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7812 not putting out 12v

falter

Veteran Member
Joined
Jan 22, 2011
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6,573
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Vancouver, BC
Not really sure what the problem is here. I have an Exidy Sorcerer 2 that I checked out the power supply on, and found that the 7812 regulator was receiving +19V but only putting out +1V on the other side. Assuming it was dead, I replaced it with a new one.. and.. same deal.

Just unlucky? Or could there be something else going on here?
 
Not really sure what the problem is here. I have an Exidy Sorcerer 2 that I checked out the power supply on, and found that the 7812 regulator was receiving +19V but only putting out +1V on the other side. Assuming it was dead, I replaced it with a new one.. and.. same deal.

Just unlucky? Or could there be something else going on here?

My understanding is that a 7812 doesn't regulate to 12v, it regulates to 12V above the ground reference. Is there anything wonky with ground?
 
Or an almost short to ground. have you checked for resistance from the 12v side to ground? the 7812 output could effectively be the middle of a voltage divider?
 
Most likely the 7812 is in shutdown mode. There is a likely near short to a low Ohmic resistance on the 12V power rail. The usual culprits would be Tantalum capacitors, but as always, there could be other causes.
 
What Hugo says!

Always test for regulated supply short circuits (with a multimeter set to a low resistance range) before powering a machine up.

I always test from each supply to 0V separately and also from supply to supply. It only takes a few seconds to perform, so why not.

If you suspect a regulator fault, after removing it from the circuit, test it on the bench. If it works now, it wasn’t your problem in the first place so you need to check the regulated supply rails further.

Dave
 
What Hugo says!

Always test for regulated supply short circuits (with a multimeter set to a low resistance range) before powering a machine up.

I always test from each supply to 0V separately and also from supply to supply. It only takes a few seconds to perform, so why not.

If you suspect a regulator fault, after removing it from the circuit, test it on the bench. If it works now, it wasn’t your problem in the first place so you need to check the regulated supply rails further.

Dave

I am currently tearing my hair out with a 12v psu with an odd switch and regulator. Has some arrangement I have not seen before to make sure 12v line is always above 5v at switch-on...
 
Thanks Gents. I didn't realize a 7812 had enough 'brains' to do a shutdown mode.. I assumed it would just do its thing and whatever was shorted would simply blow up.

I will start looking for shorts.. er... shortly and see what's up there.
 
So the issue does appear to be on the main board somewhere. When I tested for shorts from the 7812 output to ground with the psu board attached it found a short. When I removed the board and tested again.. I could see lots of resistance. As I check the tantalum in circuit I can see ground and 12 are shorted... is there an efficient way to nail this down or is it simply remove and test each until the culprit it located?
 
You can use a current limited power supply to inject voltage on the power rail with the short and then check for hot components. Start really low at like 1 volt and check for hot components while watching the current consumption. You don't want to go too high or you risk blowing traces.
 
Found it.

It was one of the 33uf tantalums near the memory. I settled on a strategy of removing one leg of each at a time until the short was gone.. and then removing the suspect cap and testing and yeah.. this one is definitely shorted.

So would standard procedure here be to replace all of them?
 
Well done fixing it !

One trick you could do to have found the defective/shorted capacitor, without trial unsoldering, would be to have left it in the state of overload with the regulator supplying 1v. (Or power the board from a current source or low voltage with current limiting) Then with the meter, look for low mV voltage readings on the board. For example with the meter placed on the terminals of that shorted capacitor, it would likely have had a slightly lower mV reading than the other capacitors on the board, because of the higher currents in the tracks and the Ohmic resistance of the tracks, leading to the shorted capacitor's terminals.

I have heard of people applying the correct board voltage from a high current capable source, then the defective tantalum lights up like an Xmas tree letting out the smoke, but I have never subscribed to that in case it vaporizes the pcb tracks.

( I can tell a funny story about Tant Caps. Many years ago I was doing a research project on neonatal breathing and had set up some breathing monitors in a neonatal unit of a local hospital. I had built the monitoring equipment. But we needed a way to record the analog breathing signals. So the Professor in charge of the project offered to loan me his Tanberg reel to reel slow speed tape recorder, I agreed, so we set it up in a nursery monitoring some babies, and we went home for the night. The next day when I arrived for work I was told that my breathing monitoring equipment had "caught fire" and they had to evacuate the unit and the fire Dept came. Gobsmacked I went to inspect the equipment, it all looked ok, except for a "smell". It had turned out that one of the Tantalum capacitors inside the Prof's Tanberg tape deck had smoked, and the smell of it was so bad that everyone panicked and assumed fire. I tried to explain it wasn't my equipment at all that had troubles, it was the Prof's tape deck but barely anybody would believe me).
 
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I am getting some cards from the 1980's remanufactured (a few thousand of them).

They are all currently going through a 400 hour burn-in process (operationally tested for a continuous period of 400 hours).

The highest mortality we are having is..... tantalum capacitors on the +/- 24V to +/- 15V regulated supply rails.

Even though they are brand new capacitors, they still have a failure rate - but the failure rate is still within the manufacturers specification for these devices.

They generally go short circuit and the + or - 24V supply shuts down - resulting in the operator pulling various cards to find the one(s) that are faulty. Just occasionally one goes short during the testing - thus bringing down the test rig.

Of course, the faulty cards go away for fixing under Warranty. I just want cards that successfully pass the 400 hour burn-in process.

Dave
 
Wouldn't it be prudent to just build a backplane with resettable fuses and LEDs for each slot so if a card fails, it just trips the slot and not take down the entire machine?

Some old camera power supplies have distribution boards with a thermistor and an LED for each output, so if and of the outputs shorts, the thermistor goes high resistance and protects the power supply and the LED goes dark. Makes it a hell of a lot easier to find the fault.
 
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It would have been for the burn-in rig!

But, hindsight is a wonderful thing isn't it :).

The backplanes were all built - and to change them would have cost me money.

As it is, we have a fixed price from our Contractor to perform all of the burn-in work (and get the faulty cards repaired under Warranty from their sub-contractor) so I don't particularly care as it isn't costing me any more money!

Each of the cards had been powered up on a current-limited ATE rig at the manufacturing and assembly plant - so this is effectively the second power-up cycle. To be honest there are not that many failures - and you can isolate the failures pretty quickly to 1 of 5 crates of 16 cards. And from there you can 'binary chop'...

There is a calculation of 'expected failures' - and we are currently within the expected limits. In fact, the most annoying thing has been a complete power failure that took out all 9 burn-in rigs. UPS backup?...

Dave
 
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