Help diagnosing alarm circuit board - turns on then immediately off

[mmarti91]

Hello!

I am trying to help my neighbor fix his alarm circuit board. It is a very old (1985) Napco alarm panel that recently went out on him. He is older and doesn't want to learn a new alarm system nor pay for a new one. When applying power to the board, the LED comes on with a click of a relay, then immediately back off (the entire cycle happens in about 1/20th of a second.) It's not a big deal if I can't fix it, but it is always satisfying to be able to!!

Does anyone have any ideas what could cause this? I know there are a TON of caps, resistors, transistors, ICs, etc that have performed their duties over the past 25+ years but narrowing it down would help me a lot. So far, I have replaced the biggest caps (2x 35v 1000uf electrolytic axial caps) and the main voltage regulator that outputs 12V. There is another 5V voltage regulator next to the 12V reg but I did not have one of these on hand to try replacing it with. I thought the voltage regulators would be the main culprits since they get so warm and have been running for 25+ years.

If anybody could point me in the right direction, it would be much appreciated!! Thanks!

 

[4pyros]

Do you have a meter to check the outputs of the regulators?

 

[tunedwolf]

I used to see a fair number of faulty bridge rectifiers in old panels, often only one diode in the package would go faulty. Also, don't assume it has to be the panel at fault, the bell sab can sometimes cause all manner of issues, if it has one fitted of course, so try disconnecting the supply to that, same with anything on the aux 12V output like beams, vipers, remote keypad etc

 

[mmarti91]

I used to see a fair number of faulty bridge rectifiers in old panels, often only one diode in the package would go faulty. Also, don't assume it has to be the panel at fault, the bell sab can sometimes cause all manner of issues, if it has one fitted of course, so try disconnecting the supply to that, same with anything on the aux 12V output like beams, vipers, remote keypad etc

Thanks for your very helpful input! So I see that 1 of the 4 diodes in the bridge rectifier has been replaced in the past. I removed all 4 diodes and tested them with my multimeter. Current is only flowing in the proper directions and my meter reads > than .500 on 3 of them and .420 on one. Do you think one of these could still be causing the issue although they seem to be fine? Thanks again, I really appreciate it!!

 

[tunedwolf]

What panel is this, BB5? I haven't seen one of those in the flesh for well over a decade

The readings you got from the bridge look good to me. What do you have on the 12V and 5V rails?

About the only other thing I really ever found in these old panels was shorted decoupling caps on the 5V rail, usually mixed amongst the logic chips. I was lucky enough to have a tone tracer at my disposal though, which got me to within an inch or so of the short. If you are into electronics anyway, a possibly cheaper alternative to a tone tracer, would be a half decent ESR meter, those are generally capable of reading down to the milliohm scale, so in a pinch you could use one of those to trace a short, if indeed that's what you have there. An early Christmas present to yourself?

 

[KeepItSimpleStupid]

This guy has recently been discussed. https://www.electro-tech-online.com/threads/nice-little-component-tester-kit.141912/ ESR or Effective Series resistance is the usual figure of merit.

I'm surprised your just "replacing" instead of first measuring the +5 and +12 V supplies first. Shorted decoupling caps are a definite possibility. These are small values caps at the power pins of IC's. With the power off and measuring ohms across the power supply rail in both directions COULD be an indicator of a short. Finding a random short is difficult It's usually a matter of luck or using a tone milliommeter, Sometimes there is evidence of a blown component.

Ripple (especially measured with a scope) or just a measure of the AC voltage on a DC supply is also a figure of merit.

 

[4pyros]

Number one, check your 5 and 12 volt supplies.

 

[Cicero]

Number one, check your 5 and 12 volt supplies.

Agree. This should be done first before any replacing. If the 5V and 12V supplies are solid and stable after everything else goes off, then its not anything in the power stage.