AXBB charge pump fault

[CharlieRam]

Here's a pic of the full board, there looked like a lot of dust on it so I've brushed it all off but it's still reading the same resistance. So are you thinking its the transistor and no the resistor? I had a look at the back of the board and it looked like a short from the pin of the led but it brushed off so maybe it just looked worse

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[john swift]

the board is very much what I expected
basically an all in one UC300 and opto isolated breakout board from what I can see

looking at your photo
the 10K resistor is in parallel with
the 200 ohm resistor that's in series with the FETs gate & drain connections

your probably seeing the 220 ohms in series with the short circuited FET gate & source



I expect the 2K resistor is probably limiting the indicator LED current

whats the 14 pin IC number that you can just see 7 of its pins ?

John

[CharlieRam]

This one says HC14
It's hard to take decent photos!

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[john swift]

that makes sence

it will be a 74HC14 which is a hex inverter very often used on breakout boards






I think this shows output 1 is fixed for use as the charge pump output


John

[CharlieRam]

Whatever file you posted I can't see it on my phone? So in your opinion do you think it's the transistor with the AA6 that's faulty? And what physically makes that port different? I mean how is that port able to provide enough current to power the solenoid when the others can't? Sorry for all the questions!

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[john swift]

yes , I think the AA6 is faulty

the first file is just a PDF copy of the 74HC14 data sheet

As the same AA6 transistor is used for all 8 opto-isolated outputs
the only obvious differance I can see is the charge pump has control of output 1
and you via mach 3/4 control of the other 7 outputs

any output should be able to control a relay

John

PS
looking back at an earlier post



I would of thought you should be able to change port 1's charge pump to another pin
for example to pin 2



John

[CharlieRam]

That's what I thought too but when I tried it didn't work....
I will have another go tomorrow and report back my findings.
We have a localish electronics shop so I might give them a ring about replacing the smd.. my hands are way too shaky and my eyes are not much better
Thanks again for the help

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[john swift]

its the sort of thing you need to have some practice before you try to repair a valuable board
I found scrap PC boards are good to practice on

the last board I worked on was this XMT-2360 speed control board
the 14 pin IC , a transistor and a few diodes had been damaged by a short circuit to the speed control potentiometer



I guess the main issue is finding the new AA6 transistors

John

PS

have you checked the relay is OK

does it work when its connected directly to the 24V DC supply ?

[Doddy]

I've been following this thread with some interest. Personally I'd RMA the board for replacement/repair, however, if that's not an option...

From the photo of the board it's feasible that some of the analogues to the left of the transistors are involved with the charge-pump - it's not clear but if I squint hard enough I can believe it. At this time I'd give-up looking for a "AA6-j" marked device - it isn't present in the top-hits of SMD-marking google hits. but the purpose of the device is clear - a logic-level N-channel MOSFET with a working voltage in excess of 24V with a commutating diode to avoid problems with back-EMF from the relay (this was one area that concerned me originally when considering this device could be a BJT). This would support the traces to OP1 being the only charge-pump output. That, in itself, is somewhat peculiar as I understand that the micro controller on the AXBB generates the charge-pump output to itself - so I'm not entirely sure of the logic behind this decision - the integrity of this safety feature is then at the lowest integrity of the board - likely the program running on the micro controller... but I digress.

In a bind, I'd look to sweat-off the MOSFET for OP1, and the three resistors for OP2, then haywire the gate signal from the landing pad from OP1 to OP2. This would transfer the charge-pump output to OP2. Or, you could carefully (without overheating) sweat off the MOSFETs for both OP1/2 and swap them - assuming that you don't require the OP2 output. It looks as though any/all of the OP1-8 have the same circuitry/options, with just OP1/2 shifted on the board to avoid the stand-off pillar.

Or replace the MOSFET with a generic with the same SOT-23 packaging and pinout - there's nothing special about it's function that requires it to be matched to the original. John's mention of the 2n7002 is a suitable device in SMD packaging.

John's analysis of the failed MOSFET is, I believe, the most likely scenario here. There is a question as to why this failed, but sometimes s**t happens. It would be worth, if possible, diagnosing this more completely before attempting a repair. OP has a meter, it would be sensible to measure the voltage at the "IC" side of the 220R resistor with the output OFF/ON (0V, 5V respectively) and at the transistor side of the 220R resistor (similarly, 0V, 5V respectively). That he mentions the LED turns on/off, however, it a pretty good indicator that this should be working.

OP asks about a sensible price for a repair shop to perform this - a blind replacement (they won't be able to easily test without a licensed UCCNC installation) would be 5 minutes work and 10 pence of component, but they run a business and have to cover their operating overheads. On a good day - price of a pint, however, if the boss is around I'd expect £30 upwards.

If the OP was closer I'd offer to help - but it's not really viable (100 miles). But if he wants to play with a hot-air rework setup with stereo microscope then the offer is there.

[john swift]

Its a pity they had not used a DIL (dual in line ) octal transistor array like the ULN2803
or a new FET array
like the Toshiba TB D62381A series DMOS transistor array
that would enable the user to plug in a replacement





If the AXBB controller was mine I would remove the AA6 FET
To prove the 74HC14 and resistors that drive the FET are OK

then either replace the AA6 FET with a
Vishay Si2306DS if it has the same footprint



John

plan B could be link the Gate & Drain PCB pads

that would enable the use of a relay and an external relay driver

for example

after checking the boards details



https://www.ebay.co.uk/itm/24V-TOP-A...592caec3ec0e9d