Before I go at it, any tips on desoldering the power transistors? My 35W
iron is not up to the task unless I cut the foil leads beforehand. A bigger
iron might be in my future.
On Tue, Oct 15, 2013 at 1:03 PM, Mike Bryce <prosolar@sssnet.com> wrote:
> excellent troubleshooting
>
> although the diodes on the swr board don't control the PA in anyway.
>
> but it could be that a spike came down the coax and took a few things out.
> It's possible.
>
> if you can get to the base of the transistors in the PA, in the lock
> posistion position and the drive all the way down, you should see supply
> voltage on the collectors, zero on the emitter and .7 on the bases.
>
> since the base resistors are toasty, be sure you replace them with like
> units, carbon flim or metal film and not wirewound.
>
> call RF parts and see what they have in stock. I'm not much for matched
> pair in simple PA designs like the ones in the Omni, so save some money
>
> mike
>
> Mike, WB8VGE
> SunLight Energy Systems
> The Heathkit Shop
> http://www.theheathkitshop.com/
> J e e p
> o|||||||o
>
> A man with one clock knows what time it is. A man with two clocks is never
> sure.
>
>
>
> On Oct 15, 2013, at 12:23 PM, Paul Harker wrote:
>
> > Well... here is where I'm at:
> >
> > Checked the 914 diodes on the SWR board and sensing diode (D1) was bad
> > (leaky reverse biased). A big smile.
> >
> > I replaced the offending diode, put everything back together, attached a
> > dummy load to the output and turned up drive to see how much current was
> > drawn. I noted a 3 amps input power increase, and smoke. Smile goes away.
> >
> > Tearing it back down I discover 3.3 ohm R5 and R8 have acquired deep
> tans.
> > These couple the impedance matching transformer to the bases of the power
> > transistors. A subsequent check verifies proper bias voltage to the final
> > stage.
> >
> > Though toasty brown, and slated for replacement, R5 and R8 are still
> > correct values.
> >
> > Unless I'm missing something major (always a possibility) I'm thinking
> that
> > this kinda seals the deal on the power transistors being bad.
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> >
>
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