Rockola 490 switched 110v to turntable

Juke-rocks · by **Juke-rocks** » Mon Oct 13, 2014 11:32 pm

I've got a new problem with this machine, and I believe it may lie in the switched line to the turntable. That said, I've no idea what to do about it. The juke operates normally, but at rest, the turntable still moves; very slowly. Making a selection or using the scan switch, the turntable rotates at proper speed. But it never quite stops when it should. I've tested the three lines going into the turntable motor: the green/yellow is properly grounded, the red/green is always hot (117v) and I'll have to admit I'm unsure of the purpose of the white wire. The wiring diagram shows it going to ground after the connectors. Any ideas?
Patrick

clones · by **clones** » Tue Oct 14, 2014 12:45 am

only guessing maybe the play control relay its sticking or needs a clean
clones

by **Ron Rich** » Tue Oct 14, 2014 1:17 am

Hi Patrick,
I am guessing, but it sounds as if the TT motor is triac (aka "BDT") driven ? A quick look at the schematic should tell you that--and if so, I suspect it has become "leaky"--
I sure don't know about the three wires/connector thing--usually green/white (or yellow) on RO's is chassis ground--
Ron Rich

Juke-rocks · by **Juke-rocks** » Tue Oct 14, 2014 2:48 pm

Clones, Ron: thanks for your replies. I failed to mention just where I started on this juke this time. The turntable motor was frozen and hot to the touch when my daughter called me over to her place where this juke was because it wasn't playing. No fuses (all properly sized) blown. After disassembly and oiling the pads top & bottom, the shaft now spins freely; I discovered the current issue where it's trying to rotate when it should not be.
The play control relay on the 490 isn't the jumbo ice cube used in earlier models. It's a small soldered component on the rectifier p.c. board. So I'm not sure how to test or clean it. The record cycle works well, so I'll remember that as a possible issue if this next item doesn't correct the problem.
Also on that board is a SC141-B triac. At this point I can probably get past this by replacing the rectifier board, but that would just kick the can down the road, since the "donor" would be a perfectly fine 494. Can this triac be tested or should I just go ahead and attempt replacement? I'm not planning on using this phonograph as-is, since I can't imagine leaking voltage to the turntable motor could be good for it, and is likely related to the first observed issue.
Patrick

mb9513 · by **mb9513** » Tue Oct 14, 2014 7:27 pm

Patrick: looking at what you said about the control board for this. i would suggest the Tiac. As they are comparable to an electronic switch. I have personaly seen these get "leaky", turning on fully when energised and letting out low voltage when turned off. they are relativly cheap too. i do not have the schematic for your box, but it should have one lead of the triac going direcly to the motor or through a resistor.
The other problem i have had with Rock ola's is they need to be plugged into a properly grounded 3 prong outlet. If there is potential between the common and ground, or if the common is weak, flakey things start happening. Mark

Juke-rocks · by **Juke-rocks** » Wed Oct 15, 2014 1:29 pm

Problem postponed. I had an extra rectifier board and swapped it out. All's well. The probable offending triac is an SC141B and is not to be found locally. I'll find one online somewhere (ideas?) and repair that board. I did check the grounds and they are all good. Thanks for pointing me in the right direction.
Patrick