SB-201/200 no output on any band

[Tom Chesek]

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 the 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.


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