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Otari MX-5050 BII Problem - Not for the faint of heart!
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DanH
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#1
22nd November 2011
Old 22nd November 2011
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Otari MX-5050 BII Problem - Not for the faint of heart!

Hey guys, apologies in advance for the wall of text, but this is a diagnostic problem and I felt I should include as much info as possible for those willing to help. You may want to zoom out on your browser by pressing CTRL-minus a couple times. The DUT is an Otari MX-5050 BII; I'm rescuing a couple of them from storage exile, and this one has me stumped. Schematics are attached (courtesy of analogrules.com).

The device powers on, and transport controls are functional. All fuses intact. Capstan spins and transport function work when the tension arm is raised; everything brakes properly when it drops. After initial cleaning, I replaced the pinch roller and VU meter lights right off the bat. The device records and plays back, but I noticed the variable pitch control and the high speed/low speed switch have no effect on capstan speed. Other switches and pots demonstrated poor contact, so I ripped the unit apart, cleaned every accessible contact and switch with Deoxit (including the H/L switch on the main control board) and gave it another try. This seemed to restore operation of the pitch control switch (as indicated by the red LED lighting up reliably), so I degaussed the tape path and prepared for alignment. Picture Otari 1.jpg shows a look at the inside of the machine and Otari 2.jpg shows the front.

My cal tape was playing back noticeably fast. Measurement showed my 1kHz tone on the MRL tape was coming out at 1.41kHz. But just for an instant it dropped down to normal playback speed, and the speed switches & pitch control DID work. So the intermittent nature of the problem made me think "bad connections." Before trying to diagnose anymore, I set about closely examining the components.

I reflowed all the relays and several other solder points, and found cap (C603) with a no-longer-existent solder joint. I tested the component out of circuit (test OK), and put it back in. Image below.



Further observation showed some resistor burn-in at the 3W R676 and R677 on the PCB, and referring to the wiring diagram they are connected in series to 38 and 59 AC volts respectively. The resistors are further connected to some plastic caps - could the caps have failed due to a short in on of the power board switches? To be safe, I swapped the resistors with equivalents, pulled all those plastic caps (test OK) and replaced them. Image "Otari 4 R676.jpg" below.



No change in operation - still no speed control. Wanting to eliminate some variables, I swapped the timer drive PCB (the one with all the ICs under the supply motor) and the capstan servo PCB (tucked underneath the take-up motor) with replacements. Still no change in operation.

I removed both the High Speed/Low Speed switches from the front, and the H/L master speed switch from the main control board. Checked for any shorts, and both tested fine. Replaced with no change in operation.

Having eliminated the some variables, it seemed there was no option but to try and trace voltages with a scope. This is an area I'm still trying to master. While I usually can tell what I'm looking at, I don't ALWAYS know what I SHOULD be looking at. I started by looking at the AC voltages to the main control board at the fuses. The AC voltage at many of the fuses were off by several volts, and also showed some serious peak distortion - not the clean AC I had expected. I don't know if this, in and of itself, presents a problem. Here are pics of the signals present at the fuses.













Starting with fuse F701 I followed the trace to its bridge rectifier D701, and observed clean DC on the output.



Followed that up to the electrolytic caps, and again found pure DC.



There was no AC component shows coming off of that cap, which is what I'd expect in an AC filtering stage.



Moving on to bridge rectifier D705, I noticed some noise present on its DC output. I thought it odd, because what little I know tells me that filtering at this stage should be pretty pure.



I coupled to AC to get rid of the DC component, zoomed in the vertical amps and saw this waveform riding the DC output of D705.



Is that bridge need to be replaced? Could these erratic voltages shown at the fuse indicate a faulty power transformer? If the circuit isn't seeing the voltages it needs, or if noisy DC is interfering with proper operation, I guess I can't expect anything to work as intended.

