Cobra 2000 GTL Volting?

[Tweek]

I'm finishing a bunch of stuff to Cobra 2000 GTL. Sweet radio, this one had a ton of issues, but they all seem to be good now (except the freq counter, that's a work in progress). I'm running the OE 2sc1969 final, have done the NPC mod, and full alignment from the syn out.

Do any of you run the final on the 12v rail, or do you keep it on the factory dc rail? I'm a little leery of volting the PA in AM, being how had it is to find good "real" 1969's. Any thoughts?

Thanks!

 

[sp5it]

NPC without 12V on final does not make sense. It is part of the mod.

 

[kopcicle]

[opinion]
Final and driver bias* is adjusted for approximately 13.8v supply and 12-16 watts pep SSB.
In AM the bias remains the same but collector supply comes from the MB3756 regulator at approximately 8v.

*30ma driver , 60ma final.

This never made any sense to me.

I usually disable the 8v trace to the driver and final and use that line for a regulated source to the top of the clarifier.

Then apply 13.8v to the collectors in AM. I just use a short jumper from

I change the resistor between the AM power potentiometer to something between 500 and 1k ohms.

SSB is tuned with ALC wide open, then adjusted for noticeable ALC action around 12 watts.

AM is tuned with AMC wide open, 3.5 to 4 watts carrier, then adjusted for 12 to 14 watts peak.

Now check it with an oscilloscope.

A two tone signal is injected into the audio line and adjustments are made to SSB if indicated.

White or "pink" noise is injected into the audio line and adjustments are made to AM if indicated.

Wash, rinse, repeat.

The usual AM result is between 3.5 and 3.8 watt AM, near 100% modulation.
The usual SSB result is 11 to 13 watts peak, two tone test.
[/opinion]

I don't often share this as you would be surprised how much "discussion" it creates.

Take it or leave it.

 

[Tweek]

[opinion]
Final and driver bias* is adjusted for approximately 13.8v supply and 12-16 watts pep SSB.
In AM the bias remains the same but collector supply comes from the MB3756 regulator at approximately 8v.

*30ma driver , 60ma final.
<snip>
Take it or leave it.

I did all the clarifier stuff and pretty happy with that. Was just concerned with switching the collector supply to 12v, and wondered what the experience was with longevity. I have the bias at 30 & 60, set the key at about 2.5w and it's working pretty well. Saw some info online somewhere where folks were running the bias at 50/100, with the final at 12v. I don't have any spare 1969's laying around, that's why I asked.

I noticed power supply sag now when SSB peaks hit about 10w, I think I saw something in a thread here about the regulator, need to search that out.

Thanks for all the info!

 

[nomadradio]

I don't have a picture of it, but there is a 10uf 50-Volt (really) electrolytic on the tiny voltage-regulator pc board.

It's wired across the output wires near the center of the board. It will fail open-circuit and cause puzzling problems with sagging voltage under load.

Got in the habit of replacing this part in every Cobra 2000GTL that we see for service, regardless of age or condition.

If it doesn't fix the 'sag' problem you didn't waste a lot to find out.

Then again, I have seen forum conversations about this fault, recommending a new rectifier bridge, large pass transistor, zener, etc.

The sad ones report having changed all that stuff with no improvement.

Never a bad strategy to try the cheapest solution first.

73

 

[loosecannon]

volting the final in these radios is 'fine', and won't degrade the final any more than operating the radio in SSB mode all the time.

there is a disadvantage though, and that is that you have taken away one modulating stage in your radio.

yes, you can achieve bigger peak numbers because of the increased voltage to the collector, but the cost for that is that you are now only modulating the driver stage.

there is a reduction in audio punch when doing this.
after all, why would CB radio manufacturers ALL put in a pre-driver, and then two modulated stages after that?
it is because of the strong AM audio that gets created when you do this, and being "loud" has always been a selling point for a CB manufacturer.

if they thought they could get just as loud of a radio out of one modulated stage, you can be sure they would eliminate the extra parts.

to really volt the final in these radios and still have both stages modulated, you need to add a transformer from a 29LTD radio in to the mix.

shockwave posted the mod years and years ago, and i tried it with pretty cool results.
i think i was swinging up to 25+ watts PEP with no problem.
LC

 

[kopcicle]

ummmm, no.

trace 145 feeds both.
It's easy to confuse DC with RF with Audio...

 

[loosecannon]

Kop,
not sure what you think we disagree on here.

