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Cobra 29 Classic No RF on TX

Handy Andy said:
Looks like the VC is a dud, that is a bad chart...

It's not that the numbers are reversed, they are transposed...

TR21 - GROUNDS the Base (Heads off to Pin 3 which then in TX mode, GROUNDS to Pin 1) - which then turns on the TX line.

So BASE goes to 0 - the Collector then rises to the 8V...
Click to expand...
Oops! I was testing TR21 but looking at TR16 on the chart. I feel dumb. TR21 looks good.
TR21 RX 7.8/0/8.6
TR21 TX 7.8/7.8/7.0

Sorry about that. I feel like I need to reset.

With this info, and the fact I’m seeing rf at IC Pin 9 @ 27.205, where do I go from here?
 
Next step downstream from there is the base of TR16. You should see a clear peak there when tuning the slug in L20. If not, L20 is suspect. And if you do have at least 1 Volt peak-to-peak RF at TR16 base, you should have a lot more RF than that on the collector (center pin) of TR16.

73
 
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nomadradio said:
Next step downstream from there is the base of TR16. You should see a clear peak there when tuning the slug in L20. If not, L20 is suspect. And if you do have at least 1 Volt peak-to-peak RF at TR16 base, you should have a lot more RF than that on the collector (center pin) of TR16.

73
Click to expand...
Well, got back to it and checked TR16. I have a scope that is junk. My Rigol is in for repair. I think I'll get on Amazon and get something to me asap. Should always have a working backup.

Here's what I can say about TR16 right now:
E / C / B
RX 0/0/.7
TX 0.1/7.9/0.8 ug, that's not right.

In the mean time I've developed another issue during key up which makes me want to use this as a door stop. I burnt my hand on the chassis. IC4 was H O T. Replaced. Same issue. Need to track that down. Didn't have that issue when I started. I think it's user error.
 
Actually, the DC voltage readings on TR16 look about right. Some problems are caused by a faulty DC circuit that you can read with a meter. And if the fault is with the capacitor inside a slug-tuned can, a multimeter can't point you towards that. Only measuring the AC signal part of the circuit reveals a AC circuit-only fault.

A hot IC4 is most likely a failed IC4. Yuck!

73
 
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