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Rev 1 vs. Rev 3 QMX Kit?

 

I have an unbuilt Rev 1 QMX kit, that I? am finally seeing time ahead to build it. But am wondering, would I be better off getting a Rev 3 kit. Are there significant upgrades or differences in the current version that would make it worth getting the new kit revision?
Tnx
Paul w2eck
 

Hi Paul,
?
if I remember correctly for the QMX+¡­
?
rev 1 to rev 2 - mainly the SMPS circuit was laid down on the main board rather than plug in modules.

rev 2 to rev 3 - only the npo capacitors changed. ?Rev 2 are axial lead, rev 3 SMD that are factory installed.
?
personally, I like rev 1 best because the plug in SMPS supplies are easier to diagnose and fix if something breaks.
?
--
73
Jeff Moore
W1NC
 

Jeff
Tnx for the help on this. I also got a reply from Hans that said changes were minor and no need to change- go ahead with Rev 1.
So that's what we will do.
?
73 & have a great weekend
Paul w2eck
?
 

Is it a QMX or a QMX+?
The QMX rev 1 had tantalum caps on the SMPS boards with power ratings that were too low.
They were susceptible to exploding if there are abrupt changes in supply voltage (as several of us found out).
If I was going to build a rev 1 QMX I would 1) replace those tantalum capacitors with the ones Hans uses on the later revisions and 2) perform the optional step (not actually shown in the Rev 1 assembly manual but found in the Rev 2 assembly manual section 2.30) of adding tape on the power connector.
?
?
73, Mike KK7ER
?
 

On Sat, May 10, 2025 at 12:06 PM, Mike KK7ER wrote:
QMX rev 1 had tantalum caps on the SMPS boards with power ratings that were too low.
humbly thinking that this statement is wrong. Wrong in so many ways...
Let us make the power ratings of tantalum caps great again!
 

On Sun, May 11, 2025 at 11:15 PM, DL2ARL wrote:
humbly thinking that this statement is wrong. Wrong in so many ways...
??????????????????
 

Voltage rating too low ???
--
73, Dan? NM3A
 

On Sun, May 11, 2025 at 02:15 PM, DL2ARL wrote:
humbly thinking that this statement is wrong. Wrong in so many ways...
Perhaps I did not include enough detail?
I'm referring to C106 and C107.
The parts supplied in Rev 1 were rated at 6V.
The parts supplied in later revs were rated at 10V.
Hans wrote a very detailed article on the overall issue here: ?
?
73, Mike KK7ER
?
 

Hi Mike, all

The SMPS step issue wasn't quite the same thing as the capacitor voltage ratings issue.?

The tantalum capacitors used were rated 6.3V. they shouldn't ever see more than 5V. Indeed there is a 5.6V zener with a sharp cut-off as final protection, so I'd be very surprised if the capacitor could ever see 6.3V and exceed its rating.?

However, there were occasional field reports of the capacitor burning out (quite literally, tantalum capacitors catch fire). And not in any known circumstances such as sudden input voltage steps. So it seems that even though the capacitor's 6.3V voltage rating probably wasn't technically exceeded, it's still too close for comfort and perhaps some devices are a little lower spec than others.?

I subsequently changed to 10V rated tantalum capacitors which are available in the same size package and have not heard of burning capacitors since this change.?

Nevertheless if you have a Rev 1 board I wouldn't rush to change the capacitors, as I recall the fires were rare.?

73 Hans G0UPL


On Mon, May 12, 2025, 17:38 Mike KK7ER via <groupio=[email protected]> wrote:
On Sun, May 11, 2025 at 02:15 PM, DL2ARL wrote:
humbly thinking that this statement is wrong. Wrong in so many ways...
Perhaps I did not include enough detail?
I'm referring to C106 and C107.
The parts supplied in Rev 1 were rated at 6V.
The parts supplied in later revs were rated at 10V.
Hans wrote a very detailed article on the overall issue here: ?
?
73, Mike KK7ER
?
 

Hi Hans.? Tantalums should meet their manufacturer's derating voltage.? It's just not enough to say you're operating below the capacitor's voltage rating.? In aerospace applications, a typical voltage derating is 50% of rated value was used, and the capacitors had to be current surge rated.? Negative effects due to high surge currents must also be carefully considered, especially at startup.? Kyocera states, for their tantalum caps, "Voltage derating is necessary for tantalum and niobium capacitors to prevent failure due to excess current availability.?Tantalum capacitors can be safely used at 80% of their rated voltage, but the MTBF will be lower and leakage current higher.?If a tantalum must be used across a low impedance source, consider incorporating a PFET integrator to reduce risk of failure."



-Steve K1RF


------ Original Message ------
From "Hans Summers via groups.io" <hans.summers@...>
Date 5/12/2025 2:03:12 PM
Subject Re: [QRPLabs] Rev 1 vs. Rev 3 QMX Kit?

Hi Mike, all

The SMPS step issue wasn't quite the same thing as the capacitor voltage ratings issue.?

The tantalum capacitors used were rated 6.3V. they shouldn't ever see more than 5V. Indeed there is a 5.6V zener with a sharp cut-off as final protection, so I'd be very surprised if the capacitor could ever see 6.3V and exceed its rating.?

However, there were occasional field reports of the capacitor burning out (quite literally, tantalum capacitors catch fire). And not in any known circumstances such as sudden input voltage steps. So it seems that even though the capacitor's 6.3V voltage rating probably wasn't technically exceeded, it's still too close for comfort and perhaps some devices are a little lower spec than others.?

I subsequently changed to 10V rated tantalum capacitors which are available in the same size package and have not heard of burning capacitors since this change.?

Nevertheless if you have a Rev 1 board I wouldn't rush to change the capacitors, as I recall the fires were rare.?

73 Hans G0UPL


On Mon, May 12, 2025, 17:38 Mike KK7ER via <groupio=[email protected]> wrote:
On Sun, May 11, 2025 at 02:15 PM, DL2ARL wrote:
humbly thinking that this statement is wrong. Wrong in so many ways...
Perhaps I did not include enough detail?
I'm referring to C106 and C107.
The parts supplied in Rev 1 were rated at 6V.
The parts supplied in later revs were rated at 10V.
Hans wrote a very detailed article on the overall issue here: ?
?
73, Mike KK7ER
?
 

On Mon, May 12, 2025 at 11:03 AM, Hans Summers wrote:
Nevertheless if you have a Rev 1 board I wouldn't rush to change the capacitors, as I recall the fires were rare.?
I laughed out loud at this? I love all of the work that you've done Hans (really), so please take this new tagline in jest:
?
"QMX - The Fires Were Rare"
 

On Mon, May 12, 2025 at 11:03 AM, Hans Summers wrote:
Nevertheless if you have a Rev 1 board I wouldn't rush to change the capacitors, as I recall the fires were rare.
Well, you're the master. But as one of those rare folks who experienced the fire firsthand, I thought it was worth sharing that advice. As I explained in this forum at the time, I had just carried the QMX through 4 airports in a roller bag and discovered after the fact that the case screws had apparently vibrated loose. The failure happened when I plugged in my CW key. I speculate that the boards shifted and the power jack came intermittently into contact with the the encoder shorting power to ground in short bursts. As I understood from your SMPS article that is sufficient to cause the failure.
?
73, Mike KK7ER?
?

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