[Heathkit] SB-220

[km1h]

It doesnt make sense IF the chokes were good. I would replace them before 
doing much operating. With direct GG you are risking instability since the 
chokes provide a bit of negative feedback due to their resistance.

If you have the old style grid capacitors, red rectangular instead of smooth 
molded, check for any cracks along the seams. They are prone to moisture 
getting in over time.

Carl
KM1H

----- Original Message ----- 
From: "Don, W4YCH" <w4ych at earthlink.net>
To: <heathkit at mailman.qth.net>
Sent: Tuesday, November 30, 2004 10:02 AM
Subject: [Heathkit] SB-220


> To: WB8JKR, K1ZOK & KM1H,
>
> Ya'll responded to my post concerning the SB-220 that I could not get any 
> output or idling plate current when the antenna change over relay was 
> energized.
>
> Well, Yesterday I found the problem...I had checked and replaced the zener 
> diode, checked all the resistors R1, R2 & R3 on the meter board, checked 
> continuity from the plate caps back through to the positive side of the 
> capacitor bank,  checked the ground side of the power supply, measured 
> plate voltage, checked the RF chokes on the grid of the 3-500's ( they 
> were 25.5 ohms).
>
> I ran out of things to look at and try so as a last resort I jumpered the 
> RF chokes on the grids and bingo! Energizing the relay showed about 90 ma. 
> plate current in the tune position, and about 120 ma. in SSB. Hooked up a 
> transceiver, tuned into the amp and with about 110 watts of drive I get 
> almost 1300 watts out.
>
> I guess either one or both of the chokes opened up with current drawn or 
> the grids were above DC ground enough to not let the tubes conduct when 
> the bias changed. Ya'll may have more and perhaps a better explanation for 
> this.
>
> Anyhow, I appreciate all of the suggestions.
>
> 73,
> Don, W4YCH
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