Value of safety resistor for TEAC Tascam 388 capstan motor - UK Vintage Radio Repair and Restoration Discussion Forum

Potless · 6th Jun 2014, 1:26 pm

Hi everyone,
A quick post on the off-chance that somebody may know the answer to this small but irritating problem!

We have in for repair a TEAC Tascam 388 Studio 8 reel to reel recorder. The original fault was non-operation of the capstan motor, and a burning smell coming from the Motherboard 2 PCB - this has since been rectified (the capstan bearing was pushing too hard against the thrust plate, so the capstan was as stiff as an old boot, causing a series resistor in the 35V supply to the motor to burn out).

Problem is, the resistor is not shown on the PCB layout or circuit diagrams, and its value bands have completely burnt off - we measured its resistance at about 47 ohms, but what should its true value be?

As usual, any help would be appreciated....

j.harv38 · 6th Jun 2014, 4:55 pm

It should be on the diagram. I see it on mine,that is if Im looking at the one you're talking about. It's listed as R3. It is a 47k metal resistor,no flame.

I had to actually replace this resistor on mine. It was burnt. It does lead to the capstans ability to spin.

ITAM805 · 6th Jun 2014, 6:56 pm

hi Potless (my middle name) the service manual might be of help

Potless · 7th Jun 2014, 12:28 pm

Quote:

Originally Posted by j.harv38

It should be on the diagram. I see it on mine,that is if Im looking at the one you're talking about. It's listed as R3. It is a 47k metal resistor,no flame

Strangely, my burnt resistor is located in a position where a wire link V4 should be (or is it W4, hard to see on the drawing), just to the left of connector P1 on the Mother (2) pcb.

I don't think it's R3, as that goes to the meter circuits via connector P1.

Maybe the resistor was added as a mod, some time after the circuit diagram was published?

Quote:

Originally Posted by ITAM805

hi Potless (my middle name) the service manual might be of help

Thanks a lot for that, it is a bit clearer than the copy I have!

j.harv38 · 7th Jun 2014, 3:22 pm

Ok.
I am for sure certain it is a 47k metal resistor. Its probably not where I originally thought it was on the diagram. I'm looking at an old photocopy of the manual. I do have the original service manual kicking around,but I can't find it. There is a wire link on some 388s and on others there aren't. I own 3 of them! Only one has a plastic sort of holder for a larger resistor.The other 2 388s have a small resistor on the bottom right corner of the mother PCB 2. If I can find my original manual, I'll try to get a pic up. The resistor is shown on the original manual and not on my copy.

Potless · 9th Jun 2014, 9:15 am

Quote:

Originally Posted by j.harv38

The other 2 388s have a small resistor on the bottom right corner of the mother PCB 2. If I can find my original manual, I'll try to get a pic up. The resistor is shown on the original manual and not on my copy.

The small resistor at the bottom right hand corner of the mother PCB 2 is the one I am after! (it's not shown anywhere in the two copies of the manual I have).

peter_sol · 9th Jun 2014, 1:00 pm

I think 47K would be too high 47 ohms fusible is a typical value for something of that sort.

ricard · 10th Jun 2014, 5:30 am

Just my two cents worth:

With a supply of 35V, a 47k resistor would have a short-circuit current of less than a milliamp, resulting in a power dissipation of 25 milliwatts. That's hardly enough to generate any noticeable heat, not to mention the fact that a milliamp is far too little to power a motor in series.

With a 47 ohm resistor on the other hand, 35 volts yields about 0.75 A, and the resulting power dissipation would be over 25 watts which is definitely enough to overheat a small enough resistor.

So as a series resistor my bet would be 47R rather than 47k, simply because the circuit would not work as intended with the latter, nor would a short circuit in the motor cause any damage to the resistor.

Potless · 11th Jun 2014, 11:23 am

47 ohms would seem to be about right then - I'll try a replacement of the correct type, and see how things go.

Thanks to everyone who helped out - it is greatly appreciated!

