Mystery of decreasing 5V

[robm]

I've pulled the next possum pin out for a refresh - its a getaway with no cab base or transformer. So i pulled (out of another possum pin) a transformer from a Dr Who, that looks pretty good visually.  Everything fires up OK except no display.

So i went searching and found low 5V. The strange thing is the CPU lights are flashing as though it booted. I tested 5V at the ROM, and it was 2.4V!!!! Yet still had the middle LED flashing as though it was booted OK???   Testing 5V test point on driver board was same reading - 2.4V. So i pulled the power driver board out (was in good visual condition, looks reasonably new) and put another one from TAF that i'd just rebuilt the 12V section of - it was testing fine in TAF for 5V.  turned Getaway on with TAF CPU, and booted OK (by CPU LEDs), and tested the 5V - it started at 4.95V, then dropped by 0.1V every second or so, until it eventually got down to 2.4.......

So i pulled all plugs off driver board except J101 -J105 and it still does the same thing - starts at 5V and then drops.

Could this be an earth problem or a transformer problem (they are the only two things i can think of - plugs look quite good)?  The thing i did realise is that the earth braid in the backbox does not have continuity to the transformer earth, and the screws holding the backbox plate in are a bit rusty.

Any help or suggestions - i'm keen to start stripping it tomorrow, but always like to fire it up 90% before hand so i know if any coils are locking on, switches not working etc as they are way easier to replace with the pf out.

Thanks

Rob

[Marty Machine]

Assuming you've removed all external connectors to isolate the problem is on the mpu/driver board?

What you have is a resistive(almost-shorting) load pulling down the 5v, hence it becoming 2.4v.

Since it happens over a period of time instead of instantly, i'd start by changing the filter cap on the 5v supply, also including any filter caps (large or small) that are on the supply that feeds into the 7805 or lm723 voltage regulator.
Capacitors after a long time will become resistive to DC and start becoming a short (or resistive-short) instead of being somewhat open-circuit to DC voltages.
There's probably still enough chemical(electrolyte) inside the cap to initially startoff as a cap, but after some current flow it starts brekaing down inside and becoming more resistive.

After all that, the time-delay for the voltage to drop could also be a faulting chip on board, breaking down internally after initial warmup/current flow starts breaking it down.
This will be harder to find, as you literally need to disconnect the 5v pin of EVERY chip until the fault goes away.
Firstly, I'd hope the CPU & PIA's would be socketed, along with ram & rom chips, at least pull all those out first and re-check the 5v stability.

Some of the unused(empty) rom sockets might just be full of dust & crap from storage maybe also causing a resistive path on the 5v supply.

my 2c  ;-)
MM

[robm]

I will check all those things out, but why does the power driver board work fine in TAF (or did up until i put it in Getaway), then showed the same behaviour as the power driver board that was originally in Getaway - hence my thinking its the transformer/wiring/earth as they are about the only other option. Plus i'm confused that the CPU stayed booted at 2.4V! - I also checked the 12V and it is stable...

[Marty Machine]

does the mpu board sitting OUT of the pinny measure 5v or 2.4v?

It seems like you're now saying when it's in various machines it's starting to fall below 5v....probably still to do with current-draw straining the cap(s) or chip(s) at fault.

If you're trying it back in a machine, ONLY plug 1 cable back in at a time, so you can isolate which loom (lamps, solenoids, switches) is causing the pulldown effect.

[robm]

OK - thanks MM for your promptings. I tried again, and with negative lead on the earth braid, it starts at 4.95V, then drops away every time i turn the machine on (with nothing bar J101 and J102 plugged in) BUT when i put the negative lead on the -ve rail of the power driver board its a solid 5.03V..... so seems to me the dodgy earth braid was causing wacky readings with the DMM.

Thanks for you help.

[Marty Machine]

Ahh, yeh ok, depends 'where' you choose to measure a ground from  :-)

So i not take it that 5v is stable once the braid is/was fixed?

[robm]

Give me a few weeks to tear down the machine, patch the cabinet, redecal it, strip playfield, then i will install the braid properly....  I'll just dodgy something up in the morning for testing purposes before getting into pulling the machine down.  Assuming now that the DMD is stuffed, so will sort that one out first