Tascam 122mkii keeps stopping on rew & ff

[beginwhereUR]

Hi Friends,

I've done maintenance on my MKII--replaced SMD caps on the motor control PCB, replaced the control belt & idler tires. Everything works as it should now, except that machine stops after a few seconds of rewind or fast-forward.

I've tried adjusting the trimmers on the transport PCB, but no improvement. Also, if it's any clue to the solution, the problem happens after the machine has been warmed up for an hour or so. When it's first powered on and cooler, or when the top of the enclosure is left open, the problem ceases.

If anyone has ideas or suggestion on how to fix this, please let me know. Many thanks!

Joe

 

[SkywaveTDR]

I seem to think there is a magnet wheel in some of these that is gear connected to the reel table on right. This feeds a Hall effect sensor and then is connected to the control board by I think a white 4 pin connector. I would check if this is properly plugged in and also the hall sensors are the right position as well as take wheel and reel table off and clean and lubricate them. I have often seen bad solder joints on a control PCB that I resolve by adding long strings of Kester 44 solder to the boards. None of this stuff is aerospace rated solder wise that is for sure.
Make sure the feedback pot is clean as when this has noise some modes are not considered to be achieved.

 

[beginwhereUR]

Thanks for your thoughtful reply Skywave!

I'll check into the adjustments you mentioned.

Reading over the service manual, I can't find anything that addresses the automatic tape shut-off mechanism. I imagine it has to do with tape tension across a sensor of some kind that activates a clutch or switch. But in that case, I can't understand why warmer temperature would cause it to misbehave.

If anything more occurs to you regarding troubleshooting that mechanism (I know how ideas can spring to mind suddenly when you have lots of knowledge and experience stored up), please drop me any hints.

Thank you again!

Joe

 

[SkywaveTDR]

The auto shut off may not be immediately obvious however in decks over the years and I have been working with them for about 46 years, the shut off is often linked to the counter. Now I know that in some decks when I clean and oil reel tables that I take off the shafts I see the optical bounce like sensors that send IR signal up and then are reflected or not reflected by white or black plastic surface. If these IR detectors are monitored in fast wind mode, it has been found that a sine wave output can be a sign of a defective device. I have had units that had intermittent problems and the reel sensor in fast wind had a bad sine wave output from the return of the sensor when all other parts where correct- things like DC power to the emitter was stable, the reel table was not warped and reflective surface at constant distance. I changed the sensor when we had them at Teac and the same signal then detected was now a constant string of square waves as any good deck would have. A study of the schematic and then a determining how the deck detects motion sensing is what you need to deal with.

The Idler tires are not the only issue- there can be lack of oil after all these years in the motors top bearing- I oil all cam and reel motor bearings with AMS oil and they spin faster and with greater detected easy after application. Dead spots in reel motors are usually found in play mode not a fast wind so this would suggest a lack or rotation detection somewhere.

Just so you know, I have had decks that after I worked on them forgot to put the Hall effect PCB in and the counter would not work or the connector was not plugged in. In hundreds of units of the same model you are bound to do this once or twice as you are focusing on a problem not related to reel rotation.

 

[wkrbee]

There are two Hall effect sensor 90 degrees off from each other.The leading edge of the pulse "tells" the counter to count up or down. If the machine does not sense the pulses, it will stop so the tape does not spill all over the inside of the mech.

 

[SkywaveTDR]

Temperature does affect some parts and when they get old they can react on this. At the same time old lubrication might be better when cool that as it heats up. I have 122 Mk II just besides me here but the answer of the operation come from the service manual and some previous knowledge. Some people have had problem in plugging the counter control buttons off or wrong.
I can not say for sure what is the cause until I see the unit not that I need more right now.

In looking at the schematic at the page where the hall wheel is in the upper left the pulses are coming from that and they are used for different purposes but one sensor goes through some diodes and there is a transistor that has a ground connection and a part that goes to stop. When this circuit does not get a string of pulses then the transistor probably issue a stop command. U504 and Q505 seem to be involved. The schematic I have on the computer is so bad as usual that a manual from Stereomanuals.com would be the best as that is what I work off of usually if not the original one.

 

[SkywaveTDR]

Additional info is that the control board of the 122 Mk II is often seen with pretty lacking solder joints particularly at the edge connector for the front panel buttons and many other places, In these decks I resolder at least 300 joints to make sure they are good. When connector on the control board get taken off and on a lot they can break so a good solder job is a good thing to have done to the board. So did you find the problem?