cb mod

My advice is to take it to a cb shop before you ruin the whole radio.

 

Let's start with a basic question... Did you ONLY replace the final? The mosfet conversion requires several circuit mods to work. You cannot JUST replace the factory final with a mosfet. Are you saying no audio on rcv, transmit, or both? Many who do the swing mod on the cobra 29 use the RF Gain knob as a variable power knob, is that how you did your mod?

 

NO on the MOSFET and heat sink compound buddy....

Here are the instructions for the MOSFET upgrade:

Replace stock final with IRF520 MOSFET transistor (pin-for-pin match when installing). Remove R56 and in its place install a 470-ohm resistor in series with a common switching diode (1n914 or similar), with the banded end of the diode towards the front of the radio. Remove C53 and solder it across C59. In C53's place solder a 3.3k resistor. Remove C 61 and solder it across C62. Remove R55 amd solder a 22pf cap in its place. Remove the slug from L14. Replace C71 with a 33-47uf cap. Turn VR4 (AMC) counterclockwise fully. Gently spread L13 and L12 and tweak the spreads for best PEP when modulated. Tweak L20 and L17 for best PEP when modulated. After tuning is finished, set VR4 to half-way.

I suspect you did not do the complete circuit mod. Once you do the swing mod upgrade, the placement of the capacitor will make it extremely difficult, if not impossible, to correctly install the MOSFET upgrade as the new capacitor placed where the jumper was will clock some of the parts that need replacing.

If I were you I would'

1. Order a mosfet upgrader kit from Dr. Duck on Ebay. They come wil all parts and instructions with pictiures. You will get a neew MOPSFET. The one you have MAY be damaged due to not being installed correctly. The whol.e kit and instructions costs $10 postage paid. I am NOT affiliated with Duck in anyway. Just a seller I know is providing reliable product.

2, Un-install the modulator kit. This will make the MOSFET install easier.

3. Follow instructions for MOSFET upgrade.

4.Re-Install modulator parts. You may want to consider the variable power option. This uses the two wires from the RF Gain knob to control your deadkey and swing. In some radios the MOSFET mod may be too loud and cause distortion when coupled with modulation mod. The variable power will allow you to adjust this.
Simply trace and cut the two wires coming from the RF Gain knob to the radio board. Leave about 1 1/2 inches sticking up from the board. Solder these two wires together.... If you forget this step you will have no RCV audio. Take the two wires from the RF Gain (extend if necessary) and solder one to each of the points where you soldered the resistor across the bottom of the capacitor. You will now be able to vary your deakey and swing using the RF Gain knob.

5. Replace covers and enjoy more power and audio!

If you read that the MOSFET is a pin for pin replacement for the factory final this simply means you solder the transistor in the same way.It still requires the above mods to work correctly. Even if you do get a real 1969 final, additional changes would be necessary to get the higher wattage output, as memory serves.

 

It is really hard to say. I have worked as a tech but NEVER a design engineer. Some suspicions:

Incorrect voltage to final.
Signal not properly fed to final, i.e. you are keying the final but not feeding the signal to it. Therfore you are, in effect, throwing a dead carrier.
Signal not properly switched. You have to add a diode when putting in the MOSFET. If I remember it is a switching diode.

WORST CASE:

You blew the audio amp by improper install of the mosfet. This ain't as bad as it sounds. It's a readily available piece and cheap. Redo the MOSFET correctly and the "mod mod". Still not working/no audio? Replace the audio amp. BUT I highly doubt this is the problem. When the audio amp is blown 99.999 percent of the time you will have no audio on transmit and receive.


To boil it down; You cannot expect the MOSFET to function normally if not installed with the additional circuit mods. When engineers design these replacement parts, they calculate the necessary changes to existing circuits to make it function correctly. They know the internals of the transistor... That's beyond my scope of knowledge.

 

Loose screw should not have been an issue unless you talked on it a long time. Being you had no audio I guess this was no a problem. The screws IS NOT a ground!@ When properly installed iot should have a plastic insulator on the screw that will prevent it from grounding the transistor. If you lost this don't worry, the kit from Doctor Duck comes with a new one. Also comes with a new clear Mica insulator. You must use the Mica insulator at the back of the MOSFET and the plastic screw insulator to insure proper long term operation of the MOSFET.

 

There is a basic adage - if you have to play with the final, just get an external amp.

Why a mosfet?

 

Well Mr. Ridgeline I do agree to an extent... If you have to play with a final and don't have no solderin skills...

Why a MOSFET? Current technology... Inexpensive... East to get. They are coming in most of the low cost amps being sold today. There is the reality that lots of folks DON'T want to talk skip, or to a trucker 3 states away. The boost from a MOSFET will give them respectable output. Coupled with a super mod type upgrade... You'll get heard. Botgtom ine... All for about $12, if you can do it yourself and no additional "box" to mount and run power to.

Contrary to what some say you can get as much as 40 watts PEP outta a Mosfet. However I prefer not to push and happily settle for 20-30 watts. Will run for a long time (much cooler) that way. I suppose it depends on local traffic where you are at but around here 25 watts and a swing mod will get you heard on 19.