SB-200

[Carlos Avellanet [kf4rrn]]

Pretty good.  I'll heed the advise.  Mark also mentioned the purpose and
effect of replacing R-12.  I am not going to mess with it today.  I think
Harbach Electronics is back open.  I am going to check with and eBay them
for replacements.

Also, I just purchased another SB-200 that someone tried to convert to 6
meters but never finished the mod.  If it is a feasible repair I'll fix it,
if not I'll use it for parts.  I should be here sometime next week.

Thanks,
Carlos.

> 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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