On Tue, 29 Dec 1998 19:00:45 -0500 "Walter Fairclough"
<wfairclo@netcom.ca> 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
>and
>replaced 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 50-100
>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! (-:
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.
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.
>
>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.
>
>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
>
>Walter - VE3EN
>wfairclo@netcom.ca
>
>
>--
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>
>
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