Stumped - WPC 5V too high

[robm]

This board is of unknown origin,  - it was working for quite a while in a Fishtales of a mates, then the sound started playing up - i measured voltages and noticed the 5V was at 5.4V or so. Swapped power driver board, and the sound problem disappeared. So i brought the high voltage power driver board home.

Obviously someone has had a go before me - and there is a burnt trace on the top side of the board - note this has been jumpered on the reverse side, and is for BR4, which is the 20V circuit (ie: not 5V)

I have since:

Replaced LM323K 5V regulator - no difference
    Note: No through hole connector to top of board (previously damaged), however there are no tracks for the 5V regulator on top, they are all on the underside

Replaced BR2 and C5 - no difference

Ran jumper between BR2 and C5 +ve as it had a previously damaged through hole connector - no difference

Reflowed solder connections at 5V header, fuse, c9, c4 - no difference.

Lightly hit header pins for AC in for 5V with emery paper - no difference

Also note: All of the above had previously been either replaced or reflowed - i suspect replaced due to a few through holes being damaged.


So basically i have replaced all the components up to the 5V regulator except c9 (0.33uf cap), and reflowed all the connections

So when i power up machine with only J101, J102, J105 - it gives 5.2V, as soon as i plug in J210 to MPU it bumps up to 5.5 to 5.6V

The only other thing i noticed is the track under one of the mounting bolts for the heat sink had the insulation worn off, hence my quick and dodgy plastic insulator under the bolt - this is an early revision power driver board for T2 as it has the relay installed for the flippers - when comparing with other boards, the track just mentioned has been moved away from the bolt.  Continuity on that track is still good.

Where to next?

[Boots]

Haven't got the schematics in front if me but are there any diodes or zeners in the the circuit?

[robm]

The only diodes i can see related to 5V section are D3 and D38 - replaced them just now with no change

[Boots]

What about the LM339?

[Retropin]

Just a thought, but have you tested for any AC present? You may have a bit of ripple throwing things out from a flakey smoothing cap C5 or problematic bridge

[robm]

What about the LM339?

There are a lot of voltages present on pins of the LM339 (or at least higher values (eg: 5.8 v 1.1)) in the problem board when compared to a known good one in the same machine. In the good board, there are 7 pins that have no voltage present, whereas only 1 pin of these on the problem board has no voltage, the others range from 0.4 to 5V.

Does this indicate the problem is th 339, or prior to the 339?

[robm]

Just a thought, but have you tested for any AC present? You may have a bit of ripple throwing things out from a flakey smoothing cap C5 or problematic bridge

I assume the cap and bridge are good, since i replaced these. I tested between ground braid and 5V test point, no AC was present

[Boots]

What about the LM339?

There are a lot of voltages present on pins of the LM339 (or at least higher values (eg: 5.8 v 1.1)) in the problem board when compared to a known good one in the same machine. In the good board, there are 7 pins that have no voltage present, whereas only 1 pin of these on the problem board has no voltage, the others range from 0.4 to 5V.

Does this indicate the problem is th 339, or prior to the 339?

Test the 339 on diode mode with the power off you might have a short internally

[robm]

OK, on diode test, LM339 tests exactly the same as a working board (pins 1 6 and 7, as well as pins 8, 9 and 14)

Replaced it anyway, still no change.

I am starting to think it has to be something to do with the grounding of the 5v regulator, since sometimes i have deliberately put a resistor between the regulator and ground to up the output from 4.8 to 5.0V (the regulators are still within factory spec, but just aren't up to the task)

Do people think it is normal for the voltage to jump from 5.2 to 5.5 or 5.6V as soon as a load (ie: plug in J114) is placed on the board. I don't think i have seen this before, and does it offer any clues?