Defender - 12v & 5v a bit low - suggestions?

Sprout

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Ok - so my Defender with it's current MPU board seems to be working fine at the moment, although it was recently throwing RAM errors I was told. It's been in foster care for some time (a few years), so figured I'd give it all a once over to be sure it's in good shape, and also go through some extra MPU boards I've got laying around.

Board that's in there seems fine - but in checking my voltages, my 12v is around 11.6v (from the PSU board, with no load), and around 11.5v on a 4116.

5v is around 4.8v on the PSU, and 4.3 to 4.5 (depending upon the MPU board used) at the RAM. Stays close to 4.8v on the ROM board otherwise.

-5 is rock solid, at both the PSU and the RAM.

Both of the extra boards have issues - one won't reset until I manually ground pin 37 of the CPU, at which point it boots, but blind - no video. Second board is just watchdogging constantly. In both cases, I suspect the low 5v is a factor, and it doesn't make much sense to start digging deeper until I'm confident my voltages are closer to spec.

PSU was re-capped a couple of years back, and game has had light use since. All headers are good - they were replaced, and still look great - no burnt pins, no cracked joints. BR2 was also replaced at the time. I've recapped the MPU boards as well.

So - seems like something's pulling the 12v and 5v down on the PSU board slightly. BR1 looks good (solder joints good, no obvious issues). Should I suspect the two 723's? Would the 18k and 4700 cap's factor into this, since those weren't swapped?

I *think* the 2N6057 was swapped, but it's been a while and I'm not positive. I've got 26.2v at pin 11 of the 723 @ IC1 - a bit lower than the 27.4 listed on the schematic, so perhaps it's BR1? TP2 has 14.2v (a bit higher than the 12.8v listed) otherwise. Pins 5 & 6 are 4.8v and 6.9v respectively on IC1 - both about .3v low compared to the schematic.

I've read about swapping out R2 for a 5k pot to allow for manually adjusting the 5v, but I'd rather get the voltages solid and where they should be if possible before considering that.

Suggestions and additional testing appreciated! I think I still have the 723's around from when I originally repaired the board, so if they're suspect, I could pretty quickly rule them out.
 
Try checking the voltage at the psu and at the mpu board. It may be the connectors. If they become corroded it can drop the voltage. The connectors can also become bent in the casing and not make a solid contact.

Another thing that can drop the voltages are the filter caps by the power input on the mpu board.

Try checking the voltages at the four corners of the RAM chips. The 4116 RAMs have all 3 voltages on the pins on the corners.

Ken
 
All covered in my original post, thanks! I try and do my homework by reviewing old posts and such before asking for help.
 
Hey Ken, feel free to get technical with Sprout. He can handle it...
 
5v is around 4.8v on the PSU, and 4.3 to 4.5 (depending upon the MPU board used) at the RAM. Stays close to 4.8v on the ROM board otherwise.

TP2 has 14.2v (a bit higher than the 12.8v listed)

have a power supply where the 12v is 14.1v and the game resets
put another power supply in where it's 12.8 and no resets
 

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All covered in my original post, thanks! I try and do my homework by reviewing old posts and such before asking for help.

Hey Ken, feel free to get technical with Sprout. He can handle it...

Sorry, just wanted to cover the bases. I've had several cases where circling back to the basics cleared up the issues.

I would look at the 18KuF. If it is showing any bulging at the ends, then replace it. Great Plains Electronics has an exact replacement. If the 4.7KuF or 18KuF caps are going out, there will be AC ripple on the regulated +5V or +12V. Just set your meter to AC and check between the regulated pins and ground or if you have an oscilliscope, slap it on the outputs.

The other common issues are usually the two small caps, C15 & C16. If they are getting weak they will drop the voltage under load.

You should also probably clean the fuse clips and make sure they are giving a solid connection. If the connection is not solid the voltages will drop under load.

If it has not already been removed, unsolder one leg of R10. This will bump the +5V a little.

It almost never hurts to swap in new 723's if the board has been socketed. There were some bad ones out a few years ago. Those do not stand up to usage and the voltages start to drift.

While I don't recommend running any of the Williams classic boards on a switcher, if you have one and can build an adapter so you don't hack the wiring harness, it can sometimes help to take the PS board out of the equation just to make sure it is the power and not the boards.

ken
 
have a power supply where the 12v is 14.1v and the game resets
put another power supply in where it's 12.8 and no resets

Yeow!
A carbon film Piher trimmer instead of a fixed resistor. These puppies aren't known for their reliability or longevity. That's asking for trouble down the road.
Better to get the voltage where you want it, remove the trimmer and measure the resistance -- then install a fixed resistor.

Ed
 
Yeow!
A carbon film Piher trimmer instead of a fixed resistor. These puppies aren't known for their reliability or longevity. That's asking for trouble down the road.
Better to get the voltage where you want it, remove the trimmer and measure the resistance -- then install a fixed resistor.

Ed

that's actually kinda creative lol. (the pot mod) but if you go fixed, won't it drift again eventually?
 
that's actually kinda creative lol. (the pot mod) but if you go fixed, won't it drift again eventually?

Over a many year span - yes, they all can drift. The drift tends to be pretty low... with a little luck, in 20+ years when you rebuild it again, you may need to find a new adjustment resistor value to tweak it back into range.

But, those carbon film trimmers have a nasty habit of getting contaminated over time as well and even have a pretty good chance of going completely open at the wiper.

In all honesty, I have never had to change a resistor value from the original design value in order to correct a voltage. Instead, I will find the culprit that is pulling the adjustment off in the first place. Often a zener diode tolerance. One thing that I have found is that old carbon composition resistors tend to change in value over the years. They absorb tiny amounts of moisture from humidty which affects the resistor value. Bake the part in the oven and presto - back to original value...for a few years. Or just easier to put in a fixed cabon film and be done with it forever.

Ed
 
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