75 meters Post by: KD6VXI on December 03, 2018, 09:37:28 PM Possible one side of C load has come unsoldered?
Some people unsolder one section to give easier / broader tuning on the higher bands. --Shane KD6VXI Title: Re: Sb 220 on 75 meters Post by: Detroit47 on December 03, 2018, 11:48:09 PM Shane
Some one had changed the plate tune cap. It is a 180 pf cap instead of 250. It only has enough capacitance to tune top end of band. I clipped a 50 pf silver mica in parallel with plate tune bingo dip. Now I have to get a new plate tune cap. Or just go back to the 4-1000. I could just pad it but that would make useless for 10 and 20. John N8QPC Title: Re: Sb 220 on 75 meters Post by: KD6VXI on December 04, 2018, 09:15:22 AM Dang, I just sold one.
K2lgo has a sb220 he's been putting up on qrz. I bought a fan and filament xformer from him. As well as a few other things over the years. He might have the cap, and he's always been more than fair. --Shane KD6VXI Title: Re: Sb 220 on 75 meters Post by: DMOD on December 04, 2018, 02:55:00 PM If you can't locate one from a fellow amateur, MFJ has variable caps part numbers: AMfone - Dedicated to Amplitude Modulation on the Amateur Radio Bands
282-2008-1 and 282-2112-1 Page 85 of the current catalog. Phil |