Uniden Madison 5 pin 8719 3rd gen No TX

[Unit 75]

Picked up this unit at a garage sale for 7 dollars. It has no tx and would like to get it going because it is in excellent physical condition and has a very low serial number #60. When you key the mic, nothing happens. The green rx light stays illuminated and does not toggle to red or go out. I have checked the voltage regulator and all voltages are correct except the pin that goes to 8V during tx mode because of something further down the line. I don't have a schematic but Im expecting the transmit inhibit transistor may be gone to allow the radio to go into tx mode. Not sure what that TR- number is. Pin 5 of the PLL is 8.25v so it is in locked condition. Any insight, suggestions welcome. PA not working, and no tx in AM or SSB modes. Thanks.

 

[loosecannon]

check TR35. that's the rx/tx switcher.
damn what a deal!!!!
LC

 

[nomadradio]

Pin 5 of the PLL will have pulses that go to zero Volts when the PLL is 'almost' but not quite locked. Makes checking it with only a meter less than reliable. The pulses are enough to shut down transmit switching.

The transmit pin 8 is not going hot when you key the mike.

But consider that the receive pin 6 is not going to zero when you key the mike. The MB3756 chip appears to be doing what it's told when you key the mike. It's not getting the command input at pin 5.

The voltage at pin 5 is probably not dropping when you key the mike. Next thing upstream from there is TR35. Voltages at the base and emitter leads are important. Those are the two outside pins.

73

 

[Unit 75]

All voltages are correct on TR-35. Thank you both for your input.

 

[Handy Andy]

Then look at your channel selector...one pin connects to the PLL to show "pin setting" for the programming uses a detent in the channel selector - if that is bad, you'd see green (RX) but it wouldn't reset the channel programming because it's "locked" to that channel originally set - that pin toggles the PLL to look for and Reset-to the channel you selected on the knob.

So it would lock into a channel - but will not let you TX or even change the channel even though you rotate the knob... try this - turn off the power, leave it off a moment or two - then turn it back on - if it's set on a new channel to the channel display (Knob setting) and you can TX then it's in the area of the channel selector.

Else you'd have to look at the MB3756 pins to see if LOCK works - heck even the PA switch can do this too - it looks at the CB PA switch to set TR35 to work or not - PAST TR35 (It's output) onto the MB3756...

 

[Unit 75]

Pin 5 of the PLL will have pulses that go to zero Volts when the PLL is 'almost' but not quite locked. Makes checking it with only a meter less than reliable. The pulses are enough to shut down transmit switching.

The transmit pin 8 is not going hot when you key the mike.

But consider that the receive pin 6 is not going to zero when you key the mike. The MB3756 chip appears to be doing what it's told when you key the mike. It's not getting the command input at pin 5.

The voltage at pin 5 is probably not dropping when you key the mike. Next thing upstream from there is TR35. Voltages at the base and emitter leads are important. Those are the two outside pins.

73

Hey, Nomad. I checked pin 5 of the PLL with my scope (50mV 500ns) and observed 10 pulses going across my screen (1.85MHz) confirming that its not oscillating between 8v and ground and that the PLL is indeed in locked condition. Pin 8 is at 4.5v Your correct that the voltage at pin 5 is not dropping when the mic is keyed. I went upstream from there like you suggested, to TR-35 transmit inhibit and have 7.8v on the emitter, 13,8v on collector and 8,0v on the base. These voltages do not change when the mic is keyed. I even replaced TR-35 with a new 2sc945 and same results. I checked D43 and have the 8v on one side and 1,5mV on the other. Signal not getting thru. I then checked D31 near the PLL and have 8,2v on both sides. I did notice that R169 was a bit overheated and replaced it with a new 22 ohm 3w. I looked at a schematic I acquired and see that when the mic is keyed, The emitter of the tx inhibit transistor is grounded, turning on the collector (goes low) causing pin 5 to switch IC4's output from pin 6 to the transmit only voltage at pin 8 and the base of TR35 being connected to the PLL lock detector (pin 6 of the PLL 8,2v) Pin 6 is not going low when mic is keyed, which also signifies PLL lock. I then looked at the PA/squelch control. I see that when in CB mode the switch needs to ground thru TR35 to Pin 5 off the mic connector. I am not getting any of this to happen as all voltages remain the same from PLL chip to TR35 when mic is keyed.

Andy... Tomorrow I am going to check your suggestions of the channel selector for pin setting and pin toggling for reset of channel programming. You can see my findings here that I have posted and hopefully it may add some new insight into this problem. I really appreciate your help and wisdom in the matter.
-Trainman

 

[loosecannon]

What is your VCO DC voltage at TP9?
on channel 1 and channel 40?
LC

 

[ExitThirteen]

Silly suggestion, check all of the wires at the mic jack and inspect the PCB mounted to the jack itself and confirm all parts and wires are soldered well and accounted for. Could possibly be something suspect at the mic jack, if you haven't checked everything there.

~Cheers~

 

[Unit 75]

Thanks, Exit. I did that when I was checking from pin 5 of the mic jack. Thought I had better check all pins, wires and traces.