Any help or insight would be greatly appreciated! I'm determined to find the cause of this irritating problem.
Attached Thumbnails
Otari MX-5050 BII Problem - Not for the faint of heart!-otari-1.jpg   Otari MX-5050 BII Problem - Not for the faint of heart!-otari-2.jpg  
Attached Files
File Type: pdf MX5050 BII schematics only.pdf (1.46 MB, 192 views)
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Last edited by DanH; 22nd November 2011 at 10:44 PM.. Reason: spelling, scale some images
#2
23rd November 2011
Old 23rd November 2011
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Hi Daniel,
No experience with this machine, but I used to fix Akai decks, component level.
Did not fully work through your findings, but the first pages, I saw you were dabbling in the take-up/supply motor section (the reels).
I understand, your problem is related to the Capstan motor (tape speed), not to the supply/take-up motors (tape tension/ wind, rewind).
It seems to me that speed control is done INSIDE the capstan motor.
All the capstan motor wires run to fixed VOLTAGES as far as I can see.
Measure the 2x+24, and +5 on the capstan motor connector.
And the 3 speed selector pins. They have to be zero or 24volt, depending on speed selection.
Check/clean switch SW801, and connector/wiring to that switch.
Pitch control has two sections. The single pole is just to turn the LED on.
The double pole part is to select between a fixed, or variable resistor.
It goes from capstan motor-pin4 to motor-pin 3, to adjust INTERNAL electronics.
As far as I remember, speed control worked like this:
Motor gets certain amount of power, and runs a bit too fast (your 1.41 times?).
Sensor (hall, tach generator) picks up RPM from the motor, compares it to a reference frequency/voltage, and feeds the difference back to a "brake" circuit.
I fixed a few pick-up coils, and replaced brake transistors.
Leo..
DanH
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23rd November 2011
Old 23rd November 2011
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Hi LeeYoo,

Thanks very much for the reply. Agreed, the problem is with the capstan motor not running at the correct speed. It seems that it's not reliably receiving the voltage it needs to see in order to set proper speed (presumably with the control selected at SW801 and the user speed switch on the power PCB.) I did pull SW801 from the main board, cleaned and tested. Also pulled the High Speed/Low Speed switch, cleaned and tested.

If I understand you correctly, I need to look more at these areas:


I'll start tomorrow by checking the voltages hitting the capstan motor, and then by checking the other pole on the pitch control shift. I presume that when off, I should be able to measure the voltage drop across the 200 ohm resistor soldered on PB-83C. When pitch control is engaged, I should see a changeable voltage dictated by the 500 ohm variable. (Pic is above).

If the capstan motor isn't seeing the 2 +24V and +5V it should, and whatever comes out of the difference between connectors 3 and 4 (ground and pitch control), then I'll know the problem lies before it, yes?

Thanks again for all your help!
#4
23rd November 2011
Old 23rd November 2011
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Hi Daniel.
Correct area.
My guess is that pin 7 on the motor is the main 24v supply. Clean, and within 0.5volt, regulated by IC702.
And pin 2 it's ground.
Pin 1, the 5 volt for the internal electronics. Clean and within 0.25volt, regulated by IC701.
9, 10, and 11 for main speed selection (24 volt combinations). This comes from lines H5 and H7 (middle of the bottom diagram), and another speed switch (check/clean).
And pin 6 for a safety go/no output for the syscon (it may give a signal to Q601 if the motor is not running, and put the deck in stop mode).
You may find the tach frequency with your scope on one of the two pins in the little cut-out on the back of the motor.
My guess, a sinewave of a few hundred herz, depending on RPM of the motor.
Try tapping the motor with the plastic handle of a screwdriver. There might be a dry joint inside the motor.
Leo..
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23rd November 2011
Old 23rd November 2011
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There are other possibilities for your speed problem. If the takeup tension is too high, or the pinch roller pressure is too low, then tape is pulled through by the takeup motor overcoming the regulation of the capstan. Also pinch roller condition is important. A worn roller can cause slippage. Give us photos of the roller.
The motor itself is probably adjustable. There is probably an end cap to take off, and there will be an electronics card in there, possibly with adjustment pots.
#6
24th November 2011
Old 24th November 2011
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Hi Radardoug.
I wanted to mention the take-up tension and pinch roller, but I didn't because the speed is 41% too high, constant I think.
Pinch roller problems and take-up reel tension are usually in bursts (slipping), and more with an empty take-up spool.
IMHO, the problem lies with the tach generator/feedback, inside the motor. Don't see why adjusting would help, because speed control worked for an instant.
Leo..
DanH
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24th November 2011
Old 24th November 2011
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Hey guys, thanks for the continued feedback. I didn't get a chance to probe anymore yesterday, but I did snap some pics.

