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[AMPS] SB-220 Problems

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Subject: [AMPS] SB-220 Problems
From: (Rich Measures)
Date: Wed, 30 Dec 1998 03:40:31 -0800
>On Tue, 29 Dec 1998 19:00:45 -0500 "Walter Fairclough"
><> writes:
>>I've run out of ideas and could use some suggestions that may help to 
>>rescue this SB-220 of  mine.
>>I had a catastrophic failure a while back due to Zener diode shorting 
>>that set off a chain reaction on other components. I have since replaced the
>>meter/rectifier board with one from Harbach, did his parasitic mods 
>>andreplaced the two
>>3-500Z with new EIMACs.  No matter what I do, the maximum power output 
>>that I can get in the CW position is about 700-800 watts (500 watts on 
>>10m), and which is about 100 watts more than the old tubes. This is with 
>>watts drive (depending on the band). 
>Assuming 100W drive period and a stiff 240VAC line you should get around
>750-800W  in the CW position and close to that on 10M. 
>In the SSB position the power should be 900-1300W, again depending upon
>line voltage.  Asssuming and as you stated the input network is tuned
>within reason. 
> > This output is achieved  with a plate
>>current of about 350ma and a HV of 2100.  Now if I do the math, 
>>something is
>>wrong. Also, I cannot seem to load to more than 350ma.
>Plate meter either broke or hitting something internally. Pull the face
>cover off and look for any needle residue that would impede the up scale
>meter reading. Also do not overlook the possibility of loose meter nuts
>since the panel was probably moved during the upgrade ( I had one 220
>that drove me nuts with that silly problem) . All early SB series Heath
>meters are known for dis-similar metal problems that can also give wrong
>readings due to added resistance. 
>When in doubt ...smack it!  (-:
The dissimilar metal problem can be fixed with silver conductive paint.  
{see *QST* article on circuit improvements for the SB-220 in 
November/December 1990 issues}.  Free, unsolicited advice:  never operate 
a SB-220:
1.   without glitch diodes to protect the meters and 
2.  the mod to remove +110v from the bias contacts on the relay.  The 
tubes will cut themselves off fine with resistive-cutoff bias.  If the 
factory-stock 110v-cutoff bias is used, and a tube grid/fil. shorts from 
an intermittent vhf parasite, it will destroy the unfused transformer.  
>In the SSB position you should have about 200ma idle current, the higher
>the better for drive and linearity up to about 230ma max. You can tweak
>by shorting out the bias diodes on the Harbach board one at a time.
>IMO, any of those so called parasitic mods are a waste of time and can
>lead to problems. Restoring to the original Heath design AND replacing
>the RFC-2 with a 20 Ohm 20W real wirewound is a better choice to long
>term reliability. 

A 20w resistor in a circuit where the dissipation is typ. 0.13a^2 x 20 
ohms = 0.34w?

>I'm sure to get the usual static but after a few hundred  SB-220 series
>repairs, conversions, etc. I have a good feeling for what works.
 And to keep having those good feelings, do not measure the resistance of 
the vhf parasitic suppressor resistors after a big bang, popped grid rfc, 
popped 0.82 ohm res., shorted zener, or unexplained arcing.  .  
>>I've checked the meter/rectifier board and all components read OK. Plenty of
>>grid and yes the input swr is 1:1 on all but 10m where I have to use the
>>built-in antenna tuner on the exciter (FT-1000D).
>I sure hope you are not using the 200+ W from the 1000D !  About 130W
>absolute max for the 220 series amp. 


>100W a lot better period.
Maybe for CW.  130w PEP drive on SSB appears to be safe.   Remember that 
the 220's HV transformer has a c.600-watt core.  
>>I suspect that with 700-800 watts out that is about all one can expect 
>>and that the plate current meter must have taken a hit and not reading
>Both meters are identical internally. If in doubt just swap them, the
>covers snap off and the scales are removed via 2 screws.
>>Any suggestions or comments would be appreciated.
>A great amp and still in demand by anyone with common sense (-:  !
>If u get real desperate I always am buying SB-220 series amps for 6M
>73  Carl  KM1H
>Amplifier Repairs and 6M Conversions our Speciality
>National Radio 1963-69    Part of NCL-2000 Design Team, Service Mgr
The NCL-2000 circuit design:
1.  used a killer screen-supply.  
2.  did not protect the 8122's unusually delicate screen from excessive 
screen current. 
3.  ran grid current (Class AB2 grid-driven) which made ssb-doodoo.   
4.  ignored a distortion-lowering circuit recommendation from RCA's 8122 
tech info. sheet.  
5.  as I recall, used no low-L screen capacitor/collet bypass to improve 
vhf stability.  

-  later, Carl.  


R. L. Measures, 805-386-3734, AG6K,  

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