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0ld style Uniden Grant. Keys when powered up?

this is very strange, and i am now more confused than ever.

there are a few possibilities for why you would have continuity from PC board ground to chassis ground.

the most common places are going to be the places where there is a metal part that bolts to the chassis that contains PC board ground connections bypassed by disc caps.

these would be the mic jack, the external speaker or pa speaker jacks, the power plug, and any IC, transistor, or other component that is bolted to the chassis.

the transistor thing may be a long shot, but i guess if you had a transistor that was shorted collector to emitter, and then the collector tab was touching the chassis, then you could get this issue.

Usually when i come upon a problem like this, i have to remind myself to go back and thoroughly check all the places that IM SURE it's not.

that switch that is hard to get to, or involves taking the face plate off yet again, the board that requires unsoldering 20 connections to take it loose, etc...

remember that depending on your antenna, it may be a DC short. if you have an antron 99 or an IMAX, and you were to put your ohm meter right at the SO-239 connector on the base of it, you would see a DC short. that's how these antennas are.

so, if that's the case, then plugging in the antenna is creating that connection between chassis and PC board ground.


it's time to stop guessing and start with a systematic approach.
disconnect everything that is mounted to the chassis one by one and check for continuity in between each part.

im afraid this one is all up to you at this point, as you are the only one who will be able to see the results of your tests.

just don't assume it's not something you already checked.

if you have a connection between PC board ground and chassis ground with the radio off, power plug unplugged, and no antenna connected, then start there and you won't need to power up the radio or hook anything up to find your problem.

good luck!
LC
 
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I have on and off this radio for a year now. I have checked everything twice if not three times. The weird thing is that when the antenna, actually dummy load. Is unscrewed from the back of the radio the TX light comes on. And it is stuck in TX. anyway as we already know.
 
i just got finished with a connex 3300 that had no audio, after removing echo board and resoldering so many spots,,,it came down to a chassis screw was touching the mike jack circuit board it would work with mike board out but in wouldnt,,,, what i did to fix was folded some black tape up and put between them and a washer on the main board to shorten the screw,,,
 
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Mike Jack on this one is not near the chassis screw. The only time I ever saw the antenna make the radio key for turn the light on anyway was when there was once no insulator behind the audio chip. Can't find a short yet
 
Did more testing Sunday and finally had time to post. This time I tested with the antenna unplugged and this problem is definitely a Chassis to board ground problem but only shows when the radio is powered up. I now ruled out Squelch/PA pot. Microphone socket and board. also ruled out the SO239 and all potentiometers. And almost forgot power socket. Still scratching my already bald head. More to come.
 
this is a lark, but have you tried measuring the DC voltage from the board ground to the chassis with the radio on?
seriously just guessing wildly here.
LC
 
.010 DC voltage to the board ground measuring from power connector ground to board ground. With radio off voltage drops to 0.

With TR35 removed Same problem and still .010V to the board ground.
 
Start from the beginning.

If you haven't already, pull TP 7 and 8, going to the Final and Driver, then it will not matter if you have an antenna or load on the antenna port, or nothing attached.

The PTT line goes through the PA switch to the Emitter of TR35, then the Collector goes to the switching input line of the MB3756, Pin 5. Remember, there are 2 halves to this switch, one half goes to the PTT etc... and the other goes from the audio amp to switch the "High Side" of the speakers from RX speaker to PA. .

If C133 is shorted, a 10uF@16v electrolytic cap from Pin 5 to ground is shorted, then it would stay keyed. You checked or isolated (desoldered) one side of this cap?

If C135 is shorted, a 0.47uF@50v, then it acts like the Lock Detect on the PLL is out of lock, so no TX, so it probably is not that.

When you pull TR35, it SHOULD unkey. If it does not, is the mica sheet to isolate the tab of IC4 intact or not, is it broken? What about the screw and mounting hardware? If I recall, this tab is Ground, so it should be at Board Ground, not Chassis Ground.

Is there a ceramic disc cap soldered from this tab to board ground? Or might have been on the audio amp chip.

Have you tried desoldering the wire from the PA switch going to TR35? Not the wire going to the mic jack.

I suspect that the PA switch on the back of the Squelch pot may be shorted or broken internally. I have had to rebuild a few of these in the past.

The RX green light should not be on in TX, unless the MB3756 is now shorted internally, so both Pins 8 and 6 are "On".


Grant TX-RX Switching.jpg
 
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I pulled TP7 and 8. C133 and C135 were replaced some time ago and the radio has been fully recapped. A week or so ago I pulled C133 and C135 both tested good. I also replaced IC4 last week. I pulled the wire fr. the pa switch to tr35 same problem. I did notice when the antenna is unplugged and the radio is in TX when the Mic is keyed the red TX light goes to green. I keyed up a radio next to this radio and it will receive while the trd TX light is on with TP7 and TP8 unplugged. I did not want to hook it to my signal generator because of the short.
 
The Red LED goes to Green, is the PLL getting the correct voltage? And the same voltage keyed or unkeyed? Is the Lock Detect pin, pin 6 high, or low?
 

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