Author Topic: DIY attempted repairs on tascam 238  (Read 2009 times)

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Offline chrish

DIY attempted repairs on tascam 238
« on: October 20, 2015, 10:24:32 AM »
My tascam 238 8 track cassette recorder that had an over speed control problem. i went online and found a repair tec talking about the capsion motor as being the culprit, so i ordered a new motor. After taking apart way to many parts to get the old motor out, i put the new motor in and soldered the 4 wires back onto the motor circut board, using a heat sink so i didn't fry anything. The motor still ran overspeed, same as the old motor. The unit does have a +- 12 pitch control circut and i originally though that might the problem, but i suspect that the new motor was just a new stock old motor subject to the same electical circut degradation which caused the old motor to fail. I guess the next step is to get a wiring diagram and check the voltages at various points, but now i'm way in over my head. The nearest pro tec is 3 hours away and i'm guessing i may be just throwing away cash. Looking for any advice? I really enjoy the ease of use, have a bunch of old recordings i'd like to review and the eight meter bars really put on a good light show.
« Last Edit: October 20, 2015, 10:32:22 AM by chrish »

Offline admsustainiac

Re: DIY attempted repairs on tascam 238
« Reply #1 on: October 20, 2015, 10:29:56 AM »

Offline AngeloEvs

Re: DIY attempted repairs on tascam 238
« Reply #2 on: October 20, 2015, 05:01:56 PM »
Looking at the circuit diagram linked above, the motor uses several components to control the speed.  From what I can see, its a case of lcating the motor speed  control circuit board and taking measurements to determine the exact cause of the problem and which, if any components, may be faulty.

 The only other solution is to have the motor connected to and controlled by an external variable DC power supply but you need to know the voltage of the motor at normal running speed.

 
« Last Edit: October 20, 2015, 05:18:38 PM by AngeloEvs »

Offline Yohanes

Re: DIY attempted repairs on tascam 238
« Reply #3 on: October 21, 2015, 02:03:52 AM »
I am not sure what is tascam 238, but if it's working mechanism is similar to cassette tape player, the speed of the tape is not solely controlled by the motor. Besides the motor, the speed also depend on the rubber roller that pinch/stress the tape to the motor capstan. This rubber roll will wear and affext the tape speed. Have you check the rubber roll and consider to replace it?
I hope this will help, otherwise just ignore it.
Thanks and wish you luck.
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Offline GraemeJ

Re: DIY attempted repairs on tascam 238
« Reply #4 on: October 21, 2015, 03:04:34 AM »
Besides the motor, the speed also depend on the rubber roller that pinch/stress the tape to the motor capstan. This rubber roll will wear and affext the tape speed.

Not so - this is incorrect.  The tape speed is solely dependent on the diameter and rotational speed of the capstan, the pinch wheel diameter makes no difference whatsoever,  Since it is doubtful the capstan diameter has changed ;) , whatever circuitry governs the motor speed is the suspect.

I think it's highly unlikely the motor itself is at fault.  My experience is they either run correctly or not at all (and that's usually a worn bearing problem).

The link to the service manual, given above, indicates your problem is almost certainly something wrong on the pitch control board.
« Last Edit: October 21, 2015, 03:06:06 AM by GraemeJ »

Offline gumtown

Re: DIY attempted repairs on tascam 238
« Reply #5 on: October 21, 2015, 03:44:13 AM »
the 4 wire motor will have 2 motor wires and two speed pulse counter wires, and interface with an analog pulse to DC voltage converter.
If the motor is running at full speed, possibly the motor drive transistor has shorted.
Either that or a op amp chip used as a comparator between the speed potentiometer and the motor speed feedback.
If you can replace all the active components associated with the speed control circuit, you will eventually sort it.

The high speed won't affect any new recordings you make.  ;)

EDIT: I was thinking of the older 4 track ones.
« Last Edit: October 21, 2015, 04:26:25 AM by gumtown »
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Offline Silas Lang

Re: DIY attempted repairs on tascam 238
« Reply #6 on: October 21, 2015, 03:55:53 AM »
It seems that this is a common failure with these wonderful machines:

http://www.thegreatbear.net/audio-tape/8-track-cassette-capstan-motor-tascam-238-syncaset/

Mine has the same problem, but eventually runs at the right speed after warming for around 45 minutes! Have you checked if yours behave the same?

