Intermittent AFP-200 Invalid Response on random devices

New here and this seems like a good place to hopefully get some direction.
I recently adopted an OLD AFP-200 with approximately 75 devices mixed with smokes, and Modules. For the last few years, they have been getting random “Invalid Response” on 10 to 20 devices at a time. The problem can last a few minutes to a few hours, then clear. This is a class A system with 16 AWG solid. At least the parts I have seen.
The trouble occurs as often as twice a week to once every few weeks.

On my first visit, I measured the SLC DC voltage averaging 15.2 to 15.4, with a few flashes at 14.9 and a few at 16.2.
I know there is an AC element and I wish I could find comprehensive information about what voltages AC and(or) DC are min/max. AC readings are 10.5 V
I went to about the middle of the SLC loop (60/40) and broke the loop. One leg (A) measured in the high 14 to 15-volt range and the other (B) in the 15 to 16-volt range. The A leg was noticeably lower by at least a volt, occasionally I saw 13.8 V.

Question: Given the constantly moving voltage, could the system be hitting a sweet spot where the voltage is dropping too low causing random devices to drop into an Invalid Response state?

I think there is either:

A) A device that is drawing more current than it should, or
B) A poor connection somewhere that is sufficient to maintain the integrity of the system to detect a class 6 configuration but not good enough to properly power the devices, or
C) The panel is going out.

The one time I personally saw the issue I did see a Class 6 error, which is why I lean a little to a wiring issue as opposed to a bad device issue.

The system has been doing this for many years, and there are no as-builts, 28 smokes are above a very high gym floor and the wires are in conduit (or are supposed to be).

The entire system needs replacement but that is not going to happen anytime soon.

Any insight someone may have would be appreciated. I am not going to throw a ton of hours at this unless I have a solid plan.


This topic has been moved to fire alarm help

good luck could be anything is a 20+ year obsolete system, need to break down the loop hopefully you will find the faulty device or devices, for sure is time to upgrade

Just as one of my friends did with an addressable 4010 before pronouncing it bad, she swapped out every device that had ever read faulty with it’s modern equivalent. after that, she determined that the 4010 was bad.

Is it the same 10-20 devices that go missing each time? If so, record these numbers and most likley it is a bad connection on the first device on the missing leg or the last working device wiring out. You may need to trace some conduits or disconnect devices briefly to map out how the circuit is wired.