The pinch roller is one of the first things I replaced, as the one on there was cracked. The pic attached is the new roller (though is has some streaking on it from tape).

The capstan servo pcb is mounted above the capstan motor. On the card there are three variable resistors to make fine adjustments to high, medium and low speeds. These adjustments are referred to in Otari's calibration manual.

When I initially saw 1.4kHz on the output I thought to make the change there. Unfortunately, none of those pots are having any effect on the capstan speed. I then swapped that whole capstan servo card with another - there was no change to the problem. This led me to believe the problem lay elsewhere.

Some pics are attaches (i think, I'm sending from my phone). Will followup when I get back to test the other parts.

Thanks guys,

Sent from my ADR6400L using Gearslutz.com

Last edited by DanH; 26th November 2011 at 09:38 PM.. Reason: spelling
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24th November 2011
Old 24th November 2011
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Hi Daniel.
I did not see a diagram of the capstan pcb. Do you have a picture?
I think you first have to concentrate on the tach generator.
The coil might be open circuit.
Leo..
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25th November 2011
Old 25th November 2011
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I don't remember how that motor works (optical sensor? Hall effect?) but if something in the feedback (servo) loop is dead the servo board would react as if the motor were running too slow, and try to run it faster. Maybe that's what's going on. I would look at the return side of the servo circuit to verify that the proper signal is there and going where it should go. Somewhere I have the hard to find schematic of the little PC board in the motor assembly, if you need it.
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26th November 2011
Old 26th November 2011
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Hi David.
+1. And exactly what I said.
No servo feedback, no braking, =higher uncontrolled speed.
From memory, those type of motors did not work with optical, or hall pickups yet. They had magnetic pickups. Some toothed gear wheel with a pickup coil.
Leo..
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26th November 2011
Old 26th November 2011
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Quote:
Originally Posted by LeeYoo View Post
Hi David.
+1. And exactly what I said.
No servo feedback, no braking, =higher uncontrolled speed.
I've been reading this thread and find it very interesting. As soon as I finished reading the original post, and DanH said he tried fixing/cleaning every switch and checked all the voltages and changed the pinch roller, I immediately thought of a speed sensor. My Nakamichi MR2 has avery similar problem: speed control pot will sometimes do nothing, sometimes work. I did not debug the problem but am quite certain it's similar.

Quote:
Originally Posted by LeeYoo View Post
Some toothed gear wheel with a pickup coil.
Leo..

Do I hear a Hammond??!?

there's just one other thing I can think of: That 500 ohm pot that controls the speed, it's mounted on a little pc board alone right? One wire goes to the speed control circuit board the other wire goes to ground. If either is not connected, the circuit will not operate properly. And I would guess it will go to one extreme or the other: which seems to be what you have.

Hang in there, you'll fix this yet!

2N
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#12
26th November 2011
Old 26th November 2011
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Quote:
Originally Posted by 2N1305 View Post
there's just one other thing I can think of: That 500 ohm pot that controls the speed, it's mounted on a little pc board alone right? One wire goes to the speed control circuit board the other wire goes to ground. If either is not connected, the circuit will not operate properly. And I would guess it will go to one extreme or the other: which seems to be what you have.
Sure, that pot (or fixed 200ohm resistor) is selected by a switch.
Things like switches, relais and connectors are the first thing to check in gear that has not been used for a while.
I think the OP has already used detox, but testing with an ohm meter (while the deck is off) is a good idea.
David, if you can find that motor diagram, please post.
It might also help people with similar problems in other gear.
Leo..
DanH
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26th November 2011
Old 26th November 2011
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I'm grateful to you guys for all the feedback; I have a ton of things on my list to check when I get back to work on Monday. In the meantime, I was able to snap some shots of the pinch roller and capstan servo pcb (didn't attach on the post from my cell phone). Will follow up with more once I get to run these tests on Monday.

Thanks,
Dan

Edit: To Do List, for my own sanity
- Verify +24V x2 and +5V on capstan servo pcb and capstan motor connector, along with IC702 and IC701
- Check SW801 for functionality
- Verify that pitch control switch does switch between fixed resistor and VR
- Probe the pins on motor cut-out for tach frequency.
- Find the feedback section and check functionality of magnetic pickups
Attached Thumbnails
Otari MX-5050 BII Problem - Not for the faint of heart!-imag0671.jpg   Otari MX-5050 BII Problem - Not for the faint of heart!-imag0672.jpg   Otari MX-5050 BII Problem - Not for the faint of heart!-imag0673.jpg  

Last edited by DanH; 26th November 2011 at 10:05 PM.. Reason: to do list
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27th November 2011
Old 27th November 2011
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I've just noticed on one of the first pictures: Good on you for wearing an anti-static bracelet. I suppose some of the ICs in there could be static sensistive, especially if it's an early eighties model.

I just thought of something: You might check "COUNT" signal at pin 8 of IC2. I'd tell you to check the pin on IC3, but there is no pin number!
this seems to be the feedback signal from the tachometer (or tachometers, as it seems there are two signals up and down) I can't see any other pulse source, even on PB-46Q schematic, where the speed selector switch is located.

Maybe as Mr Kulka pointed out, you have to look for the tachometer in the capstan motor itself. You might have to take it apart...
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27th November 2011
Old 27th November 2011
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Hi Daniel.
In the middle of the PCB, right above/in the discoloured spot, I think I see a dry-joint.
Leo..
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#16
5th December 2011
Old 5th December 2011
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quest continues

Ok guys, hope you're still following. I finally got to put some more time into this. Here's what I found:

I started by scoping both sides of the connections here:



I was able to verify +5V on pin1, and main +24V on pin 7. While in stop, pin 6 shows a few hundred mV of noisy AC, which continues to ride the +22.5 DC voltage when the machine is put in play. It's 1.5V less than it should be, so I plan to trace that back to it's supply and see if there's a filtering issue or something. However, I don't know if that voltage difference is sufficient or relevant in the malfunction the machine is experiencing. Here's a pic:



Pins 9, 10 and 11 each show the +24V it should, dependent on what speed is selected. So with SW701 in High, and the front panel switch set to high speed, pin 10 (15ips) shows +24V while 9 and 11 have no signal. With SW701 in Low and the front panel set to low speed, pin 9 (3.75 ips) shows +24V while others have no signal. This was a good sign, because it would seem to eliminate all the switches and relevant relays from the suspect list.

Finally, I checked pin 4 (pitch control). With the pitch control switch OPEN, pin 4 presented 1.5V DC, nice and clean. With the switch closed and turned full left, pin 4 presented 3V, and swept smoothly down to 200mV as the pot was turned full right. Another good sign, as it would seem to eliminate the pitch control circuitry from the suspect list as well.

It looks like the capstan servo PCB is getting everything it needs - internal power and speed selection info. This leaves the feedback circuit as a prime suspect, right? There are two little screws cutout on the back of the capstan motor housing; no signal was present there.

I'm thinking I need to find any connection coming back from the motor to the servo PCB and verify. And going down this road will likely lead to opening up the motor itself to find the feedback pickup coils. As a first step, I may swap the motor with another on hand to further isolate the motor group as the suspect.

Thoughts and feedback are appreciated as always, guys. Thank you!
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5th December 2011
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wow, now that's what I call an informative post, or thread, rather.

My bet's on the tach in (on?) the motor.
#18
6th December 2011
Old 6th December 2011
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Hi Dan.
Don't worry (yet) about the lower voltage and the ripple on the 24volt line.
A higher current draw from the faster running motor could do this.
(The filter cap (C704) is only 470uF.)
Still waiting for a picture/diagram of the servo board.
Leo..
DanH
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#19
6th December 2011
Old 6th December 2011
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Hey Leo.. cool, I won't stress the ripple yet. Incidentally, when I first started looking at this unit the filtering caps were the first thing I wanted to check. Unfortunately, they are all epoxied to the damn board, making testing them out of circuit very difficult.

Anyway, here are the pics you've asked for. I don't have a schematic unfortunately - it may be lumped into the control pcb mat.

