[AMPS] SB-220 HELP

[km1h@juno.com]

Hi Denis;
As you are aware the SB-220 is a very simple amplifier. 

A few questions and suggestions:

- With the 100W of drive, what is your input VSWR? 
- With no drive, but just grounding the relay jack do you have any idling
current?
- Does the exciter work properly into the antenna in by-pass but thru the
220 relay?
- I have worked on several 220's that have had relay problems due to
weakened spring tension. They   work fine manually but fail to make full
contact in operation. Remember there are 3 poles that have to   make
contact.  Yes, the spring tension does increase in TX.
- Burnish the relay contacts.
-  Measure the DC relay voltage. A leaky C-4 in the relay/bias supply can
drop the voltage low enough to inhibit full relay pull-in.
- I assume that you have verified HV all the way to the plate connectors?
With relay on and off? Not just continuity. That SB220 wiring is brittle
and the PC board connections are subject to intermittents also. 
- The zener and the .82 Ohm resistor are OK?

I asked many of those questions just to get a handle on what you have
checked sofar. I have up to 6 SB-220's on the bench here at any one time
so I can work along with you as necessary.

73...Carl   KM1H  Ex -Navy  ET


On Mon, 17 Mar 1997 12:24:05 -0500 CATALANO_DENIS_E@hq.navsea.navy.mil
writes:
>     I have several SB-220's which I have worked on over the years to 
>keep 
>     going.  I'm not an amp expert, but have fixed several.  Most of 
>the 
>     time, the problem is in the HV power supply.  This time it's not, 
>the 
>     meter shows about 3,200 V, so I have HV, and the tubes are lit.  
>The 
>     problem is there is no power (0 watts) out, with 100 watts drive 
>on 
>     all bands.  There is continuity between the HV power supply and 
>the 
>     plates of the tubes.  Also, the relay appears to be working, ie. 
>they 
>     click when energized, and a continuity check from the input to 
>the 
>     grid caps look good when keyed by hand.  One other thing, this 
>amp 
>     failure was without sparks/ smoke/ fire as my other enlightening 
>     experiences have been.   Any ideas or suggestions appreciated. 
>                                                 73 Denis W4DC 
>(x-WD4KXB)
>
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