Tascam 424 MkII capstan motor running fast

[GOYA551]

I haven't had the time to find a voltage regulator yet but I checked the resistance between pin A and pin B. There is something in circuit like a charging cap that is increasing the value to more than 20MΩ (limit of my DMM). Then if I switch polarity I read a steady 15.75MΩ.

I have to admit a level of ignorance regarding that schematic. I usually work with tubes. Big, dumb, point to point tubes. Transistors reduce my level of confidence greatly. But the 12V on that schematic appears to parallel through R7 and then to a transistor/diode something or other with the other parallel leg going off that board. That transisor/diode something would have to do with regulation would it? I'm going to see where the off board leg goes as soon as I have the time. I would guess it will lead me to the power board.

[Enzo]

But the 12V on that schematic appears to parallel through R7 and then to a transistor/diode something or other with the other parallel leg going off that board.

I don't mean to sound harsh, but if I described a tube circuit as:

There seems to be four or five resistors all solderd together at one end and then three of them are soldered to a tube socket...

would you recognize that as the phase inverter?

We really need to see the schematic of the circuit you describe.

I just made a note to dig that manual out next time I go to my shop. Hopefully sometime soon. if you have a scanner or access to one, maybe grab the power supply circuit in question for us. At least we will know what/where R7 is.

[GOYA551]

It is the schematic in my first post.

[Enzo]

I am sorry, looking at that does fit your description, but I was confused because I thought we were chasing down the power supply. (to answer your question) no, the R7 stuff has nothing to do with voltage regulation. The post #1 drawing has no power supply circuitry in it, it only has the +12v and ground entering at top right, but coming from an unseen power supply. Your concern was over the 12v being 15v, so we need to look at the power supply, the actual source of the 12v. The HEAVY line indicates that is ground n that board. So the negative side of 12v is ground.

The triangles are "digital transistors" whch means a transistor with a couple extra resistors in it. it is made for off/on use like a switch, not really for audio use. See this data sheet for what is inside.
DTC124X pdf, DTC124X description, DTC124X datasheets, DTC124X view ::: ALLDATASHEET :::

R7 is what we call a "pull up" resistor. It is connected to the +12v, but has no direct affect on the motor. Just happens the motor and the R7 circuit use +12 as power. A pull up resistor has a high enough resistance that it won't draw appreciable current, but it is enough to bias a digital input up to a logic 1, and allow something else to ground that same input to a logic 0 without drawing a lot of current. Those four transistors are involvd in switching those speed controlling resistors in and out. That is why we were concerned that all the resitors and controls were intact. If you disconnect the three cables to that small board, then you can measure the resistors in circuit.

Unplug the motor and connet a DC volt meter to A and B. Give the motor a spin. do you see a little spike of voltage on the meter?

[GOYA551]

Thanks for the info. Manually turning the motor does generate voltage on the A-B pins.

The 15V doesn't connect to the power board. It connects to the main board. I hope to remove that board and follow it back soon.

[Enzo]

It may take a route through other places, but the 15v comes from a 15v supply SOMEWHERE.

That AB is your servo output, it is fed back into the speed control circuits.

[GOYA551]

If A and B are output pins, what actually makes the motor run at a specific rpm? I still haven't pulled the big board to trace the source of the motor supply voltage. But have looked over the tops of the boards for a voltage regulator. In my experience, they tend to be mounted on heatsinks. Or at least have a place to mount. I don't see one. There is one to-220 component but it isn't a voltage regulator.
R8 on the speed board is shown as 3.3k and there was a 2.3k resistor there. I changed it to 3.3k. No speed change.

[Enzo]

Whether you can post it or not, do you have the complete schematic? If so, find the power supply on the drawing. That will tell you where to look for it on the boards. I don't recall this model, but one place TASCAM liked to use as a heat sink was the transport deck chassis.

Hopefully I will make it to my shop in the next few days, and I can open my manual.

[GOYA551]

I don't have a schematic. I've looked for more information and have some up with some things. The 15V to the motor shouldn't be a problem. There is an internal motor speed regulator that should handle that voltage. The problem must be on the A/B pin circuit.

[Enzo]

OK, I have my manual in hand. Sure enough, the 12v supply is not regulated, and so 12v is only an approximation.
The schematics are huge foldouts, and I have no way to scan them. But we can work with the bit you posted.

There is a speed button on the panel. In the high speed position, it shorts HVR to HIGH. Those are on P1 of the speed board. And in normal speed position, it shorts HVR to both NORM and NVR. You can verify that is happening with power off and an ohm meter.

Power off, and disconnect the cables to the speed board. Find Q3 next to the speed pot. From Q3 center terminal, measure resistance to NORM and HIGH. To NORM, we should see from 0 to 470 ohms, depending upon the setting of trimmer R3 - run the trimmer back and forth to see. To HIGH we should see from 1k to 3.2k depending on R2. Run R2 up and down to see.

From the B terminal on P2, measure to NVR on P1, and expect something like zero to maybe 1k as you sweep the speed pot. Then B to HVR, expect something like 0 to 1.5k as you sweep the speed pot. These measurements are in the schematic section you posted.

All connected and powered, there is control signal MMS coming into pin 1 of connector A. It controls Q2, which in turn controls Q4, and via Q1, Q3. As you start and stop the transport, a volt meter should tell you if those transistors are turning on and off.

[GOYA551]

All pots measure on the money with the exception of R3 which measures 8Ω to 530Ω.
BTW, all pots were cleaned with Deoxit.

I'm not sure if you mean to check voltages elsewhere but with the transport stopped I get 4.9V on pin 1 of connector A relative to pin 2 (ground).
When the transport is started, voltage drops to zero.

[Enzo]

That sounds like a reasonable logic transition to me, 5v to zero. So more forward to the cathode or the diode D1. (The collector of Q2.) Does that toggle too? Pin 1 of connector A is a control signal from the system controller IC. So those four transistors are more or less on/off switches.

Pots are never precise, so 530 for a 470 pot is close enough. I was worried more about open that value drift.

However, when you say the pots measure right on, it makes me think you checked each part. That is fine, but remember, this is about circuits, not parts. A dead on pot is useless if the circuit around it is broken. That is why I suggest things like measuring from the middle of Q2 to pins 1 and 2 of P1. And from pin B of P2 over to pins 3 and 4 of P1. By measuring from the edges of the board so to speak, and through the board, we are testing not only those parts, but also the connections between them. For example, if there were a broken solder joint or a cracked trace at P1 or P2, you would never find it by checking the resistors and pots.

And with power off but the board connected, at least P1 connected, you can verify with your ohm meter that the appropriate pins on P1 are closed and opened by the speed switch on the panel of the unit.

[GOYA551]

Sorry for the delay in response.

Transport off: D1 Cathode 30mV , D1 Anode 650mV
Transport on: D1 Cathode 13.9V, Anode 10.6V

With speed switch at normal speed - continuity between pins 1 and 3 and also pins 1 and 4.
With speed switch at high speed - continuity between pins 2 and 4

Tape speed definitely increases when switched to high speed.

My earlier measurements were taken at the points you indicated.

[olddawg]

Is there ANY adjustment in the speed at all? If the speed adjusts, try what I said earlier. Forward bias a diode or two in the 2 wires going directly to the motor. It won't hurt anything and reduce the voltage to the motor by .6-7v for each diode. If it takes two put one on each side. If it gets you in the adjustment range and is stable, call it a day. Also... Have you checked the ripple voltage on your DC rails? This thing is old. It would not surprise me if it has a 2200uf or 4700uf cap going south. Especially if it has sat unpowered for a while...

[GOYA551]

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.