Cobra 2000 GTL Hi-Fi questions:

[Hawkeye351]

Got a Cobra 2000 GTL on the desk today.

Issues:
Weak receive in all modes.
Modulation/SWR meter won't fall back to 0. Stops at 30% scale.
Owner wants Hi-Fi installed.

Evaluation:
Receive alignment is out of whack. No real issue.
"Zero" adjustment on back of modulation/SWR meter needs freeing and reset.
SSB output too high, 29w PEP. Checked for missing AMC, yep it's yanked out.
Clarifier strapped (properly for once).
Expo kit installed.
Variable key (done the right way for once).
No other mods that I can see.

The above issues are of no real concern for me to handle.

The Hi-Fi I do have questions on.
For the mylar type caps, which I don't have any, can I use polyester film (dark green shiny)? I have an entire assortment of those up to 4700nf, which I think is 4.7uf correct? I also have an assortment of BP (bi-polar) electrolytic caps which I was told I could use as non polarized caps, is this true? I ordered an assortment of non polarized, but they sent me an assortment that has short leads and long leads with no stripes and has the BP markings, the container states Bi-Polar.

Are the Hi-Fi cap changes still the same as was posted on WWPDX by gold finger or has it been revised?

Any recommendations on this Hi-Fi project that would be an improvement to the current info available?

This is my first attempt at doing Hi-Fi, I want to make sure it's right for him.

Note:
I have no EQ, keyboard or any way to test the capability of the finished product, just a stock Cobra coffin style handmic. What should I listen for on xmit to be sure the xmit caps are working properly?

 

[Shadetree Mechanic]

Has he had a "Hi Fi" converted radio before so he knows what to expect? If you do the Gold Finger mods, then he will definitely need to run an equalizer. If he is running a hand mic, then there will be way too much bass. For a hand mic you would not want to change the cap values as much as Gold Finger.

 

[Cable Guy]

The Hi-Fi I do have questions on.
For the mylar type caps, can I use polyester film (dark green shiny)?

I'm no expert,I have, but some would say that it can effect tonal qualities.

I also have an assortment of BP (bi-polar) electrolytic caps which I was told I could use as non polarized caps, is this true?

Afaik, yes, I have done this with this mod.

Are the Hi-Fi cap changes still the same as was posted on WWPDX by gold finger or has it been revised?

That is the basis I use, altering values as I go to taste, but you can very easily muddy it up using the max listed value. I start middle of the road.

Any recommendations on this Hi-Fi project that would be an improvement to the current info available?

Check your schematic as you go.

 

[KneeBiter]

Mylar is a brand. Not a type. Those green caps are fine.

 

[dave457]

If he doesn't do ssb, I'd say direct injection is a better way for hifi am

 

[Hawkeye351]

Waiting on response from him on concerns above.

 

[Hawkeye351]

He got back in contact with me. This is what he told me.

Shure Beta condenser microphone,
Mixer with built in EQ.

Said he's ran this same setup on his 148 for some time before the 148 PLL shot it's wad. He said he knows he'll have to tinker with settings when running through mic socket.

As for direct inject,
He said he's never ran direct inject before but wanted the option available on the 2000 so he could learn how to use direct inject, he said as long as I put it on a switch so he can switch back and forth from DI to mic socket.

Primarily AM, maybe some sideband every once in a while.

 

[dave457]

If you do direct injection on that radio you'll lose ssb tx.... but you can still rx.
But maybe you can do a switch of some kind... dunno

 

[Shadetree Mechanic]

Direct injection needs an audio limiter of some sort. People normally do this with a rack gear processor using the compressor function. Turn the compressor up high to act as a brick wall limiter. Even then it's tempting to keep edging up the level because the audio doesn't give you a clue when to stop. We get used to listening for the raspy or crunchy sound when running through the mic jack, direct injection sounds good all the way up to the point of failure. At least that's the way my Madison is, I had a big learning curve....

NU9N AM Transmitter Hi Fi Audio Modulation Processing

 

[dave457]

Direct injection and Properly setup audio
Sounds just as good as any high-end sdr

 

[nomadradio]

before the 148 PLL shot it's wad

And this 2000 hasn't yet?

Don't tell me, let me guess. Date codes on the chips inside the counter shield all start with 78 or 79, or a lower year.

If it still has the stock MB8734 PLL chip, it usually has an "open" four-digit date printed on it. First two digits for the year, last two for the week 01-52.

Even the newest Cobra 2000 is hovering on that "awkward" age when electrolytic caps start going south like geese in the autumn.

As for weak receive all modes and a S-meter that won't zero, this sounds like trouble with FET2 in the AGC circuit. This would affect both AM and sideband. AGC voltage drives the S-meter.

We have seen FET2 fail to deliver sufficient base current to TR10. TR10 is the source of AGC voltage to the rest of the receiver. FET2 has a sort of "backwards" effect on AGC voltage. Max current through FET2 and into the base of TR10 takes place with no signal. As the signal level rises, diodes rectify the signal to put a negative DC voltage on the gate of FET2. This reduces FET2's drain current, reduces TR10's base current, and the AGC voltage from TR10's emitter falls, reducing receiver gain and deflecting the S-meter.

The FET we use to replace FET2 when it goes bad is an American JEDEC-numbered part, like 2N5951, maybe. It's written down at work, so I'll see if my memory is skewed on that detail later. If FET2 fails to deliver enough base current to TR10 with no signal, the radio will act as if the RF gain knob has been turned down.

I don't have a reference AGC voltage that indicates a go or no-go for the AGC circuit. But a 1k resistor with the leads touching across Source and Drain solder pads on the pc board will jazz up the receiver's sensitivity if FET2 is involved with the fault.

Just the same, seeing reduced sensitivity on all modes and the S-meter won't zero add up to AGC trouble of some sort.

73

 

[Hawkeye351]

I spoke to him about all that was pointed out from you guys/gals. He wanted me to thank you to all that is helping out with info on this project.

Nomad, no sir, not yet on this one, lol. His previous Hi-Fi 148 setup was worse than just the PLL. The VCO block leads were eaten up by that Sony bond, 10.240 crystal had that Sony bond splattered all over it and around it. Basically, the entire VCO/PLL section was eaten up, it looked as though they spilled that glue in that area. Surprised me after opening the cover and seeing all that mess, but yet the thang still worked until the PLL shot it's wad.

As for the s-meter, this meter works fine, drops back to zero like it should, shows proper signals, although it is a little floppy (happy, wants to bounce too much). It's the SWR/Modulation meter that won't drop back to zero, it stops at 30% when dropping back.

So, you're saying I could strap a 1k ohm resistor across the drain and source pins of FET2 and improve receive sensitivity? Interesting, thank you.

He said he wants the direct inject on a switch so he can switch between mic socket input or direct inject input. He stated he's mainly going to run through the mic socket, but wants the option to tinker with direct inject when he wants to learn how to use direct inject properly.

Any recommendations for his mic of choice going through mic socket? His mic of choice is the Shure Beta condenser mic. He never told me the brand name of the mixer/EQ he's using though.

 

[Shadetree Mechanic]

One of the things about the electret condenser mic is that it's very sensitive. It will pick up the dog crunching on its food across the room. Without a piece of rack gear that has a noise gate, it will be even more problematic. I would run a dynamic mic instead.

 

[Shadetree Mechanic]

Also, one of the best ways to listen to your transmit audio is with another radio that has no antenna and headphones.

 

[dave457]

Your gonna need an adapter to run xlr to 5 or 4 pin connector