SB-200

[Ed Lambert" "K1ZOK]

A few comments.....

First, on the 33 ohm resistors.  I'd suggest replacing them again with
the one watt style as called out in the original design.  They act as
"fuses" and can prevent a fault from traveling into the rest of the
circuitry.....  as it did.  When you replace them with a higher (much)
wattage rating, then the resistor cannot blow and protect the rest of the
components, including the tubes....  might be very expensive!

The one ohm resistor R12 is the current monitor resistor.  If you replace
it with a two ohm, then all of the current readings on the meter will be
twice as high as the actual value, and the resistor will dissipate twice
the power at the same current level.

You MIGHT have damaged the original R12 when the capacitor let go.  You
should make sure you monitor the plate current when you do the next
testing.  If you have a couple of the two ohm resistors, you could place
two in parallel FOR TESTING PURPOSES.

Ed
K1ZOK

On Sat, 22 Feb 2003 20:09:35 -0600 "Carlos Avellanet [kf4rrn]"
<kf4rrn at COX.NET> writes:
> Hi again.
>
> 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
>
>

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