Problem with Tascam Portastudio 464 pitch control

[Manu]

Hi everyone !

I need some help with my Tascam Portastudio 464 : it gathered dust for a few years in a closet, and now I try to run it again. I insist that it litteraly gathered dust.

I had to change the belt that drives the capstan, as it was completely loose and wrapped around its axis. Now I bump into something else : when I play a tape, the speed (or pitch) is erratic. It starts slow, then goes faster for ten seconds or more, then a bit slower etc., with no particular pattern.
I measured the voltage of the capstan motor, and it appears that the variations of speed correspond to variations of voltage. So I deduce that it's not a lubrication problem, and that it is not a problem from inside the motor. But I am not a specialist, so... yeah...

I dismounted the "pitch control" electronic board, on which are soldered the two pitch controllers : one button to choose between high and regular speed, and a potentiometer for adjustments. Both seem to be clean.

Details that I think might help, about those two pitch controllers : during my first attempt of running the machine after I changed the belt, the pitch potentiometer seemed to work (as far as I remember, cos it was a brief attempt). Now it does not look like it changes much of anything when I turn it.
Something else : strange, but the regular/high speed button does not change pitch, but when I change its position it sounds a bit like there is a change in sound filtering. Nothing huge, it's just slightly more "muffled" when it's on "high". I don't know if this machine applies a specific filter when it's on "high speed" ; if not, it might be a lead to the problem...

Any idea ? Thanks in advance !!

[52 Bill]

Welcome to the place.

I dismounted the "pitch control" electronic board, on which are soldered the two pitch controllers : one button to choose between high and regular speed, and a potentiometer for adjustments. Both seem to be clean

Something else : strange, but the regular/high speed button does not change pitch, but when I change its position it sounds a bit like there is a change in sound filtering. Nothing huge, it's just slightly more "muffled" when it's on "high". I don't know if this machine applies a specific filter when it's on "high speed" ; if not, it might be a lead to the problem...

Have you tried cleaning the speed switch and pitch control with some deoxidizing spray like DeoxIt?

Even though they might look clean, they both could be in need of cleaning.

The frequency response of most tape recorders changes with the speed of the tape. The audible sound change you hear may be a done for a reason.

[Enzo]

My 464 stuff is on paper back at my shop. Do you have a schematic or manual online you could link us to or post?

DOn't overlook mechanical as well. Like a hardened pinch roller or loss of roller tension.

My memory may be faulty, but seems to me the speed control is just pots and resistors in the motor servo loop, and as such can be checked with an ohm meter out of circuit.

[Manu]

Alright, thanks for your answers.

Have you tried cleaning the speed switch and pitch control with some deoxidizing spray like DeoxIt?

I did !

The frequency response of most tape recorders changes with the speed of the tape. The audible sound change you hear may be a done for a reason.

This is what I suspected. If that's the case, then it's a relief, I guess. I was afraid that the problem was not only about the pitch regulation.

DOn't overlook mechanical as well. Like a hardened pinch roller or loss of roller tension.

Everything looks clean on that side, and in a pretty good state.

I have the user manual of the 464, but there seems to be nothing helpful in it. It explains how to use the machine, nothing more... As for online material, I found nothing. I took two pictures, one of the small pitch controller board, and one of the bigger board to which it's connected.

The pitch controller board, with the selector and potentiometer :



The bigger board. The pitch controller is connected through the 5 pins connector on the bottom of the picture, right top of the big capacitor:



I measured the resistance of the pitch controller potentiometer. It has 4 pins ; I identified the ground and the two other pins connected to the circuit (the fourth pin is connected to nothing). Between the ground and one of the connected pin, turning the pot all its course, it goes from 0 ohm to 15.3k ; between the ground and the other pin, it's 7.7k at an end, 0 ohm in the middle then back to 7.7k at the other end. From what I remember about pots, it looks like there is no problem here !

[Enzo]

So trace out the small board, it is just a pot and a few resistors and a switch. Then verify at the connector that the circuit paths are operable.

