Cobra 2000 direct injection problem

[Draxx2755]

I acquired this 2000 about a year and a half ago. I set it up for direct injection at c174 using a 10uf tantalum cap with the positive side going into the plus side of c174. I also set up a switch so I could switch it back to stock and use a Shure 526T. The jack in the back is isolated from the chassis and the ground is going to one of the cans. When I acquired the unit and started looking around inside I realized that it has an NPC Mod and the final was volted. I "un-volted" the final and put it back to stock. No problem..... Works great. I can set the dk to 2 watts and it swings to about 12 watts on am. When I switch it over to use it direct injected, there is almost no signal. Had it hooked up to my Yaesu y301 inline scope and there is barely any alteration from the dead key. I'm driving it with a Behringer mixer with a built in preamp and a EV PL 80a studio mic. Ive double checked that the tantalum cap is going in the right direction and that the switch is working properly. Is it possible there just isn't enough to drive the audio. I've heard of other guys using Behringer mixers and not having a problem. I thought it might be a problem with TR41 or TR42, the am regulators but then I figured if that was the case it shouldn't have worked when I switch it back to stock and use the power mic. any ideas?

Also, I noticed when I key down my frequency changes a couple hundredths of a kc. I tried adjusting VR5 which is the frequency adjust, but there is no change whatsoever. I also noticed that R95 has been clipped which comes off of the vco circuit so I'm wondering if that's the problem there. Anyone know why R95 would be clipped ? Not sure it's part of the NPC Mod but I see no advantage to clipping it. Any and all ideas or theories welcomed. Thanks for reading my post.

[Grinder74]

Turn the cap around.

(edited by MDY, removed double post)

[Draxx2755]

Thanks...... I'll give it a try although it should be facing correctly now, but who knows....lol.

[Deleted User 14541]

Your cap is going the right direction. Positive leg in the positive hole and negative leg of the cap to your audio plug. I use a non polarized electrolytic these days.

I've driven one with only a behringer 802 mixer. It was barely enough. The preamp was maxed out and it seems like I had to turn a couple of balance knobs all the way to one channel (left or right) to get enough juice.

I don't know what mixer you have but you might compare specs to the 802 if you have a different one. Double check your switch and all wiring with an ohm meter.


[Double post edited by Slyguy]

[Draxx2755]

The audio amp ic in the 2000 generates about 4 to 5 watts before going into the balanced modulator. I found that if i use a 5 watt audio amp in the circuit prior to the injection point it is enough to drive the finals properly. The mixer is not enough to drive it alone. So far this has worked for me.

[lydianmode]

I hope this helps

[Deleted User 14541]

The audio amp ic in the 2000 generates about 4 to 5 watts before going into the balanced modulator. I found that if i use a 5 watt audio amp in the circuit prior to the injection point it is enough to drive the finals properly. The mixer is not enough to drive it alone. So far this has worked for me.

A lot of guys can't get enough drive out of the mixer alone. I guess I just had the right combination of junk. The Behringer 802, a shure sv100 and a Malaysian cobra 148 was all I had in the beginning. The mic was connected to CH1 of the mixer using an XLR connection. Preamp maxed out, pan all the way to the left, then set the level for 100% mod. I had no idea how much easier life could be with a peak limiter.

[TheCBDoctor]

Good afternoon Draxx2755

Have you tried the Cobra 2000 on SSB? On AM the carrier envelope is produced at the Driver and the Final with the carrier and audio mixed at these points. The collector voltage is anywhere from 4 volts to 6 volts. On SSB the audio and carrier are mixed at the Balance Modulator. The Collectors on the Driver and Final are about 12 volts on SSB.

I would start with that test to see if there are any issues. As for R-95 it is a 1.5K ohm resistor, which has 8 volts on one side of the resistor coming from source 2 of the power supply. The other side goes to Pin 1 of the PLL, which controls the output voltage going to the VCO tank circuit. It also is tied to R-96 (33k Ohm) at the same point; the other side of R-96 goes to pin 2 of the PLL.

This creates a voltage dividing network and stabilizes the output voltage on Pin 1 of the PLL, which should be about 3.4 volts on Channel 19. Cutting R-95 allows Pin 1 to exceed the circuit's minimum and maximum voltage. Your signal injector may not work with R-95 in place.

I would put it back. If the engineers didn't think the resistor was needed they would not have put it in the circuit. You will also see R-98 (33K) in parallel with R-96 except it has a capacitor in series and only feeds back any AC signal to Pin 2. DC voltage is blocked by this .1 (Point 1) capacitor C-79.

Try putting R-95 back. You can't make an omelet unless you crack a few eggs.

Respectfully

[Deleted User 14541]

If you do any testing in SSB mode you will have to connect a stock mic and use the radio normally. Injecting audio at c174 will only give you modulation in AM.

[Draxx2755]

Thanks Rick. I'll try that. I also repair that this mod is for am only. I do love experimenting though....lol.

-- Monday, 01 December 2014, 18:40 PM --

Realize, not repair. I hate auto correct.....lol.

[fat boy]

Where would i inject the audio in order to make it work on ssb

[Draxx2755]

That's a whole different ball game. This would not be easily accomplished. Your best bet is to read the threads concerning ESSB. That's what you're looking for.

[Deleted User 14541]

Use a low level modulated rig that uses a balanced modulator, there are a couple of cb type rigs like this. My direct injected kenwood works on AM and SSB. Depending on the radio you may have to do some crystal filter changes to pass more high frequencies. If you are a SSB guy you'll really appreciate the stability of an HF rig too.

[smoke blower]

Could someone tell me where to get a 5 watt audio amp or can I make one. The direct inject on a cobra 29 dose great but the ranger I have doesn't hardly have any out put. I am using a berhinger 804 eq

-- Wednesday, 15 February 2017, 18:40 PM --

Please tell me where I can get a 5 watt or more audio amplifier for my cobra 2000 for direct inject

[Draxx2755]

It's been a long time since I looked at my post. A lot has happened. To make a long story short, by experimenting and taking a close look at the schematic, I found that injecting the audio at C99, at the plus side, I was able to use the internal audio amp in the radio to drive the darlington pair just fine. The audio amp has very good frequency response too. Basically I used a dpdt switch to be able to switch in the DI or use the front panel mic. Also, by injecting at C99 the signal splits there and goes to the am transmit switch and the balanced modulator, so when injecting, the rig also works in ssb. For those who inject at C174, the mod is an am only mod. I had a problem finding a decent small audio amp to run it from there and that's why I started taking a look at the schematic to figure out where I could inject to use the internal audio amp and have it work in ssb mode. If you like, you can also increase the values of C174 and C175 to increase the bass response, but I would see how it sounds first on another radio before I changed any cap values.

This is just my 2 cents worth but it's worked beautifully for me and everyone says I sound like I'm sitting in the room with them. This was my ultimate goal, to sound as natural as possible. Hope this helps someone out there. Take care and ROCK ON dudes!