Cobra 2000 mod

[BAGEBOY]

I just got a cobra 2000 I see a mod I have never seen before.can someone tell me what is the mod and also the capacitor value. See picture attach

 

[Handy Andy]

Lookis like it';s on the AM Regulator the TR42 / 41 - 1419 - the cap being it's size, looks around 4.7uF to as much as 10uF but not much more.

Schematically it SEEMS to be a power supply or tone filter - the Leg in question is the Collector - capped across to ground.

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As these chassis age, any work done to them - when you reassemble and put them back together, you can get a hum noise - this may be an attempt to lessen the noise.

The effort in cleaning up these bonding points where you pull out the board or remove panels - putting them back together dirty - you get a poor bond, causing a mini-antenna noise loop for form and these radios pick the effect up big time.

I see another cap off to the side - again a similar cap - looks like that had a noise or hum in the power supply.

Now, if it's connected to the BASE

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This may be an effort to provide swing - but until you tell us if the Base or Collector - we're only guessing...

 

[DoctorAudio]

@Handy Andy
That pad isn't ground, it's the junction of R228, R194 and D63

 

[BAGEBOY]

It is connected to the Base of the transistors

 

[Handy Andy]

Slow down folks, there's a two-edged sword here...

A blurry photo and to me, bad soldering - so that mod - may be the reason the radio is showing up on @BAGEBOY desk...

I don't know if the transistor is even a 1419 or something else, the real pin out should be - when invented like that, ECB not BCE - I did this on purpose to get more details before I start making too many wrong guesses...

@BAGEBOY - The photo then, that part is on the LEFT-most terminal - then the part is the Emitter - that is the OUTPUT of the AM Regulator - before it heads to the front panel MODE switch.

To put a cap on it, tells me there may have been a squeal or noise they "smoothed" out - which to me - to place it there, makes no sense. It doesn't need to be there.

There is a part further downstream - back by the TP where the MODE switch's' output heads to - is a cap that is about 1uF to 4.7uF C191 - it's a 50V part

I'd remove it the cap on on the transistor - then review the results.

 

[BAGEBOY]

Thanks you Andy..I have 2 cobra 2000 and never saw that mod also I will show u the mosfet mod that was done u can share your thoughts

 

[Handy Andy]

How is the Gate's voltages on the MOSFET's holding up?

 

[BAGEBOY]

So far it seems ok I didn't check it in terms of voltage but I m see 13 avg into the dummy load on bird 43. Wha are your thoughts om this particular mosdet.mod

 

[Handy Andy]

Running fixed values - not knowing how it handles - SSB drive without some form of a variable to control or fine trim the Gate - the main issue here is did you set by voltage then run by watts (AM Power setting) or did you go the mA route?

I guess once you put the thing in, it does work, but your results may favor one mode over another.

Usually in "fixed" modes, unless the Biasing is just right, you're stuck mostly in Class C - for too much Biasing the SSB side will "latch" and too little - it' cuts off. In this scenario, in your effort, I presume you're running, injecting - voltage off the 8V TX - which is regulated and is not exactly at 8 volts - it can be "dropped" down thru several means - including using the current limiting resistors like the 150 ohm and 220ohm since they're present as a means from the Bipolars Base trimming, to start the dropping process then let the divider do the rest.

At least if you haven't tried, I just gave you several directions...in which to try...

 

[BAGEBOY]

I got the radio from a local who came back with the radio but he had some blown bulbs so he asked me to check it out.never done a mosfet mod on a cobra 2000 before so I was just asking for knowledge. BTW mosfet is RT1 bias volts 3.85

 

[Handy Andy]

IF it isn't making the AM Regulator too hot when in AM - that's' your call. It's a risky venue...

But if it's working....

Don't Fix it...

 

[BAGEBOY]

What is the ideal bias voltage on the mosfet

 

[BAGEBOY]

Andy have you ever triedthr mosfet mod by 813 of the mb8719 chip radio cobra 148 washington etc

 

[Handy Andy]

IF you're doing the one I think you're doing - with the trimmer, then yes...

Just don't run the trimmer OPEN -
Run a 2.2K onto a 3.3K Resistor to GROUND off the Trimmers
open leg - look above - you might want to just
use the resistor divider to the FERRITE BEAD jumper
not direct to Gate - that's just me...IMHO
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I'm surprised you haven't done much research - do a search here...much of what's going on here has been discussed to length in other posts.

So by any means you have problems ... these links might help...

https://www.worldwidedx.com/threads...g-mosfet-conversion.220760/page-3#post-634713

https://www.worldwidedx.com/threads...58-and-mb8719-boards.95853/page-9#post-689859
https://www.worldwidedx.com/threads/u1-diodes-in-mosfet-linears.238363/#post-656522

Don't take my word for it, check out these posts too...
https://www.worldwidedx.com/threads...companion-part-info.238796/page-2#post-658477

 

[Handy Andy]

What is the ideal bias voltage on the mosfet

The Ideal voltage?

That's the problem.

Although great strides have been made in fabrication technology - you still have the inherit problem of "migration" a form of clumping caused by the "seed" and the impurity of the doping element to make the pure Si into a Semiconductor.

Schottky designs used just 1 doped region bonded to a pellet - but it still is a "mass migration" problem when you try to control the weld - too many electrons drag their host donor with them making the N or P layer - a layer of a given thickness - but to measure it exactly you have to take it apart and examine and test the structure to get actual values.

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Why does this matter? Read thru the posts in many of the link threads and search for others using MOSFET - the bane of the MOSFET is everyone thinks they know the secret formula - I don't - and in some ways, I'm thankful - I am forced to use the older Bipolar method because I don't have the scanning microscope to view the thickness and work accordingly - I know a much simpler way...

I just measure the mA draw and work to 50mA - for pretty much any mode.

It may seem archaic, but if you want to save your radio...?

IF I go too high in the trim voltage, another event takes place "latching" there's been several posts about that too.

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Here's one...can take out your power supply in the process..

https://www.worldwidedx.com/threads/galaxy-dx-2547-died.246426/

The problem has been addressed - all the way from the Datasheets and their data all the way down to the tenth's of a volt differences that can keep a radio alive to see another day.