[Heathkit] sb200 R15 question

[KHeck73 at aol.com]

I'm sending this a second time as the first one wasn't plain  text...
 
Many years ago (more than I care to think about), my high school station  had 
an HW-101 running into an SB-200. Once or twice a year somebody would try  
loading up the amplifier full power with the bandswitch set in the wrong  
position vs. the band set on the transceiver. This would blow one (or two?)  'grid 
bias' resistors. I don't have a schematic in front of me, but I recall  them 
being 1/2 watt carbon. This situation happened more than once. It didn't  seem 
to hurt the amp, and probably prevented further damage elsewhere inside.  So, 
if it was R15 that I remember, I'd consider the 1/2 watt rating a safety  
feature not to be changed It inexpensively tells you if something is set wrong  
(with a pop and then the smell, because I did it once  ;).

-Karl.

In a message dated 9/25/2008 10:48:40 A.M.  Eastern Daylight Time, 
_w5rkl at yahoo.com_ (mailto:w5rkl at yahoo.com)   writes:
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_ (mailto:ka9cch at arrl.net) > 
wrote:
 
From: labernathy <_ka9cch at arrl.net_ (mailto:ka9cch at arrl.net) >
Subject: [Heathkit]  sb200 R15 question
To: _heathkit at mailman.qth.net_ (mailto: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.
 





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