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QMX mid-band - back from the dead

 

My mid-band QMX (rev 4) is finally back up on its feet and running nicely after a few repairs. I posted a thread a while back with the initial fault(s) and progress with its diagnosis / repair.
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To cut a long story short, a fault on one of the PSU boards took out a few of the ICs on the main board. After fixing the PSU board (faulty zener), I went about ordering replacements for the 5351 and IC's 501, 502 and 503. These were all fitted, along with a fresh set of BS170's and a quick rewind ?of transformer T501, just in case. The end result is a fully working radio that puts out a minimum of 5W on all bands running on a 12V supply.?
?
Now for a quick heads-up:
There was one small issue in the repair. The 74ACT08 chips that I ordered from Farnell were a tiny bit too long for the tight space on the board. The metal screw for the BS170 retaining washer prevented the IC from aligning properly with its pads. I ended up having to sand the body of the chip down very slightly in order that it would fit the space. The chips I ordered were manufactured by ONSEMI - part number MC74ACT08DG. They are the correct package and pin spacing but the chip body is just that tiny bit too long for the available space without modification.
Here's a photo of the shortened chip in place, showing the clearance between it and the screw.?
Hope this helps someone who may have similar issues with their radio. It's a great little box and I'm glad it's back in action!?
?
Cheers,?
Ronan
MM0IVR?
 

I've built a QMX HB and 2 QMX+ units.

In all three I found the screw head to be too close for comfort to that nearby chip.

I simply filed a flat on the screw head right down to almost the ends of a pair of the Phillips head slots. Used a hand file for two then remembered I had a nice Dremel tool and used a medium coarse sanding drum for the third. That last one took about 2 minutes going carefully.?

You can also shift the screw body a bit away from the IC when doing the final snug down of the screw/washer/nut. Hold the screw head with a screwdriver and tighten the nut carefully with your fingers or a small pair of pliers while adjusting the position of the washer over the BS170's. No need to use a lot of torque!?

73 GREG KI4NVX?


On Thu, Oct 3, 2024 at 6:23 AM, Ronan Cantwell
<ronancantwell@...> wrote:
My mid-band QMX (rev 4) is finally back up on its feet and running nicely after a few repairs. I posted a thread a while back with the initial fault(s) and progress with its diagnosis / repair.
?
To cut a long story short, a fault on one of the PSU boards took out a few of the ICs on the main board. After fixing the PSU board (faulty zener), I went about ordering replacements for the 5351 and IC's 501, 502 and 503. These were all fitted, along with a fresh set of BS170's and a quick rewind ?of transformer T501, just in case. The end result is a fully working radio that puts out a minimum of 5W on all bands running on a 12V supply.?
?
Now for a quick heads-up:
There was one small issue in the repair. The 74ACT08 chips that I ordered from Farnell were a tiny bit too long for the tight space on the board. The metal screw for the BS170 retaining washer prevented the IC from aligning properly with its pads. I ended up having to sand the body of the chip down very slightly in order that it would fit the space. The chips I ordered were manufactured by ONSEMI - part number MC74ACT08DG. They are the correct package and pin spacing but the chip body is just that tiny bit too long for the available space without modification.
Here's a photo of the shortened chip in place, showing the clearance between it and the screw.?
Hope this helps someone who may have similar issues with their radio. It's a great little box and I'm glad it's back in action!?
?
Cheers,?
Ronan
MM0IVR?
 

Yep, it's tight alright. In the photo, the screw is already shoved as far clear as it can possibly go.
Still, it worked out ok in the end so I'm a happy camper. It's my second QMX build. The first was a rev 1 which is still running well.?
 

On Thu, Oct 3, 2024 at 06:22 AM, Ronan Cantwell wrote:
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the chip body is just that tiny bit too long for the available space without modification.
?
An alternative would be to substitute a socket head cap screw (which I prefer in general, Philips heads are designed to "cam-out"!) or similar fastener with a smaller head or file down the existing screw.? ?But removing excess mold flash as you did will work as well.
I al glad your build worked out well and with nice power output also.
73, Don N2VGU

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