Slowly advanced the voltage from my PSU, current started rising as usual but then went up too much so I immediately backed off.  I again checked the connections on the RPi power plug, particularly for possible solder whiskers but nothing and connections are correct.  I think the LEDs on the board coming on indicates that the polarity is correct.  Next I decided to try a USB PSU and the message on the monitor cleared and the usual text scrolled up the screen and stopped.  Of course, the USB PSU only supplies 0.5A and the RPi can take 0.6A or even 0.7A when starting up.  This corresponds to the 0.36A current input with input voltage of 13.8v, to the buck converter when running the RPi from it.

All this seems to point to the plug wiring (checked umpteen times) or the bench PSU (unlikely).  I'm stumped - again!!!

Been testing almost everything in sight.  The output voltage of the bench PSU was within 30mv as read by my 4 digit DMM so that can't be the problem.  The negative terminal of the PSU is/was linked to earth by a link on the terminals and that is mains earth.  I tested the voltage on the HDMI connection to the TV set I'm using as monitor and found 110V AC on the connector but this was just leakage and when connected to earth through the DMM on AC Amps, mA and µA the DMM did not register any current.  Using another bench PSU with a USB power output and without HDMI connected, the RPi red LED came on.  I tried touching the HDMI plug casing to the casing on the RPi and all fine but after connecting the HDMI, the red LED did NOT come on when the PSU was switched on.  So the problem would seem to be the HDMI TV set   I guess next thing is to test another HDMI display as monitor.

I thought before doing that I'd check that my plug connections were alright so I connected the RPi to the terminals on the PSU from which the ammeter works and the voltage can be controlled (the USB socket is 5v fixed).  Switched the PSU on and gradually increased the voltage while watching the current (ammeter goes up to 1A on this PSU).  Everything went fine and current reached 0.8A as the OS loaded then went down to 0.3A or so, varying as the system loaded.  This has proved that my plug wiring was fine and the whole problem with HIDs attached was the HDMI monitor.  I'm relieved that I haven't blown another RPi but it's a nuisance that I can't use this display.

Thought I'd try one last test with the TV set as monitor.  With 0v on the PSU, I connected the HDMI and raised the voltage.  RPi started up and the display showed the usual boot sequence but the current went up well past the 1A limit and the PSU cut out.  With the volts at 5.1, this process repeated but, of course, the boot up couldn't finish.  I don't know why the RPi draws over 1A with the monitor connected but only 0.8A without - the display shouldn't draw current from the RPi.  Don't know if I dare plug in the HDMI after the OS has loaded.  Probably not - I think there has to be something wrong.

From the RPi web page

Quote

the HDMI port uses 50mA

Nowhere near 3-400mA that the RPi is drawing!

Had a thought - I have a 5" RPi HDMI display   So I connected it to the RPi and powered up - it works   Current never went anywhere near 1A.  Could probably see it alright with reading glasses and desk magnifier   Precludes the use of the HAT when connected though as it uses the GPIO, though I don't know what for.

Well, it works and I can see it alright with just my reading glasses but it won't let me change the display resolution so the image is far too big for the screen.  The Wi-Fi isn't working and I don't know why so I'll have to run a cable from my router - I do have a long one.  Then I may be able to run Firefox and install TeamViewer to enable me to run it remotely on a decent size screen.

Hmm - seems there isn't a TeamViewer version that works with the RPi   But the LAN connection for command line works now I have a wired connection.

I now need to see where I am with all this... 

1. The RPi power plug and wires are alright.
2. The latest RPi works from my 1A bench supply.
3. The LAN connection works alright over Ethernet.
4. Wi-Fi isn't working.
5. The HAT is still suspect.

Guess I need to dismantle the current setup and "regroup".  Tested the RPi from the 1A PSU again just to make sure it's still working after removing the LCD screen.  I think to eliminate one part of the HAT I'll wire the RPi power directly to the buck converter rather than via the HAT.  Makes it easier to test the RPi without the HAT.  I think this will be the first test.  I can still use the 1A PSU to run the RPi via the buck converter.  The 1A PSU has 5v and 15v ranges.

