I have a Connex 3300HP with IRF520s and a 2SB827 in it, radio is only putting out 1/2 watt carrier on high power and no modulation. After a few attempts at keying the radio and drifting the frequency to be on, I found the finals and AM chip to be extremely hot. I pulled the 3-IRF520s and the AM mod chip and all check good. What could be causing the low output carrier and extreme heat?
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Connex CX-3300HP Hot Am Mod and Finals
- Thread starter David Hunter
- Start date
if a "golden screwdriver" tech got in there and adjusted the
RF Amp bias pots incorrectly, it would cause this.
VR 10/11/20 seen that before......
RF Amp bias pots incorrectly, it would cause this.
VR 10/11/20 seen that before......
if them IRF520s are still out of the radio, then power up, and key up
the radio. measure the voltage at the place on the circuit board where
the gate is soldered in place. Make a note, write down these voltages.
let us know. then also after this, turn each bias adjust pot back and
forth and while watching the voltage go up and down from zero to
over 5 V. if this is OK, set each one to 2.5 volts for now. I found a
problem on the 2950/2970 where you cannot adjust this properly
on the driver IRF520.
the radio. measure the voltage at the place on the circuit board where
the gate is soldered in place. Make a note, write down these voltages.
let us know. then also after this, turn each bias adjust pot back and
forth and while watching the voltage go up and down from zero to
over 5 V. if this is OK, set each one to 2.5 volts for now. I found a
problem on the 2950/2970 where you cannot adjust this properly
on the driver IRF520.
ooooh! just took another look at some pix and the schematic again,
on CB Tricks. just noticed there is no "mirror board"
"shorting board" on the schematic or the picture. This would
make it a little more difficult for some, to set the bias on the
IRF520s. somebody messed it up. I bet that is your problem.....
the "golden screwdriver" treatment.
put that 2SB827 ( is that TR51?) back in. put in AM mode, key up, see if the
voltage at the drains can be adjusted, say 3 to 8 volts or so,
the AM power adjustments. if you have a scope, you can
modulate that signal. talk into the mic.
put them IRF520s back in. with the gate voltages adjusted to
2.5 V or less, so they do not draw current.
measure the total input current to the
radio to adjust the bias on them IRF520s. key-up in SSB.
no mic audio. turn VR10 then VR11, then VR20 for 30 to 50 mA
increase in current at each pot .... EZ PZ ! ! ! !
I just started working on these Ranger/Galaxy/Connex radios
several months ago. I like them.. neatO. Fun. the transmit
section is very similar in most models. and I used IRF510s
in my pirate transmitters years ago....
on CB Tricks. just noticed there is no "mirror board"
"shorting board" on the schematic or the picture. This would
make it a little more difficult for some, to set the bias on the
IRF520s. somebody messed it up. I bet that is your problem.....
the "golden screwdriver" treatment.
put that 2SB827 ( is that TR51?) back in. put in AM mode, key up, see if the
voltage at the drains can be adjusted, say 3 to 8 volts or so,
the AM power adjustments. if you have a scope, you can
modulate that signal. talk into the mic.
put them IRF520s back in. with the gate voltages adjusted to
2.5 V or less, so they do not draw current.
measure the total input current to the
radio to adjust the bias on them IRF520s. key-up in SSB.
no mic audio. turn VR10 then VR11, then VR20 for 30 to 50 mA
increase in current at each pot .... EZ PZ ! ! ! !
I just started working on these Ranger/Galaxy/Connex radios
several months ago. I like them.. neatO. Fun. the transmit
section is very similar in most models. and I used IRF510s
in my pirate transmitters years ago....
the zener diodes D112/D88/89 will keep the bias voltage from going over
5.6 V. And that would be way to high anyways. Usually it is around 3.7 V,
but this must be adjusted for current through each IRF520.
hopefully you are measuring in the correct spot. the gate lead spot
is to the far left when looking down at the front of the IRF520.
You have a problem if the gate DC voltages are 6.89, when keyed-up
TX, in SSB mode, mic turned off, so no RF. And you cannot adjust these
voltages. You should be able to go to zero volts at the gates by
adjusting VR10/11/20 according to the schematic.
I have not had one of these radios in front of me, so do not know
what is there on the board, any errors or mistakes.
But the radio worked when it was new.
there are errors/mistakes in some places on these diagrams/designs,
on these Galaxy/RCI/Connex radios. But an experienced tech can
get around this. this EPT360015C diagram does not show the
complete modulation section. On all these radios, the IRF520s
are drawn incorrectly at the source and drain, and also they
are enhancement mode, not depletion mode FETs. No biggie,
they can only be soldered in place in one way.
So we need someone that had one of these in front of them,
and did repairs. maybe something is odd.
what about that NOMAD, or "Spit" guy.
here we go! correct diagram, and pix. Too hot outside here in
southern Louisiana for me to go outside. I got nothing better
to do! but drink ice tea, cold beer, and work on guitar
amplifiers and radios.
The may seem simplistic, but with the RCI build quality issues lately, look for bad solder joints in the output tank circuit. I've seen a few of these come through that were basically brand new with tank circuit coil connection failures. Any breakdown in the tank could cause the problems you're seeing. I agree about the bias levels being important, as well, and you need to make sure the mount screws are tight. If not, the transistor heat sinks won't be making proper contact with the heat sink insulators and won't properly transfer heat to the radio chassis. I've seen AM regulators hit 250+ degrees F when viewed with a thermal imager, so make SURE these mounts are tight!
I'm stuck against a wall, have been for a year or so, regarding the bias adjustment for these, as the 3300hp doesn't have SSB, so am or fm only. I wish someone would detail the procedure for setting idle bias current in an am/fm only model of this board. I haven't found this yet, aside from setting gate voltage, so what am I missing?
Last edited:
So here's my method. A little convoluted, but the radio's "blow it up soon" design makes it necessary.
First find TR46, typically a 2SC1906. Unsolder the center pin. This is the collector, the output of this stage.
This simulates keying the mike with the gain down on a sideband radio and removes the carrier drive from the power stages.
Put a 10-Amp meter in line with the radio's power connection and key the mike. Turn each bias trimpot to the end where the current draw is lowest. This is the end of the pot where each MOSFET is drawing no current at all. The current reading you see is now the baseline, or the reference point. Now turn up the driver bias pot until the reading has increased by 50 mA above the reference reading. Turn up the final trimpots for another 50 mA each above what you saw with only the driver drawing current. A two-final radio should now show a current draw that's about 150 mA higher than it did with the bias pots all set to zero.
Resolder TR46 collector and you're good to go.
And if anyone has an easier method, I'm game.
73
First find TR46, typically a 2SC1906. Unsolder the center pin. This is the collector, the output of this stage.
This simulates keying the mike with the gain down on a sideband radio and removes the carrier drive from the power stages.
Put a 10-Amp meter in line with the radio's power connection and key the mike. Turn each bias trimpot to the end where the current draw is lowest. This is the end of the pot where each MOSFET is drawing no current at all. The current reading you see is now the baseline, or the reference point. Now turn up the driver bias pot until the reading has increased by 50 mA above the reference reading. Turn up the final trimpots for another 50 mA each above what you saw with only the driver drawing current. A two-final radio should now show a current draw that's about 150 mA higher than it did with the bias pots all set to zero.
Resolder TR46 collector and you're good to go.
And if anyone has an easier method, I'm game.
73
Thank you, Nomad, that's what I've been looking for. I figured forcing a "SSB-like" condition was going to be necessary, I just didn't know how. Never cared for setting MOSFET gates by voltage, there had to be a better way. Thanks again!