FB on the troubleshooting Bill. Sometimes it's the simple stuff that gets
overlooked and causes hair loss. An local friend of mine told me about
picking up an old TT (Omni or Corsair?) at a hamfest for a parts rig price
because it was dead on RX. He got it home, opened it up, and found that
someone had broken a terminal off the speaker taking the cover off. He
replaced the speaker and the rig ("magically" HI HI!!) came to life....
73, Al
On Thu October 29 2015 1:25:48 pm Bill ACITO, W1PA wrote:
> Think I located it.
> Finally had a chance last night to
> 1) check the ALC diode on the SWR board -- it’s ok
> 2) open up the PA module and take some voltages/transistor checks
> Check both drivers and PA transistors using meter diode and resistive
> checks and no power – look good. Connect the power to the PA on the solder
> lug with the electrolytic and go to check diode drops on the PA board-- no
> 13.8; none. huh?
> Check the outside of the lug – 13.8. Check the PA board – none. Repeat.
> Check the inside nut of the lug. None. huh?
> Let me say that again: one bolt, head outside reads 13.8, nut inside PA
> reads 0v. Tilt head to side like confused puppy.
> Power off, disassemble.
> From what I can tell (and suspect), either at build, or on a previous
> repair, a “black metal” (anodized?) nut and screw were used to secure the
> lug to the PA cover. Over time and temp/current cycles, the nut corroded,
> and the subsequent current heating slightly melted the inset plastic
> spacer. Material and corrosion filled in between the solder lug and the nut
> on the inside cover, and isolated the lug from the nut and bolt threads...
> no 13.8. Could have pushed me over with a stick. Replaced hardware.
> Unfortunately, in the process of diagnosing, I cracked the 100 ohm bias
> trimmer inside the module. I am waiting on the replacement before I close
> it up and apply power.
> Cross your fingers.
> Bill
>
> >>On transmit with drive turned down, rig goes from drawing 1A to about
> >> 1.4A... bias on finals seems ok. Putting an ammeter in line with the
> >> drivers, no current (supposed to be 4 to 8 mA)... bias adjust has no
> >> effect. Looks like I lost the MRF476's, or the passives around them.
>
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