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Connex 3400HP, no tx after replacing finals

TruckerKevin

KD9NTD
Nov 9, 2017
234
142
53
58
Buckner, Illinois
www.qrz.com
I have a barely used 3400HP that started blowing the inline fuse whenever I keyed up on the mic.

I suspected the finals were blown. I removed them. They didn’t have any markings on them. I have a small bag of finals labeled c2078 in my parts bin. They looked identical to the ones I removed as far as shape goes. I didn’t really have anything to lose and I was just playing around, so I went ahead and soldered these into the radio. (And why are there what appears to be 3 sets)

Well the fuse blowing problem was solved. But now I have no transmit. When I key up on the mic, The needle on the meter goes up a little bit but stays put when I speak and it doesn’t modulate. I tried another known good microphone, same thing.

We have another radio in the house, on a different antenna in a different room. You can just vaguely hear some faint scratching from this radio to that radio when I talk and you can hear when I let up on the key like a small static pop. But that’s it.

Any pointers? This is just a hobby I’m learning
 

What finals were in the radio?im not sure that the 2078 finals are a direct replacement.the pre driver transistor might’ve also blown.Just need more info.Lots of knowledgeable folks here to help
 
Yeah those aren't the correct finals, that's an Irf-520 Mosfet chassis. Bias and companions parts in the circuit won't jive together. You should have a 520 pre driver driving 2 520 finals. Gate voltages no more than 4 volts. I would also look for cooked caps and resistors in the circuit along with the diodes under the Mosfets
 
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pointers?

yeah, don't just arbitrarily replace parts in a radio without knowing what came out of it.

just messin' with ya.

the best pointer i can give you is to start reading some books on the subject, and make sure you understand the electronic fundamentals involved in radio.

you don't need to become an engineer, but you should understand how each component works, what a time constant is, the difference between being at DC ground as opposed to RF ground, etc.

then you need to learn how to read a schematic, and be able to trace a signal or voltage flow through a radio.

if you knew how to do these things, then you could have gone over to cbtricks.com and looked at the schematic for that radio, and it would have given you the part numbers of the finals in that radio.

what's that you say?
you tried to go to cbtricks and there was no schematic for this radio?

well, that's where the experience kicks in.
those in the know will find the board number, and then find another radio with the same board in it that does have service info and schematics, like this one:
http://www.cbtricks.com/radios/galaxy/dx55hp/graphics/dx55hp_sch.pdf

this is the same board that is in your connex radio.

trying to learn how to fix radios without learning electronic fundamentals first is a sure path to becoming a certified golden screwdriver with a pile of radios relegated to dust collecting because they are considered 'unfixable'.

if you really want to learn how to do it, start with the fundamentals and apply them to radios, not the other way around.

the problem most encounter is that they find that learning electronics is like buying high quality coax.
its just not as fun as buying radios and amplifiers, but using cheap coax will cripple your station, just like only trying to learn about the fun things will cripple your repair abilities.
LC
 
Ha ha, I knew I was gonna get my ass kicked. But it’s worth it to get some experts opinions. I ordered the right transistors for it, when they get here in a few days I’ll try it again. If they didn’t work, I’ll be out $4.95 and still have three left over for my next butcher job
 
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Ha ha, I knew I was gonna get my ass kicked. But it’s worth it to get some experts opinions. I ordered the right transistors for it, when they get here in a few days I’ll try it again. If they didn’t work, I’ll be out $4.95 and still have three left over for my next butcher job

LOL just like in any respectable workplace, when people are messing with you, you know you're ok.
it's when they start ignoring you that you have f'ed up.

be aware of the static sensitivity of mosfets as opposed to bipolar transistors.
im not saying they are going to pop if you drop them on the floor necessarily, but if your carpet is staticy, and you grab the leads of a mosfet after walking across it, you could damage it.

before i started using anti-static mats and wrist straps, i used to 'ground' myself to my metal shelves before handling static sensitive components.
was it effective enough? i'll never know, but it seemed to work.

good luck on the repair.
LC
 
I meant here not at work. At work I’m worshipped like an old battleship, they couldn’t function happily without me.
 
mosfets are more static sensitive than their bipolar counterparts.

if your antenna socket took a static electricity hit, there is a chance it could have popped a mosfet.
also, overdriving them can literally pop the front of the transistor off LOL.

congratulations on getting the radio working.
LC
 
Good to hear it worked out.

The IRF520-type MOSFET transistors were not originally designed to be RF power transistors.

They are used in those radios only because they are cheap. They are not designed to withstand a high SWR on the antenna.

Doesn't take a terribly high SWR on the antenna to pop that kind of final transistor. A poorly-grounded coax cable inside a connector or a loose bolt on the antenna mount can damage a transmitter, even if the antenna checked okay on SWR. Anything that's loose and can break the antenna connection with vibration while driving can cause trouble.

73
 

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