31vDC from 40vAC...how?

[Alex R]

I'd appreciate your views on what you thought was the best way of going about the following little task:

Now and then I fix amps for people's jukeboxes. But this isn't about the amps.

A jukebox power supply has to run a small 28v motor (this psu just supplies the motor and some relays; the amp has its own psu). However these motors are old and the restorer would like me to get something in the range of 30 to 32v out of the power supply 'to encourage the motor along'. The power supply has a 20-0-20v power transformer running into a full-wave centre-tap (ie two-diode) rectifier. This with a 1000uF filter produces 28vDC. Like I said, the engineer would like a few volts more.

A bridge rectifier would give 40vDC of course, which would need dropping somehow.

The motor is on a 1A slo-blo fuse; the relays on another. Given the max current draw of 2A how would you go about getting the requested 30 to 32v?

By the way there is in the standard circuit a 500ohm 10W resistor from +28v to earth, across the filter. Not sure I know why...?

[Alex R]

...hey let me know if this is too far off-topic & I'll go find a jukebox forum somewhere.

[Bruce / Mission Amps]

Lift the 500 ohm resistor and see if the little <60ma load changes the voltage a little.
I bet it is there to drain off the cap and give a little poor man's regulation to the 28VDC rail.
The DC resistance of the motor windings are probably Way way less then 500 ohms so when the motor starts, the voltage will drop to something pretty low anyhow.
Without mounting a new transformer somewhere, I can't see how to get something for nothing.
Using a FWB will double the unloaded DC voltage but also cut the DC amp load rating of the supply almost in half.
Doing that without knowing what the winding rating is, could lead to trouble.

A 10,000uF@100v smoothing cap might charge up to a larger peak voltage (20vac (times the 120vac line voltage % error) x 1.414 = +28vdc) and it might be enough to act like a storage battery for a few moments while the motor starts.

[Alex R]

Thanks Bruce. Lifting the resistor does very little to the voltage. Good advice about the 'motor start' cap, thank you, I hadn't thought of that. And I won't put a bridge in and burn out the windings either. The guy told me today that the motor only draws about 250mA, but it is on a 1A slo-blo. I think he can make do with a nice big cap. He says the tech who used to do this stuff for him could 'find a few more volts', I was wondering if I was missing something (why do I spend so much time doing that?).

[Tom Phillips]

A jukebox power supply has to run a small 28v motor .... However these motors are old and the restorer would like me to get something in the range of 30 to 32v out of the power supply 'to encourage the motor along'. ...

Alex,
Just a thought here but...
1) Seems like a bad idea to apply more voltage to an old motor to "encourage it along". That's just treating the symptom rather than repairing the cause of the problem and will cause the motor more distress.
2) In my experience with other small old motors, they are usually just gunked up and need cleaning, new brushes, commutator service etc.
3) Even more likely is that the mechanism the motor runs is putting too much load on the motor and it needs cleaning, lubrication and/or adjustment.
Just my 2 cents.
Regards,
Tom

[Alex R]

Fair enough. I must say though that the guy does do a really meticulous job on these jukes. The motors are 50+ years old remember, even spotlessly clean they're going to have a bit of play and maybe be a bit harder to start.

AMIs he does, mainly. Fabulous amps in the old ones, very like the Beam-Echo UK hi-fi models. 2 x 6L6 outputs, bags of wallop.

[Enzo]

Well, I been in the juke box game for 33 years. I'd have to agree that adding a few volts is not the way to encourage the motor. If the motor bearings are that worn, you are going to get an armature pole against the field locking up sooner or later.

If you use the whole winding instead of the center tap, now you will be wasting current in your Vreg.

The motor itself having trouble starting, or is the wear in the mechanism more drag than it likes?

The guy may be a great restorer, but has he done this voltage increase before? Or is he assuming this is the answer to his dilema?

[Alex R]

What it is, he had a partner that used to do all the electronics; he did the mechanics and the cosmetics. Now he buys in help with the amps, and also the power supply. I suspect the few-more-volts was the partner's idea. They disagreed and parted ways, and as the restorer is a thoughtful and reasonable guy I would not be surprised if his faith in the for-a-few-volts-more idea turned out to be the result of residual misplaced respect.

