Honeywell Accenta fault

[greenhouse24]

I am trying to diagnose a fault on a Honeywell Accenta alarm that's used in a church. A while back the external sounders started going off, although the alarm itself wasn't triggered. The alarm itself and the keypad seem to work fine, it's simply the external sounder that is continually going off.

Disconnecting the D and B terminals on the main PCB stop the external sounders (presumably since there's no 12v running to it any more) and the rest of the alarm system continues to work fine.

Poking the TADB terminals on the alarm (while not alarming) with a multimeter shows:
A to D 11.68V
A to B 300mV but occasionally jumps to 3V
A to T 0V
A to Strobe 600mV

If the alarm is triggered, there is no difference in the A to B reading, it's still 300mV with occasional jumps.

I would expect the B terminal to be high when there's no alarm and low in the alarm condition, unless I'm misunderstanding how it works.

Any ideas what might be wrong, or is the PCB just fried and needs replacing.

 

[sparkymarka]

Probably it’s lost 12v +….or tamper switch issue and the bell tamper in the panel is linked out ….also check battery charging resistor as they can burn out with a faulty battery ….post pictures of pcb..

 

[greenhouse24]

Thanks for the reply.

I'm fairly confident it's not tamper, as when the external sounder is disconnected the system all works correctly - ie the internal sounder goes off if I open the case and the spring-loaded button is no longer depressed.

I'll grab some photos of the PCB next time I'm in the building.

 

[sparkymarka]

ok post pictures

 

[greenhouse24]

A couple of PCB pictures attached.

There's no fuse in the speaker socket as I moved it into the bell/strobe fuse, just to check, and didn't replace it in the speaker holder to save it going off next to my head.
The red and blue wires for the bell are detached.
The blob of blu-tac is holding the spring down for the tamper, to stop it going off.

Thanks in advance for any advice.

 

[sparkymarka]

Oh dear ….. bin time as the battery resistor is burnt out

 

[carlbidwell12]

It does sound like the bell output side of the panel may have failed. On an Accenta, the B (Bell) line is normally held at a higher voltage and is pulled low to trigger the external sounder, so seeing only around 300mV all the time would explain why the bell box is constantly sounding.

Before condemning the PCB, I'd disconnect the wiring to the external sounder completely and measure the B terminal again. If it still sits near 0V with no load connected, the bell driver transistor or related circuitry on the panel is likely faulty. If the voltage returns to normal with the bell disconnected, the fault could be in the bell box or cabling causing the line to be dragged low.