Rock-ola Jukebox Audio Hum Help?

Question

This is a Model #481

The jukebox works completely, but whenever it isn't playing a record (which is most of the time) it emits a hum. It's not "loud", but it disrupts a room's quietness enough that I turn it off every time it's not in use, which is a hassle and makes it feel less authentic/vintage, since jukeboxes in the 80's didn't need to be constantly powered on and off.

I've inspected the amplifier, and the Muting & Squelch board looks fine, with nothing blown. The power supply board in the amplifier also looks okay, but has some apparent burn marks behind several resistors. The appropriate resistors still have the right impedance though, and all the diodes are working.

My guess is that the power isn't being rectified or filtered properly, but since I have no means to test capacitors, I'm at a loss. I've already redone the grounding, to no avail, and tried a noise filter on the 120V input. Any ideas?

Answer 1

A low frequency hum, near the 60hz harmonic or a fundemantal harmonic of it, is usually a sign of a poor ground, or a bad cap in the power supply, or audio amplifier. If this model is a unit that is for records, be sure to check the tonearm ground and the preamp ground connectivity, these are notorious for picking up AC hum if the ground fails.



If the unit is more than 15 years old, I would recommend replacing all the filter caps in the power supply and amplifier. These units are notorious for heat generation, and heat kills caps by allowing them to slowly dry out inside. So a shotgun approach to it by replacing them all would cost about the same as a cap tester 50-75 dollars. Good enginering practice when restoring an old machine would be to replace these anyway.



You mentioned hot spots near resistors...you should change those as well ( very inexpensive ) excessive heating might not cause failure right away, but you can put good money on a bet that they will before too long. Check all the solder connections near the hot spots for "iffy" connections as well, be sure to check the the bottom of the board for lifted traces, near the burned areas and make repairs as needed. Burned resistors can in some cases be a sneaky indication of a weak component in line with the resistor that is increasing the current flow though the component. Perhaps your bad caps are downcircuit from the resistors that are getting too hot !



It's also a good idea to check the grounds on the flourscent lights in the cabinet that are used to light up the exterior and panels, those light ballasts can make some hum as well, and if they dont have a ground...ground the case of each ballast to the AC mains ground ( via the ground wire in the power cord ) with a #16 solid insulated copper wire.



Dont forget when you check the rectifiers that you must pull the diodes out of the circuit before you test them. A full wave rectifier can show a ok test with a basic meter, even while defective, a factor caused by the other components still in circuit. A good diode should show less than 5 ohms forward resistance and at least 20meg reverse resistance on a basic meter.

Answer 2

Rock Ola Jukebox App

Answer 3

If the hum is comming from your speakers..then you got an amp problem....could be a filtering cap...Cap checkers aren't very expensive.