New User G7ELK Fareham Hampshire

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8 years 3 months ago #1806 by mw0uzo
RH recommends washing the PCB with IPA to get rid of any flux traces that might become conductive under HV.
I get a toothbrush with IPA on it and scrub the areas containing HV.

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8 years 3 months ago #1807 by G7ELK
Thanks for the suggestions. Count level still very high after some hours in the sun, so unlikely to be moisture related as I first thought.

Flux on the board is a possibilty, though it was built pretty carefully using commercial "no clean" solder and has been working very happily on my desk at work for several days, so unclear why it should go wrong now. More inclined to beleive that I may have damaged the tube, though not sure how.

Will get it off line as soon as I get home and report back when I work our what I have done wrong!

Regards, Rob

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8 years 3 months ago - 8 years 3 months ago #1817 by G7ELK
Spent some time troubleshooting. The excess counts are definitely down to an EMC issue relating to the solar PV system. Panels (likely carrying inverter noise) are around 2m form the tube and the inverter itself (in the loft) is 3-4 metres away. Excess counts stop if I trip the breaker on the solar inverter and return when I switch it back on. However, they also stop if I disconnect the USB cable and power the counter from batteries.

Pretty confident that this is happening because of the way i have remoted the GM tube. Have already moved the anode resistor to the tube (which improves things a bit and is anyway the right thing to do). But still, with the counter receiving a ground reference (essentially mains ground) via the USB connection, it would seem likely that interference from the PV system is being picked up on the tube cable (miniature 50R co-ax about 75cm long). At the moment the cable outer is connected to the Kathode and inner to to the Anode. It strikes me now that this was a mistake, as the counting circuitry at the Kathode can easily be triggered by a voltage induced at the cable outer, which is at a high impedance relative to ground.

Best thing to do I'm sure would be a two core screened cable (like professional mic cable, with inner cores for anode and Kathode and an overall grounded screen). Unlike the co-ax there woudl be some risk of insulation breakdown at 400V, but I think I'd be surprised if this happened. Having a genuine ground at the tube would also allow local decoupling of both anode and Kathode.

First, though, I think I will try swapping round the coax core and shield.

Busy the rest of the weekend so I guess this will be a job for Monday lunch time at work...

Regards, Rob
Last edit: 8 years 3 months ago by G7ELK. Reason: gibberish

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8 years 3 months ago #1818 by Sonarflash
Hi,
Atomic Dave built my GK+ 4 custom using twin lead shielded mic cable and 3-hole push on connectors for the probes. Works fine. I have an LND712 probe, an LND7317 probe, and a Soviet SBT-11a home-made probe that connect to the GC through 1 metre of quality twin-lead shielded cable. No break-down problems at 400 VDC. Sounds like that might be the best way to shield your GM tube line. Also might be worth the trouble wrapping the inverter in grounded screening, or improving the shielding on the whole solar panel system?
Interesting problem. Maybe adding shielding braid and an earth ground to the USB lead and computer might be in order?
I'll mention this one to ham club members. Our mountain-top VHF-UHF repeaters have solar panel/inverter/battery systems.
Regards,
Brian Gage, VA7BDG

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8 years 3 months ago - 8 years 3 months ago #1820 by Juzzie
ordinary 1mm speaker cable works too. I've used 10m (with the anode resistor at the tube) and had no noticeable problems.... If you are using an inverter (12VDC to 110VAC ?) , that is more likely the problem. They are usually cheap and dirty :woohoo: I'd try ditching the inverter and use a DC-DC buck converter, straight off the battery, instead. You'll save power that way too.

Owner and operator of "southofhobart" monitoring stations.
Last edit: 8 years 3 months ago by Juzzie.

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8 years 3 months ago - 8 years 3 months ago #1830 by G7ELK
Thanks to everyone for your suggestions on this.

I replaced the coax cable to the tube with two core + screen professional audio cable, with screen grounded at the counter end and left unconnected at the tube. This sems to have resolved the issue of interference from my solar PV installation - PV is currently generating approaching 3KW (and of course powering the whole house) with no apparent disturbance to the counter. Ideally I would also have decoupled the Anode and kathode wires to ground at the tube end, but I did not have suitable high voltage caps to hand and touch wood it seems unnecessary.

I still see the odd "zero reading" every few hours both during the day and at night. This is statistically pretty unlikley I think, particularly since there are no other readings below 6 cpm, so I suspect some glitch in the USB serial comms between the counter and Radlog, but think I will ignore for the moment..

Now just need to get my broadband working reliably so I can upload to the web - for some reason it has been up and down constantly over the last few days....

Regards,

Rob (G7ELK)
Last edit: 8 years 3 months ago by G7ELK. Reason: illiterate

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Solar powered Raspberry Pi 4 server stats: CPU 47% Memory 11% Swap 9% CPU temp=61.3'C Uptime 3 Days