AR2 assistance with -5v

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Im bench testing my rebuilt AR2 board with just the power brick and the AR2; no game PCB, coin door, etc.. Everything seems within spec except that my -5v test spot is showed -14v. Pulling J10 and testing pin 9 also shows -14v...

That seems quite a bit off from what should be -5v..

Can anyone suggest the sorts of things that might cause this?
 
Replace the 7905 regulator on the right side of the AR. They do go bad.


And make sure you use a silicone insulator pad behind it to isolate it from the heatsink, if it's missing one (which if so, could be why it's damaged). It also requires a nylon screw, not a metal one (which will also cause a problem).
 
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The 7905 regulators are the ones on the heatsync, correct? They area all brand new as I shotgun-rebuilt the PCB..

I just continuity checked them; all isolated from the heatsync...

 
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The 7905 regulators are the ones on the heatsync, correct? They area all brand new as I shotgun-rebuilt the PCB..

Ill re-check the sil-pads; I may have just thermal-pasted them. ooops.



The 7905 is Q9, which is the -5V regulator. Q8 is the 7812 +12V regulator. They are not the same part, and you can't swap them. They each have what they are printed on them. The same goes for the TIP32 transistor at Q2.

You need a silpad on the 7905, and a nylon screw. And you don't use thermal grease with a silpad. The 7812 doesn't require a silpad (but it's ok if it has one, as it won't hurt anything), and can have a metal screw, though they're usually nylon as well. The TIP32 at Q2 does need a silpad and nylon screw.
 
With J10 unplugged, the rightmost pin of the 7905 should read -5V. If it isn't, then it's most likely bad, unless you have a solder bridge somewhere, shorting stuff together that shouldn't be. There isn't much to that side of the AR, as the 36V AC comes in, gets rectified and filtered to make +22 and -22 DC (which are usually closer to +/-24V), then those get regulated by the two regulators to -5V and +12V.

What are you measuring on the +22 and -22 test points?

And where did you get the rebuild kit? Maybe someone got a batch of bad regulators. I honestly I haven't ever seen a new one that was bad, but anything's possible.
 
Ya, sorry.. edit my original reply with a pic.

Q9 has a new 7905 Q8 has a new 7812. Both sil-pad and nylon screw isolated. I continuity checked them both.

Perhaps one was DOA.

I'll volt-check the rightmost pin on the 7905.

the +/- 22 show 24V. (23.9something)

Its an Ian Kellog rebuild kit.
 
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Wow.. this is super interesting.. Mine are ST's. Wonder if Ian knows/knew about this?

http://www.jammaplus.co.uk/forum/forum_posts.asp?TID=5993&title=warning-bad-7905-regulators

I've tested a total of 6 L7905's by St from 2 different batches and they are ALL bad! (well they are good if you want -15v)

I have just been out and picked up 2 Ferranti 7905's and they both test good, so I am 100% confident that it is not my testing procedures.

If you have any ST made 7905's I suggest you either chuck them out, or if you are going to use them at least make sure you test your powersupply WITHOUT your pcb tested to make sure you dont smoke your rams and amps.

You have been warned.

Batch numbers

WCC8N0311
WCC0L0327

I assume there are going to be more then just these 2 bad batches

Can a mod please make this a sticky
 
Some 78xx and 79xx three terminal regulators have a minimum load requirment of 5ma, to regulate properly. Output regulation with currents below 5ma are not always guaranteed. If you are testing it unloaded, try connecting a 1k resistor from the output to ground and then measure it.
 
Some 78xx and 79xx three terminal regulators have a minimum load requirment of 5ma, to regulate properly. Output regulation with currents below 5ma are not always guaranteed. If you are testing it unloaded, try connecting a 1k resistor from the output to ground and then measure it.

Ha! Bingo. -5.0v on. the. money.

Wow.. thanks Kngcbra! (and Andrew as always. :) )
 
Some 78xx and 79xx three terminal regulators have a minimum load requirment of 5ma, to regulate properly. Output regulation with currents below 5ma are not always guaranteed. If you are testing it unloaded, try connecting a 1k resistor from the output to ground and then measure it.


I test all ARs that I rebuild both with no load, and full load, using a dummy test load I made. I've heard about the no-load issue, but I've never seen a 7xxx regulator actually behave that way, as the ones I source have no problem regulating properly with no load. But it sounds like yours are different.

You could just solder the 1K resistor to the output, somewhere on the board where it's convenient (like across C24), as it would bug me not to see the correct voltage without the game board plugged in.
 
