HRB Troubleshooting

This is the troubleshooting page for the HRB series of horn relay boards. This little board was designed by our founder to simplify wiring up lights and horns on model locos. Perhaps unfortunately he included many options on this board so that it could be used in a variety of configurations by cutting various links on the PCB. Whilst this does make for a very useful and flexible design, it does sometimes lead to a little confusion and so this page is a starting point to figure out if it is working correctly.

We’ve created these voltage tables to help check if the relays are behaving as they should.

If you are using the HRB-324 with 24V lights and horns [i.e. without link K being cut] use this table.

If you are using the HRB-324 with 12V lights and horns [i.e. link K has been cut] use this table.

We’ve created this video that explains how the HRB works.

And this video that shows how to wire one up.

We have found that the relays on this board are really reliable [apart from the odd occasion where the contacts have been vaporised by a short circuit].

The most common form of damage is that transistors Tr1 and Tr2 get blown. These can be tested using a multimeter on the underside of the board. We’ll add some values in here soon. The transistors are easy to change if you are handy with a soldering iron.

*Note. In May 2025 the RBT was changed, the old 2 position rocker switch for reverse was changed to a 3 position switch that gives forward / off / reverse operation. With the rocker switch in the center [off] position, only one of the horn relays will operate. To get the other horn relay operating switch to either forward or reverse [so that the LEDs come on].