[TenTec] Omni C + new Ham = Oops.

Mike Bryce prosolar at sssnet.com
Tue Oct 15 15:26:16 EDT 2013


while every tech has his or her own style, here's how I do it.

you're 35 watt iron should be fine. You don't want to lift the copper from the board!

while the soldering iron is warming, draw, photograph or take some kind of notes so you know what goes where. Most likely you'll need to unsolder feedback capacitors and resistors, so make damn sure you know where they go.

you'll need some, make that lots, of solder wick. Get the good stuff and not from china. You want it at least 1/4 in wide.

suck up as much as you can and then try and get even more off the pcb.

then when a sharp knife (I use an xacto) slip under the tabs and with heat applied to loosen any left over solder, gently lift the tabs up from the pcb

the collectors have the tip of their tabs cut at an angle. Make sure you document how the transistor (S) go in. They can easily be flipped 180 deg and you'll never know it.

I don't recall what they used for the pa, but get some thermal goo and apply a thin layer onto the heatsink and device. Thin is better than a big glop.

I go for  the base first. I can then use a VOM to do a resistance check on the remaining leads. Bet you'll find a base to collector short. 

And while I'm in there, I'll lift the base leads of the drivers and look at them as well.

good luck.

mike, wb8vge

Mike, WB8VGE
SunLight Energy Systems
The Heathkit Shop
J e e p

A man with one clock knows what time it is. A man with two clocks is never sure. 

On Oct 15, 2013, at 3:08 PM, Paul Harker wrote:

> 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 at 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

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