Tape deck motor speed/voltage inconsistent

[Ian_99]

Hi all!
Been struggeling to find the cause of an old tape deck's motor having inconsistent speeds, causing horrible music. Basically the 12V (electronically governed) motor is struggling to keep it speed the same, going up and down constantly.
Measuring it's input voltage, while running, is giving me around 11.3-11.8V it's constantly changing. It should be 13V according to the service manual (has the whole circuit in it so it's 'relatively' easy to figure it out).
When the motor is off it's a stable 13.8V
My biggest question is, is this an electrical problem in the power supply, or a mechanical problem where the motor needs to deliver more torque than it's designed for?
Thanks to anyone helping with this!!

[tunk]

Never done anything like this, but here is what I would have done:
Find out (approximately) how much current the motor draws. Replace
the motor with a suitable (high wattage) resistor. If the voltage
varies, then I guess it's the power supply. Otherwise, I would guess
it's mechanical, like high friction in a bearing.

Edit: Maybe you could hook off the drive belt and see if it still varies?

[Ian_99]

Thanks for the reply!!
And that could indeed work! Problem is, I can't find any data about the motor only that it's rated for 12V, so measuring might be the only way to get an idea of the current, then hope i can find a suitable resistor, or maybe a potentiometer.
Gonna try hookin off the drive belt next, or just remove and inspect the motor altogether...
I'll keep this post updated!

[Smoky]

I think your best bet is to take the motor apart and to clean it good. The armature needs to be polished to remove the carbon build-up and the bearing surfaces need to be cleaned and lubricated

[Ian.M]

I think your best bet is to take the motor apart and to clean it good. The armature needs to be polished to remove the carbon build-up and the bearing surfaces need to be cleaned and lubricated

Caution: DO *NOT* dismantle the motor unless you know exactly what you are doing.  e.g. old motors with high flux AlNiCo magnets, and some old Ferrite magnets require a soft iron keeper^* of the same diameter as the rotor, to be pushed in as you pull the rotor out to avoid weakening the magnets.  Failure to use a keeper that closely follows the rotor as you remove it can weaken the magnets by up to 50% leading to a motor with half the torque and approximately double the unloaded speed!

If the end bells/caps are bolted, not crimped, its generally OK to remove them one at a time to inspect the bearings and commutator, provided you don't attempt to pull the rotor.  You can do a fair bit of cleanup without removing the rotor.   Beware of brushes mounted to the rear end cap catching on a lip on the commutator as you remove the cap, which can easily wreck them.

* The keeper needs to be hollow to fit over the shaft and should actually push on the end face of the rotor core so there's minimal gap as you insert it.  Its end may need to be notched/hollowed to fit over the windings where they emerge from the core.  Improvising with a piece of mild steel heavy wall pipe that's a good match to the diameter, possibly skimmed to fit, is far better than no keeper, but if the wall thickness is too low or gap too high you'll get some loss of flux.

[Ian_99]

Thanks everyone for the replies! And sorry for my very late reply
I just dissasembled the the deck to get the motor out, surprisingly easy. I added some pictures below i took to this reply of the motor itself.
As you can see it's looking very hard to dissasemble the motor itself correctly, so i didn't risk trying.
I blew out the motor with compressed air but that didn't help anything.
Without belts the voltage that supplies the motor is about 13V as it should be, quite high though.
And after testing again it's as if the motor slowly goes faster...
I ordered a tool to remove the wires which are wrapped around a metal pin, so i can remove the electrical connection as well, and maybe replace the motor, for which i sadly can't find an exact replacement sadly

Caution: DO *NOT* dismantle the motor unless you know exactly what you are doing.  e.g. old motors with high flux AlNiCo magnets, and some old Ferrite magnets require a soft iron keeper^* of the same diameter as the rotor, to be pushed in as you pull the rotor out to avoid weakening the magnets.  Failure to use a keeper that closely follows the rotor as you remove it can weaken the magnets by up to 50% leading to a motor with half the torque and approximately double the unloaded speed!

It feels as if it's a brushed motor, as i can feel it brushing against something while rotating the rotor, so i'm guessing it's not one of those? Hopefully the pics help

Pics:

[Gyro]

Yes, that's a standard brushed motor with a simple linear speed controller board on the back. Typical of the time.

