HF Amplifier MX-P50M issues

Equipment Radios & Power
21

Hi Ron
I used your guidance to perform some Resistance measurements using a DMM
Since this is a packaged, 2x MOSFET device and has a common source (the gold color base), the gate1 (A) and gate 2 (B) and the top two terminals, the drain1 (C) and drain2 (D) are the bottom two terminals, here are the values i get -

R(source-drain1)=8 ohms [this doesn’t seem to be high value resistance]
R(source-drain2)=1 ohm [this doesn’t seem to be high value resistance]

R(gate-source1) =infinity [this is open circuit]
R(gate-source2)=568 ohms [doesn’t seem like open circuit]

R(gate-drain1) = 1326 ohms [doesn’t seem like open circuit]
R(gate-drain2)= 439 ohms [doesn’t seem like open circuit]

Seems like the FET has ended its life?
I have already pulled it out and awaiting a new FET arrival.
I will repeat the above measurement on this new device, before i install.

3 Likes
22

My one unfortunately blew up! There is a mod that needs to be done to these to protect your rig.

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23

Hi Jim,

Thanks for the clarification.

Using a multimeter is not infallible for FET testing but it’s still useful.

Drain to Source should be at least 1 Mohm. Test with Gate connected to Source to clamp its voltage at 0. This may not be necessary but ensures stray charges do not cause the FET to be partly turned on and is a good lab technique.

Gate to Source should also be at least 1 Mohm.

As FETs are sort of symmetrical I would expect similar results with the Drain and Source reversed. Some have protection diodes which can give different readings.

You can see why I invested in a smart semiconductor tester.

However it does look very much like a damaged FET.

Your idea of testing the new one is good.

I look forward to hearing of a successful repair.

73
Ron
VK3AFW

1 Like
24

No need to reply but just by way of confirmation. It’s taken me 3-days but I finally found my two new and unused spare MRF-186’s. (I found my spare MX-P50M and my MX-P50A hiding in the same place too).

The DMM test findings of 2017 are confirmed but there are different values for A and B forward, depending on the DMM used to measure them (as you’d probably expect).

The 2017 values of .57 Volts forward drop in my first post were for a quality Beckman DMM.

Today using two cheapo DT830D DMM from ebay, the values were .41V as measured by the yellow DMM and .77V by the black DMM. This proves that when it comes to DMM’s colour matters! (This would likely be the case with diodes too).

Just kidding about the colour of course but for for A and B forward on a new device you can probably believe (accept) anything from .41 to .77V +/- a bit. Both MRF-186’s measured the same as one another.

I must agree that this is certainly not the best method for checking these things but from what we know so far, it can distinguish a new MRF-186 from a blown one.

Hope you mend your amp OK Jim,
73, John

1 Like
25

Hi John,

Thanks for the input.

Two terminal measurements on resistances under 10 ohms are notoriously inaccurate even after zeroing the meter. You can get a lower reading by pressing a little harder on the probes.

But when it doesn’t matter if the reading is 0.1 or 0.5 ohms accuracy is less of an issue.

Radio amateurs only occasionally need four terminal measuring ohm meters.

73
Ron
VK3AFW

2 Likes
26

Following up, after the new PA was installed.
Bought this for 10 bucks on ebay (china)…they shipped fast.

So here are the measurements…

As received new PA…

1 = A = gate 1
2 = B = gate 2
3 = C = drain 1
4 = D = drain 2

Diode measurements between common and 1,2,3,4 = all open ckt
Reverse Diode measurements between (common and 1=700mV), (common and 2=700mV), (common and 3= open),(common and 4= open)
Resistance measurements between (common and 1=open), (common and 2=open),(common and 3=open),(common and 4= 1195ohms)

Now to RF measurements, after i installed it, using Peak Power mode and RTTY from radio at 5W.
Battery at 13.9V to MX-P50M.

Filter switch position : (80m) (40m) (30m,20m,17m) (15m,12m,10m)
Power output when radio is on 80m : 35W, 40W, 36W, 31W
Power output when radio is on 40m : 0W, 40W, 50W, 40W

Power output when radio is on 20m : 0W, 0W, 41W, 37W
Power output when radio is on 17m : 0W, 0W, 37W, 35W

Power output when radio is on 15m : 0W, 0W, 22W, 37W
Power output when radio is on 12m : 0W, 0W, 2W, 34W
Power output when radio is on 10m : 0W, 0W, 0W, 28W

Are these harmonics i am seeing on adjacent bands?
Are the filters busted?
Perhaps band switch is not working ok?
Any thoughts on how do i test this?
I lowered the input power but the harmonics don’t go away.

1 Like
27

John,
Congratulations, you fixed it. It’s all good.
Sorry for short response. More later.

Use it with correct band selected.

73
Ron
VK3AFW

2 Likes
28

.Deleted.

1 Like
29

This seems to be crazy if the amplifier is using a high pass rather than a low pass filter on the output as it will let the harmonics through.

John are you sure your documentation is the correct way around - i.e. when you say transmitting on a certain band the TX is on that band not the amplifiers filter switch?

Normally on amplifiers they have a low pass filter on the the output so that, if you have the output filter set to say 40 metres, everything above 40m will be blocked but everything below (e.g. 80m) will be let through as will 40m. This is normal.

The “40m” LPF will be resonant at something like 8MHz so that it lets 7MHz through but not 14 or 21 MHz (the strongest harmonics).

In any case, you should make sure you select matching bands between the radio and the amplifier. if you for example transmit on 80m but have selected the 40m filter, the second harmonic of the 3.5MHz signal at 7MHz will get through to the antenna.

73 Ed.

1 Like
30

Hi John

I have had time to look at your post more carefully and your results are consistent with a working amplifier with low pass filters. I deleted my erroneous post on which Ed had commented. Sorry.

Just make sure you use the amp with the right band selected for the frequency in use.

Enjoy using the repaired amplifier.

You aren’t seeing harmonics by the way, just your signal passing through a miss match when you have the wrong band selected…

73
Ron
VK3AFW.

3 Likes
31

Thank for for the explanation of harmonics passthrough, this makes sense of the data.
73
AB1PJ

32

Yes - there are a lot of harmonics…

I once forgot to change the band. While calling CQ on 40m, it was still on the 10m band… I got a response from RNB as a spot at Sotawatch that I had a good signal on 15m. :grimacing:

73 Armin

5 Likes
33

Hi Armin,

I would expect the third harmonic without filtering to be -20 dB compared to the fundamental but that’s still maybe 300 mW so enough for a report.:grin:

73
Ron
VK3AFW

1 Like