Tascam 48 Capstan Motor

[moellhoven]

Hello,
I am looking for advice on putting back together a Teac/Tascam BM-5 capstan motor from a Tascam 48.

Recently I bought a Tascam 48 in the Kleinanzeigen that was reasonably priced but otherwise in poor condition.
I like to restore the machine into proper working condition to use it in my home studio.

Unfortunately I hit a road block with the capstan motor.
The capstan did not start by itself and needed a helping hand to get started.
When slowly turned by hand it also made a clicking noise.

Getting the motor out was a piece of cake as the Tascam 48 is well designed and very service friendly.
I've took the motor apart cleaned it, replaced the two bearings but I have a bit of trouble putting it back together.

I am confident that everything is in its right place but when the motor is put together the axle can not be turned.

The outer gear in the endcap seems to be fixed. The inner gear on the axle is turning. When the two gears go together and the cap is screwed in the inner gear is fixated.

Unfortunately I don't know how the motor works in theory and have currently no idea what to change.
Is there somebody familiar with the BM-5 motor?

Also I am wondering if the motor looks okay on the inside?
There is a blackened area on the inside and I am wondering if this is normal?

best regards from Berlin,
André

 

[BeatleFred]

If you are on Facebook, there is the 'Reel to Reel Clinic' group which you can join.

Sam Palermo is a long time Teac/Tascam "guru", who knows these machines and their parts inside out.

If you post over there, he can properly advise you.

 

[sweetbeats]

Sam is also a member here under handle SkywaveTDR. To the OP I have never dared to take apart a DC servo capstan motor because of the tight tolerances so I don’t know what to tell you…I do a lot of stuff that is adventurous relative to my skill level, but opening up one of those has also been on my “nope” list. My plans have always been if I have something like that seems to need service to send it to a shop that specializes in servicing those types of motors.

 

[moellhoven]

I'll check out the "Reel to Reel Clinic".
Thanks for the tip.

 

[moellhoven]

I was able to get the motor back together again.
There needs to be a small gap between the gears.

The bearings are replaced and the capstan turns nicely without any noise now.
However it does not work in the machine yet. The voltages from the servo board do not look correct.

Next steps are to check the PSU and Servo board.

Can I check the capstan motor with an external power supply to make sure that it is fine?
There are multiple voltages on the connector.

 

[sweetbeats]

Hi. Congratulations on getting it back together.

I’m really hesitant to comment here because DC servo motor control systems are not an area of expertise for me. I believe powering the capstan motor without the servo feedback and control signals present will cause the motor, if healthy, to spin up in a runaway state and I don’t know how long it’s okay for that motor with those internals to spin at high RPM.

You have the service manual, yes? The power supply has a servo system power output that goes to the sync interface PCB, and then that interfaces the capstan servo board to which the motor is connected. You could have a power supply issue, or a sync interface PCB issue, or a capstan servo board issue. If I had to guess I’d say you have a servo board issue, or a power supply issue.

You can find the theory of operation information for the servo on pages 5-22 & 5-23 of the service manual.

I would start by isolating the power supply output to the sync interface PCB (7-pin connector P10), measure the voltages with no load using a DC voltmeter (COM probe to pin 3 and positive probe to pin 1 should be +15V, and then to pin 2 should be +5V, then COM probe to pin 5 and positive probe to pins 6 or 7 should be +24V), and then check for AC volts across the same pins and in all cases it should be at or very close to 0.00V. Do that with no load, then do the same checks at P10 with the system connected including the sync interface and capstan servo PCBs and the capstan motor. If the voltages remain essentially the same at P10 with the system load, then I would suspect a capstan servo PCB issue. If there is a significant change in the voltage measurements under load I would suspect a power supply issue, or a power supply and servo PCB issue.

Step 1 is always to get the power supply working correctly without and with load, then see where you’re at.

You have the SPEED MODE switch set to FIX, yes?

Did you check fuses? F6 for the +24V supply, F7 for the +15V supply and F8 for the +5V supply…?

If there seems to be a power supply issue you can also check to make sure the main transformer isn’t at fault under load…measure for AC volts across the following power supply input wires from the main transformer under no load, and then compare to the same measurements under load and if significantly different I would suspect a main transformer fault:

P5 pins 3 & 4 (these power the +24V supply)
P5 pin 2 and P4 pin 4 (these power the +15V supply)
P5 pin 1 and P4 pin 4 (these power the +5V supply)

ALSO…if the +24V supply seems to be loading down at the power supply output, don’t skip checking the power transistor Q2 on the JOINT (L) PCB…this is a 2SD716 part that is part of the +24V supply and critical to the load capacity of the supply.

If you determine you have a main transformer and/or power supply PCB issue, I have both here, pulled from a working machine, would sell either or both for a reasonable price.