When will I ever learn? (SB-101)

[Kenneth G. Gordon]

On 8 Mar 2010 at 15:36, Kenth B Astrom wrote:

> Noticing that this VR tube was not lit in any way, I switched it out
> for one I had in a Drake TR4C, which I know works, and it still did
> not light up.
> 
> In doing this though I noticed that the R305 (2500 ohm, 7W) next to
> the regulator tube V18, was getting EXTREMELY hot.

Although that resistor does get awfully hot in normal use, it 
should not get hot enough to melt solder....which it does if 
something in the rest of the circuit is not right.

If R-305, that 2500 ohm 7-10 watter, is getting VERY hot, 
then you have a short circuit in the +150 volt regulated line.

Among other things, that line feeds the LMO. You might 
wish to check to see if you have ~+ 150 VDC B+ at the 
terminal on the LMO labeled B+ :-)

I'll bet you don't.

With no LMO, neither the receiver nor the transmitter will 
work.

There is another resistor, I believe it is 1000 ohms 7W, just 
to the north and west of R-305 on the schematic which is 
physically very near R-305. That one too can give 
problems.

In this case, it too may be getting unusually hot as it is in 
series with R-305 and the +300 VDC from the power 
supply.

If it is OPEN, which I have found to be the case with 
several HW/SB allbanders I have restored, there will be NO 
B+ to the receiver at all.

> It also seem to have been 'upgraded' at some point to a 10W resistor,
> instead of the original 7W.

That is because the original 7W jobs were, quite simply, too 
small to safely dissipate the heat over a long period of time, 
even when mounted slightly above the circuit board, and 
therefore it burned out. 

What I routinely do is buy those metal-encased precision 
gold colored 10 watters, and mount them solidly to the back 
panel or to the chassis somewhere close by for a heat sink 
with heat-sink compound between them, and run a pair of 
wires from each resistor to the appropriate places on the 
circuit board. If you do this, keep the wires as short as you 
can.

Doing this precludes that nonesense from ever happening 
again.

To repeat, it is quite clearly obvious that there is a dead 
short circuit somewhere in the +150 VDC regulated line, 
and I am betting that it is in or near the LMO. It may simply 
be that the wire from the +150 volt source to the LMO 
broke off and shorted to the chassis somewhere.

Please fill us in when you find it.

Ken Gordon W7EKB

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