Tag Archives: EQ2B

Federal-Signal EQ2B Amp Repair

The Federal-Signal Q2B is the classic mechanical fire-engine siren, essentially a scaled-down version of an air-raid siren. The EQ2B is the electronic version, and F-S was so intent on getting the sound right that they put a DSP chip in an otherwise very old-school big-iron transformer-coupled amplifier.

But the fancy-pants front end doesn’t concern the Determined Enthusiast, because this one had a blown-up power stage. Here are the basics:

Green engineering paper = I definitely know what I’m doing

Looking not at all unlike a tube amp from the 50’s, this is your typical Class AB push-pull transformer-coupled topology. Four NPN BUT92’s are used as power devices, driven by 2x TIP35c transistors. Being an automotive amplifier, B+ in this case is essentially filtered battery voltage. A separate winding delivers negative feedback to the preamp.

In this specimen, the two 25A fuses were blown. The first thing you look for in solid-state amps like this is shorted power devices (usually FETs). Even though the BUT92s are BJT’s, three of the four were shorted anyway. They were nice enough to take out the fuses before burning up the 0.025-ohm balancing resistors or the transformer windings. Score one for Federal-Signal.

BUT92 isn’t a common part anymore, but a quick trip to eBay produced a set of four for short money. While the possibility of counterfeit devices on eBay is always there, in this very-not-audiophile application I’m guessing the specs aren’t too critical.

BTW, if you find yourself into one of these, they need to be biased. Federal-Signal kindly provides the procedure, but basically you measure the bias current across the fuse terminals (fuse removed, obviously) and adjust the handy multi-turn pot for 70mA per side.

photo credit: Federal-Signal

Another note: to save wear and tear, the B+ is kept off the power stage by a relay unless the knob on the remote head unit is switched to one of the several positions that make noise. If you find yourself with a dead amp, be sure to turn it on.