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Re: [Amps] SB220 step start blows fuses

To: amps <>
Subject: Re: [Amps] SB220 step start blows fuses
From: Warren Volz <>
Date: Mon, 18 Sep 2017 12:51:28 -0600
List-post: <>
Just to close this out, as various people suspected the QSK circuit was
broken. I ended up replacing 4 components (Q7, Q6, Q4, U1) on the W7RY
QSK board and now the amp is working. I believe that a HV arc took out
these components prior to me (used QSK board). 

Thanks everyone for the help! 

-Warren KD5YPH 

On 07/12/2017 12:40 am, Warren Volz wrote:

> Well I have a major update!
> The amp shipped with 2 3-500zs that are "not full output" according to the 
> seller. Since the amp was in shambles I didn't have a way to test them until 
> now.
> One of the 3-500zs is unmarked and missing the phenolic spacer below the 
> glass envelope. The other is clearly labeled as a Eimac. I tested each 
> tonight with the HV plugged in. The unmarked worked without issue but the 
> Eimac arced right after turning the amp on!
> So, I guess I'm in the market for a new 3-500z. Based on what I see online, 
> there isn't really a way to fix this gassy tube but I'm going to let it sit 
> with no HV and filament voltage for ~30-60 min to see if that has any impact.
> Tonight I also removed the fan and oiled the bearings + the surrounding 
> fiber. The fan is a lot louder than I expected, but with the oil it's 
> slightly quieter.
> Thank you all for your help!
> -Warren
>> On Jul 11, 2017, at 6:35 PM, Warren Volz <> wrote:
>> Again thank you all for the advice and guidance. 
>> Replacement fuses showed up today so I'm being a little more
>> adventurous. As multiple people pointed out the dim bulb tester I have
>> as built won't work unless I can find at least one 300W bulb. 
>> So, I remove the dim bulb tester and tried each tube in the amp. Good
>> news is that both filaments light up just fine! To me that rules out the
>> filament supply and paints a target on the HV supply as many of you have
>> suggested. 
>> The amp as I received it was a pile of parts, and so I have no reference
>> in terms of working/not working prior to my mods. Both the QSK and the
>> RM220 were used pulls off of an auction site. The RM220 is squarely in
>> my sight now since it had a burned out metering resistor when I got it.
>> When I checked previously the diodes were all ok. Also, all components
>> (caps + resistors) on the filtering board are new. 
>> I'll likely try reconnecting the HV primary without the plates connected
>> to see what that does next. 
>> -Warren
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