SB-200 issues.

[Kenneth G. Gordon]

Today, I brought home an SB-200 amp that a blind friend of mine recently sold 
me. She had been using it for quite some time, but needed something 
that didn't require sight to tune.

I have discovered several issues with it:

1) One of the tubes was not lighting. This appeared to have been going 
on for quite some time. I replaced the tube socket and in the process 
discovered several no-solder joints in the filament circuitry. 

2) The grid-to-ground connection of that non-working tube was ALSO 
not connected.

3) After replacing the socket and redoing all the no-solder joints I could
find, it appears that the amp is working. Power output on 80 meters is 
around 750 watts. 

HOWEVER, one tube, apparently the tube that had been doing all the 
work, gets a very red plate even if the tuning time is kept to a minimum, 
AND the filament is only about 1/2 as bright as the other tube. 
Furthermore, there is the number "40" written on that tube in black ink. 
Sounds to me as though that particular tube is pretty flat. Your 
thoughts?  

4) The final "loading" knob is over against the fully-closed stop, even 
with an SWR of 1:1. Sounds to me as though the 500 pfd cap that is 
supposed to be paralleled with the loading cap on 80 meters isn't being 
connected. What do you think about this?

5) There is no grid current showing on the meter with the meter switch 
in the grid position.  I have not investigate this any further. However, it 
appears to me that the switch needs a good shot of DeCoxit at the very 
least.

6) It appears that the SWR input is quite high, as the loading control on 
the driver, an SB-102, has to be set fully to the right-hand stop for 
maximum drive. I know how to fix this...I think...

I would be pleased to hear comments from you all about these issues, 
to confirm my own ideas on the subject, and/or to hear suggestions 
from you about how to fix these.

At this point, it looks like I need a pair of matched finals, the input 
circuitry needs to be worked on, and I am going to have to CAREFULLY 
examine the final plate bandswitch to determine why the added 
capacitor is NOT being switched in. I am also going to have to take a 
careful look at the meter switch.

Are there common problems I should be aware of?

Thanks,

Ken W7EKB

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