AMI MM-1 amplifier

A project from the past autumn

 

This is AMI’s first solid state amplifier. It is a stereo amplifier and the electronic design is much like a tube amp, with drive transformers, output transformers and two NPN transistors in each of the output stages. It was completely dead when I got it and it was a quick fix to get it up and running again. It had been in the hands of another repairer who didn’t manage to sort out the problem before I got it. And he had actually found the fault, but made a little mistake when fixing it. The problem was that the 26.5 v supply was missing and the cause was the 27V zener diode, Z1. But when replacing it he turned it the wrong way round and all I had to do was to de solder it and mount it the right way. I played the amplifier for a couple of hours using an MP3 player as signal source. It worked fine and I returned it to the customer again, case closed.

But I got it back again a couple of days later with the complaint that there was way too much background hum in the sound. One disadvantage to have only the amp and not the rest of the machine is that it is hard to be sure everything is OK. I run the amp at the workbench again, and there are some hum in the sound when playing at full volume, but not more than I would expect. I checked the ripple of the +-38 voltage and it was OK.

Since I couldn’t hear any unnatural hum at my workshop I brought it back to the customer and connected it to the box, and there was quite a lot of hum in the sound when playing a record. I also noticed that the volume control had to be turned up quite high to get normal listening level.
We inspected the pick-up together and the stylus didn’t seem to be pushed in to the pick-up properly and the cause turned out to be the wrong stylus in the pick-up. With a proper stylus the pick-up gave a much higher signal and there was no need to turn up the volume control as high as before and the hum was gone. Another thing we found out when inspecting the pick-up was that it was mounted the wrong way round. This sound a bit strange but the tone arm is mounted at the left side of the turntable in this jukebox, at a normal record player the tone arm are to the right instead.

The pick-up has to be turned 180 degrees at the tone arm if it is mounted at the left side of the turn table. Else the stylus might do a pole vault and break while playing a record.