I'm beginning to think that the motor kind of fell between two stools; between the mechanical engineer and the electronics technician neither of them fully understood it. I mean apart from anything else I can't work out a way of getting 2-3 more volts out of the supply without going to a bridge rectifier and halving the current capability of the supply. And I am thoroughly persuaded that I don't need to anyway. So he's getting 28v and a bigger cap, and if I fit a bleed resistor (or whatever it is) I'll maybe use a bigger value than 500ohms.

But would that big resistor help to protect the 28v relays that this supply also runs?

[Enzo]

I don't think the relays are in much danger from anything.

Which AMI box is this?

I mostly rebuild Seeburg amps - there were a ton of them in mid Michigan. And the occasional Wurlitzer. I hate the CObra pickup, but what can you do? I have a little cobra tonearm here to plug in - the RFoscillator wont run without it. DOn't see too many AMIs For a while I was repairing a bunch of ROckola SS amps, but the operator retired them years ago. I have a box of dead Rockola gripper motors on my back porch.

I do work for a few collectors, and now and then I get pre-WW2 amps, like a 1938 Rockola and a Seeburg something not long ago with 2A3s in it.

[Alex R]

It's for a model G I think, one of those with the shoebox-size amp with 2 x 6L6 outputs. But I think it's the same psu as the one they put in the Continentals with the 6973 amps. Little box about 8x3 inches with a transformer on one end and various sockets. Originally they had a black selenium rectifier with fins inside.

Boy I've been learning a lot about jukeboxes. There seems to be a big market for vintage ones over here, many thousands of pounds paid for restored items. This guy I'm talking about seems to be able to make a decent living out of selling and repairing them, mainly repairs in people's homes. The Amis do sound great. He gave me one of the Continental stereo amps to fix on a long deadline. The guy says you have to balance up the channels well or you lose the bass 'because' the LF outputs are combined into a central bass speaker. Know anything about that?

[Bruce / Mission Amps]

Without re-reading the whole thread again, does this one use two Se diodes?
If so, using two Si 1N400X diodes should boost the voltage up.

[Alex R]

No, thank you but it doesn't make a lot of difference. Anyhow I'm persuaded that a big fat cap will help the motor start, and upping the volts is not such a good idea.

I take the old item to be selenium, but maybe not given their performance - they're 50s units, black inch-square items with two thin metal wafers joined at the middle by... something, and comprising two diodes.

[Enzo]

Selenium rectifiers are generally squares of sheet metal stacked with spacer washers between. SOlder terminals come sticking out here and there. To me they look like hot water baseboard heaters from someones house.

COmmon woofer and separate high drivers. The ear doesn't hear stereo at bass freqs anyway.

My company actually was formed for this in 1974, and we did home service on Jukes, pinballs, arcade games, etc. Lansing, Michigan has multiple huge General Motors plants, and tons of highly paid UAW workers. And I think half of them got the idea - HEY, why don't I duplicate the bar we hang out at in my basement. I made a living going around fixing jukes in people's basement rec rooms. And of course I made magic on their pinball machines.

I have a certain number of schematics, check with me if you are looking.

[Alex R]

Yeah definitely selenium then. Don't really know whether there's a silicon advantage as I've never fired up the selenium ones due to fear of the stink they make if they go off, who needs that.

Understand about the common woofer but not the reason why the bass allegedly disappears if the channels aren't balanced. More apocryphal wisdom from the departed partner I expect.

Yeah sounds like the UK caught up with the private bar idea a bit later; still going strong it would seem. Me I'd soon get bored with the same guys in there all the time (and probably suicidal if it was just me and the jukebox playing Misty for me), but whatever... I was very impressed with the sound the big 50s AMIs make though, haven't heard old singles sound so good for years, with big fat clear bass. Took me back to my misspent youth in in the Angel and Elephant, by the park in Widnes, Lancs, used to be a fantastic old jukebox there around 1972, forget what sort... so I guess it's a nostalgia trip for some. Great way to store your old singles collection.

The Queen's Head had a good one too.

Thanks for the offer on the schemos, I'll try not to hassle you too often.