[km6xz]

If I remember right, it has been over 10 years since I have seen one, there is a hi and lo speed calibration trimmer on the main board and the speed selector switch on the pitch pot board. Deoxidize the speed selector switch and the speed calibration trimmers. The external circuit is very simple so mos of the speed problems related to the regulator pc board inside the motor itself. I have repaired those in the past and even traced out a circuit but that was a long time ago. We were the warranty station Tascam and did a dozen every week, and tons of DA-88 and DAT decks since few people were authorized for the digital. If the trimmers and speed selector are deoxidized and has not impact on the speed variation I would look for mechanical drag as measured by a Tentelometer, or the motor internal regulator. A tiny bit of deoxidizer in the slip ring area inside the motor could help.
s the +12volts to the motor steady?

There were two thermistors on the speed control board, one of those might be a problem. If so, I would expect speed to be fine in one mode and vary in the other speed mode.
Is the speed control voltage steady even when the motor varies? Watch it with a scope on DC input on the motor terminals A and B. Does it drift when the problem occurs? If it does, pull off the capstan belt and monitor the same voltage, does it vary? If not, look for a mechanical drag problem with the transport

[Manu]

Okay ; first I'll confess that it's the first time I try to repair an electronic device, and that I rely on the little knowledge I remember from electronic courses I had to follow years ago, and in which I was not so tedious. I walk a bit in the dark, to make it short...

So trace out the small board, it is just a pot and a few resistors and a switch. Then verify at the connector that the circuit paths are operable.

By tracing out, you mean checking pole by pole that the circuit is closed where it should be ? I checked that, it looks totally fine. I guess the two round components (one black, the other red) are some types of resistors, judging by the symbol on the board ; in any case, when I check they react like resistors. No problem with the connector too : the circuit isn't cut where it should not.

[km6xz]

What test instruments do you have available? A Digital voltmeter or analog volt-ohm-meter, oscilloscope?
If you have a meter pin 1 and 2 of the motor board should have 12 volts DC. Does it and is it stable even when the motor varied. How does the speed vary? In discrete jumps to another speed and back or drifting up and down seemingly randomly? If the 12 volts is present and steady, measure the DC value on A and B connections of the motor board. They should be a lower voltage and change voltage when the speed selector switch if switched. Is it stable? Measure those items and report back. If these are not known, no one can help you, randomly checking connections and boards will just confuse the issue. Those two measurements about should isolate the problem to the motor or external circuit.

[Manu]

Hehe, okay : so I have a digital multimeter ; the voltage at the pins 1 and 2 of the motor (red and black wires) is not 12V, and is not stable. It goes from around 11.2 to 11.5 or more, drifting up and down randomly. I can clearly see (and hear !) that the speed variations are corresponding to the voltage variations. Sometimes it stays stable, let's say at 11.2, then it suddenly rises to 11.4 and keep stable a few seconds, then it goes a bit down, etc. It can keep stable on the same value for like 5 or 10 seconds, sometimes... But most of the time it's quite erratic.

I removed the capstan belt : still the same problem.

I deoxidized the trimmers and selector, also. And as I wrote earlier, the speed selector and speed pot have seemingly no effect. Or let's rather say, no effect I can hear...

[g1]

This is all I could find for schematics. It does not show where the main 12V comes from. In the pic of the mecha. control board from post #4, there appears to be a regulator to the lower left.
You said that the speed switch is not changing the speed?

[km6xz]

Is the voltage between the motor board A and B wires vary with the speed control pot when the variable pitch switch is activated? A and B wires go to a motor interior speed governor.

[Manu]

Well, something just changed, for some reason I don't know :
Now, when the speed selector is on "regular", the speed pot does work. But when the selector is on "high", it doesn't.

The voltage between A and B wires (blue and yellow) is zero, all the time.