6th Jun 2014, 1:26 pm	#1
Potless Tetrode Join Date: Feb 2014 Location: Norwich, Norfolk, UK. Posts: 53	Value of safety resistor for TEAC Tascam 388 capstan motor Hi everyone, A quick post on the off-chance that somebody may know the answer to this small but irritating problem! We have in for repair a TEAC Tascam 388 Studio 8 reel to reel recorder. The original fault was non-operation of the capstan motor, and a burning smell coming from the Motherboard 2 PCB - this has since been rectified (the capstan bearing was pushing too hard against the thrust plate, so the capstan was as stiff as an old boot, causing a series resistor in the 35V supply to the motor to burn out). Problem is, the resistor is not shown on the PCB layout or circuit diagrams, and its value bands have completely burnt off - we measured its resistance at about 47 ohms, but what should its true value be? As usual, any help would be appreciated....

6th Jun 2014, 4:55 pm	#2
j.harv38 Retired Dormant Member Join Date: Jun 2014 Location: Cambridge, Ontario, Canada. Posts: 2	Re: Value of safety resistor for TEAC Tascam 388 capstan motor It should be on the diagram. I see it on mine,that is if Im looking at the one you're talking about. It's listed as R3. It is a 47k metal resistor,no flame. I had to actually replace this resistor on mine. It was burnt. It does lead to the capstans ability to spin.

6th Jun 2014, 6:56 pm	#3
ITAM805 Nonode Join Date: Jul 2010 Location: Folkestone, Kent, UK. Posts: 2,172	Re: Value of safety resistor for TEAC Tascam 388 capstan motor hi Potless (my middle name) the service manual might be of help

7th Jun 2014, 3:22 pm	#5
j.harv38 Retired Dormant Member Join Date: Jun 2014 Location: Cambridge, Ontario, Canada. Posts: 2	Re: Value of safety resistor for TEAC Tascam 388 capstan motor Ok. I am for sure certain it is a 47k metal resistor. Its probably not where I originally thought it was on the diagram. I'm looking at an old photocopy of the manual. I do have the original service manual kicking around,but I can't find it. There is a wire link on some 388s and on others there aren't. I own 3 of them! Only one has a plastic sort of holder for a larger resistor.The other 2 388s have a small resistor on the bottom right corner of the mother PCB 2. If I can find my original manual, I'll try to get a pic up. The resistor is shown on the original manual and not on my copy.

9th Jun 2014, 1:00 pm	#7
peter_sol Octode Join Date: Aug 2003 Location: Welwyn Garden City, Herts. UK. Posts: 1,906	Re: Value of safety resistor for TEAC Tascam 388 capstan motor I think 47K would be too high 47 ohms fusible is a typical value for something of that sort.

10th Jun 2014, 5:30 am	#8
ricard Octode Join Date: Mar 2006 Location: Lund, Sweden Posts: 1,632	Re: Value of safety resistor for TEAC Tascam 388 capstan motor Just my two cents worth: With a supply of 35V, a 47k resistor would have a short-circuit current of less than a milliamp, resulting in a power dissipation of 25 milliwatts. That's hardly enough to generate any noticeable heat, not to mention the fact that a milliamp is far too little to power a motor in series. With a 47 ohm resistor on the other hand, 35 volts yields about 0.75 A, and the resulting power dissipation would be over 25 watts which is definitely enough to overheat a small enough resistor. So as a series resistor my bet would be 47R rather than 47k, simply because the circuit would not work as intended with the latter, nor would a short circuit in the motor cause any damage to the resistor.

11th Jun 2014, 11:23 am	#9
Potless Tetrode Join Date: Feb 2014 Location: Norwich, Norfolk, UK. Posts: 53	Re: Value of safety resistor for TEAC Tascam 388 capstan motor 47 ohms would seem to be about right then - I'll try a replacement of the correct type, and see how things go. Thanks to everyone who helped out - it is greatly appreciated!