Tascam 424 MkII capstan motor running fast

[GOYA551]

After replacing a dead motor in a Tascam 424 MkII, the new motor is running too fast.

The old and new motor have the correct part number.

There is a thread about a 464 with pertinent information here: http://music-electronics-forum.com/t40362/

The most important info in that thread is the 424 speed board schematic:


My 424 is getting a little over 15V between pins A and B. (Should be 12V)

Where should I begin to find the cause of the high voltage.

BTW, my first post here. Nice forum :-)

[GOYA551]

My 424 is getting a little over 15V between pins A and B. (Should be 12V)

My first post and I'm already screwing things up.
I meant between pins + and -

[olddawg]

To begin with... Are you using the original pulley and the correct belt. How fast is fast? Do you have a reference tape to do the speed adjustment procedure in the manual?

[GOYA551]

Yes, original pulley and belt. I can't quantify the speed increase but it is obvious. I can make a reference tape but I would think the first efforts should be to correct the motor supply voltage.

[Enzo]

You didn't post the power supply schematic, so we don't know where the 12v comes from. It may not be regulated, in which case it is not adjustable, and probably doesn't matter. The speed is controlled by those external circuits which A and B inform.

If the old motor was running slow, anything you recorded then will play back fast now if the speed has returned to correct. So it is a valid question: is it now running fast or at correct speed. If it is running at correct speed, then your 15v doesn't matter.

[Mark Hammer]

I have a service manual for the one I own. If you can wait a few days, I can scan the schematic; assuming one doesn't show up here before then.

[GOYA551]

I don't have a schematic other than the one already posted. I found one for sale but hoped to avoid that added cost. Mark, thank you for your offer. It will be very helpful and appreciated.

Enzo, I haven't recorded anything on this machine. Low speed is the cassette standard 1 7/8 ips. High speed is 3 3/4. At low speed, with the pitch control at it's slowest and the low speed adjustment pot on the speed pcb at its slowest, thr machine runs too fast. This was an ebay purchase and the old motor quit very shortly after I got it.

[g1]

Is the pitch control working? Is the pinch roller fully engaging the capstan?

[GOYA551]

The pitch control works but it seems to be less of a speed change than it did when the old motor worked. If that isn't clear, I think the pitch is supposed to vary +/- 12%. It now seems to vary no more than a couple of percent either way. Just enough to notice.

The pinch roller appears to be fully engaged and the tape speed is steady.

[olddawg]

Are you sure the motor is an exact replacement? I've played games like this before. The internal regulator in he motor may be different if it has one and is a generic replacement. If you find there is nothing wrong with the power supply and speed adjustment circuits, you can always try forward biasing a diode in series with the motor supply wires. Each diode will drop the supply voltage by .6 to .7v. Might get you to an adjustable range.

[GOYA551]

I'm sure that the part numbers were the same. I suppose it could be mislabeled. I wouldn't consider it likely but anything is possible. I know that the correct motor is a 12V motor and my machine is supplying more than 15V.

[Enzo]

personally I am convinced the voltage is not the issue, the speed is controlled externally. But if you are convinced it IS the problem, then a simple test is to disconnect that 15v and connect a bench supply of 12v in its place. If the motor slows right down to proper speed, I'll tip my hat to you.

You will still need a standard to calibrate the speed when you are done. I use commercial test tapes, but a poor man's test tape is made by recording something on another deck that we know is correct speed, then playing it back and beating the signals together. For example play an A on a keyboard, then move the tape to your deck and play it back along with the original keyboard and beat the notes to zero.

[GOYA551]

Enzo, I defer to your expertise. I actually thought of connecting 12V to the motor but if you feel it is a waste of time, I won't. As far as calibration, my plan was to do something very similar. I was planning to play the known frequency back into an electronic tuner. I thought that would be more accurate than the beat method. But coincidently, I just discovered that the capstan motor on my main Nakamichi deck died. When it rains it pours.

If the speed is controlled by external circuitry, how should I begin troubleshooting? Measure voltages at the A and B pins?

[km6xz]

It has been years since working on a 424 but I remember the supply for the motor is through a 12 volt regulator. The speed regulator is independent internal with the only external variable being the current determining series range resistor and trim pot and variable speed panel pot.
Disconnect the two wires for speed control on the motor and measure the resistance between the disconnected wires. Make sure that the value adds up to the value of those three resistances in the schematic. If it is supposed be 12v on the motor trace the regulator circuit to see where the problem is.

[g1]

Make sure that the value adds up to the value of those three resistances in the schematic. If it is supposed be 12v on the motor trace the regulator circuit to see where the problem is.

Are you referring to the schematic in post #1 ?