Yep, rightmost of Q9 reads -14.14v. I guess its bad.

Looks like its this guy..
http://www.mouser.com/ProductDetail...=sGAEpiMZZMtUqDgmOWBjgBuyVw5VvOi725xyyf3BP3I=

You need to read Bob Roberts My Chicken Little Story
http://www.therealbobroberts.net/7905.html

In a nutshell, I am willing to bet their is nothing wrong with your -5volts. Are you checking the voltage without a load ? If so wire up a 5 volt light bulb to the -5volts and ground and check the voltage again and I'll bet you will have -5 volts unless you do have a bad 7905 which is always possible but check it under a load first.

Copied from Bob Roberts Atari Audio/Regulator Board Notes
Q9 7905 -5 Volt Regulator
http://www.therealbobroberts.net/aar2.html
TO220 Insulator must be used.
Nylon 6-32 Machine Screw must be used.
Do not overtighten screw. Overtightening of nylon screws will strip the threads, so tighten just till the screw snugs up.
Unloaded output -15 volts. Many Atari games do not use the -5 volts even though their manual and/or schematic may show it & hence an unloaded -15 volts at the test point even when the game bd is in use. Pin 9 of P10 connector is the -5 volt output & many game harnesses don't even have a wire in this position.
If you want to see it on video check out my Red Baron video where I test the voltages on the A/R 2 board and about the 26 minuet mark into the video I test the -5 volts and get -15 volts so I put a 555 bulb in line and with the load I get -5 volts.
 
This.
Ignore the output reading on the 7905 unless there is some type of load.
Even a 1K resistor from the output to ground will force it to produce a regulated output.

It isn't bad.
 
OMG. This just got me and it feels like deja vu. I tested like 5x AR2 on the bench and 3 of them -5v regulated with no load so just assumed the 2 showing -13v were bad. I ordered some 7905s, replaced them, Doh! -13v... WTH. Got wise and hooked up a small load and they worked fine then went trashcan diving to grab the <$1 parts.

Sadly, this is probably not the first time I replaced a working 7905 on an AR2.
 
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OMG. This just got me and it feels like deja vu. I tested like 5x AR2 on the bench and 3 of them -5v regulated with no load so just assumed the 2 showing -13v were bad. I ordered some 7905s, replaced them, Doh! -13v... WTH. Got wise and hooked up a small load and they worked fine then went trashcan diving to grab the <$1 parts.

Sadly, this is probably not the first time I replaced a working 7905 on an AR2.

There's a small mod I'll do on AR-II-02's now, when I get one that has one of the 7905's on it that needs a load to read properly (as some do, and some don't). It's one of those 'meant to prevent customer confusion' mods, which I would prefer not to do if everyone was aware of this behavior with 7905's. But the fact is most people don't know, and this will prevent anyone in the future from making the same mistake.

There's a spot to the right of the 7905 where you can add a 1k resistor between -5V and GND, using one of the existing unused holes that is already connected to GND. You just have to drill one small pinhole to connect the other end of the resistor to the -5V trace, which you can solder to by scraping off a tiny spot of solder mask. I bought some teeny-tiny drill bits on ebay so I can make a hole that's basically the size of the resistor lead, so it looks pretty clean.

I don't love the idea of drilling a hole in any board, but I figure it's better than someone needlessly replacing the 7905, or thinking the -5V isn't correct when they go to check it before installing the AR.

Neg5 1k resistor mod.JPG
 
There's a small mod I'll do on AR-II-02's now, when I get one that has one of the 7905's on it that needs a load to read properly (as some do, and some don't). It's one of those 'meant to prevent customer confusion' mods, which I would prefer not to do if everyone was aware of this behavior with 7905's. But the fact is most people don't know, and this will prevent anyone in the future from making the same mistake.

There's a spot to the right of the 7905 where you can add a 1k resistor between -5V and GND, using one of the existing unused holes that is already connected to GND. You just have to drill one small pinhole to connect the other end of the resistor to the -5V trace, which you can solder to by scraping off a tiny spot of solder mask. I bought some teeny-tiny drill bits on ebay so I can make a hole that's basically the size of the resistor lead, so it looks pretty clean.

I don't love the idea of drilling a hole in any board, but I figure it's better than someone needlessly replacing the 7905, or thinking the -5V isn't correct when they go to check it before installing the AR.

View attachment 690146

Can we call this "the AR-II load mod?"
 
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