CAJA, amplificador de madera!

The Box

The brand of this amplifier is quite easy to work out, Incluso para mí. Una vez dicho esto, there seems to be little information on this make. It came in with a report that the volume was either nothing or full, same for treble and bass. The reverb suffering the same symptom. The more worrying issue was that there was a rattling in the cabinet and sometimes if it was nudged, there would be a loud buzz and banging noise?

The case was an all wooden construction, somewhat homemade. But with the chassis removed, what I thought to be a home brew amplifier was quashed. The Insides looked very ‘70s with a PCB silk screen with BOX. Fairly standard set up with a couple of 2N3055s for output. The first thing that hit me, literally, was a make-do mains filter suppression lash up. Someone had soldered 4 x 0.22uF on flying leads across the mains switch. I guess to stop a pop when switched on-off. These were left to dangle in mid air, with mains power hanging off the ends. They would have been touching the frame of the speaker. No wonder there were some strange noises. At least they were X2 rated! These were removed and a genuine snubber wired across the switch.

On power up, there was quite a loud background hum and all the potentiometers were gummed up and so were given a goodly squirt of magic fluid. Now we had variable volume, bass, treble and reverb, after sorts. The reverb tank had been hard wired, but bad soldering meant signal feeds were shorting to chassis.

Hum was due to the power supply filter capacitors being almost open circuit, so these were changed. Every now and then, there was a bit of instability when the input was touched. This was traced to noise on the plus and minus supply lines to some OP amps. The filter capacitors, old Philips types had wire out leads almost corroded away and had no farads left in them. So changed.

With everything tided up and a good clean, the amp worked really well and the owner sat playing his guitar through it in raptures. Happy days.

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