Im talking about AM mode, and when you volt the final, you pull the collector feed for the final off of trace 145 and put it to VCC.
it does not get modulated when connected to VCC.
LC

 

[Shockwave]

Don't waste your time trying this on a 2000GTL. You've already seen the power supply sags. This is not an easy fix because the power transformer itself doesn't have the headroom. Its voltage to the regulator is low enough that any extra load will cause it to fall out of regulation on word peaks. Once that happens, you have defeated the entire purpose of the mod.

Secondly, don't ever remove modulated DC from an RF stage and replace it with pure DC. This cripples the audio capabilities of the rig. Don't believe this? Put the mod on a switch and listen to the before and after effects. The right way to do this is to copy the modulated DC stage they use in the 29LTD. Notice the transformer in that radio does not use two windings in transmit.

It is not used as a transformer in the TX mode. It is used as a choke to provide a high impedance path for the audio so that it can be mixed with DC without loading the audio stage down. The transformer can be replaced with an alternator whine filter choke. On a 2000GTL, this will require using an outside power supply or replacing the internal parts to provide the needed current. You should also remove the 8 volt bias line in AM so that the final can be fully modulated in class C. That will make the final run a lot cooler too.

 

[Handy Andy]

I tend to agree with Kop on this...

AM is the only loading TR41 ever sees in AM, so SSB modes should already get full voltage - but to bypass? Questionable...you already have some bypass in there - Tr41 is an underrated part for the whole strip and they knew it.

Look...


I attached part of the Audio Chip section that drives the AM side - but is COMPLETELY bypassed in SSB modes...


It's why you have that 220 Ohm resistor in there, keep it in there - or tailor that resistor value to a higher ohmic value and change C173 to a LARGER value else go with Exit 13's 130% or more mod...for AM...

Don't risk damaging the 2166 either - that Driver is all you've got for Modulation when you jumper the Finals...

 

[Tweek]

Thanks for all the info! I'm working to replace the DC supply with a nice switcher I have now, working to correct the power supply sagging issue. I'll be running it behind a little Kris box I've converted to 6P45's, so probably not that important to squeeze every drop out of the PA.

 

[Handy Andy]

There's a secondary issue too...

I noticed power supply sag now when SSB peaks hit about 10w, I think I saw something in a thread here about the regulator, need to search that out.

Thanks for all the info!

That may not necessarily be all "power supply" issues, because if you're getting good swing out of that 2.5W setting for AM - you'll have enough power for much higher PEP than the 10W you're getting out of SSB modes now...

• unless of course your lights are dimming and the Frequency Counter bounces all over the place when you transmit. - Then you have NOT ONLY power supply issues but overmodulation - causing the counter to jump as it tries to find something to "count". Again a tune up issue.

Did you, or do you need to, have more wattage in SSB modes? I mean if some audio mods have been done to the 2000's Mic amp - they may "slam" the AN612 with TOO MUCH audio causing more of a saturation or filtering issue that can make the SSB audio more "Bassy" and hard to tune into - so if you're a SideBander - then you may want to re-tweak the values of the mods or remove them and start over - to the Mic Audio amp strip in there.

 

[NZ8N]

In AM the bias remains the same but collector supply comes from the MB3756 regulator at approximately 8v.

Not correct, in unmodified AM mode, the voltage to the collectors of both the Final and Driver are set by the AM Regulator circuit, TR41 and TR42, not by the 8v regulator MB3756.
When in LSB or USB, the collectors are "strapped" to the 13.8v rail through the mode switch.
When bypassing the mode switch for the Final when volting or "strapping" it, this also takes some of the strain off of the AM regulator circuit, and the mode switch.

What LC said in the above post is correct.

Has anyone thought about modulating the predriver (TX Buffer) in AM Mode so you have 2 circuits being modulated as before, after "strapping" the final?

 

[Handy Andy]

Has anyone thought about modulating the predriver (TX Buffer) in AM Mode so you have 2 circuits being modulated as before, after "strapping" the final?

I caught h*ll every time I mentioned it...

 

[Shockwave]

Modulating the pre-driver is a great idea and here is how to do that very easily. Notice the 2SC1973 pre-driver is getting 8 volts VCC applied through L44 on circuit trace 12. Simply break the DC feed to L44 and replace it with the same modulated voltage that was feeding the final and still is feeding the driver. One break and one jump to complete the job.

Now, keep in mind that the AM carrier control could be set to apply more than 8 volts to the pre-driver. Doing so will increase its gain and severely overdrive that final you just put on 12 volts. The 2SC2166 driver will probably handle it but the final will not. Just don't turn the carrier up to more than the final can handle and this should work well.

PS: There is one problem with this simple mod that I just realized. Going to SSB will place 12 volts on the pre-driver. That would be bad and will need a relay to revert back to the stock 8 volt line for SSB use.