Just a thought, does gently tapping (not turning) the wiper of that carbon preset speed control pot make the motor speed waver?


P.S. It will have ordinary ferrite magnets rather than AlNiCo, but there's little to be gained by pulling it apart and you might damage the brushes trying to put it back together again.

[andy3055]

You should be able to lubricate bushes from outside. Also, there should be a pot on that board. Make sure there is no dirt on the wiper.

[floobydust]

Where is the tachogenerator?

I've fixed similar cassette deck motors. I would desolder the two motor-to-PCB tabs and remove the board.
When the speed control board is removed, you can power the motor alone with a DC power supply at say 9V and see how it is for torque. It can be as if the motor has good torque, then goes low torque, wimpy
Intermittent speed problems I've seen the brushes worn or needing cleaning/alignment. So I take the motors apart, but this done best requires pulley removal.
You can't pull off the back cover of the motor without trashing the brushes! although there might be disassembly pins/holes not sure.

Those blue Matsushita electrolytic caps fail low value, if it's leaking from the bung, ones from the 1980's.

[m3vuv]

that mauve collered cap looks like its buldgingto me in the pics,check its esr.

[m3vuv]

the one next to the trimpot

[1audio]

The holdback tension on the feed reel and the takeup tension could all be way off. Every friction element in that will have changed, a lot, since it was made. if you run it with no tape is the voltage steady? If you can check the tensions on the drive/pickup spindles that may be a clue. Is the head badly grooved? Tape is pretty abrasive and it could have worn grooves that the tape is sticking in.

[kreutz]

In the 70's I repaired a lot of tape decks, the most frequent problem for those were the belt and the motors, I found that most of the cheap tape decks' motors had wear on the collectors and the brushes were metallic thin plates that also had a lot of wear.
Solution get a toy motor, disassemble it and use the collector and thin plates as spares. Be careful and mark the relative position of the brushes respect to the magnets, so when assembling it back it won't turn backwards.

[HB9EVI]

some time ago I repaired an old high end Dual tape deck dating from the late 80'; the motor was not able to drive the mechanics anymore due to the grease which was entirely resinified; the whole mechanics had to be disassembled, cleaned and freshly greased.
even if the motor has a problem, check the mechanics properly too

[Ian_99]

Thanks all again for your replies! Really gave me ides of where to look.
Meanwhile I seperated the motor from the controller board to get a better look at it, below are some pictures of it, looks like a relatively very simple circuit and everything looks alright from a visual inspection.
Also ran the motor from an external 12V/2A power source that definitely ran the motor at a stable 12V and it was still behaving the same way...

Just a thought, does gently tapping (not turning) the wiper of that carbon preset speed control pot make the motor speed waver?

Yes that is something i noticed, when i put a screwdriver in it and just bumping into the pot makes it's speed either go up and down quite a bit, sometimes more sometimes less. Maybe the pot is bad?

Pictures of the controller board:

[Gyro]

Yes, the pot looks well oxidised. You might be able to bodge it with some switch cleaner (although it's the plating on the ends of the track where the leads are crimped that look the worst, rather than the wiper).  A more permanent solution would be a new 1k ohm  replacement. 

[floobydust]

Nice job pulling the board out 
If there's something mechanical affecting the PCB, then go after that. The trimpot wiper looks oxidized, I would give it a clean - I pull a small piece of printer paper through under the wiper between the resistance track, as a form of soft sandpaper to rough off the oxide. You can also measure it's resistance and then just turn the pot back and forth many times too, and put it back to the same resistance reading afterwards. I use DeOxit cleaner on potentiometers the PPE seems to work well.

The board seems to have mystery components, maybe a weird stabistor diode as the blob with green/yellow strip, and what might be an inductor? as the dark green blob with red/brown stripes.
I have seen damaged or weak power transistor cause speed controller problems too.

[Ian_99]

A more permanent solution would be a new 1k ohm  replacement.

Thanks for the asap reply! And just removed the pot, was kinda annoying because it's legs were bended on the PCB, almost ripped a pad off. Also cleaned the pads a bit.

