Alarm panel won't drive bell current

[blueman <NOSPAM@xxxxxxxxxx>]

I have a DMP XR200 alarm panel with the bell output driving an MPI-11
siren driver. The panel bell output supplies 12VDC @1.5A max via an
output relay.

When the siren driver is connected, I can hear the bell output relay
switch on but then within about 1/2 second the relay immediately switches
off -- this occurs in both pulsed and steady alarm modes.

The siren driver itself seeems to be working and the panel itself seems
to have sufficient amperage, because when I connect its inputs directly
to +12VDC on the panel, it properly drives the alarm speakers at full
volume, drawing only about 0.4Amps. My oscilloscope confirms that the
output is the expected pseudo-sine wave.

This led me to think that the problem must be with the panel bell output
circuitry.  However, when I disconnect the siren driver, the relay stays
on and the voltage to the bell outupt is ~12VDC as it should be. I then
tested it under load using a 10ohm 10W resistor and even under load, the
relay still stayed closed -- using my DVM I verified that the panel was
sourcing ~1.2A without tripping the overcurrent protection. So, at least
with a static load, all seems to work fine on the panel.

I then noticed that the siren driver has a 2200 uF cap across its
input. When I put a similar 2200uF cap across the panel bell output, it
caused the relay to switch off. Similarly, when I temporarily removed
the 2200uF cap from the siren driver, the siren driver board no longer
caused the relay to shut off. (I also tried swapping a new 2200uF on the
siren driver and it still caused the relay to shut off)

So I can only conclude that the transient current draw used to charge
the 2200uF input capacitor is somehow tripping some internal overcurrent
protection circuitry on the XR200 panel. Note that tripping only causes
the alarm output to stop -- the rest of the panel functioning is
unchanged.

Until now, my alarm has been working reliably for the past ~10
years. So, it seems like something has gone wrong in the current
oversensing circuitry for the bell output.

Has anybody experienced similar issues before? Any thoughts on how to
further troubleshoot & fix?

One other aside which may or may not be related. The alarm a few days
ago started showing a battery trouble which by itself didn't seem
worrisome since the batteries are about 9 years old. Meanwhile, I have
new batteries on order.

But this still leaves me to wonder whether the two problems may somehow
be related. Did the battery issue trigger the bell output issue or
vice-versa? Are the batteries even bad or is the 'trouble' really just
triggered by something wrong with the board. (Note: I only discovered
the issue with the bell output after I noticed the battery trouble but
it may have preceded that too)

(note: disconnecting the batteries did not stop the problem with the
siren driver -- I thought initially perhaps that bad batteries could be
drawing too much current but removing them had no effect).