0ld style Uniden Grant. Keys when powered up?

[Low_Boy]

I have been messing wit this radio on and off for a very long time. Kees up when plugged in but the RX/TL light does not change and when you key the Mic and talk the radio has forward swing. If you take the PA/Squelch out(same Pot.) radio works like it should but RX/TX light stays red and turns green on TX.

No grounds from chassis ground to board ground.
Caps on mic. socket board changed.
parts checked
swopped pa/squelch.
TR47 TX Sw.
TR 27 PA Sw.
C115.
TR 35. Tx inhibitor.
TR 41 and 42 regulator
TR13 TR28 squelch amp.
TR 31 Sw.
TR 33 Sw.
D55 protection diode.

I am now searching for ideas.
Thanks

 

[nomadradio]

Too many radios named "Grant".

You said "Uniden" and not "President", so it's a 5-pin mike right?

Doesn't say "XL" or "LT", does it?

73

 

[Low_Boy]

Five pin. Just Grant. Probably the generation after the president grant. I am going pretty much by the photofacts with the President Grant because this must have been when uniden started not putting out information.

 

[999]

Schematic
http://www.cbtricks.com/radios/uniden/grant/graphics/grant_sch.pdf

Block diagram. It's best to download this one because it needs to be rotated.
http://www.cbtricks.com/radios/uniden/grant/graphics/grant_block.gif

 

[999]

Could C133 pull TR35 down and key the radio if it shorted?

 

[midnight special]

https://www.qsl.net/kf6foz/transitional/transitional.html

 

[Low_Boy]

I forgot to mention all electrolytic capacitors have been changed.
thanks

 

[oldtymer]

View attachment 28818
Five pin. Just Grant. Probably the generation after the president grant. I am going pretty much by the photofacts with the President Grant because this must have been when uniden started not putting out information.

My favorite mobile radio from the 70's, mounted up on dash of the truck with all the
controls you needed to operate on the left side in easy reach.

 

[999]

I forgot to mention all electrolytic capacitors have been changed.
thanks

Did you check all of the voltages on IC4?

 

[nomadradio]

Carefully unsolder pin 5 from the MB3756 chip and lift it away from the pad. Only need to break the connection to the foil pad. Don't wan't to break it off.

If this restores receive mode, I would suspect either a solder bridge on one of the newly-installed capacitors, or maybe one that was turned backwards when installed.

First question to ask here, is whether the MB3756 is misbehaving, or if a "sneak" path is feeding transmit voltage around the output from pin 8.

Pin 8 feeds 8 Volts for transmit mode only. If unsoldering pin 8 shuts down the transmit, the 3756 (IC4) may be the problem. If not, this means some other circuit fault is energizing the transmit circuits, NOT a bad 3756.

If this fault appeared after the caps were changed, it's probably not a coincidence, and the fault was introduced inadvertently while changing out caps.

73

 

[Low_Boy]

I wish it was that simple as a bridge or backward cap. This problem happened before the cap change.

I will pull some pins and see what happens. Hope it is IC4.
Thanks.

 

[Low_Boy]

Well first i pulled pin 5 and TX went away. Hooked 5 back up tx was back. Unhooked pin 8 and Tx went away. Changed the MB8756 and same problem.? I bought this radio with this problem. Changed capes after I got the radio.

 

[nomadradio]

Pin 5 is the keying input to the 3756 chip. If isolating it restored receive mode, this means the problem is outside the 3756 chip. C133 is the first suspect leading away from pin 5.

Upstream from there, this leaves TR35 and the mike jack. Not much else left to blame.

73

 

[Low_Boy]

I will have to look. If C133 is a electrolytic it has been changed. I think I remember the radio unkeying when the Mic. socket was removed and when the squelch was removed. This has been a on and off project for about a year now. I found no shorts in either and changed ceramic disc capson mic board.

 

[sonoma]

have you pushed on the 4 buttons on the front to see if that changes any thing. years ago I saw one that had a controlled carrier mod and they used the mod button to control it. it had to be in a certain position for the radio to function as it should. I have a controlled carrier mod on both of my Jackson radios and you have to leave the mod button out the way I have it wired for the radio to function properly.
just a thought.