SB-200

[Mark Graalman WB8JKR]

Carlos, that 1 ohm resistor has to be one ohm or your plate current
meter will read twice as high as it should, it is the plate current meter
shunt.
 I have a suspicion the tubes may be a little gassy, I would run them
at lower power levels for a few hours before cranking her up, also I'd
look for some different 33 ohm resistors, those 5 watt jobs are more
than likely wirewound and you don't want that.

Mark  WB8JKR


On Sat, 22 Feb 2003 20:09:35 -0600 "Carlos Avellanet [kf4rrn]"
<kf4rrn at COX.NET> writes:
> Hi again.
>
>  I have received and overwhelming response to my request for advise
> and
> information while I try to repair my SB-200.  Is along those same
> lines that
> I please ask you all to reply to the reflector, not just me, so we
> can all
> share the information being provided.  If you like to personal
> e-mail me,
> that's fine.  I would just like everyone reading and following along
> the
> benefit of the information flow.
>
> Now, back to the 200...the one area I received the most advise on
> was also
> the one I was most inclined to check; the 33 ohm resistors in line
> with
> C-14.  Sure enough, though they still look good physically, the one
> on the
> V-2 side was wide open.  These are 1 watt resistors.  Luck so have
> it that I
> had a couple of 5 watts 33 ohm resistors in my tool box so I
> replaced both
> of them even though the V-1 side was good.  I ohmed the rest of the
> resistors and capacitors in the circuit and they all read OK.
> Another
> suggestion received via e-mail was a wise way to get around the lack
> of tube
> tester--install one tube at a time in the amp and check it's
> performance
> individually.  Well, being I found the 33 ohm resistor open, I
> replaced that
> first before testing the tubes so I missed the chance to see what
> the
> results would had been.  At any rate, both tubes, checking one at a
> time,
> check exactly the same.  I set the FT-920 for 20 watts output and
> each
> individual tube output was about 150 watts on CW.  I didn't even
> look at the
> amps ma reading (should have).
>
> Smoke test...350-400 watts SSB on 20 meters, 450-500 on 40 meters,
> 500 to
> !POW! smoke and sparks on 80 meters!! Didn't get a chance to test 10
> or 15
> meters.  Inspection revealed blown 1 ohm 5 watt R-12 resistor on the
> power
> supply board.  I didn't have a 1 ohm resistor but I did had a single
> 2 ohms
> 7 watts and I went ahead and replaced R-12 with it.  Before I try
> again, I
> want to go thru the schematics once more and see what could have
> caused this
> resistor to go.  However, I've been working this amp a good 8 hours
> today
> already and I am not getting paid to fix my own gear, so I am
> quiting for
> the night.
>
> I'll keep posting updates for all of you who care.  Thanks for all
> the
> e-mails and all the great suggestions and advise so far.
>
> 73,
> Carlos. KF4RRN
>
>
> >Hi gang,
>
> >Here's an update on my SB-200 for all interested.  I replaced all 6
> power
> >supply caps and bleeder resistors. The amp turned on fine and was
> providing
> >about 600w output on CW and about 450w SSB.  Shortly after
> capacitor C-14
> to
> >pin 3 of V2 blew out with a fancy flash and a trailing plum of
> smoke.  The
> >amp still works but power is down to about 300w CW and around 150w
> SSB.  It
> >seems it is working on only one tube, though both tubes light up. I
> double
> >and triple checked my newly installed parts and inspected the board
> for
> >other damage--none found.  I replaced the cap and the power output
> is still
> >down.  I wanted to check the tubes but discovered my B&K 707 tester
> does
> not
> >support testing 572B's.  Troubleshooting is on hold until I can
> verify
> tubes
> >condition.
>
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