Cobra 2000 AMC and ALC Issue/Question

[Brian G]

So I was restoring this 2000 back to stock in the audio by replacing TR24 so I can adjust the AM modulation.
After putting it back I still could not adjust the modulation and then I Noticed that R131 was also missing and and R124 and 126 were changed to 4.7k. So I replaced 131 with a new 10k but left 124 and 126 as is. But when I tested it, the audio was like “machine gun” sounding. All chopped up, kind of hard to describe. The only way I could stop it was to pull R131 out again.
any thoughts on why that would cause that issue when restoring those parts? Should I change 124 and 126 back to stock? Maybe that’s it?

also the SSB ALC pot doesn’t seem to do anything either. Is that disabled the same way the AMC is by removing TR24 etc or if not why else would that not seem to work?
thanks.

 

[Handy Andy]

ALC and AMC are two separate SENSORS...which go into an AMP that then works the LIMITER.

There's a lot here
- but remember to take in the big picture first,
Then break it down
​

So you have 3 pieces

• one the AMC - samples the AM Regulator pipes it to TR26 which R131 works with onto TR25...
• Two, the ALC - which TR34 samples, looks at, the RF output and sends it to the TR25 - directly
• Thirdly - the MOD LIMITER Amp - takes in the signal that exists - on it's base, from either or, and acts on the LIMITER TR24

The process is simple enough, but in trying to visualize it, takes a bit of a stretch that most could care less in understanding - they just want it to work.

The AMC side, works with the AM regulator, which sees the amount of power on the output of the AM Regulator TR42 and TR41 form - along with it's CARRIER (or BIAS power a DC voltage) power setting.

That's where R131 VR12 and D54 work...In AM mode, you set limiter by injecting a level of modulation into the Mic socket, AFTER you set the AM Carrier to proper wattage. You then adjust the power flowing thru into the MOD Limiter AMP TR25 by adjusting VR12 to allow a percentage of this audio power to pass thru TR24.

You adjust this percentage using VR12.

It also has D54 - a diode set up so that power flows out of the Base on the TR26 AMC Amp, thru Vr12 and D54 into board ground.

D54 lets power flow across it into ground but it can not return to the Base of TR26 that way.

Instead, it forces TR26 to obtain a way to recover this power thru R131 or when the Emitter voltage rises back to refresh or turn off using an equilibrium set by VR12's power flowing out of, and power flowing into - TR26's Base using R131.

So the key here is R131, D54 VR12 and the Emitter.

Where does the Emitter get it's energy to work?

Thru the Divider "tap" from TR41 / TR42 -

​

But ... There's always a condition...

The level that VR12 is adjusted for, lets only so much change (your modulation) to happen before the circuit engages and starts to work the Modulation AMP and it works the Limiter.

It's set up to ignore DC values - but work heavily on voltage changes to the DC level - so you'll always have carrier power, just the amount of power that is available on it depends on the level of carrier DC bias that VR10 sets.

This is why Audio presence tends to be louder when you turn down the Carrier power to lower than 4 watts - but going higher, the mid-point voltage then puts the audio signal non-symmetrical and tends to lower its presence in volume and sounds poor - muffled pinched up. You only have 12 volts to work with, so audio symmetry is obtained (your 4 watts) thru VR10 at about 5.6 volts - less voltage provides more upward room and cushion but not symmetry.

 

[Brian G]

Good info and great explanation of how it works but not sure it helps me fix my problem. R131 and TR24 are new and spec so its not that. Not sure why the diode would be bad but I suppose that could be it. And given that it doesn't appear that the AMC or ALC do anything and you said that both feed into the limiter then I would guess the problem is in the limiter circuit and not at the AMC or ALC itself.
As I mentioned in the original post, R 124 and 126 have 4.7k values instead of spec 10k so I was wondering if that could be the issue when I try to restore it back to stock by putting TR24 and R131 back to stock?

 

[Handy Andy]

Oh, Check R166 and R165 - they're buried in the board...They work AMC - if the feed is right, well, you get the blasting peak effect.

​

R166 and R165 are close to the Power Choke.

As I mentioned in the original post, R 124 and 126 have 4.7k values instead of spec 10k so I was wondering if that could be the issue when I try to restore it back to stock by putting TR24 and R131 back to stock?

They put 4.7K to add more audio, R124 is the input from the Mic socket, Thru Dynamike - into the Mic amp R126 also, but it is at the Limiter - you'll need some resistance to offset the Limiter action - TR24 is working more from TR25 the Mod Amp - driven by either TR26 or TR34 - the limtier is limiting whatever is present after R124 and just before TR23 - C104 a blocking cap to keep Bias from TR23 from leaking into TR24s ability to limit the audio signal.