Removing an MXCOM voice inverter from an M1225

[sataraid1]

I picked up an M1225/20 channel model secondhand. After hearing some really odd and amusing audio out of it, it was discovered that it has an MXCOM MX014LH voice inverter board in it.

How do I go about removing this and restoring the radio to normal operation? Unfortunately, simply unplugging the card results in NO audio at all, not even a startup beep. If I turn the volume up 100%, I can just *barely* make out the audio signal. It's the same via the internal speaker or the external speaker output via the accessory connector.

It seems odd, because each wire is only tapping into an existing circuit. I would think that unplugging it should restore normal function, unless there's been some other component removed or altered that I haven't noticed.

I've read and programmed the codeplug many times, everything to my knowledge appears to be in the default configuration. The radio behaved properly with the board in place, screwy audio not withstanding.

The card was wired in like this:

Yellow wire to pin 3 of the accessory connector
Blue wire to pin 9 of the accessory connector
Red/White wire to pin 5 of U501 (the audio amplifier)
Purple wire to C409 (just to the right of the audio amplifier with the LCD board facing you)
Brown wire to pin 6 of J13 (the Advantage board connector)
Black wire to pin 8 of J13
White wire to pin 10 of J13
Gray wire to pin 12 of J13

If anyone is familiar with these boards, and how the installation may have modified the radio, and if it can be restored to normal operation, I would surely appreciate the help.

[d119]

Have you traced along where those connections were made to see if a trace has been cut? Perhaps checked MXCOM's website to see what the installation instructions are so you can reverse them?

[Bill_G]

Yep. No doubt there will be an instruction to remove a resistor or jumper on the main board so audio would pass through the inverter.

[sataraid1]

Unfortunately, MXCOM appears to be out of business, or merged into another company that I haven't found yet.

I found a datasheet for this particular board, but the URL included on it (www.mxcom.com) just has a blank page with the logo on it.

The datasheet has specs and diagrams, but no instructions on actually installing a board or model-specific alterations. I'll keep digging.

I looked in the general areas where the connections had been made, looking for cut traces or other signs of alteration, but haven't found any yet. At least, that stood out to me, but this is first time I've handled an M1225, so I'm not very familiar with them.

[Bill_G]

Try this Midian instruction sheet for M1225 radios to see if it guides you in what functions go where, and what audio paths were broken.

http://www.midians.com/pdf/appnotes/TVS ... tested.pdf

[sataraid1]

Try this Midian instruction sheet for M1225 radios to see if it guides you in what functions go where, and what audio paths were broken.

http://www.midians.com/pdf/appnotes/TVS ... tested.pdf

Thanks, I'll use those references as a starting point, and if that doesn't pan out I'll just try to systematically check both sides of the board.

I hope it's not riddled with removed components, though. I've done board-level work on Maxtracs before, but this M1225 may be a bit too intricate for my meager soldering skills.

Had I realized the board couldn't simply be unplugged, I'd have probably sent it back. But now that I've been inside it knocking around, that's not a realistic option.

[sataraid1]

Ugh. After finding R551 and R654 on the board diagram, I realize they're right where the wires were connected to the traces coming out of the option board connector. I had wondered why there would be open pads there, but didn't put two and two together. I don't have the radio in front of me right now, but I'd bet they're both gone.

As if that weren't enough, while I was doing some tests on the radio, I disconnected and reconnected the board a few times. At one point, a wire leading to one of the open pads came off. At first, I thought the solder had simply broken, but when I looked at the end of the wire under a magnifier, I realized the pad had actually come off as well.

Sigh. Anybody got a main board they want to get rid of?

[sataraid1]

Try this Midian instruction sheet for M1225 radios to see if it guides you in what functions go where, and what audio paths were broken.

http://www.midians.com/pdf/appnotes/TVS ... tested.pdf

The PDF you provided a link to was right on the money. R551 and R564 had been removed. I bridged R551 just long enough to see if it would bring back the audio, and it did.

Since this radio won't be used to transmit, all I have to do is put R551 back. I wouldn't be able to replace it with a surface mount because of the broken pad, but it shares a trace with C554, so I may be able to tie in there with an axial instead.

Unless someone thinks it's safe to just bridge it with a straight wire. I can't tell exactly if R551's function is tied to the use of expansion cards, which I won't be using.

Thanks for the help, I appreciate it.

[Bill_G]

R551 is 30k in the rx audio path from the AFIC to the vol control. So, it is necessary, but almost any value 10k or greater will do. R564 is 15k and is further down in the rx audio path. It only drives pin 11 in the accy plug. It probably is not necessary.

I think you meant R654 which is a 24k in the xmit path. Again, you should be able to use 10k or greater.

[sataraid1]

Thanks again for the information, and yeah, I got my numbers a little scrambled.

I took a close look at the board last night, and got all the surfaces cleaned up. I should be able to get an axial installed on a wire jumper without too much trouble.

I appreciate everyone's help and input!

[sataraid1]

Well, FWIW, I put a 27k resistor on a jumper and managed to get it installed without incident. The radio seems to work great, and I'm thinking it may well pick up better than my Maxtracs do ... but with the crazy VHF propagation the last few mornings, it's hard to be sure.

Thanks again to all for the assistance.

[Bill_G]

Excellent. Glad to help. Cheers!