SMPS high output voltage after recap

[baritonomarchetto]

Hi all,

after recapping an old switch (+5V, +12V, -5V), the 5V line output minimum level (with load) is 4.9 V, which is higher than it was in origin. If i turn clockwise the voltage regulator pot, it even go in protection (tic-tic-tic) at middle position... what could it be?

It's like the minimum 5V line voltage rise of about 0,5-1V with respect to the normal operation condition...

Consider that i replaced all the caps with the exeption of the 2 big line filter caps AND that i used general pourpouses JH and Samwha caps (better choices where not at disposal at the local dealer )... could the high ESR being affecting the output thus causing the problem?

thanks

[Th3_uN1Qu3]

Re: SMPS high output voltage after recap

Um, a 5v line is supposed to be 5v. That's why it's called a 5v line, duh. 4.9v is already "low". If it were lower before, it means the old caps sucked even harder than what you have put in now. Btw, ATX tolerances are 4.85 - 5.25v for the 5v rail. Which is +/-5%. Same goes for 3.3 and 12v.

General purpose caps suck for this application. They're only gonna be good for a few months or so. The ESR could well be affecting the output, but you aren't going to see it if you don't have a scope.

[370forlife]

Re: SMPS high output voltage after recap

If this unit dosen't have a 3.3v line, it sounds like an AT unit, in which the tolerances were actually wider at +/- 10% for the 5v and 12v.

[Pyr0Beast]

Re: SMPS high output voltage after recap

It's like the minimum 5V line voltage rise of about 0,5-1V with respect to the normal operation condition...

You mean 'abnormal' right ?

Voltages usually are higher when capacitors are ok.

[baritonomarchetto]

Re: SMPS high output voltage after recap

Um, a 5v line is supposed to be 5v. That's why it's called a 5v line, duh. 4.9v is already "low".

No, this is not correct: the pot allows you the control of the 5V line voltage from lower values to higher (this is because some PCB works at 4.80, another at 5.20 and so on) with respect to the "mean" 5V that should be obtainet at the half position.

I don't think the prob lies in using new caps... at least if the project was a good one.

Is it possible that higher ESR caps affect the circuit thus giving wrong output voltages?

Thanks

[brethin]

Re: SMPS high output voltage after recap

No, this is not correct: the pot allows you the control of the 5V line voltage from lower values to higher (this is because some PCB works at 4.80, another at 5.20 and so on) with respect to the "mean" 5V that should be obtainet at the half position.

I don't think the prob lies in using new caps... at least if the project was a good one.

Is it possible that higher ESR caps affect the circuit thus giving wrong output voltages?

Thanks

I am assuming you are referring to a ATX PSU, if not then you need to state what PSU it is and what its for. For a ATX PSU the trim pot is there so you can adjust the output to 5v and stay in tolerance of 5v +/-5% ATX spec. It is not there for you to adjust down to 4.8 or up to 5.2. Half position on the pot means nothing and does not mean it will be 5v at half position.

[baritonomarchetto]

Re: SMPS high output voltage after recap

I am assuming you are referring to a ATX PSU, if not then you need to state what PSU it is and what its for. For a ATX PSU the trim pot is there so you can adjust the output to 5v and stay in tolerance of 5v +/-5% ATX spec. It is not there for you to adjust down to 4.8 or up to 5.2. Half position on the pot means nothing and does not mean it will be 5v at half position.

thanks for your answers guys.

The switch is not a PC switching but one used on amusement cabinets. It's not an ATX since is from 1989.

Almost all the other similar switch i have give 5V at medium position (or close to it), and this one gave too before the recap.

I can assure you that some PCB needs 4,8V to work correctly, others 5,2, others PCB values between those.

[Th3_uN1Qu3]

Re: SMPS high output voltage after recap

I can assure you that some PCB needs 4,8V to work correctly, others 5,2, others PCB values between those.

Then there's something else wrong with the boards themselves. Arcade cabinets are still computers, just that they were programmed for a single purpose - running a game. Here is a table that shows the voltage ranges for each type of logic, to clear your doubts.

5v logic won't give a rat's ass whether you feed it 4.8, 5.0 or 5.2 volts. In fact it works just fine down to 4.0 volts, tested by me on a Z80 system. Some of the peripherals might start acting up but the main CPU is still alive and well at 4v. On the other hand if there are bad caps on the boards themselves, transitions from a low to a high level will make a big downward spike in their power supply, which may bring the signal below the threshold of 2.5v. However that's another problem entirely. Certainly, a voltage change from 4.8 to 4.9 volts... um, i hardly see how that would change anything in any kind of digital circuit.

[baritonomarchetto]

Re: SMPS high output voltage after recap

Then there's something else wrong with the boards themselves. Arcade cabinets are still computers, just that they were programmed for a single purpose - running a game.

Yes and no

Generally what you stated is true, but especially very old boards had some voltage that need special tuning... maybe because of bad engineering

Anyhow, what can cause an abnormal high voltage on the 5V line on your experience?

[Th3_uN1Qu3]

Re: SMPS high output voltage after recap

Maybe they have some onboard regulators that went bad if they need tweaking to that level. Other than the analog circuits (namely CRT and audio) nothing should be critical about power supplies. Doubt there's any bad engineering in arcade cabinets - otherwise they would've gone out of service much sooner.

Higher than normal voltage can be produced by a feedback loop disturbance. The ESR of general purpose caps is significantly higher that what was on there originally - depending on the compensation scheme, this may cause the supply to oscillate and no longer achieve correct regulation. I strongly suggest you hunt for better caps.

[baritonomarchetto]

Re: SMPS high output voltage after recap

Maybe they have some onboard regulators that went bad if they need tweaking to that level. Other than the analog circuits (namely CRT and audio) nothing should be critical about power supplies. Doubt there's any bad engineering in arcade cabinets - otherwise they would've gone out of service much sooner.

Higher than normal voltage can be produced by a feedback loop disturbance. The ESR of general purpose caps is significantly higher that what was on there originally - depending on the compensation scheme, this may cause the supply to oscillate and no longer achieve correct regulation. I strongly suggest you hunt for better caps.

Thank you very much

Arcade machines sometimes where really badly enginneared: one of the most citet case is Namco Pole Position: even if there are tons of those PCBs it's really hard to find a working PCB nowadays

[Th3_uN1Qu3]

Re: SMPS high output voltage after recap

Eh, maybe they were simply played a lot.

[baritonomarchetto]

Re: SMPS high output voltage after recap

Eh, maybe they were simply played a lot.

hehe, not more than a joust or other williams boards... still working most of the times

[Scenic]

Re: SMPS high output voltage after recap

if you're working on arcade machines you might want to send maxxarcade (forum nickname) a message. he fixes those as his job IIRC.