Rockola 442 went from buzzing to dead latch bar solenoid

[jukerocks]

    The RockOla 442, from 1970, has the alphanumeric keyboard. The latch bar solenoid should pull in when connected through the credit switches and relax after credits are exhausted following letter then number button selection. I have the service manual and wiring diagram, but some of this might as well be in a foreign language for all the sense I can make of it. All connections seem secure.

    This phono has been working now for a couple years, with the latch bar solenoid sometimes buzzing after credits are established. I’ve never let it buzz very long, since this heats the sand bar resister. The buzzing could be stopped by either pressing the reset button on the keyboard, pressing any letter button, or by manually pulling the solenoid plunger.

    This has been an intermittent problem that I’d never been able to solve and now it’s worse. It started buzzing very weakly, and then the solenoid stopped working altogether.

    These are my observations: I have checked resistance on that solenoid at about 13 ohms; same as another on a working machine. The 25vac is being supplied through the 3a fuse when the credit unit is moved, and there is continuity on the other side to ground. The diodes above the solenoid are warming but the sand bar resister is not, but this makes sense as its purpose is to prevent overheating of the solenoid which is not energizing. Measuring voltage with wires disconnected to the coil, I see 12vdc until they are connected to the coil, then nothing. I have tried connecting another solenoid that checks out with that same resistance but it does not energize either. I’m uncertain how and where this gets from 25vac to 12vdc.

    I’d greatly appreciate help with this. I’m weak at the component level here so any suggestions are welcome.

Patrick

[Ron Rich]

Hi Patrick,
I am guessing here as I do not have a schematic-- check the switch in the pricing unit--be sure it makes well--use an Analog meter to test it-- Check the sand block resistor ( they are supposed to "get hot" when current is present). Check continuity with that meter on both sides of the solenoid--looking for a poor solder joint all the way from the solenoid to the power supply transformer. HTH Ron Rich

[jukerocks]

Thanks, Ron. I'll check all of that. You wrote, "Check continuity with that meter on both sides of the solenoid". I checked for solenoid continuity by probing the two leads on the coil with each other. Is that incorrect?
Patrick

[jukerocks]

It appears all parts in the credit unit are working correctly. 25VAC is being supplied through the ADR and the credit switch closes the ground. The trouble seems to be in the array at the solenoid. I’ve probably got this wrong, but I believe, according to the manual and wiring diagram, the completed ground should pull in the solenoid, which opens a switch sending power through a loop involving a sand bar resister that prevents the solenoid from overheating. There are also two “plastic rectifiers” with the positive ends facing each other and the connection in between supplying 12VDC.
The solenoid itself works just fine if I jumper a hot connection to it. But will not work in the machine. Before complete failure, the solenoid buzzed. The resister used to generate a lot of heat (there is a shield over it). Now it does not, but the two rectifiers get a little warm. The switch involved tests OK.
I’ve spent a good deal of time with a meter to try to figure this one out but no luck. I do not have another phono with this type of selector system to try out parts against, so any advice will be appreciated.
Patrick

[Ron Rich]

Patrick,
What value is the resistor ? Is it good ? Are the diodes good ?
Ron Rich

[jukerocks]

I’ve got the selector out. Looking at the leaf switches it looks like I’ll need to make some adjustments before reassembly. The resistor checks good at about 15 ohms . It’s 15 ohm, 10 watt. But the two diodes differ. The one on the left shows voltage in one direction only but the other shows voltage both ways. The Rockola part number is 44875 “plastic silicon rectifier”.
I went to an electronics store where they did their best to help with no legible markings on the original parts. They sold me a pair of “1N4743 Zener diodes 3.3-30v.”
The originals convert 25vac to 12vdc. Should these be the right replacements? If so I’ll take that all apart, swap parts then try it out. Patrick

[Ron Rich]

Patrick,
How are you "checking" the diodes--in or out of circuit--with an ohm, or diode meter (or setting on a multi-meter )?
A "Zener Diode" does not replace a "--rectifier" ! Take that back, and purchase a 1N4004 or better--Replace both of them--
Blade adjustment, is covered above in the FAQ's-- Ron Rich

[jukerocks]

Ron: I have tested them in place with all wires disconnected with the diode function of a multimeter. I have ordered a package of 1N4004 rectifiers. I have the manual's specs on blade adjustments and I've read the stickies. Going forward, how do you spec out replacements for small electronic components in these when there is so little information given?
Patrick

[Ron Rich]

Hi Patrick,
Gotta test those diodes out of circuit-- specs. are tricky--just need to know how the component should work, and make an "educated guess" if no cross ref. has been provided. Ron Rich

[jukerocks]

Holy smokes, it works! The buzzing, then nonworking, solenoid was caused by the failure of one of the two rectifier diodes. Thanks, Ron, for the part cross-reference. I removed and replaced both diodes (1N4004) (along with the large resister since I had it off to replace the diodes) after photographing each connection to make sure I'd get everything back in place. In spite of the fact that I'm truly unskilled at soldering small parts (I burned the insulated connection board in two), it's all working now. Prior to reinstalling the selector I cleaned and adjusted all leaf switches. I've already observed that the solenoid operation is quick and silent and that the whole assembly generates a lot less heat than it did before failure.
Patrick

[Ron Rich]

Hi Patrick
Phunny how replacing a defective part makes it "work" ?? Glad you got it enjoy your music ! Ron Rich