Another common problem mine also has is that the counter resets during recording and/or playback and the motor stops. Very annoying:



If somebody knows where these machines can be serviced in Europe (and I mean people specialized in these machines), please let me know!

Offline Elantric

Re: DIY attempted repairs on tascam 238
« Reply #7 on: October 21, 2015, 08:23:24 AM »
Remember this is now a 25 year old unit. All failures described are precisely a result of bulk Electrolytic capacitor leakage and failure

Replace them all ( not a trivial task )

Offline chrish

Re: DIY attempted repairs on tascam 238
« Reply #8 on: October 21, 2015, 10:02:21 AM »
Thanks for all the advice, links and wiring diagrams. From reading those links, it appears that the caps on speed control board are the problem. That board is attatched to the motor and was included in the 100 bucks i paid to the tascam parts people for the new capstan motor assembly. When i soldered the positive lead to the board, i noticed that the cap closest to that connection got warm to the touch even with the heat sink. Is that common or did apply to much heat or for to long (it seemed quick) The iron i purchased said on the package 'up to 750 degrees F' so i assumed it had a varriable heat control. It turned out it did, but only if you unplug it and plug it back in and check if either the iron is hot enough to flow, or too hot to smoke, the solder. Ha. I got fooled yet again. Or the other possibility is that the caps on the new, old stock motor were bad. I guess i need to call tascam again.
« Last Edit: October 21, 2015, 10:06:41 AM by chrish »

Offline Elantric

Re: DIY attempted repairs on tascam 238
« Reply #9 on: October 21, 2015, 10:09:42 AM »
Dive deeper to the main Power supply board , and use an oscilloscope to measure presence of any AC ripple on all internal DC voltage power supplies - should be no AC  there )

Offline chrish

Re: DIY attempted repairs on tascam 238
« Reply #10 on: October 21, 2015, 11:15:21 AM »
Dive deeper to the main Power supply board , and use an oscilloscope to measure presence of any AC ripple on all internal DC voltage power supplies - should be no AC  there )
When i was a kid, i built a few simple guitar effects and even made some etched circut boards and an oscilloscope was on the list to purchase. But then the girl thing started happening and suddenly it was fixing cars and folding seats. :-). The best i can do now is measure voltages and check continuity. I'm going to try plugging it in for 45 minutes as suggested above in the thread. Sometimes the repair gremlins find the problem and fix it for ya. :-). I will check the voltages though, i think your suggestion to check the power supply is a good one.

Offline chrish

Re: DIY attempted repairs on tascam 238
« Reply #11 on: October 22, 2015, 11:31:44 AM »
Renewed hope after reading the service manual that was linked to in this thread. '' caution: after replacement of castan motor assembly, be sure to short-circut(by soldering) the points on the assembly shown by the arrow in fig. 4-10. Otherwise, correct  tape speed adjustments are not ensured''. Apparently this motor assembly has two applications. One for a tape deck that runs at 4.8 and one for 9.6 cm per second. You have to make the right 'shorted out' connection to get the desired tape speed. I'm going back in.

Offline chrish

Re: DIY attempted repairs on tascam 238
« Reply #12 on: October 22, 2015, 01:22:49 PM »
Success! Making that connection worked. Lesson: don't worry about getting in over you head, but bring plenty of backup. Thanks for the assistance.

Offline Elantric

Re: DIY attempted repairs on tascam 238
« Reply #13 on: October 22, 2015, 01:28:30 PM »
Very good to hear your success story


I still have "the big magilla" Tascam 688 that works great for tape saturation "squish" 

Offline vanceg

Re: DIY attempted repairs on tascam 238
« Reply #14 on: October 31, 2015, 10:12:06 PM »
Very good to hear your success story


I still have "the big magilla" Tascam 688 that works great for tape saturation "squish" 
(Image removed from quote.)
Wow!  in 1987 I wanted an 8 track Cassette recorder SO SO SO badly!

Offline chrish

Re: DIY attempted repairs on tascam 238
« Reply #15 on: November 04, 2015, 10:45:05 AM »
I had forgotten just how good tape can sound. In following many links around the web concerning looping techniques, i came upon a story about a musician who was buying up some used 4 track porta-studios. He was taking cassett tapes (or trying to find old answering machine tape cassetts which are loops) apart and making tape loops and using those for tape eco loops in the porta-studios.

Offline admsustainiac

« Last Edit: October 26, 2017, 12:20:09 AM by admsustainiac »

 

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