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Help I Installed A RFX75 Cobra 29 Squeals & No Modulation

S

steve25537

Member
Sep 22, 2012
44
2
18
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I got the directions from CBT and followed it to a T. Well got it all installed and has receive, will key up but no modulation and it squeals really really bad.

I ran a short jumper from R142 to the audio transformer ground pad still no Mod and Squeals.

Also for some reason I cant get the dead-key down. Its dead keying 50 watts way to much.

Can anyone please help with this problem, please.

Thank You In Advance.
 

I've spoke to you on craigslist, small world!
Anyway, check to see if you have the RX out and RF in connections reversed.
 
No cause the directions I went by told me to cut out C10 and the RX Out Cable. What did we talk about on CL? Yeah it is a small world after all, lol. Below is the directions I went by.

RFX75 – Cobra 29 Installation
1. Locate the capacitor at C10 on the RFX75 and carefully cut it off the board.
2. Cut the Yellow and Blue wires off the RFX75 board.
3. Cut the RX OUT coax cable off the RFX75 board. Take care to cut it close enough to the PCB so that it
does not short out on anything.
4. Drill and mount the RFX75 on the back of the radio using the supplied hardware.
5. From the radio, remove C8 (keep part for later use), C53, C57, R56, L12, and the final transistor.
6. Route the RF IN coax through the hole where the final transistor was so that it can easily be soldered to
the solder side of the radio PCB. Solder the coax center conductor to the front hole of R123 and the coax
shield to DC ground.
7. Solder the Red and Black wires to the back of the DC power jack (observe polarity!).
8. Solder the Orange wire to the front side of L15.
9. Solder the RF OUT coax center conductor to the rear hole of L12. Solder the coax shield to the chassis
ground tab on the back of the radio’s antenna connector.
10. Using the 15pF capacitor that was removed from C8, solder one end to JP24 and the other end to the
rear hole of L12 on the solder side of the board. Make sure to insulate the leads. You will probably have
to lengthen the leads of this part to make it fit. This re-connects the RX circuit in the radio.
11. Add a short jumper wire from the ground area at R142 over to the audio transformer’s ground pad. This
eliminates a ground loop that can cause the radio to squeal on transmit.
12. In the radio, replace R48 with a 470K ohm, ¼ watt resistor.
13. In the radio, replace C42 with a 3300μf, 16volt electrolytic capacitor.
14. Retune the transmit and receive of radio for best performance. IMPORTANT: If the carrier is higher
than 15 watts you should take the following steps to reduce the carrier to below 15 watts.
a. After confirming that the radio is functioning properly, remove JP36 and install a 1000μF
electrolytic capacitor in its place with the negative lead in the rear hole and the positive lead in
the front hole.
b. On the solder side of the radio’s circuit board add a 10 ohm - 47 ohm, ¼ watt resistor across the
two points where you installed the 1000μF capacitor. This will allow you to adjust the carrier -
The higher the resistor value, the lower the carrier. Typically, 33 ohms is about right.
 
The reason you're getting the high "deadkey" is because of the squeal. Has the modulation limiter (D11) been clipped? If so, you'll want to re-install D11. You don't want to run the radio with the mod limiter removed or clipped. This will cause a squeal issue. Wouldn't hurt to make sure all grounds are properly soldered and secure to eliminate all possible ground loop faults. Bump your mic gain back a little too, and see if that helps. let us know how you get on.


~Cheers~
 
Update, I found out why it doesnt have any modulation the audio chip was bad, but the bad part is it blows everyone I put in. I have put 2 in so far and as soon as I power it up it pops the chip.

Any ideas why?

Thanks
 
That what is was a bad Cap, but now I can't get the deadkey below 40 watts and it squeals still yet and it just dont have the swing it should have.

Does anyone have any advice?


Thanks

P.S. The squealing is really bad with the mic gain turn all the way up when I turn it down its not so bad but its always there.
 
I still cant get the deadkey down, it stays around 40 watts. Any ideas?


Thanks

Ive tried different resistors, caps but nothing works.
 
You did do the swing mod that was in the directions didn't you? If so increase the resistor. Also try removing that 82 pf cap you installed by the driver.
 
Do u run the ground wire to the transformer? I had the same problem and forgot to run the ground, but once I did that everything worked great.
 
Heres the info, add a small ground jumper from the ground trace of T1 to the
ground trace of C72.

The ground should stop the problem, at least it did for me. Let us know how it goes.
 
Thanks, it's the variable mod, I had to open the radio and find my delta tune(taped up and floating around) turned that back and it's good now. I appreciate it guys.

This thing sure gets hot fast!
 
surveywaters said:
Thanks, it's the variable mod, I had to open the radio and find my delta tune(taped up and floating around) turned that back and it's good now. I appreciate it guys.

This thing sure gets hot fast!
Click to expand...

Put a fan on it; I did.
Cold as ice now.
Unless you want to replace the RFX-75 again . . .

Another thing . . .
The variable mod will burn out the Delta Tune control eventually. Too much current for it. The proper way is to have a voltage regulator installed so that the control will survive. One day you may turn that radio on, and you will have no output because that control smoked.

The variable power mod is surely one cause of squeal . . .
 

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