FS Heathkit SB-220 Amplifier

[Charles Hugg]

FS Heathkit SB-220 Amplifier. This is the one for which I have asking seeking help.  From all the input that I have received, it appears that there has been a grid to filament short in one of the 3-500Z tubes. When this happened, it took out R3, the 82 Ohm Shunt Resister in the Grid Current Metering circuit, and the 1 mH RF Choke in the Ground to Grid circuit of the tube that shorted out. Everything else looks good. There is no evidence that the filament transformer was harmed. I have a new R3 Shunt Resister, but the local electronic parts place does not have the RF Choke. I am sure I could order this part, but am not crazy about risking a new 3-500Z to test it out.

This is a one owner amp that I am selling as part of an estate. It has the original tubes with 7611 date codes. It has not been used much. It looks very good. There are several small stains on the front panel. I am not sure what caused them. The cabinet had a ding on the rear corner, but overall the amp looks great. Knobs and Meters are all original and look good. I am going to offer this amplifier to someone who is more technically inclined than I. It will need at least one new 3-500Z and an RF Choke. I don't think much more. I understand that the Zener Cathode Bias Diode is often taken out by a grid to filament short, but I have not tested it.  Also, the reason the grid to filament short occurred in the first place needs to be looked at. It appears that the main cause for this is unwanted oscillations at around 110 Mhzs. There are some cures for this, but I am not sure that I understand how to  perform them. If the amp were working, I figure that it would bring around $475 to $525 , so I am going to price it at $250. I will also consider offers.  Thanks, Charlie Hugg, K5MBX  

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