Marantz PM94A long standing customer came in today, carrying this Marantz PM-94. I didn’t know what to attend to first, the amplifier or the owner’s hernia.
The complaint was that one channel was dead. Thinking to myself, that will be an easy fix, probably a selector or mode switch being dirty. I powered up the amplifier and left it whilst I deal with another valued client. Happened to be an attractive young lady, not that that had any priority. I went back to the amplifier and found that one of the large heat sinks out of the two was cold. This meant that we had a bigger problem that I first thought.
With input signals, dummy loads and an oscilloscope hooked up, we found there was a small distorted output on the faulty channel, but as the volume was increased, a horrid output appeared and the amplifier went into protection mode. It seemed that the DC conditions were all wrong. These amps are not the easiest to work on live, almost impossible unless you have the extension harnesses that the manufacturer made at some point. So, the best you can do is to remove the faulty amplifier module and make loads of cold tests.
This is a sort of two amps in one idea. Starts out as Class A then when the demand for some noise is there, crashes into its reserve section via a synchromesh gearbox to give you plenty of I squared R. All the big transistors seemed to be okay. But, one in the driver stage seemed to be open circuit, unusual. A bit more investigation and we found a couple of Zener diodes and associated capacitors and resistors were. These were responsible for the DC conditions in the driver and output stages. Replacing the components and making up the correct Zener values using several Zeners in series, we were ready to reassemble.
Once all reconnected, I powered up, having no confidence that things would work. However, joy of joys. The speaker relays clicked in and all was good. Setting up of the quiescent levels done, the amp worked very well. Another happy customer….until he sees the invoice.