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Vintage Tape (Audio), Cassette, Wire and Magnetic Disc Recorders and Players Open-reel tape recorders, cassette recorders, 8-track players etc.

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Old 19th Nov 2020, 5:31 pm   #1
Magical
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Default Motorola 251 8 Track Motor Resistor Burn Out!

Hello,
Im after a bit of help again. Ive refurbished a Motorola 251 8 track, which runs great....mostly. The motor supply circuit has a 2.7 ohm resistor in series. When running the unit everything is fine and the motor pulls about 60mA then, for no apparent reason, the motor starts to pull 1.5 to 2 amps and fries the resistor. This can take up to an hour to happen. Everything is running free, no binding. Ive measured resistance across the motor through 360 degrees of rotation and get between 5 and 10 ohms, so nothing seems awry there. There is a 1000mF cap from live to earth in the circuit too, but this appears to be ok. Replacing the resistor restores motor function until this happens again.
Anyone any ideas what I need to look for please?
Thanks again.
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Old 19th Nov 2020, 9:24 pm   #2
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Default Re: Motorola 251 8 Track Motor Resistor Burn Out!

The 1000mF must be a 1,000F (micro Farad).

When you say live I guess you mean battery live as against mains live because I think this is a car unit.

Sounds like something is breaking down/shorting down, maybe after it has heated up. If you have a Megger suggest you check the motor circuit isolation resistance to earth, starting with the lowest voltage setting on the Megger, with 1,000uF disconnected.

David
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Old 20th Nov 2020, 12:33 am   #3
Magical
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Default Re: Motorola 251 8 Track Motor Resistor Burn Out!

Thx for your help. You're right I meant uF not mF...doh!

I'll do what you suggest. It is a car unit. I'm beginning to suspect something breaking down inside the motor as the external circuitry to it is very simple...so simple that me as a mechanical engineer can understand it. I have another motor that was duff so I took it apart. There are a couple of capacitors, 3.3uF and resistors on the armature. It's rather strange how it occcurs. The motor spins happily, maybe even up to an hour then wham...full melt down!
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Old 20th Nov 2020, 1:08 am   #4
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Default Re: Motorola 251 8 Track Motor Resistor Burn Out!

In some older literature F (micro Farad) is written as mF, but technically mF = milli Farad with 1 mF = 1,000F
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Old 20th Nov 2020, 4:34 pm   #5
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Default Re: Motorola 251 8 Track Motor Resistor Burn Out!

Several posts moved to a new thread here:-

https://www.vintage-radio.net/forum/...d.php?t=173330
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Old 24th Nov 2020, 2:34 pm   #6
Magical
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Default Re: Motorola 251 8 Track Motor Resistor Burn Out!

Well!!!
I voltmetered it, ohmetered it, ammetered it, meggered it, ESR'd it and who knows what else it and everything measured fine. Fairly confident it was the motor from my investigations and the kind input from David I duplicated the circuit and ran the motor off the 8 track. After numerous stop/startsF, holding the drive pulley to induce and current, no melt down. So I carried on and on and eventually it did it!!
Feeling pretty confident the windings were right I swapped out the lower part of the motor with the brush set with the knackered one I have. Even though the carbon brushes on this were damaged I ran it for a couple of days, various stop starts etc and all ok so I figured it must be a component here.
I swapped them out on the correct brush set one by one and I believe the culprit to be the 4.7F capacitor bridging across one of the brush sets to earth. Even the meter tells me it's fine I suspect an intermittent short. Everything running fine up to now. Am I right or talking shi!te?
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Old 24th Nov 2020, 2:36 pm   #7
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Default Re: Motorola 251 8 Track Motor Resistor Burn Out!

So this is the brush assembly.
It's the one I used to test my theory. I know the brushes on this are damaged but the other components appear to be ok.
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Old 24th Nov 2020, 10:32 pm   #8
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Default Re: Motorola 251 8 Track Motor Resistor Burn Out!

Those capacitor leads look like they are hard up against the edge of the can and in hot conditions could work their way through the sleeve to contact the edge electrically.
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