Space Invaders 5v drop under load

[DayOff]

Hi all,

I'm working on a couple power supplies for a SI... I have both PS working with the PCB disconnected and 5/-5/12v dialed in on both. On the first one, it works great and game is fine. On the second one, the +5v just drops to near zero.

Everything is pretty much replaced on this board from a cap kit and things look on track, but what components would be involved in the 5v just dropping completely? I read a few past posts that it may be related to something on the actual game board, but I'm not getting that issue with my other PS, so I'm thinking it's got to be on this power supply.

Any suggestions are appreciated...

Pete

 

[DayOff]

Alright, running in circles on this one...replaced LM305, 2N3055, 2N2905, some of the caps (Again), the L7905 for kicks, the 5w ceramic resistor, nothing changes. I even put the arcadeshop pcb with a switching power supply on the board and it boots right up..

Any other thoughts on what may be happening on the power supply board to not be able to carry a load without the +5v falling apart?

Pete

 

[mdeslaur]

Have you reflowed the connectors?

 

[ArcadeTechGW]

You might be tripping the current limit on the LM305. Look at the resistors that line up in the diagram, R8, R9 and R10. Lift a leg and see how they look. One of them may have failed, causing the LM305 to shutdown on high current.

That, or check R5 in the feedback part, which can cause a Compensation Shutdown.

 

[DayOff]

How are you referencing R8/R9/R10? (My diagram doesn't reference that....are you meaning the four resistors between the 5v adjuster and the big cap, or the ones above it?

 

[andrewb]

Where are you measuring the 5V? Try measuring on the PS itself, not the game board.

You might have a cracked trace or other bad connection on the PS somewhere. There isn't much to these, they're basically the same as an Atari AR. If you've replaced the 3055 and LM305, those are the main parts that will cause low 5V. But you might have something else like a bad pot, bad joint, dirty connector pin, or cracked resistor that isn't obvious.

(One trick to finding cracked resistors is to push on them sideways. Healthy resistors can tolerate pressure. Cracked ones will break.)

Tone out all connections on the PS with your DMM. Don't trust your eyes.

 

[DayOff]

I'm measuring 5v on the PS and it looks ok, then hook up the board and still measure it on the PS, where it's now low. (I'm measuring it on the metal strap by the black connector).

 

[DayOff]

I tried reflowing the connector pins, doing the deoxit magic on the pins, checked all the continuity, tried some magnification to look for any other questionable areas, flexed all the resistors/other bits to see if they were toast....nothing... I hate throwing money at this thing by replacing stuff haphazardly, especially since the switching solution is $10 than a cap kit and and afternoon of soldering...but kind of the principal of it not working is bothering me more now

 

[andrewb]

Where did you buy the parts that you used as replacements?

 

[VectorCollector]

(please don't say ArcadeShop, please don't say ArcadeShop ...)

 

[VectorCollector]

Is the SI PSU the same as the SI DLX PSU?
If so, suspect the 0.18ohm current sense being higher value than spec'd (but your average DVM won't measure this low ohm correctly).
And review the 27ohm, the 2N2905 and 4R7 resistor.

As noted previously, when these type of linear power supplies are not regulating with load, it is typically caused by the 2N3055 pass transistor or the LM305A controller.

If you have a working unit, you can do a side-by-side comparison of impedances to "possibly" find the fault or narrow down the area where it exists.

 

[andrewb]

Yeah, VC has a good point. Making sure all resistors are correct per the schematics is another good idea.

If you've never seen this PS work correctly, it's possible someone put a wrong-value resistor somewhere.

Scrutinize any parts that look like they don't have the original solder on them (i.e., anything that has been replaced).

 

[Alpha-tron]

Every time I think I have a bad power supply it is something else.

Check all tantalum caps on the power rail lines.

Check all of the all caps on sound pcb and not just the freaken electrolytic caps.

I can't tell you how many idiots will assume when I say all the caps they will just assume I mean just the freaken electrolytic ones. Speed reading techs really need a butt kicking. Let the beating begin. Laughs

Replace the power supply with a switcher version is just a temp fix and it will fail after a while. None of the L shape space invader pcb style got a permanent fix by adding a switcher.

Fix it right the 1st time and it will keep running for decades. Do something stupid and don't call me cause you cannot follow the simple instructions of Check all of the caps.

 

[KazooBR]

Sharing this thread with interest. I like a good mystery. We all learn from the troubleshooting tips that get shared.

And, I have a Taito SI with a smoking power supply so I need some inspiration.

 

[DayOff]

Where did you buy the parts that you used as replacements?

Actually, I'm not even sure... not sure if that's worse. Probably Bob Roberts..this board has been in a box for a while, and I'm not sure how it even got there... I'm sure it was some generic cap kit from a while back and probably didn't work off the bat, and I canned it until I had more cycles to get creative...

Just a few notes from the other comments (all appreciated!):

-All the solder joints look like they've been touched now, pretty much... it's quite a workshop at this point.
-I'll circle through the resistors and see...there's definitely some non-original ones in there...small blue ones and such..so I'll compare it with my working board and see if it helps narrow it down

Question...the comment about the switching power supply masking a problem on the sound or game board...wouldn't that come up with my working power supply? Seems to me like the switching supply wouldn't just be able to strong arm itself through a game-board based issue causing a 5v voltage drop... I also have a couple game boards that work and the one bad linear PS acts up on both of them..

Will circle back with details once I have some updates..thanks all!
Pete

 

[DayOff]

Ok, got it going! It ended up being the resistors... with a componded problem. For one, a few of the resistors colors didn't line up with the schematic. Even MORE fun, is that the bag the resistors were in from the arcade vendor were mislabled! So I probably started into this years ago and replaced a few things and the schematics matched the label on each resistor bag, but whomever sent me these things was probably a bit color blind...as it was usually a blue instead of a gray, a orange instead of a red, etc. etc..


So...moral of the story is to make sure you're actually putting in what you think, and also make sure when they don't match what you're taking out, you're at least checking another reference to get it on track.

I think these were the ones involved int he 5k
910ohm
1.8k ohm
4.7ohm 1/2watt

...and I replaced these two as well...the 1K and 2.2K next to the single 1N4004 diode on the top of the board.

Thanks for the ideas and making me check those resistors! On to the next repair
Pete