No RX/ TX Uniden Washington

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No RX/ TX Uniden Washington

Postby chrdij » Tuesday 10th Jun 2008, 11:37

Hi All,
I just bought a Uniden Washington at a local Hamfest for $10 bucks and to no surprise it has no TX/RX. The Radio has the MB8719, board # PC-385AG. Everything lights up with no problems and tested the mic on my other washington and is OK. The TX light will not light up when I keyup. I tested the MB8719 and there is no voltage greater than .25 on any pin.
Anyone have any ideas?

Thanks a bunch!
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Postby chrdij » Tuesday 10th Jun 2008, 13:37

Anyone?
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Postby timrim » Tuesday 10th Jun 2008, 14:45

Those radios have a voltage regulator IC mounted on the side of the radio, it should be an 8 pin regulator MB3712. Those are a common failure on that board. If you can give me the voltage readings on each one of the pins, in both receive and transmit, I will let you know more.
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Postby The Defpom » Tuesday 10th Jun 2008, 15:29

it is a MB3756 and is IC5 if I remember rightly.
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Postby chrdij » Tuesday 10th Jun 2008, 16:07

Ok, I put the voltmeter to the 8 pin IC and here is what I have...
With the radio upside down front facing me, starting with the 1 st pin closest to me..

RX and DX mode all read the same volts..no change..seems dead.

1) 0
2) not used
3) 8.0
4) 13.6
5) 0
6) 8
7) 13.6
8) 0
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Postby The Defpom » Tuesday 10th Jun 2008, 16:19

OK, I have repeated your voltage list below, with the pin numbers:

pin 8 - 0
pin 7- not used
pin 6 - 8.0
pin 5 - 13.6
pin 4 - 0
pin 3 - 8
pin 2 - 13.6
pin 1 - 0

Pin 1 should have a constant DC voltage of 8V, so the IC appears to have failed (as mentioned, this is a common problem on radios that have been reverse wired / over voltaged).

You may also want to check D60 (on the side near the IC but a little further back) as it can sometimes blow, resulting in either a direct short, or (as in your case because it still powers up) no longer providing protection for the IC.
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Postby timrim » Tuesday 10th Jun 2008, 17:34

Sorry, I stand corrected, it is MB 3756, The MB 3719 is the audio chip. For some reason I get those 2 numbers mixed up. Looks bad from readings, as The Defpom said, be sure to check or replace the diode when replacing the IC.
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Postby chrdij » Wednesday 11th Jun 2008, 3:20

Great, thanks a bunch guys. I have the parts on the way and will let you know how it all turns out :)
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Postby chrdij » Thursday 12th Jun 2008, 9:37

Ok, just installed a new regulator MB3756 and D60 checkout just fine on the meter. I keyed up the mic and still nothing :x The radio seems dead, no RX or DX. The TX light doesnt even come on and no needle movement. I replace the Aduio chip TA7222AP to see maybe if I would get anything life at of this thing with no such luck.

Anyother suggestions possibly? It seems as if the TX/ RX circuit is dead all together before the regulator?
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Postby The Defpom » Thursday 12th Jun 2008, 15:54

The MB3756 regulator is also the main TX/RX switch, do the voltage measurements again to see if there are any differences.

I suggest that you replace C164 and C133, just in case they are leaky.

Pin 1 of the MB3756 should have 8V, as that is the main DC supply, if not, then there could be a short circuit somewhere in the radio which is shorting out the supply, easiest way to check for that would be to unsolder that pin and see if it comes high once it is no longer attached to the track.

Other bits of info:

The TX/RX switching control is provided by TR37, D44, R194.

TR37 connects to pin 5, when pin 5 goes low it switches the IC into TX mode, making it turn off the RX only voltage at pin 6 and turn on the TX only voltage at pin 8.
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Postby The Defpom » Thursday 12th Jun 2008, 16:08

If pin 1 of the MB3756 shows 8V when you de-solder it, try the following:

there are only a couple of parts that I have come across, which can kill the MB3756 devices constant 8V supply, as follows:

Replace C66, it is a 10V elect. cap, I have found that shorted out once.

I also once had a radio where the VCO module had shorted out internally, which then shorted out the DC supply, try desoldering pin 8 of the VCO module (UHIC007) and re-test after re-soldering pin 1 of the MB3756, you wont have TX/RX but you may get the constant 8V at pin 1.

The capacitor is the most likely fault, however the VCO is something else to consider.
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Postby woody » Thursday 12th Jun 2008, 16:10

check the 10.240 crystal this is located over by the mb8719 pll chip I have found this crystal to be bad or it could be a bad solder joint.
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