Portastudio 424 MKii Warble

[samgraysound]

Hi All,

I'm trying to fix a Tascam Portastudio 424 MK2. It has a very significant warble on playback. Very consistent and rhythmic, about 3-4 times per second.  It sound like a vibrato effect.

I tried replacing the belt set and if anything it made it more pronounced.

Any idea what else this could be? Motor? Is there some sort of capstan tension sensor circuit?

Sam

 

[JohnRoberts]

If the motor has a speed control/trim look for a problem in that area,

JR

 

[samgraysound]

Don't suppose anyone has a schematic? Tascam will sell it for $15 but it is on backorder.

Sam

 

[radardoug]

Could also be pinch wheel. Get a new one from Terrys Rubber Rollers. Hes on the web.

 

[samgraysound]

I don't think a pinch roller problem would create such a consistent rhythm to the pitch shifting. Is that a poor assumption?

I tested all the components on the pitch control pcb and they seem ok. The transistors are all digital transistors so my transistor tester couldn't tell me much about them.

 

[JW]

I think Teac parts still sells pinch rollers for their 4 tracks? Could be wrong, but I got one for my 244 a year ago.  They're only $25 or something I think. It probably needs a new one by now anyway, though maybe not.

That might be a poor assumption you speak of, but I'm not a cassette repairman or anything. Pinch rollers can do some weird stuff.

 

[radardoug]

I don't think a pinch roller problem would create such a consistent rhythm to the pitch shifting. Is that a poor assumption?

I tested all the components on the pitch control pcb and they seem ok. The transistors are all digital transistors so my transistor tester couldn't tell me much about them.

It certainly will!

 

[samgraysound]

Okay doke. I'm going to borrow a friends working machine and swap in the pinch roller to check.

Sam

 

[radardoug]

Not really a good idea, swapping in a worn pinch roller for a worn pinch roller. Also look at the rotation time of the pinch roller and compare it with the frequency of your wobble. Another possible issue is a bent capstan.  Normally the capstan is small, 1/8 inch or so, and the roller is 3/4 inch. So their rotation speeds can give you clues when compared to the frequency wobble on tape.

 

[samgraysound]

Swapping the pinch roller did not change the warble at all.

 

[samgraysound]

Not really a good idea, swapping in a worn pinch roller for a worn pinch roller. Also look at the rotation time of the pinch roller and compare it with the frequency of your wobble. Another possible issue is a bent capstan.  Normally the capstan is small, 1/8 inch or so, and the roller is 3/4 inch. So their rotation speeds can give you clues when compared to the frequency wobble on tape.

If i had turned out to be the problem I would of gotten a new one. But just to diagnose it seems okay...I don't see why a pinch roller that works fine in one machine would show the exact same warble in a different machine.

 

[samgraysound]

Not really a good idea, swapping in a worn pinch roller for a worn pinch roller. Also look at the rotation time of the pinch roller and compare it with the frequency of your wobble. Another possible issue is a bent capstan.  Normally the capstan is small, 1/8 inch or so, and the roller is 3/4 inch. So their rotation speeds can give you clues when compared to the frequency wobble on tape.

According to the manual the tape speed at Normal is 1.125 ips.  I measure the capstan at about 1/8" diameter and the pinch roller about 3/8".

So multiplying by pi to get circumference:
Capstan - 0.3925"
Pinch Roller - 1.775"

Dividing speed by circumference to get rotations per second:
Capstan - 2.86hz
Pinch Roller - 0.63/hz

Did I do that right?

The warble sounds to be between 2 and 3 times per second so a capstan issue seems more likely then a roller one.

 

[JohnRoberts]

Not really a good idea, swapping in a worn pinch roller for a worn pinch roller. Also look at the rotation time of the pinch roller and compare it with the frequency of your wobble. Another possible issue is a bent capstan.  Normally the capstan is small, 1/8 inch or so, and the roller is 3/4 inch. So their rotation speeds can give you clues when compared to the frequency wobble on tape.

According to the manual the tape speed at Normal is 1.125 ips.  I measure the capstan at about 1/8" diameter and the pinch roller about 3/8".

So multiplying by pi to get circumference:
Capstan - 0.3925"

0.125 x 3.1415= .3926

inch Roller - 1.775"

.375 x  3.1415=  1.1780

Dividing speed by circumference to get rotations per second:

1.125/1.178=0.86 rotation per sec

1.125/.3926= 2.86 rotation per sec

Capstan - 2.86hz
Pinch Roller - 0.63/hz

Did I do that right?

The warble sounds to be between 2 and 3 times per second so a capstan issue seems more likely then a roller one.

perhaps bent motor shaft or bearing issue? but still could be coincidental and electrical.

JR

 

[samgraysound]

Just swapped in the motor from the working machine and problem disappeared!Also apparently it was playing at a super high speed before.  Now I just need to find a replacement motor. Which I believe tascam doesn't stock anymore.

 

[radardoug]

The fact that it was playing at high speed would indicate a motor control problem.  So focus your search on the motor drive electronics. Bent shafts are possible but not common. Also disassemble the motor, and clean and relubricate. You are looking for something happening once a rev, that could be mechanical or electrical. If there is a test point on the motor, look at that with a scope, an error in the rotation speed should show up.

 

[samgraysound]

Any advice on opening up the motor to clean and lube?

 

[radardoug]

Its straight forward. Just make sure you know the order you took it apart, and reverse to re-assemble. Clean out the bearings with Q tips, once assembled put a little light oil on the bearings. You can usually do this once the motor is re-assembled. The important thing is to get all the gunk out of the bearings. Once re-assembled the motor should spin freely.