CONNEX TX 200 AMP

[doctor]

Anyone know anything about this amp??..age...filtering or???
DOCTOR/795

 

[Rooster427]

Anyone have a schematic for this amp? Thanks

 

[Decoder]

duel 455 cb amp doesabout 180 out SSB delay

 

[Rooster427]

I still can't find a schematic for my Connex TX-200 after 3+ years. Maybe someone here can fix this thing, it has an metal oxide input resistor by the first yellow relay in the input circuit that was burned accidently by over-driving it. I also have a NEW OEM set of Toshiba red dot 2879's which can be used if the mrf455's are bad, but I changed one, and both were good last time I tested them on a transistor tester. I read somewhere 2290 or 2879 are direct replacements for the upgrade, so I am ready to have this amp serviced, and possibly keep the 455's since they still read good. Thanks in advance, I hope I get a reply without waiting another 3+ years to get this thing fixed or I will have to sell it for parts.

 

[nomadradio]

The 2SC2879 is not a good direct substitute for MRF455.

The impedance feeding into and out of each transistor type are pretty different. Getting good performance will require changing capacitors on both input and output circuits, maybe also the winding ratio of the input and output transformers. If the amplifier was built without negative feedback, this should be added no matter what anyway.

Unless you have a 'cookbook' setup for that circuit board that's already optimized for a different transistor, working it out is a cut-and-try proposition.

And if you're not careful, it can be a "cut-and-poof" proposition.

73

 

[9C1Driver]

Agreed, they are anything BUT a direct replacement.

 

[Rooster427]

I wonder what should be the value of the burned resistor next to the yellow relay in the red circle. I cant read these chinese colors, but I do think the other one in the circle is good, and might or should be the same value, it's a 2 pill with mrf455. I never fix these things before but after pulling out the resistor by the relay, all I can say it was a PITA because these's solder on both sides of the board. Out of desperation, I tried to use a metal oxide @ 82 ohms which I found in my parts boxes. The 82 ohm got very hot in about 5 or 10 seconds. After that, I called it quits, and put this whole mess in my junk box for about 3 years. I'd like to get this thing fixed.

 

[nomadradio]

Just goes to show the practical value of a schematic diagram. Just one problem. Contraband products built out of sight from the authorities just don't always come with a diagram. And even if it did, a schematic would seldom get passed along when the amplifier changes hands down the line.

The price of hiring someone to trace down the circuit and draw up a schematic would be enough to buy a few cases of these when they were new.

One back door is to find an identical unit that hasn't been damaged by excessive drive power yet, and copy the markings from undamaged parts.

But those are the three options I see, an original diagram, or have one made, or copy directly from another unit of the same type.

Welcome to the information age. Sometimes the information about a product is worth more than the product itself.

73

 

[Rooster427]

Just goes to show the practical value of a schematic diagram. Just one problem. Contraband products built out of sight from the authorities just don't always come with a diagram. And even if it did, a schematic would seldom get passed along when the amplifier changes hands down the line.

The price of hiring someone to trace down the circuit and draw up a schematic would be enough to buy a few cases of these when they were new.

One back door is to find an identical unit that hasn't been damaged by excessive drive power yet, and copy the markings from undamaged parts.

But those are the three options I see, an original diagram, or have one made, or copy directly from another unit of the same type.

Welcome to the information age. Sometimes the information about a product is worth more than the product itself.

73

----------------------

I did manage to get a man in Canada to send me this photo a few years ago, but I still cannot make out the color bands on these metal oxide, I am more into the OLD carbon ones, all this looks like japanese paint, maybe you can make out the colors on a working unit from the enclosed pic. It's the one closest to the big maroon capacitor. I also included another persons sample, but the metal oxide is of white color, hopefully making the colors more readable. To me it looks Brn/Grn/Blk (15 ohms is my guess) OR Brn/Grn/Brn 150 ohms. Earlier, I stated I put a 82 ohm metal oxide in there but it got really hot in 10 seconds. And I was sure both mrf455's were good because I changed one of them which tested bad on a tester. Thanks for any help you can give me while I try to figure this out without damaging the new pill

 

[Chevboy0167]

Your pics of your amp show the resistor to be a 39 ohm, 2 or 3 watt metal oxide flame resistant with a 1% tolerance. The other pics from other amps show to be different values than yours.

Have you put an ohm meter on said resistor to see if you get any reading?

 

[Rooster427]

This is the resistor that reads as open (in the circle nearest relay. I don't think it's a 39 ohm, it would have to be grn/wht/blk. Look at reply #7, I have it circled. I don't see anything thats 29 ohms in there. Pics of the other amps show me what might be either 150 or 15 ohms.

 

[Rooster427]

This is from someone elses amp....it looks like 150, not sure. Hope it zooms good enough.

 

[Chevboy0167]

Most of the pics of the other amps get too grainy when zoomed in. I say 39 cuz you said you tried an 82 and that "got hot fast". More resistance to current flow = more heat.

 

[Shadetree Mechanic]

Less resistance means more current. More current means more heat. Generally speaking.

 

[Chevboy0167]

LOL... tree ya got me! That was what I was thinking but my fingers typed it in reverse! Communication from brain to fingers breakdown.... sheesh!!! Hahahaha