[Amps] SB-200 Problem

[W6NQ]

Hi Chuck:
Previous comments have given you have a wealth of good info to explore.
One thing not mentioned is the possibility that you might have a
shorted or gassy tube at this time.
Shorted tubes usually fail due to broken filament.
You can check for this and possible direct grid-filament short with an
Ohmmeter.
A gassey tube you can't tell for sure without a hypot tester.

Did both tubes light?

If tube looks good--

Replace the 33 Ohm resistor before further tests.
Install one of your original tubes into the socket  that lost the resistor.
Also run the new tube that didn't loose it's resistor.

See what happens.
Then sub the second new tube in place of the the first new one.
Any change?

This might be clue--  i.e.:
Does the one tube that oscillates (or could be shorted/gassey)
carry it's problem along where ever it goes?

Regards,  Merit  W6NQ




----- Original Message -----
From: "Chuck Mills" <millscl+@pitt.edu>
To: "Gary J. Ferdinand" <W2CS@bellsouth.net>
Cc: <amps@contesting.com>
Sent: Monday, March 04, 2002 6:36 PM
Subject: Re: [Amps] SB-200 Problem


> Yikes....lots of good info here.  I replaced all of the blown components I
could
> find (RF Choke, the 33ohm from lug to ground) and threw the antenna switch
...
> it still pegs and the 33 ohm resistor will blow again (or throw the
breaker at
> the panel...).  This is on the bench with NO rf into it.
>
> Chuck
>
> "Gary J. Ferdinand" wrote:
>
> > > Heathkit SB-200 Built circa 1977.  Currently in need of a minor
rebuild
> > > for maximum life which I plan to undertake (Harbach power supply, new
> > > antenna relay, replace some ancient caps, etc...).  Currently has new
> > > 572B's with < 5 hours operating time on them.
> > >
> > > Symptom:
> > >
> > > While transmitting a while back, I let the smoke out of a resistor
that
> > > is part of the RF choke on the anode clip (one of the 80 5w wirewound
> > > guys with the coil wound around it).  Obtained new resistor and
soldered
> > > it in.
> > >
> > > Proceeded to put amp back in service.  Upon hooking it up, found a
clear
> > > frequency and got ready to transmit.  As soon as the antenna relay
> > > clicks shut, the plate and grid current peg and another resistor self
> > > destructs.  This one is off of lug 2 of V2 to ground (right most tube
> > > socket when looking at the back of it).
> >
> > Chuck,
> >
> > Were you feeding power to the amp when it pegged or did you just throw
it
> > into transmit position but without the exciter putting any power into
it?
> >
> > This sounds like a classic amp-taking-off parasitic problem. It would be
> > interesting to note if it were happening by its lonesome or whether you
fed
> > 100W into it when it happened.  It would also be interesting to know
what
> > band it was being tuned to.
> >
> > Others here are far more expert than I, but if I were you (and ONLY if
the
> > amp misbehaved when feeding power to it), I would feed a very small
amount
> > of power to it (barely cause the plate current to go above resting Ip)
and
> > determine if it is possible to find a point where it is resonant.  Also
> > determine whether it resonates on ANY band.  One failure mode of the
SB200
> > is the bandswitch wafer.  Another are tank circuit connections (solder
joint
> > failures).  If the amp is not seeing a 50 ohm load (because of a screwed
up
> > tank circuit due to either of the above), all bets are off as to
> > whether/when it remains stable.