Add FM Mode to Cobra 29 LTD / Uniden PC-76 / Cobra 25 LTD

[LeapFrog]

Hello all, I've made an image with install details for Cobra 29 LTD or Uniden PC-76 radios.
it's for the Spectrum Communications FM 455 Mod/De-Mod PCB
kit that is used to add F.M. Mode to your A.M. only radio (for Export or HAM use only)

Make available a DPDT On-On Switch for switching between FM /AM.

**Lift the leg of R40 that isn't connect to C33

[J1] = FM side of switch for Mic audio path
[J2] = Provide 8+ Volts during Transmit only.

[J3] = FM TX uses Varicap Diode D13, attach one end of 100K Ohm resistor to the end of the diode that is not connected to PCB Gnd.
(keep leads as short as possible).
The other side of the resistor is decoupled with a 10nF "103" capacitor.
A wire then goes from the decoupled junction to J3 of the 455 FM PCB.
(If you don't have TX audio in FM mode use a 1nF "102" capacitor instead, or try without the cap)

[J4] = Connect to low impedance winding of the last I.F. Can (L9), the collector of
TR10 is connected to this winding.

[J5] = Provide 8+ Volts during Receive only.
Orange or pink wire on Delta-Tune Knob has 8V during RX only
(if no clarifier mods have been performed).

[J6] = FM side of switch for FM RX AF
[J7] = D.C. Ground (PCB earth or can)

I didn't show in the schematic the 8+ Volts sources that are needed,
one during receive only (J5) and the other during transmit only (J2).
If anyone would like to add specific locations for (J2), (J5), feel free to do so.

I found 8+ Volts during TX only (for J2) at an empty hole near the buffer/driver P.A. area.
I found 8+ Volts during RX only (for J5) at the unmodified clarifier (might be a pink or orange wire).
for clarification I ran a wire to PCB where the clarifier gets 8 Volts, I did not connect directly to the potentiometer.

All the best.

 

[LeapFrog]

Here is the cheat sheet for Cobra 25 LTD.

Beware, I did not verify the two voltage rails:
(J2) TX only 8+ Volts
(J5) RX Only 8+ Volts
So double check those as you go along if you're using the image as a "how to".

Make available a DPDT On-On Switch for switching between FM /AM.

**Lift the leg of R15 that isn't connect to C32

[J1] = FM side of switch for Mic audio path
[J2] = Provide 8+ Volts during Transmit only.

[J3] = Varicap Diode D11, attach one end of 100K Ohm resistor to the end of the diode that isn't grounded.
(keep leads as short as possible).
The other side of the resistor is decoupled with a 10nF "103" capacitor.
A wire then goes from the decoupled junction to J3 of the 455 FM PCB.
(If you don't have TX audio in FM mode use a 1nF "102" instead, or try without the capacitor)

[J4] = Connect to low impedance winding of the last I.F. Can (L7), the collector of
TR4 is connected to this winding.

[J5] = Provide 8+ Volts during Receive only
[J6] = FM side of switch for FM RX AF
[J7] = D.C. Ground (PCB earth or can)

I hope this is coherent enough for anyone who may need help installing one of these kits.

All the best!

 

[grimace]

I found this post when i was doing a google search for something else but I was looking for it a while back and I did a search on the forum here and I couldn't find it. Thanks for posting up the information

 

[jtrouter]

I found this post when i was doing a google search for something else but I was looking for it a while back and I did a search on the forum here and I couldn't find it. Thanks for posting up the information

Now if you could find it for the Cobra 2000/148gtl that would be awesome!

 

[TM86]

Now if you could find it for the Cobra 2000/148gtl that would be awesome!

Mr. Maurer has you covered.

EPR Funkservice - FM-modules - CB-radio modifications - Repairs - Frequency counter - Programming - AVR-firmware - Developement - GSM-switchbox - Rogerbeep - Repeater - PLL - DDS - FM - SSB - module - Transmitter - Graz Steiermark - Österreich

cb-radio repair, avr firmware programming, Elektronic, repeater, modification, frequeny extension, transmitter, rogerbeep, echo module, FM, FM-module, PLL-module, DDS module, ssb-module, ts340

funkservice.at

 

[grimace]

I'm looking for a recommendation on type of wire I should use for the shielded wire if anybody's got a link that I can go to purchase it or take a look at it. Or should I use RG316 I remember some FM boards from years ago using RG316