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PostPosted: Wed Sep 12, 2018 2:38 am 
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Joined: Sat Jul 20, 2013 11:00 pm
Posts: 22
Hi Jim & group,

I've just finished assembling HR50 #1902 with rev G board and QSK option. It's failed all 3 of the ohmmeter tests as follows:

Test 1 0.6 ohm
Test 2 0.6 ohm
Test 3 MOSFET 1-3 all have 50ohms between Gate and Ground, MOSFET 4 has 170 ohms minimum, with VR4 as low as it will go.

I took the front panel off and measured resistance between 13.8V and ground and it's 5.4kohm

At the power switch header on the RF board, resistance between one pin and ground is 400 ohm and the other pin and ground is 3 ohm.

I believe this all points to a short on the main board somewhere. There's also the issue of MOSFET 4.

Any advice about how I should troubleshoot this further?

Thanks and 73,

John
VK7JB


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PostPosted: Wed Sep 12, 2018 5:09 am 
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Joined: Mon Jul 23, 2012 6:25 am
Posts: 1143
Hi John,

Sorry to hear that Hardrock50 #1902 is having trouble.

Our board manufacturer had a defect in some of the PCB, there is a short between Pin 1 of RLY15 and ground. I thought that we had found and corrected the issue, but maybe your unit was shipped before we made the discovery. Unfortunately, you can't see the short without removing the relay.

Let's see if the picture shows up, if not, view it here https://www.dropbox.com/s/zf97c3ld8ikreu9/RLY15%20Short.jpg?dl=0

Image

If MOSFET 4 is showing is showing a high resistance the most likely culprit is VR4. Does the resistance at the gate of Q4 change when you adjust VR4?

73,
Jim WA2EUJ


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PostPosted: Wed Sep 12, 2018 7:10 am 
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Joined: Sat Jul 20, 2013 11:00 pm
Posts: 22
Hi Jim & thank you.

I removed R15 and the area around pin 1 looks as follows:

https://www.dropbox.com/s/vdwcdyjuhi38p ... 2.jpg?dl=0

It looks OK to me and testing resistance between pin 1 of the relay and ground, I find 1900 ohms.

With the relay removed, there is still a short between the 13.8V line and ground at the DC input, so I guess the short is elsewhere on the board, but I don't know where to look next.
There are no obvious solder bridges anywhere I can spy.

Any tips?

73,
John


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PostPosted: Wed Sep 12, 2018 6:31 pm 
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Joined: Mon Jul 23, 2012 6:25 am
Posts: 1143
Hi John,

Sorry that you had to pull that relay to find nothing.

It sounds like the trouble is on the unswitched power side which feeds the drain circuit of the MOSFETs. There are a handful of caps which are very low-failure items. L5 is also connected and it you cross the windings it won't work but usually doesn't short to ground. Next there are the MOSFET themselves. It's easy to get a solder bridge between the drain and source and also possible that one of them is shorted.

If you pull L5 off it will separate the circuit into 3 sections, then input connector, Q1/Q2 and Q3/Q4 that would help us narrow it down further.

Jim.


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PostPosted: Fri Sep 14, 2018 11:56 pm 
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Joined: Sat Jul 20, 2013 11:00 pm
Posts: 22
Hi Jim,

Just to let you know the outcome. I decided to go the route of the probability diagnosis and replaced the MOSFETs - I had a spare set, which I ordered with the amp kit (?prophetically :).
With that, the short was resolved and the rest of the alignment proceeded smoothly and the amp is alive and doing well. I didn't isolate which of the MOSFETs had developed the short.

Thank you for you help. Now on with the ATU build.

73,
John
VK7JB


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PostPosted: Sat Sep 15, 2018 7:55 pm 
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Joined: Mon Jul 23, 2012 6:25 am
Posts: 1143
Hi John,

I'm glad that you resolved the problem. An ohmmeter test from drain to source should reveal the offending MOSFET. We'll be more than happy to replace it.

73,
Jim WA2EUJ


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PostPosted: Sun Sep 16, 2018 3:30 am 
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Joined: Sat Jul 20, 2013 11:00 pm
Posts: 22
Thanks Jim, but no need to replace the MOSFET. Such things happen. Today I made up the ATU, installed it and the whole package is playing perfectly. I LOVE the silent switching on CW - the QSK module is perfect. Thanks again for a great product, excellent assembly instructions and all your support.

73,
John


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