I've got a dead Mk2 unit here. When connected to +12V AC adapter, it measures about 5V across the 12V socket (ouch!). Something is internally shorted somewhere.

After much fussing about, removing/replacing components as I traced the power circuitry, I found two very hot F7413 chips. These are the two regulators for the +5V supply (2.5V each?), controlled by the Maxim MAX1631EAI chip.

Disconnecting the next stage from the chip outputs had no effect, so I desoldered the F7413 chips. +12V now measures +12V again, which is good, because it means there's no internal short in the PCB! But the 3.3V still measures approximately zero volts, which is weird.

So I've ordered "samples" of the MAX1631EAI chips from Maxim (Digikey doesn't stock them), and also some new F7413 chips from Digikey (# IRF7413-ND, $4.65/ea).

Unless Patrick or Hugo have better advice, my plan is to first replace just the F7413 chips with new ones, and if that still fails I'll swap out the MAX1631EAI chip as well.

Hopefully that'll do it. Everything downstream from there looks fine -- no big shorts that I can find -- rather easy to isolate it there, as that's where the red jumpers are located for cutting off the +3.3V and +5V.

Cheers

Measure the off resistance across the FET (source-drain) in the IRF7413s? At least to compare against the new ones you've already ordered before soldering replacements in?

From what you're saying I'd probably be blaming the MAX1631 actually. i.e. it's gone a bit mental and turned on both FETs resulting in essentially a dead short on the input supply.

That said the heat from the "induced" short may have since killed the IRF7413 chips.

I don't know if there's anyway you could diagnose the MAX1631 whilst installed with no FETs. I would expect the "top" FET would be turned hard (since the controller will not be seeing any output). At least I'd expect the "bottom" FET to be off in that situation.

Yeah, that does seem a bit more probable than two separate chips suddenly both going bad at the same time -- except I suppose if one of them did go bad, the MAX1631 would likely crank up the other one and try to fry both..

Mmm... okay, I just measured the two that were removed: one shows about 10.5 Ohms, whereas the other shows infinity (my meter only goes up to 2MO). Since they should be identical, this means that at least one of the two has gone bad.


Yeah, they do get rather hot. There's a 2.5A fuse on the 12V supply, and the IRF7413s are rated for 3.1A, though where all that wattage is supposed to go is beyond me!


Yeah, I can check that. It is outputing the "permanent +5V" line properly now, though, so portions are still okay. But the 0V on the 3.3V outs has me a bit stumped -- perhaps I have to trigger the MAX1631 to "on" or something to see that ?

Thanks

Well, being the sort of person that I am, I decided not to wait for the Digikey and Maxim shipments later this week. I actually have another not-completely-dead player around -- a Mk2a -- and so I pinched one of the IRF7413s from that, to use along with the likely-good one (infinite resistance) from the dead Mk2.

The player now powers up and works perfectly!

So I guess the Maxim chip is more robust than I thought.

Once the Digikey shipment arrives tomorrow, I'll put one of it's new IRF7413s back onto the Mk2a board, to restore its former not-quite-dead status. It's useful for debugging and tracing circuitry.

The photo below shows the working Mk2.
And the a.png link shows the relevant PSU components, for reference.

Cheers

What, me, bored? Never!

Just updating one of my external diaries here now. The Mk2 unit pictured/repaired above ("ruppy"), now sports a new rotary encoder, and a full button/knob LED illumination job. Looks rather smart!

Cheers