SB-401 Resistor R17 correct wattage?

[Michael Waldrop]

Thanks to Bob and others, I have progressed some in the repair of my SB-401.
  I now have plate current. The problem was not R17 although I wanted to reduce
  the resistor to the proper size. The no carrier problem was caused by an opened
  RFC in the plate of the 6CL6 driver. With that replaced, I now have plate current.
   
  The 401 will tune up and produce about 90 watts of output. However, I can't adjust
  the output down to zero with the front panel CW gain control. Why? Well, from what
  I have found, CR-1 and CR-3, 2 of the 4 crystal diodes in the balance modulator, are
  blown. That could be the reason I can't reduce the output to zero. Instead with the
  blown crystal diodes the lowest I can get the output is 30 watts. 
   
  Now I have to find replacements for all 4 of the balance modulator diodes. I have what
  appears to be 1N210's but they do not have the PIV and current rating the original 1N191
  crystal diodes have. The 1N191's are rated at 90V PIV at 500ma. The 1N210's are rated
  at much lower PIV and current rating so they won't work.
   
  Does anyone have an 1N191's?
   
  73's
  Mike
  W5RKL
  
W5EUQ <w5euq at verizon.net> wrote:
  Michael,
I have an original manual on it. Pictures show R17 as being 
1/2 watt. Calculations also show that 1/2 watt would work for 680 
ohms. There is 250 Volts on the input of R17. It drops the voltage to 
the screens to around 150 to 200 Volts. Screen current runs from 7 to 
11 ma per tube. Therefore, for two tubes, max current will be 22 ma. 
Using ohms law, P=I2 times R take .022 squared times 680, which 
equals, 0.32912 watts. Therefore a half watt resistor will work fine 
here. 
What do you think? Good luck. HNY, Bob W5EUQ Tampa

Michael Waldrop wrote:
> Good afternoon,
> 
> I'm working on an SB-401. The final has a resistor, R17 680 ohm, that is in the
> final amplifier's screen supply. A previous owner installed a 2 watt 680 ohm 
> resistor, did a horrible job. The resistor has very short leads and it's soldered to
> a piece of thick wire and I'd like to replace it and clean up the mess. I don't have
> the original assembly manual. I only have the bama manual which does not include
> the assembly portion. The schematic is a bit fuzzy. I have a 1/2 watt resistor which
> I think is the correct wattage but I want to make sure. No I don't want to smoke a
> resistor just to find out :)
> 
> Anyway, any suggestions and does anyone have an original SB-401 assembly manual
> that can verify R17 as being a 1/2 watt 680 ohm resistor? R17 is the resistor shown
> on figure 2-3, one lead on terminal strip "P" and the other end on the wire between V10 and
> V11 pins 3. 
> 
> Another problem it has is no plate current. There's high voltage on the plates of the finals
> and all the oscillators are working; I can hear them in a seperate receiver loud and clear.
> I've removed the driver tube, 6CL6, to make sure it's working and it is. I can adjust the signal strength of the SPOT signal in SPOT plus the PLATE control also varies the SPOT signal strength. But, in TUNE, there's no plage current. The relays close, both of them, so it's
> not the relays (not closing). I suspect both finals have bit the dust.
> 
> Any suggestions and if someone has a complete assembly manual that can check R17 to
> verify it's a 1/2 watt resistor. It's the resistor shown in figure 2-3 in the alignment section. I
> don't have a page number but you'll find it in the alignment section.
> 
> Happy New Year and thanks.
> 
> 73's
> 
> Mike
> W5RKL
> 
>
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