[Amps] SB-220 help

Mon Jul 15 11:13:42 EDT 2013

The full list of SB-220/221 bulletins is on the Web and at
https://puck.nether.net/pipermail/heath/1998-January/028165.html
Bulletin 27 is the pertinent one. It took one hit to find after typing 
"SB-220 service bulletins".

A different search term came up with this.
http://www.nostalgickitscentral.com/heath/Service_Bulletins/sb220.txt

Those other links do not address the question and are not changes I would 
suggest.

The factory way puts a full 120-130VDC into the cathode at all times in 
standby.

The "alternates" do not protect for a sudden gas arc that may have been 
prevented if the full 120V was applied, instead of only 25V, ALL THE TIME 
since many shorts/arcs occur suddenly when in standby. While the resistor 
bias method has been common for many decades long before Measures, the 
resistor is sized for a higher cutoff, usually a 33-47K. OTOH with the 
prevailing problems with 3-500 gas from all manufacturers it does not go far 
enough. The AL-82 and 80B (as well as out of production amps from Ameritron 
and others) are notorious for letting loose in standby with a lot of 
resultant damage a good part of the time. Hard grounded grids just expedite 
the damage wheras the choke usually opens; or use a 24 Ohm 1/2W resistor.

Adding a fuse to T2 is a good idea as well as replacing RFC-2 with a glitch 
resistor and the slow circuit breakers with fast acting fuses. Grid fuses 
are another possibility.

C4 is a weak link in those amps as very few bother to monitor the standby 
bias on a regular basis and as the cap ages and leakage develops the voltage 
goes down and hum ripple goes up.

To minimize gas arcs run the tubes with color on a regular basis, a dark 
orange for Eimac and other sheet metal anodes and a bright red on the lower 
2 rows on graphites. An easy way to do this is to operate as an AM linear 
with 20W of carrier and about 250W out which is the safe limit for the 
rather wimpy SB-220 transformer. An hour or so of enjoyable ragchewing once 
a month is sufficient; DO NOT do long winded old buzzard transmissions. The 
original Eimacs are still in the LK-500ZC I have had since new in 86 and 
with the optional external HD Dahl xfmr PAC-5 I can run 350-400W carrier as 
long as I want with the very quiet EBM Papst 4600N fan. Many modern SS rigs 
sound decent on AM once set up properly and the SDR's sound great.

Carl
KM1H

----- Original Message ----- 
From: "Jim W7RY" <w7ry at centurytel.net>
To: "mike stokes" <mike9v at gmail.com>; "Amps" <amps at contesting.com>; "R 
Johnson" <rjohnson at tmlp.net>
Sent: Monday, July 15, 2013 1:47 AM
Subject: Re: [Amps] SB-220 help

> Here:
>
> http://wy6k.com/homebrew_stuff/essential_mod_sb220.htm
>
>
> And here:
> http://www.somis.org/sb220ci_diag.gif
>
> 73
> Jim W7RY
>
>
> -----Original Message----- 
> From: R Johnson
> Sent: Sunday, July 14, 2013 11:33 PM
> To: Carl ; mike stokes ; Amps
> Subject: Re: [Amps] SB-220 help
>
> Hi Carl:
> I remember the problem, but I can't find the "Heath App Notes" I had saved 
> on one my old machines.
>
> Maybe you could give a little more info for the "NEWBIES" on the list.
> I have 10 old HD's I would go thru to find my old "Heath App Notes" 
> directory !!!
>
> Maybe you can do a better retrieval job than me :-) !!!!
>
> 73 Bob, K1VU
> FDN42ma
>
> At 08:22 PM 4/23/2013, Carl wrote:
>>Early 220's had a problem with shorting the filament/bias transformer if 
>>it was wired for 115V and the zener shorted. There is a change notice on 
>>the web.
>>
>>Carl
>>KM1H
>>
>>
>>----- Original Message ----- From: "mike stokes" <mike9v at gmail.com>
>>To: "Amps" <amps at contesting.com>
>>Sent: Tuesday, April 23, 2013 6:28 PM
>>Subject: [Amps] SB-220 help
>>
>>
>>>I just picked up a used non working SB-220.  Suppoesdly the filament
>>>transformer smoked.  Still have to pull it apart and see what is going
>>>on inside.  Tubes are supposedly still good.
>>>
>>>The question I have is it possible/likely that a tube failure caused
>>>the filament transformer to smoke ?  What is the best way to check the
>>>tubes to insure they don't cause more problems ?
>>>
>>>Thanks in advance.
>>>
>>>-- 
>>>Mike Stokes
>>>KK9V
>>>_______________________________________________
>>>Amps mailing list
>>>Amps at contesting.com
>>>http://lists.contesting.com/mailman/listinfo/amps
>>>
>>>
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>
>
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