[AMPS] SB-220 Problems

km1h@juno.com km1h@juno.com
Wed, 30 Dec 1998 11:39:59 -0500


>>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 suggest you consult  the schematic to determine the location of RFC-2
Rich. 


>
>>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. 
>
>agreed
>
>>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
>>>correctly.
>>
>>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
>>conversions. 
>>
>>
>>73  Carl  KM1H
>>Amplifier Repairs and 6M Conversions our Speciality
>>National Radio 1963-69    Part of NCL-2000 Design Team, Service Mgr
>>1965-69
>>
>The NCL-2000 circuit design:
>1.  used a killer screen-supply.  

Separate winding on the xfmr, common useage for the times. Also was per
RCA recommendations including the surge and fuse resistors used.

>2.  did not protect the 8122's unusually delicate screen from 
>excessive 
>screen current. 

Screen trip ckt was in the prototypes. RCA really tucked it to National
with the early 8122 mfg problems. 

>3.  ran grid current (Class AB2 grid-driven) which made ssb-doodoo.   

With ALC connected it was simple to limit grid current and give
acceptable doo doo. There is only 1dB difference in IMD3 between AB1 and
AB2 with the 8122.

>
>4.  ignored a distortion-lowering circuit recommendation from RCA's 
>8122 
>tech info. sheet.  

Not according to RCA, suggest you get reading glasses.


>5.  as I recall, used no low-L screen capacitor/collet bypass to 
>improve 
>vhf stability.  

That is on the prototypes also.

As I stated here and to you in the past, what was designed in Engr is not
what got built after the Marketing VP screwed around. One engr quit in
disgust. I transfered to the Svc Dept, wrote NCL service bulletins and
was soon promoted to Svc Mgr. over the objections of the idiot in
Marketing. He left years before I did.

73  Carl  KM1H

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