The trimpot wiper looks oxidized, I would give it a clean

Thank you for the recommendation! Sadly i already removed it, and can't put it back on now, cut it's legs off to remove it without ruining the pads completely.
And won't be going to reverse engineering this board with it's weird components (i don't even own an RLC meter)... Unless i have to

[Ian.M]

Do you mean that running the motor from your bench supply without the board still gives very unstable behavior?  It *should* run steadily at around 9V, obviously without speed regulation, i.e. it is expected to slow down if you run the pully against your finger.  If it is at all erratic, then the motor has an electrical or mechanical problem, and further dismantling (or an equivalent new motor) will be required.

If the motor is erratic, lets have a good closeup look at the back of the motor now the board is off to see if there's any way to get at the commutator without wrecking the brushes.  Its fairly obvious that if the three clips are pushed in, the back plate should come off, but:

You can't pull off the back cover of the motor without trashing the brushes! although there might be disassembly pins/holes not sure.

 
*IF* you can gain access safely, at the minimum the contactor can be cleaned and the brush springs retensioned hopefully resulting in smooth running (if the brushes aren't so worn they are a lost cause) and if they are too worn, we may be able to suggest a source for replacement brushes.   If the commutator is FUBARed you'll need a replacement motor and to transplant the pully.

Lubricating the front and rear bearing bushes with ONE SINGLE SMALL DROP of sewing machine oil, spindle oil, or other non-gumming light machine oil would be a good idea.  Turn it by hand to distribute the oil, blot up any excess, run under power then blot again.  Its especially important not to overlubricate the rear bearing as if any oil gets on the commutator when its running, it will rapidly kill the motor.  For the front bearing its less critical, and you can usually run some oil in down a toothpick or similar behind the pully then thoroughly clean up any that got on the pully or body. *DON'T* USE WD40 or similar spray lubricants, as they almost invariably get where they shouldn't then dry out and turn to goo!

[Ian_99]

Do you mean that running the motor from your bench supply without the board still gives very unstable behavior?  It *should* run steadily at around 9V, obviously without speed regulation, i.e. it is expected to slow down if you run the pully against your finger.  If it is at all erratic, then the motor has an electrical or mechanical problem, and further dismantling (or an equivalent new motor) will be required.

I used a 12V adapter, it was easier than to move my old and heavy bench supply, but the behaviour is not noticable by eye. When I insert a test tape that should give a constant 1350Hz signal, it constantly changes and never stays the same after adjusting the trimmer on the motor. So I'll order a new replacement trimmer/pot first and also take a look at the motor itself.

Lubricating the front and rear bearing bushes with ONE SINGLE SMALL DROP of sewing machine oil, spindle oil, or other non-gumming light machine oil would be a good idea.

I'm guessing the big hole on the top of the motor is used to lube it? I don't see any other way of doing it. I'm definitely not taking it apart though, not without having a replacement.

[floobydust]

In the shop, we just put a finger on the motor pulley as a load test using friction. You can see if the motor has some torque and does not cut in and out.
Don't lock the rotor long the motor (or speed controller if connected) will overheat. Use lower voltages even 6V to play around with it.

Lube is only applied at two ends of the shaft, a drop of motor oil 20W or 30W. Don't put oil inside, yuck - or on the commutator/brushes they don't need lube/oil.

[Ian_99]

That's exactly what I did, and it ran pretty smoothly, not too rough. And had quite a bit of torque but not that much, I have no real idea of what it should be but but i think it's good enough for a small motor like this. And I'll try to lube the shaft a little after getting a new pot, still looking for a suitable replacement

[Ian.M]

N.B. Oilite bushings are made of sintered BRONZE, or other copper containing alloys so the oil must be non-detergent with no EP additives that may react with the copper.

[lazyman]

Try with new idler and rubber parts. With tape deck, all this parts begin to return in original state, oil

[Ian_99]

So a quick update on this tape deck
I got a new potentiometer/trimmer and installed it on the motor controller board (forgot to take any pictures) it was an extremely tight fit but it did fit. Now after putting everything back together it was extremely promising at first! Stayed at a stable speed for a long time, music played nicely too!
But of course it had to start acting up again soon
It once again slowed down after a while, even when I set the motor speed with the new trimmer on it.
Motor voltage too was still at around 10-11V might try sourcing a new motor entirely

Edit: Here's a picture of a spectrum plot of about 1mins of a 3kHz test tone playing: