Tascam 424 MkII capstan motor running fast

[olddawg]

I have begun to think the problem is with the new motor. I have read that there are counterfeits out there. There is a hole in the back of this motor but there is no adjustment pot under it. According to what I have found, this motor can operate with a voltage supply of about 9 to 15 volts so I don't think lowering the voltage will change the speed. I have not replaced any caps or checked ripple but it sounds fine. I don't want to put anything else into it until I can get the speed right. I may look for a single speed motor if I can't get this one working.

The reason I suggested the diodes is because I believe it may be the motor. I very much doubt if new OEM motors are being made. As has been suggested before.... If you have a variable DC power supply hook it directly to the motor and and vary it between 9 and 15v. See if the speed changes. If you don't have that, use a 9v battery. Then a 1.5v battery in series with that and then another 1.5v battery in series with that, etc. If you find a voltage near the accurate speed (using a reference tape) you will know what range the DC voltage must be in to use that motor. Then modify the circuit accordingly. With obsolete stuff you have to get creative sometimes. Diodes are much cheaper than motors. Honestly, I don't understand why you would refuse to even try that approach. But que sera sera.

[GOYA551]

You misundertand me. I'm going to do what I can to get the motor working correctly but I'm not going to start recapping the PS. I do have a bench supply and I will give it a shot. But I'm not expecting it to make a difference. At least it shouldn't.

[GOYA551]

Well that was interesting.
If I feed about 8.8V to the motor and then press play, the voltage slowly drops to about 4.4V and the tape speed is in the right ballpark. (30-45 sec)
After pressing stop the voltage slowly ramps back up.

[olddawg]

What are you using for a power supply? What is the DC current spec at that voltage? Is it adjustable? How much current does the motor draw? You may be exceeding the current available from the power supply.

[Enzo]

Would that 8.8v be a 9v battery?

[GOYA551]

The voltage is from a sweep marker generator that has 4 bias voltage generators . Three are +/- 25V fully adjustable. I thought you might have a point about low available current but a quick check with a 100 ohm resistor as a load and I measure 155mA. Specs for a very similar Mabuchi motor draws less than 100mA. I don't know what the motor was drawing during the earlier test. I guess the voltage was the speed control circuit in action.

[olddawg]

Does the speed of the motor vary as you vary the voltage? That supply may be marginal for testing.

[GOYA551]

The speed drops as voltage drops. What could be marginal about the supply? It has the voltage and the current.

[olddawg]

It's a bias supply. Not sure about the regulation and filtering. Most bench DC power supplies are at least 1 amp. So you might be able to do my diode trick now to get it in the adjustable range? Anyway I was a Teac/Tascam authorized tech for over 20 years back in the day. I worked on a lot of this old stuff. See if it has a 100v/120v switch or is a Japanese domestic model. Japan is 100v/50hz. Sometimes we had to change pulley sizes or other modifications and such to make their gear work here. Did you ever see it operate at the correct speed? Difficult to trouble shoot without the complete service manual and bulletins sometimes.

[GOYA551]

I finally found some exact data re: voltage supply range and current for this motor here:

ftp://ftp.jameco.com/archive/previou...lectromech.pdf

Voltage range 8.4 - 15.6 and current 80mA.

My Tascam is 120V 60Hz. No switches.

I'll try a different supply but I really think it's wasted effort. I have something else that I know is well filtered because I built it. It supplies 180mA.
I'm not worried about regulated because the Tascam supply isn't regulated.
I would imagine in your experience working on Tascams that if you had a bad motor you just replaced it rather than repair the motor? I think the internal regulator is screwed up.

And BTW, do you or anyone else know how the regulation circuit works in this type of application? Is it a varying resistance between pin A and pin B?

[olddawg]

Again. Have you ever seen this unit work properly? Before anything else you need to know if the b+ going to the circuit is clean and correct regulated or not. That's why I said check the ripple. You can get a false reading on some meters and the voltage will appear high with the ripple component.

[GOYA551]

I have been busy replacing the motor on my other deck. When I can, i'll run the supply into my oscilloscope.
Yes, it worked correctly before the old motor quit. Never with the new motor.

[olddawg]

I would suspect it's the wrong or a defective motor then.

[GOYA551]

That's what i've been saying. Do you have any knowledge of the internal regulator?

[olddawg]

You need to look at the old motor. Did it even have an internal regulator? If memory serves me, most of these motors had an "adjustment hole" whether there was a circuit there or not. If it was a standard deck without an external speed adjustment circuit, the motor motor always had an adjustment circuit. That's how you adjusted the speed. But... Many times the units with external speed controls used a plain motor with no internal regulator which still had the adjustment hole. I've seen techs damage these new motors by sticking a screwdriver in the hole scraping up the armature.