SB-201/200 no output on any band

[Kenneth G. Gordon]

On 18 Oct 2008 at 17:40, Tom Chesek wrote:

> I have an SB-201 which was converted to an SB-200 equivalent with a
> complete bandswitch swapout. I cannot tell you if it ever worked after
> the swap. This amp has the Harbach power supply mod and antenna relay
> including the resistor change that comes with the relay kit. The tubes
> are a pair of Cetrons which are soft but verified as working in my
> other SB-200 amp. Using the Taylor tubes from my SB-200 make no
> difference at all. There is no output on any band. I've cleaned the
> bandswitch with Deoxit, verified that the wafers are aligned with each
> other corectly, and verified that each contact has good wiping action.
> I have verified the -2vdc bias by measuring with a DVM and get a
> reading of -1.78vdc. The filament voltage measured at the tube side of
> the bifilar choke measures 6.72vac. The HV reads just under 2400vdc on
> the built in meter. The idling plate current reads about 98ma. PC1 &
> PC2 both have NOS carbon resistors. R21 and R22 are new also.
> 
> I've tried drive on each band from 30 watts to 100 watts with no
> output measured on the amps' meter or on the watt meter inline to the
> 50 ohm antenna load. If I drive it with higher power, such as 100
> watts, the tubes will start to "cherry up" in color if I keep the key
> down for 30 seconds or so. It almost appears to me that no rf is
> passing beyond C24 but I have swapped that cap with the one in my
> working SB-200. All screws have been tightened to be sure the ground
> paths are intact. 
> 
> Nothing is going up in smoke so where is my output? This is driving me
> nuts!
> 
> 73 and Tnx,
> 
> Tom K3TVC
> 
> ps: for what it is worth I know that at one point this amp was
> modified for 11 meter use and then changed to the current SB-200
> configuration.

Hmmmm....most odd.

OK. Let's start at square one:

1) Assuming that your meter circuits are OK, when you drive it and the 
tubes start to "cherry up", what does your plate current read?

2) Under the same conditions (drive), what does your grid current 
read?

3) Under those conditions, what does your +HV read?

4) Have you verified the center frequencies of the input tuned circuits 
with a grid-dip meter or some other means? Or you could check the 
SWR between the driver and the amp. If it is above 2:1, there may be a 
problem.

5) Have you checked RFC-3 1.1 mH (under the chassis) to see if the 
jerk who modified it for 11 meters burnt it shorted? That RFC is to 
prevent +HV from appearing on the antenna if C-24 shorts out. You 
might try lifting the ground end to see if you have output then.

6) Have you CAREFULLY checked the plate coil L-6 and L-7 for 
purposely shorted turns? Under the coils, perhaps?

(Actually, the inductance of RFCs 1, 2, and 3 are really too low for 
proper RF filtering at 80 and even 40 meters, and for maximum output 
on those band, really need to be replaced with RFCs of higher 
inductance.)

7) Have capacitors C-14 and C-15 been changed from their original 
value of 200 pfd? (BTW, this value is pretty critical: it makes the grid 
circuit series-resonant to ground, tuning the inductance of the grid and 
connecting wires. Collins' 30L1 uses a similar method to properly 
ground the grids. If you want to change this for what you think is a 
more "reasonably high" value, you have to do some OTHER things to 
make that work properly).

8) Are you sure that RFC-1 is OK? If that was burnt shorted, your RF 
could be dumped into the power supply, but there would probably be 
other noticeable (!) effects.

9) Is C-28 your dual-section 437 pfd/section output tuning capacitor 
shorted? Have you checked to make sure it really IS 437 pfd/section?

Let us know what you find out after following as many of these 
suggestions as you feel worthwhile.

BTW, for those interested, this site is possibly the best I have found for 
SB-200 mods.

http://www.xs4all.nl/~pa0fri/Lineairs/SB200/sb200eng.htm

Ken Gordon W7EKB

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