USB - A cautionary tale....

[radiogareth]

I've been making progress at last on a combined 9cm and 6cm transverter using a Toshiba 40W amp, Ionica front end, one of the Marconi/Ferranti 10W 6cm amps and a home-brew RX strip. The time had come to test the 6cm PA system so I booted up my Portsdown and went to Langstone mode to generate a carrier on 6cms. This PA draws about 6 amps from a 13.8V PSU, the same one that was running my Portsdown. With current flowing and no evidence of RF emerging at high levels I decided to investigate lower down the driver chain and pulled the earth lead from the amp power socket.....BAD IDEA as there remained an earth path for the 6+ amps via the coax to my Portsdown/Pluto, which promptly caused a re-boot....PLUTO not found, no blue activity lights, no Nextion screen which runs a mouse and data facility on mine), no output to the spare outside world USB port.

Lesson(s) learnt....don't unplug the earth lead first....solder on a plug like I've done on all my other stuff....so if you do yank the power, at least they will both disconnect at the same time.

Now browsing for RPI4 USB circuitry to see if its is simple fix.....no distinctive 'magic smoke' was detected (I have a well-trained nose for that) but I'm not convinced these tiny components hold much of it these days.
If anyone can suggest a good use for a USB-less RPI4 I'd be interested. The Portsdown touchscreen all seems to work, as does the Network port (just tried it with Putty) but with no USB.......
Gareth
(Fortunately I do have a spare RPI4...)

[radiogareth]

Looking at the diagram here https://www.raspberrypi.org/app/uploads ... s-R1.0.pdf it looks as if all the 5V is via self-resetting thermal fuses. But the USB ones trip at 280mA, so I'm puzzled how it can power a Lime or Pluto adequately. My PSU also feeds directly to the GPIO +5 and 0 pins, bypassing the earlier fuse from the USB-C power socket. Seemed a good idea at the time.....
More investigations required....
Gareth

[radiogareth]

Initial explorations seem to point to a problem with the Pluto as with it unplugged the Nextion and everything else seems OK, including trying an external Pluto which was recognised. Actually I'd prefer to replace the RPI4 as its a lot cheaper than the Pluto, but further dismantling now required to assess the problem
Oh well.....
Gareth

[radiogareth]

No visible signs of duress on the Pluto board but referring to the PSU section here https://wiki.analog.com/_media/universi ... c_revb.pdf the power is getting as far as U9, which is enabled, but nothing comes out the other end. Jumping it with an amp-meter shows the Plug-top PSU is tryimg briefly, shutting down, so something upstream is presumably short to ground.
Not looking promising at the moment.......
Gareth

[g4eml]

Hi Gareth,

I had a very similar occurrence about a year ago. Several parts had failed in the power supply section Including the main regulator U8 and the input filter. I couldn't find a source for a replacement U8 and it would have been very difficult to replace anyway.

I did manage to get the pluto working again by bypassing the original power circuitry and fitting several of the little adjustable switched mode modules available on eBay, one for each of the voltages. I wasn't an easy job though and would not have been worth it if the Pluto was less expensive.

Colin.

[radiogareth]

Thanks for the info Colin. Yet further investigations led me further up the chain to the main regulator so I applied 5V across C10 on its input. Basically a short circuit and no life beyond. In the short term I have my Satsagen shack mule Pluto that I will repurpose and than maybe get round to sorting out regulators as you suggest/did. I think the lack of smoke is probably thanks to the limited current in the USB chain of events and that the damage stops at the PSU chip.
I have now soldered on one of my XT60's to the amplifier in question....like I should have done from the off.....
Gareth

[M5TXJ]

Hi Gareth,

If you decide to go down the separate regulator route, Bitsbox in Birmingham have some very small 1.8A adjustable ones for 84p each, I've used many of them with no problems.

https://www.bitsboxuk.com/index.php?mai ... ts_id=3477

73 Dave

[radiogareth]

Good spot Dave, cheaper than even eBay even after adding in postage.
Worth a few quid and some time rather than finding another £236!!
Gareth