AMI Continental 2 "popping" sound when making selection

Hello All,

I have a Continental 2 (XJDB-200) that has been in my small collection for more than 20 years. I have worked on it, on and off for that time. I classify it as a Grade 2 restoration. It has new Chrome, cabinet painted, new LED lighting. I rebuilt the main power supply, re-capped preamp and amplifier. I set bias of output amp and replaced some marginal resistors. The tubes are in good condition.
I also re-capped the crossover module.
Side speakers are re-coned. Bass and treble drivers are good. The amp, preamp and crossover test ok on an oscilloscope with sine and square waves. There is some slight oscillation in the lower frequencies but not too noticeable to the ear. I have disabled auto mix. There is no coin mechanism. So the jukebox runs on free-play.

There is a new Audio-Technica magnetic cartridge installed, and it sounds good when In play mode.

Here is the problem:

When making a selection, there is a “POP” sound when the sprag relay picks up and the latch relay de-energizes and the latch bar solenoid resets. So, while a record is playing, if another selection is made there is a noise though the speakers when the search wipers find the selection. If I unplug the RCA leads from the control amp, the pop goes away. With shorting plugs also there is No noise. With an oscillator plugged into the control amp, the noise is not there. Originally, I thought it was coming from the cartridge. Maybe the sensitivity is higher than the original GE cartridge. But with the cartridge leads disconnected and shorted, I get the same noise. It seems to be coming from the tonearm wires and persists whether the tonearm wire shield is grounded or not. I rebuilt the wiring harness from turntable chassis to RCA inputs of control amp. That did not help either.

This is a nice playing and sounding jukebox except for this one issue. I’m stumped. If anyone has any ideas, please respond to this post.

Mitchell,
Pennsylvania, US.

Hello Mitchell,
the popping noise is usually caused by switching off relays or solenoids which generates, due to self induction, a high voltage peak in the wiring harness or sparks at the switching contacts. Have you checked the .1uF and .05uF capacitors in the relay back plate assembly? They are connected across switches or solenoids, their purpose is to reduce such voltage peaks. Symmetric wiring (2 twisted and shielded wires per channel) all the way from the amplifiers rca plug to the cartridge can reduce the sensitivity to electomagnetic interference and hum. Avoid ground loops.
Best regards
Jürgen