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President Grant DX stays keyed up even with mic unhooked

Jan 6, 2017
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President Grant DX is staying in transmit mode even with microphone and cord unhooked from radio. It has the 999 board. When turning radio from off to on, it takes about a second then goes into transmit mode, also I can heard what sounds like a roger beep tone, which stays on continuously. Radio does show wattage output on the watt meter when hooked to it through antenna. What's your thoughts? Diode, transistor, roger beep circuitry? Has anyone ran into this issue before? Thanks for your time!
 

While you're at it...

Look around the bottom of the board, see if any works been done to "Strap" (or ground) the Chassis Case of the radio and the Board itself.

Take a Resistance load test, check the Case screw to the NEGATIVE leg of the power connector, if it shows there's a short, you'll need to investigate some more inside.

Why, because of how the Mic audio and Switch circuit works.

upload_2020-3-26_10-0-29.png

The Mic is wired as a 5 - pin with SEPARATE ground return for the Keying circuit - the MB3756 uses its' own set of contacts inside the handset to RX or TX - on top of - the RX side uses that ground for SPEAKER using the MB3756s' ground return - NOT THE Audio Side - or Pin 1 and 2 combo. IT needs to be separate and on the MB3756 side. Pins 3, 4 and 5.

Well why would this matter...?
upload_2020-3-26_8-52-1.png

It depends on how they grounded the pins in the Mike (Mic) handset - If you Ground Pin 4 back to Pin 3 and it still stays keyed u[p, you have a Mic jack wiring issue - you gotta' fix - or rewire - that Mic plug and Jack back to OEM. Look above to help you - just follow pins #'s...

It is my fear that they grounded to Pin 2 - which when you use Pin 5 SPEAKER ground, Pin 2 is not a perfect ground - hence the "Strapping" check. The audio power going thru the circuit that Pin 4 is to ground when you unkey - and eddy currents can flow thru shield and chassis ground which RAISES pin 3 enough to trigger MB3756 to toggle on.(your tone or squeal from the ground loop)

Worse case scenario is the MB3756 has to have point-to-point continuity checks to make sure all the triggers it needs to switch RX and TX and handle PLL LOCK enable/disable functions are working right.
 
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Gosh Handy Andy, what a great reply.

Thanks for explaining in detail, with pics and great reasons why this radio may be having the issues. I really appreciate your time. We will explore the grounds, check for shorts, and look at the mic vs speaker ground routes, especially noting pin 2 due to the squeal from possible ground loop.

Thanks again for your in depth reply.
 
It is an Export Grant DX PC999, which is a 4 pin mic socket, not a 5 pin.

Check IC4, the dual Op Amp, pin 7, it should be at around 8v when unkeyed, or near 0v when keyed. The one side is the mic preamp (pins 1,2,3) and the other side (5,6,7) is the keyer / delay circuit for the Roger Beep.
When pin 7 is at around 8v (RX mode), it turns on TR36, which turns on TR35, to provide +8v RX voltage, then when pin 7 is near 0v, it turns off TR36 and TR35, turns on TR37 which provides your +8v TX.
If pin 7 switches ok from RX to TX voltages, then I would check TR37 to see if it is shorted, which will cause the TX voltage to be on, and because the PTT line is not pressed in the microphone, causes the Roger Beep to be on unless the mic is keyed.

GrantDX PC999 key circuit.jpg
 
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OMG it's a 999 board - then it's only a 4 pin - oh geeZ - I thought it was an XL...

The DX uses 4 pin - shieesh!

Ok too many things on my desk today ...this whole week...

Sigh...Sorry NZ8N...you're right...

Well, look into it when you're ever come across a 5-Pin Grant ... :whistle:

Then that goes back to the 4558 chip...and follow NZ8N's lead on this - it's the keying circuit he's taking you to.

Here's some help from Graphics I've done up myself...
GalaxyUniden4558Chip.png

For overview...

The 4558 serves two functions,

In Galaxy, Connex, Uniden - President... a Mic amp and a RX/TX switch

The RX/TX switch side
  • - designed to look at 3 things - Power Good, PLL LOCK signal and if you want TX or RX - the pins 5-8 dose just that; using a resistor feedback principle of "threshold" to make the amp work more like a switch but with reservations
    • - I will toggle to this state, if these two are met first - kind of logic so it's kinda treated like an AND gate
    • - but the AND is for a looking at a change of state as well as Power and LOCK remaining in their original condition..
So if the thing wants to start TXing, then the Resistor divider network that parses the signals arriving - make sure the diodes that route the RX/TX sides to the INPUT are working - AS WELL AS THE OUTPUT.

This isn't for the Grant 999 board,
but it is a good flowchart of things going on in there...
4558TX.png

Sorry about identifying the WRONG radio...
This (I hope) should help...
4558Chip.png

The above graphics give different VALUES for the RESISTORS used, but their pinout functions are still pretty much the same.​

So with the above graphics, find a resistor network that works for you - they did different values of the types of outputs the PLL chips they served with and the functions of how their lock functions were checked.

While you're there, look for C173 - a 10uF cap - looks at the PA switch - see if that's gone, shorted or issues with the WIRING of the PA switch...
 
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NZ8N thank you! Wow this site is great, such generous folks! I will study your replies and see what we can do on this radio. Thanks for the time of posting the pics and reading my quest, your all great! I like the way you guys communicate with one another!
 
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