[Heathkit] sb200 R15 question

[Michael Waldrop]

R15 along with R16 and the bias supply through the relay is part of the standby cut off bias as well as the normal transmit -2VDC operating bias. Changing the values of one or both resistors can and will affect the cut off and operating bias voltage to the grids of the tubes.
 
I would not substitute the original 1.5 ohm 1/2 watt resistor with a 2 ohm resistor. A 2 ohm resistor is an increase in resistance of "40%" (1.5 + 40% = 2.1ohms) of the original value. I would order a new resistor from Mouser or your favorite parts outlet instead of using a 2 ohm resistor.
 
Why did R15 burn open? A shorted tube could cause R15 to burn open..
 
73
Mike
W5RKL
 
 

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

From: labernathy <ka9cch at arrl.net>
Subject: [Heathkit] sb200 R15 question
To: heathkit at mailman.qth.net
Date: Wednesday, September 24, 2008, 8:35 PM

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.