Flex 30A Internal Fuse blown twice in three days...

AE6BH

Well as the bands have heated up here I've been hitting the radio pretty hard with long FT8 sessions on everything from 40 to 10m... Generally at 100W, occasionally at 60W-ish into my old SB-200 linear. However three days ago while running 60W of FT-8 into the SB-200 on 20m with a radio-linear SWR of about 2.2:1 the 30A fuse blew in mid-transmission. Never seen that before. The radio fan was not screaming so it wasn't particularly hot. So, I replaced the fuse, which looked a little funky as it blew near one of spades not in the middle of the link, and the radio was again working fine. Fast forward to today, again a long FT-8 session with a few ATNOs on 15, 18 and 20m, but after several calls on 20m at 100W directly into my OCF antenna with about 1.2:1 SWR pow the 30A fuse blows again! Weird! The radio is powered from an Aston VS-50M power supply and the ammeter typically shows about 10A when transmitting at 100W. Anyone got any clues? Maybe the radio really is getting too hot? In the past the internal fan speeds up to the point where it's so noisy I just slow down and let it cool to be quiet again. Am I just pushing it too hard for too long? Do I need to accelerate my time line to getting a better solid state amp so as to not run the radio so hot?

KD0RC

That does not seem normal at all. I would submit a help desk ticket.

AE6BH

Thanks Len, I changed the fuse and again the radio appears to work fine; so, that's a relief. Curiously the green 30A fuse plastic case was a melted blob indicating the fuse itself was quite hot for some time. Thereby tending to indicate a substantial voltage drop across the fuse as necessary for the fuse to dissipate enough power during high current operation while transmitting to get very hot. Not good fuse behavior. Too bad I can't upload pics here of the molten fuse... To be fair these little 30A fuses are the mini-automotive models and I've got a mixed box of them from Amazon which I presume come from China... but they were cheap... but the original fuse was melted somewhat as well. I'll update the software tomorrow from its current 2.6.2 to the latest 2.7.6 just in case that has anything to do with it, but I seriously doubt it. I'll also inquire about a help desk ticket. Thanks again.

KD0RC

Try posting a pic of the fuse now.

AE6BH

OK I can drag and drop pics... cool...
This morning I noted that the J3 side of the fuse holder was very loose with a new fuse, as in the fuse blade could rattle around in it, not good. The J4 side is quite a tight fit as it would be expected. On closer examination it appears that the J3 fuse holder is missing the spring contacts. I tried to identify the J3 part on the internet as a SMD Mini-fuse holder to no avail; it would be a simple repair job to replace it. In mean time however I was able to squeeze what would be the fuse contacts on J3 with a dental pick to get a very tight fit on the fuse blade, and ensured the fuse blades were very clean. One powering up I measure a 15mV drop across the fuse with the radio receiving only. On TX into a dummy load at 90W (according to the Flex Meter app) I measure a voltage drop of 190mV across the fuse with a power supply current reading of about 6A (as read from the Astron PS ammeter (so not a particularly precise measurement...), but about 1.2W fuse power dissipation. And following a 30 second transmission the fuse is noticeably warm/hot to the touch, but only a 5degC rise from 30 to 35 with my IR thermometer.

KD0RC

I think that you nailed the issue... I would submit a help desk ticket. Be sure to include the pics and description you gave above.

AE6BH

Well I initiated a help desk ticket and the issue was resolved. It is an interesting saga though...

The ultimate issue comes down to excessive contact resistance in the 30A mini-fuse holder, this resistance when coupled with high power supply current during high power (100W) transmissions results in a large voltage drop at the fuse holder which in turn manifests as a temperature rise of the fuse holder and in turn fuse itself, obviously getting hot enough to melt the plastic. Additionally per the fuse data sheet, at higher temperatures such as the 40-60C temperatures measured at the fuse holder the maximum fuse current is reduced to 24-22A (see attached figure).

Given the measured power supply current of 23.2A when transmitting at 100W, it's no wonder the fuse eventually blew well below its 30A rating. (In the course of all this it was also determined the Astron PS ammeter was far out of calibration, so that was also fixed by an easy adjustment within the PS.) During testing I initially measured the voltage drop across the fuse holder at the PCB J3/J4 solder pads of around 225mV with a 23A current during transmit (so about 5.2W power dissipation). However at the fuse itself I measured only 18mV of voltage drop. So the additional voltage drop was taking place in the fuse holder itself. I attempted to clean the fuse blades and connector oxidation with DeoxIT D100L to no avail. I then lightly sanded the fuse blades and cleaned them with alcohol then reinstalled the fuse; this resulted in a voltage drop of only 40mV at the PCB solder pads and greatly reduced heat build up during long transmissions (still about 1W though...). I then removed the fuse and added a small amount DeoxIT in hopes of preventing further oxidation; however, upon retesting the voltage drop had surprisingly increased to 65mV. On that note I removed the fuse and cleaned the blades with alcohol again and the voltage drop returned to about 40mV.

The moral of the story is that minimizing contact resistance of the 30A fuse / fuse holder is critical if intending to use the 6500 for extended high power operation on digital modes such as FT8.

Mike-VA3MW

Ohms law strikes again! :)

Glad you got it figured out

KD0RC

Thanks for the update!