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QMX+ PA

 

I have one question, which is possibly totally silly, about the QMX+.
If I add a second layer of 4 BS170 to the current layer, squeezing the 8 FETs with the bolt, nut and washer with some heat paste, will the PA stop working because the QMX+ components have been calculated for 4 BS170, not for 8 or will I get roughly give 10 W output instead of 5?
73 - Pierre - FK8IH
 

Another question : what are these two diodes D503 and D504, on the schematics_rev3 on the PA between Q5, Q6 and T501. They are not listed in the parts list of semiconductors? I guess they are Zener diodes and maybe they are not anymore used?
73 - Pierre - FK8IH?
 

Adding 4 more BS170's would add capacitance to the load
that the 74ACT08's see when driving all those gates.
This would cause them to not work well on the upper bands.
?
If all you are interested in is 80 and 40 meters, you might be?
able to get this to work, but that would require T501 to be redesigned
to present a lower impedances to the BS170's.
The band switching scheme and the LPF's may not work well
at the increased power level, and the firmware written to
deal with the higher power levels seen by the SWR detector.
?
"what are these two diodes D503 and D504, on the schematics_rev3 on the PA between Q5, Q6 and T501"
?
On? ?
it says this file
has the "Schematics for PCB Rev 2 and 3 (published 10-Oct-2024)"
?
I see no D503 and D504 in those schematics, nor do I see Q5 or Q6.
?
I think you are looking at this schematic for the QMX, not for the QMX+
?
I assume Q5 and Q6 are Q505 and Q506
The BOM in the assembly manual for "rev2 and rev3" of the QMX states that D503 and D504 are "unused".?
Rev1 of the QMX assembly manual does?not mention them at all.
It seems those two diodes were never stuffed in any version of the QMX
They are not drawn as Zeners but rather as regular switching diodes, perhaps 1n4148's.
?
Jerry, KE7ER
?
?
On Fri, Feb 28, 2025 at 07:07 PM, Pierre - FK8IH wrote:

I have one question, which is possibly totally silly, about the QMX+.
If I add a second layer of 4 BS170 to the current layer, squeezing the 8 FETs with the bolt, nut and washer with some heat paste, will the PA stop working because the QMX+ components have been calculated for 4 BS170, not for 8 or will I get roughly give 10 W output instead of 5?
73 - Pierre - FK8IH
 

On 01/03/2025 03:07, Pierre - FK8IH via groups.io wrote:
will the PA
stop working because the QMX+ components have been calculated for 4 BS170, not for 8 or will I get roughly give 10 W output instead of 5?
Pierre

As Jerry says, you are altering capacitances and matching.
A lot of experiments will need to be done to optimise, even then you are unlikely to achieve your aim.
Do not forget Q507, it could take twice the current and need changing.
And it would probably not suit Hans' SSB operation when it comes. That is optimised for the standard PA.
And previous discussions have questioned whether doubling power is really worth the effort. It will make a small difference in marginal conditions. 1/2 an S point.

73 Alan G4ZFQ
 

You will not get where you want and increasingly so as the frequency of operation increases?
?
Your idea is moving away from an already optimised position?
 

I don't think that you will get more power because I don't think the FETs are what controls the current, it is the things around them, L502 and T501 which controls this.
The increased gate capacitance will affect things, maybe overload IC503, especially at higher frequencies.? The reason for two FETS is because the required current is too high for a single FET and two halves the current in them.
?
One way to try this would be to only fit one pair of FETs and run at a lower voltage, maybe 9V for the 12V version.? Measure the currents and power output, then add the second pair of FETs and see what happens to the current and power out.? The power might increase a little but I don't think it will double.
?
Chris, G5CTH

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