Rock Ola 473

I have been working with the amplifier during the day and the first thing to check was the fuses. The fuse for the left channel driver board was blown and the main fuse for the left channel had been substituted with a 20A fuse instead if the 3A that should have been there. It’s obvious that somebody had tried to repair it before, without success. The service solution used was instead to reconnect the woofers and play the sound at the right woofer and left mid/high range speakers. I suppose it was done to not make it to obvious that one channel was quiet. Since the rated current of main fuse was way wrong the fuse for the driver board blow instead. Luckily nothing was harmed by the wrong fuse, if I swap the right driver boards to the left channel I have sound in the left channel, so the power transistors are still OK. When measuring through the faulty driver board I found that the transistor Q6LR was shorted from the collector to the emitter and everything else seems to be OK. Unfortunately I didn’t have a replacement for it in stock, but it is on order from my supplier. I will replace it by a TIP42C and since it is not the original transistor I also will replace the Q5LR with a TIP41C to have a reasonably matching pair.

 

Since I couldn’t finish the repair of the amplifier I instead took a look at the display problem. It’s a quite interesting construction where the display segments that should not be lit are shorted through a matrix of diodes and the selection indicator PC board. When measuring through the diodes I found five that was shorted and five that was leaking in the back direction. Once the diodes were replaced the display gave the proper record numbers.