[Heathkit] sb200 R15 question

[Glen Zook]

R-15 is the meter shunt for the grid current position on the meter.  It really needs to be the same value.  If you change the value then the meter will not read properly.  You can get replacement resistors from Mouser 

http://www.mouser.com

and they are inexpensive.  You need at least a 5% resistor and a 1% or 2% would be even better.

If one of the 572B tubes got a grid to plate short this resistor could definitely go.  You have to be VERY careful as to which brands of 572B tubes are used in the SB-200 since a number of the Chinese manufactured tubes have had problems especially when mounted horizontally like in the SB-200 and SB-201.  What happens is that the grid "sags" and comes in contact with the plate.  When that happens all sorts of "fireworks" can happen.

Glen, K9STH

Website:  http://k9sth.com


--- On Wed, 9/24/08, labernathy <ka9cch at arrl.net> wrote:

From: labernathy <ka9cch at arrl.net>

I'm working on an old SB200. In the process of changing out the bias cap (C3) and C19, I found that r15 (1.5 ohm 1/2 watt) was in two pieces.
 
I have some questions:

1. Any ideas as to what would cause this resistor to overheat and break?
 
2. How critical is the resistance, will 2 ohms work just as well?
 
3. Is this resistor expected to open as a "safety device"? I know that in some Kenwood xcvrs, a resistor doubles as a fuse and changing to a higher wattage unit can shift the failure to an expensive part.