On "regular" speed position, now the voltage between 1 and 2 pins seems a bit more stable, most of the time around 11.9 ; but it still can jump to 11.8 or 11.7 and stay around these values. Just a bit more stable than before.
On "High" position, same problem as before. The voltage between 1 and 2 is more around 11.5 and goes up and down.

LAST MINUTE UPDATE : I deoxydized the A and B pins, and took measurements again ; now the tension between A and B goes from -0.09 to 0.09 when I move the speed pot, and when the selector is on "regular" speed position. That's when the pot works, because now there are moments when it does not have any effect anymore... Back to the previous situation, so.
On "high" position the voltage between A and B goes randomly between -0.07 and 0.07, approximately.

[Manu]

Okay, I start to be a bit confused with the erratic functioning of this capstan motor. For the sake of clarity, I'll try to sum up my observations here :

When I turn the portastudio on, the capstan motor starts ; most of the time, i won't here any speed change when I use the pitch controllers (pot and selector), except a slight change when the selector is set on "regular" speed and when I move the pot around its minimum position. If I just leave it and wait, the motor speed can be stable a few seconds, then suddenly it will go faster ten seconds, etc.
Still when the selector is on "regular", it can happen that the motor suddenly responds a bit better to the pot ; the motor speed changes when I turn the pot, but I can hear that it's not an homogeneous variation, and not along the pot's entire course. Then, sometimes, it happens that suddenly the motor reacts well to the pot rotations. But that's still on "regular" position. If the selector is on "high", the pot won't have any effect whatsoever. But when I am on "regular" and if the motor seems to react well to the pot, switching to "high" makes the motor run faster.

Even at the moments when the motor reacts well to the pot, it may happen that it suddenly goes a bit faster or slower.

Something I observed : trying to measure the voltage between the motor's A and B pins, it can happen that I briefly make contact between one of these pins and the motor's body. The motor stops, then starts again, and a couple of times it was at this occasion that it went from "not reacting to the pot" to "reacting well to the pot". i don't know if it's a coincidence, but I report it anyway, in case

-Between the motor's 1 and 2 pins, the voltage is never at 12, always lower. It can go from 11.2 to (rarely) 11.9. I can hear that this voltage variations have direct effects on the motor speed (or inversely).

-Between the A and B pins, well, this is complicated : sometimes it goes from 0.14 to 0.22 V, sometimes from 0.12 to 0.16... The other day I even had -0.07 to 0.07. But in any of these ranges, this voltage reacts to the pot rotation, whatever position the selector may be at. When the motor goes in one of its "random" speed jump, the AB voltage range changes too. The slower the motor goes, the lower the voltage is.

-I deoxydized the connections and components between the speed control board and the capstan motor. I checked, it all looks fine.

-I observed that the motor can get quite hot sometimes. My question would be, can it be a problem from the motor itself ? As Km6xz wrote,

The external circuit is very simple so mos of the speed problems related to the regulator pc board inside the motor itself.

I opened the motor and deoxydized the circuit in it, but it did not change the motor's behavior.

[Enzo]

A and B are servo inputs. If the speed control they represent isn't working, the motor will just spin however fast it wants to. I still suspect a broken connection in the speed circuit.

[km6xz]

The A-B voltage is the key to speed. At normal speed there would be about 1.5 volts difference between the terminals. If the pitch control is simply a resistance that establishes the nominal speed in the pitch adjustment "off" mode. All the active electronics for the speed regulation is in the motor itself. You still need to see if the resistance if changing or the regulator in the motor is changing. The motor getting hot is not a good sign.
You can skip all the connectors, switches and pots if you just connect a 1.5k resistor between A and B and disconnect the small speed/pitch board at the connector. If the motor runs steady and relatively on speed(it will not be exact, just ball park, since disconnecting the speed/pitch connector disconnects the speed calibration trimmer also. The voltage will vary slighting across the resistor with varying load such as from one end of tape versus the middle or using an old tape with tape lube worn out of the tape backing. If your connect that resistor and the voltage across it is very low like some of the values you posted, (0.07 etc) I will bet a pint the problem is a bad motor.