#109293

I built one of the early QMX rigs, S/N 338, and it's done great ever since, about 2 1/2 months, until last week. I gave it the ultimate test by transmitting without an antenna and not being observant so did it a couple of times. It took it at first, but eventually gave up and shorted one of the finals.

I replaced the finals and D106 zener diode it shorted also. It came up and did fine so put it back in the case. It worked for an hour or so then the same thing, short causing the power supply to current limit to 1 Amp. This time D106 was fine. I replaced all the finals and to be safe the driver IC503, 74ACT08D, chip. Tested and all ok so back in the case. Again it lasted about an hour with FT8 and bang. Same short.

What could be causing it to repeatedly fry the finals? Actually only one of the finals shorts. The other 3 all test good. The antenna is connected with SWR of 1.0. Power out on 20M is 4.5 watts and 40M is 4 watts. Now this evening with it out of the box and just replacing the one bad BS170 it's worked for an hour and a half and still is ok. Could being in the case causes too much heat build up? I don't see anywhere where the case touches anything.

Noticed today an oddity, the QMX display kind of flickers as it powers up. No flicker once it's up, but I don't remember that occurring previously. ??? It makes me think that there is something that is putting a heavy load on the voltage as it powers up so the display isn't seeing the full voltage. I don't see that indicated on the meter on the power supply. 

Ideas?

Help!

73,
Cliff, AE5ZA

#109295

Hello Cliff,
I suggest you take a look at the clearance between the lower case lid edge that? is used for the threads of the blacks
screws for the cabinet and the BS170:s soldered close to the edge of the board.
To do this you need to remove both side panels.
Now put the bottom lid back.
Now look at something moderately bright (computer screen?) through the QMX.
You will se very little, if any, clearance between the soldering and the above mentioned edge of the lower case lid.

I used a sharp chisel to remove some material from the lid, and covered the now exposed aluminium with kapton tape.

I hope my explanation is understandable...

73 de SM5EIE /Gunnar

#109316

Cliff, you might have introduced an intermittent short somewhere when you took the unit apart and reassembled it.  I would carefully take it apart one more time and reinspect all the boards.
One thing I like to do as a final inspection is  to take a smartphone photo of each side of all sub-assemblies, then, using Microsoft Paint to view it and edit the photos, add small red circles around anything suspicious looking using the shapes tools.  After any inspection and possibly touch-up, I then put a red line through the red circle to indicate that particular joint has been looked at and touched up as necessary so you don't lose track of what you inspected.  (See attached).    It's some more work to do, but at least you are ruling out a newly introduced issue caused by mechanical disassembly and reassembly such as a hairline short from a tiny piece of stray clipped wire or a loose solder ball.

-Steve K1RF

Show quoted text

------ Original Message ------
From "Cliff" <ae5zaham@...>
To [email protected]
Date 9/4/2023 11:55:39 PM
Subject [QRPLabs] Ideas please

I built one of the early QMX rigs, S/N 338, and it's done great ever since, about 2 1/2 months, until last week. I gave it the ultimate test by transmitting without an antenna and not being observant so did it a couple of times. It took it at first, but eventually gave up and shorted one of the finals.

I replaced the finals and D106 zener diode it shorted also. It came up and did fine so put it back in the case. It worked for an hour or so then the same thing, short causing the power supply to current limit to 1 Amp. This time D106 was fine. I replaced all the finals and to be safe the driver IC503, 74ACT08D, chip. Tested and all ok so back in the case. Again it lasted about an hour with FT8 and bang. Same short.

What could be causing it to repeatedly fry the finals? Actually only one of the finals shorts. The other 3 all test good. The antenna is connected with SWR of 1.0. Power out on 20M is 4.5 watts and 40M is 4 watts. Now this evening with it out of the box and just replacing the one bad BS170 it's worked for an hour and a half and still is ok. Could being in the case causes too much heat build up? I don't see anywhere where the case touches anything.

Noticed today an oddity, the QMX display kind of flickers as it powers up. No flicker once it's up, but I don't remember that occurring previously. ??? It makes me think that there is something that is putting a heavy load on the voltage as it powers up so the display isn't seeing the full voltage. I don't see that indicated on the meter on the power supply.

Ideas?

Help!

73,
Cliff, AE5ZA

#109366

开云体育

Thanks Steve and Gunnar,

I've looked it over and found a couple suspect spots that I've cleaned up. Am back on the air now and we'll see how it holds up. I'm going to run out of the case for some time to be sure all is well then try the enclosure again. I appreciate your input very much.

73,

Cliff, AE5ZA

Show quoted text

On Sep 5, 2023, at 05:39, Steven Dick, K1RF <sbdick@...> wrote:

Cliff, you might have introduced an intermittent short somewhere when you took the unit apart and reassembled it. ?I would carefully take it apart one more time and reinspect all the boards.
One thing I like to do as a final inspection is ?to take a smartphone photo of each side of all sub-assemblies, then, using Microsoft Paint to view it and edit the photos, add small red circles around anything suspicious looking using the shapes tools. ?After any inspection and possibly touch-up, I then put a red line through the red circle to indicate that particular joint has been looked at and touched up as necessary so you don't lose track of what you inspected. ?(See attached). ???It's some more work to do, but at least you are ruling out a newly introduced issue caused by mechanical disassembly and reassembly such as a hairline short from a tiny piece of stray clipped wire or a loose solder ball.

-Steve K1RF

------ Original Message ------
From "Cliff" <ae5zaham@...>
To [email protected]
Date 9/4/2023 11:55:39 PM
Subject [QRPLabs] Ideas please

I built one of the early QMX rigs, S/N 338, and it's done great ever since, about 2 1/2 months, until last week. I gave it the ultimate test by transmitting without an antenna and not being observant so did it a couple of times. It took it at first, but eventually gave up and shorted one of the finals.

I replaced the finals and D106 zener diode it shorted also. It came up and did fine so put it back in the case. It worked for an hour or so then the same thing, short causing the power supply to current limit to 1 Amp. This time D106 was fine. I replaced all the finals and to be safe the driver IC503, 74ACT08D, chip. Tested and all ok so back in the case. Again it lasted about an hour with FT8 and bang. Same short.

What could be causing it to repeatedly fry the finals? Actually only one of the finals shorts. The other 3 all test good. The antenna is connected with SWR of 1.0. Power out on 20M is 4.5 watts and 40M is 4 watts. Now this evening with it out of the box and just replacing the one bad BS170 it's worked for an hour and a half and still is ok. Could being in the case causes too much heat build up? I don't see anywhere where the case touches anything.

Noticed today an oddity, the QMX display kind of flickers as it powers up. No flicker once it's up, but I don't remember that occurring previously. ??? It makes me think that there is something that is putting a heavy load on the voltage as it powers up so the display isn't seeing the full voltage. I don't see that indicated on the meter on the power supply.

Ideas?

Help!

73,
Cliff, AE5ZA

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