Actually, I decided to check the camera hooked up to the RPi and running KStars/Ekos/INDI.  RPi powered directly from bench PSU and LAN wired connection.  Here's a screenshot from the KStars FITS Viewer.  Rough focus manually.

At least this proved I haven't cooked this Raspberry Pie - it's still RAW   That is Ready And Working   I'll get my coat...

Now have RPi attached to its bracket on the ASC and wired to the buck converter which is being supplied with 13.8v via its long power cable.  Everything still working.  No HAT on yet.  I think that's as far as I'm going tonight - going to stop while ahead

Can't put it off any longer - today I plug the RPi in to the HAT and power up and fingers crossed...  Just the HAT to start with without external power and no motor driver modules.  One small step at a time.

Passed that test

Next to shut down again and connect the 13.8v Gnd.

13.8v Gnd was fine and I went on to connect the +13.8v wire.  That wasn't   I now have yet another cooked Raspberry Pi   So what do I do now??!!  This is getting (got) expensive!!  The only thing I can think of is to buy another HAT and wire that up.  The only difference from when this HAT was working fine was that I added a second position for an A4988 stepper driver module, removing some components and moving the first module position to make room for the second.  The dew heater and camera cooling circuits have not been touched.  I just don't understand it - there is no circuit to the RPi except via high value resistors and the drive transistors which are in turn controlled from GPIO pins and +3.3v.  The resistors and transistors are in the dew heater circuit.

Looks like it's time for a different project

I hate just giving up - makes me depressed!!  And yes!  I'm depressed!   And I refuse to believe I'm past it!  But it does take a lot longer to do things and solve problems and I guess I make more mistakes and cost myself money!   With the ASC so near to working and all the structural work done on the ASC itself I can almost see the end in sight.  I need to re-do the dew heater but that's a simple little job.  I think I shall omit the circuitry for that for the next attempt.  Oh yes there has to be a next attempt - I won't be defeated by some silly electronics!!

Searching for RPi HAT I can only find them on Amazon at a fiver but delivery towards the end of January   Guess I'll order one but won't solve my present problem.

I keep peering at the HAT through my 3x magnifier and testing with my DMM but just can't find anything wrong.  I connected the bench PSU to the yellow +13.8v power wire and green Gnd wires (13.8v Gnd and RPi Gnd) with HAT on duff RPi to see if there was any current - nothing significant, about 4µA even at 15v.  That shows that the transistor is not shorted out as the collector resistor is 22K which would conduct around 700µA from 15v.  The 4µA would be the leakage current of the transistor.  So how on earth could 10v or so blow up the RPi???

HELP!!  Has anyone any ideas, please?

A new Rpi 3 has arrived and I've ordered a spare - I must be stupid, I really should give up but I can't!  OTOH I also can't apply "juice" to the 13.8v power wire with HAT installed or I'll blow up another RPi!

The new RPi works with camera but without HAT.

Here's a screenshot of the KStars FITS Viewer zoomed in to cover a bit more than half the image.  Focused by hand as accurately as I could with motor removed.  A few degrees rotation of the lens on its thread makes a difference to the focus.  Camera Gain of zero and Exposure of 1ms (1/1000th).  ASC sitting on outer window sill.  Visibility is very poor today and the far hills can barely be seen.

Been reading the book I have on interfacing the RPi and it seems advisable to use optocouplers so I've ordered 10 off EL817 Optocoupler from Amazon with Prime delivery Wednesday.  Best I can do with the dreaded bank holidays!

Further very close examination of the HAT has shown broken connections on the second stepper driver module so I've moved the connections onto good ones GPIO23, GPIO24 and GPIO25.

// Lens Cover Motor
#define IN1 RPI_BPLUS_GPIO_J8_16	// GPIO23  DIR
#define IN1 RPI_BPLUS_GPIO_J8_18	// GPIO24  STEP
#define SLEEP RPI_BPLUS_GPIO_J8_22	// GPIO25  SLEEP

Screenshot of image as seen in KStars FITS Viewer.  Just started to rain again so had to close window.  Two raindrops on lens.  ASC on window sill just inside.