Back of servo PCB. This is actually the board that was ORIGINALLY in the machine. Recall that I swapped it with the board currently in the machine while doing broad brush troubleshooting. Note the same discolored patch towards the top near the 11 pin connetor - there's a 1W or so resistor on the other side.




DanH
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#20
6th December 2011
Old 6th December 2011
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Here's a pic of the capstan servo pcb currently installed. It shows all the wiring from PCB to motor.

DanH
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6th December 2011
Old 6th December 2011
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Capstan motor disassembled


All connections from servo PCB seem to be intact, and continuity checks out ok


What I believe is the tach generator.


Side view of tach generator. Hard to see, but its covered in filth. Will clean it off and check continuity.
DanH
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#22
6th December 2011
Old 6th December 2011
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Well, this looks like it. Continuity between the servo pcb and the pickup is good. But no continuity between the two poles on the tach pickup coil. In fact, it shows a resistance of 2.4 kohms. I really hope this is the root of the problem. Going to grab another one for comparison, and hopefully, replacement.

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6th December 2011
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Just tested another coil from an identical motor. Again, no continuity, but 1/2 the resistance. In my mind, a coil is just a wound piece of wire, and therefore should have continuity from one pole to another. I'll remove them from circuit completely and try again.

Edit: removed from circuit, still no continuity. In trying to see whether or not the coil responds to a magnetic field, i hooked up two probes to the two poles, then observed the change in voltage as I brought a degausser close and backed away. Seems to work in theory. I'll put this one in the motor, and see what happens.

Edit 2: Replaced the tach gen coil with another I had on hand. Not sure how to test these for proper function (both show no continuity?). Upon reassembling the motor, it looks like there's been no change.
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6th December 2011
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Here's the capstan servo board schematic. Remember that DVM's can give erroneous readings when measuring inductors, because hum pickup can interfere with the meter circuitry. Sometimes you need to use an analog meter for transformers, motors, coils.
Attached Thumbnails
Otari MX-5050 BII Problem - Not for the faint of heart!-otari.capstan.jpg  
DanH
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#25
6th December 2011
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Thanks for that schematic. I re-mounted the motor and loaded up a tape to observe the device in full operation. Now instead of constant fast playback, I get variable speed playback!

Don't be mistaken - the pitch control pot and hi/low speed selector switches still have no effect. But playing back a tape recorded a constant (but fast) speed yields playback that slowly rises and falls in pitch. I'd have to get tones and freq counter to tell what factor.

I guess though that this is good news, as making the replacement did have an effect. I just need to figure out what should be coming off that tach pickup and where I can measure it, to make sure feedback is actually coming back into the servo PCB. Any help would be appreciated.

I dont think the problem is on the servo PCB itself, since I switched that whole board early in the troubleshooting process with no change to operation. One possibility: the replacement tach pickup is also bad?
#26
7th December 2011
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Hi Dan.
And thanks David.
Good info here. I think you're very close to solving this.
I would scope the output of IC1 (pin1) with a running motor.
Pin1 of IC1 should have an amplified full swing 20v t/t wave of the pickup coil (connected to pin 6).
This switches Q1 and influences the speed control pot voltage on C4/C29. (Q1 is the brake.)
If there is no wave on pin1, expect the coil to be the problem.
From memory (not sure), coils like that are only a few hundred ohms.
Could also be mechanical, dirt on the gap/ distance to the motor/ magnets on the side? Or Q1/ D1/ IC1, in that order.
Leo..
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7th December 2011
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Not to spoil the technical challenge or anything, but it shouldn't be that hard to find a used motor that works...and you'd have your life back again. Just a thought.
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7th December 2011
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But.... it might be something else.
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#29
7th December 2011
Old 7th December 2011
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Haha David you're right.. I have 5 motors here. But the truth is I'm learning far more by trying to fix this than I would by unscrewing a motor and screwing another in.

Its always an option as a last resort!
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7th December 2011
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Since you have all those spares, have you tried installing one to see whether the motor is actually the problem?

I serviced tape machines for 25 years but had a rule that I would never disassemble a motor. I tried it a few times over the years and found that I always made things worse. (Same with VU meters.) I used to either send them to a motor shop or replace them. You may well be successful, but for me attempts at motor repair always backfired.
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