Yamaha KX-1200u plays / winds for 2 seconds and then stops

TheMarco

TheMarco

Thrift Store Audiophile
Subscriber
So I bought this Yahama KX-1200u on FB Marketplace (BAD move, Marco. BAD move, as it turns out)
The deck arrived not working. Opening it up revealed that the belts were shot. So... I went and gave it full service, new belts, lubed the capstans, cleaned everything, new idler tire.

I'm 99.999% certain I did not mess anything up.

After all this work the deck plays for a few seconds and then stops. Same for fwd / rewind. When playing it looks like the right reel doesn't seem to want to spin right.
When powered up the capstans are both spinning smoothly.

I'm assuming this means the reel motor is broken. Or is there anything else I can try to do with it?
 
Register to hide this ad
Hi. I'm assuming that it was not a broken reel motor. It sounds like one of the hub rotation sensors is not working. This is used to detect the end of tape during Play, FF, REW and stop the transport.

In other transports, there is a disc with slots or black/white stripes that interrupts the LED light beam to a photo sensor. As the hub rotates the slots generate a series of pulses that are detected. When the rotation stops, a capacitor is usually charged and will trigger the Stop.
If you have a scope, you can look for the pulses.
1728582608372.png

1728582507515.png
It might be located here, but not sure?
1728583001146.png
 
dankik said:
Hi. I'm assuming that it was not a broken reel motor. It sounds like one of the hub rotation sensors is not working. This is used to detect the end of tape during Play, FF, REW and stop the transport.

In other transports, there is a disc with slots or black/white stripes that interrupts the LED light beam to a photo sensor. As the hub rotates the slots generate a series of pulses that are detected. When the rotation stops, a capacitor is usually charged and will trigger the Stop.
If you have a scope, you can look for the pulses.
View attachment 3316460

View attachment 3316459
It might be located here, but not sure?
View attachment 3316476
Click to expand...

Unfortunately I don't have a scope nor the skill to use one. Assuming for a moment that this is the problem: can this be fixed or would it require unobtanium parts? Even motors for these decks seem very hard to source.
 
If it is the rotation sensor itself then it could be:
  1. mechanical rotation of the slotted wheel between sensor and LED (sometimes it uses a magnetic or hall effect sensor) - you can check the rotation if you can find the sensor
  2. LED is bad and not emitting light (sometimes it may be IR light, not visible to the eye, but if you view it with your phone camera you might be able to see it)
  3. Bad photo sensor, really can't tell unless you have a scope to see the pulses
  4. Do you have any test equipment at all (i.e. digital multimeter, soldering iron, etc.)?
Do you know if it was working before you changed the belts?
When it stops, both reels stop rotating... correct... and the head retracts like you pressed the Stop button?

Here is where the two sensors go to the Main PCB, G2/G3:
1728586120396.png
 
When it plays, reels rotate, same for fwd / rewind. Heads move properly when pushing play and move back to their resting position when it shuts off.

I don't know whether or not it worked fine before the belts failed. I think the deck has been non-operational for a long time given the shape the belts were in (turned to goo).
 
When it plays, reels rotate, same for fwd / rewind. Heads move properly when pushing play and move back to their resting position when it shuts off.

I don't know whether or not it worked fine before the belts failed. I think the deck has been non-operational for a long time given the shape the belts were in (turned to goo).

Another important observation: When I hold down the play button it will keep playing. Steady speed. Good sound. All good. It will not however do this with fast forward or rewind.
 
When you changed the belts, did you disconnect any wires going to/from the sensor board or to the 10-pin connector on the Main board? Maybe you did not reconnect them or did not reconnect correctly.
 
dankik said:
When you changed the belts, did you disconnect any wires going to/from the sensor board or to the 10-pin connector on the Main board? Maybe you did not reconnect them or did not reconnect correctly.
Click to expand...
I just double checked. Looks good to me.
 

Attachments

  • PXL_20241010_192535110.jpg
    PXL_20241010_192535110.jpg
    92.1 KB · Views: 14
  • PXL_20241010_192642522.jpg
    PXL_20241010_192642522.jpg
    90.8 KB · Views: 15
It may well be that the issue with the tach\sensor is what took it out of service decades ago. Given the age, it might be that the light source is actually an incandescent lamp. If that is the case, it might be possible to replace it. I have a Beogram 4000 TT that uses a simple incandescent bulb to provide light for it's sensor.

If you find an incandescent lamp, all you need to do is a basic continuity check to see if it's bad.
 
I
oldvinyldude said:
It may well be that the issue with the tach\sensor is what took it out of service decades ago. Given the age, it might be that the light source is actually an incandescent lamp. If that is the case, it might be possible to replace it. I have a Beogram 4000 TT that uses a simple incandescent bulb to provide light for it's sensor.

If you find an incandescent lamp, all you need to do is a basic continuity check to see if it's bad.
Click to expand...
Interesting for sure. I can't seem to find any kind of bulb in the service manual though :(
 
My biggest Q at this point is: could this also be the dreaded "dead spot in the motor" thing a lot of decks have or no? Asking because I know it's not exactly trivial to fix that or get another motor.
 
TheMarco said:
could this also be the dreaded "dead spot in the motor" thing
Click to expand...
It looks like the transport has 3 motors (see diagram below):
  1. Reel Motor = You said in a previous post "When it plays, reels rotate, same for fwd / rewind." So, I would assume that is OK.
  2. Capstan Motor = You said "When I hold down the play button it will keep playing. Steady speed. Good sound." This indicates that the capstan motor is spinning.
  3. Assist Motor = I think this motor is used to move the cam for going from Stop, Play, FF, Rew and heads. "Heads move properly when pushing play and move back to their resting position when it shuts off." Looks like that is also working. You can verify that the Assist Motor works by seeing if it moves when you go from Stop to Play or FF/Rew.
  4. Sensor Circuit Board = Looking at the description again, I think it is for tape type detection (see below).
1728609085241.png
I take back my previous comment regarding the Sensor Circuit Board. It might be for detecting CrO2/Metal tape and the REC Protect tabs on the cassette case.
So now I'm not sure where the rotation sensor is located, probably near the reel hubs?
 
dankik said:
It looks like the transport has 3 motors (see diagram below):
  1. Reel Motor = You said in a previous post "When it plays, reels rotate, same for fwd / rewind." So, I would assume that is OK.
  2. Capstan Motor = You said "When I hold down the play button it will keep playing. Steady speed. Good sound." This indicates that the capstan motor is spinning.
  3. Assist Motor = I think this motor is used to move the cam for going from Stop, Play, FF, Rew and heads. "Heads move properly when pushing play and move back to their resting position when it shuts off." Looks like that is also working. You can verify that the Assist Motor works by seeing if it moves when you go from Stop to Play or FF/Rew.
  4. Sensor Circuit Board = Looking at the description again, I think it is for tape type detection (see below).
View attachment 3316880
I take back my previous comment regarding the Sensor Circuit Board. It might be for detecting CrO2/Metal tape and the REC Protect tabs on the cassette case.
So now I'm not sure where the rotation sensor is located, probably near the reel hubs?
Click to expand...
Yes you are right those sensors are to auto-detect the tape type. It knows what tape you put in right away. That feature is working properly. I dug through the service manual and was unable to find anything about hub rotation sensors :(
 
By the way, holding the play button makes it keep going. Holding the fwd / rew button does not. But I assume that's because holding it triggers another function: extra fast winding. It has two speeds. So it will go in 'extra fast' mode and still quit after 2 seconds.
 
TheMarco said:
So... I went and gave it full service, new belts, lubed the capstans, cleaned everything, new idler tire.
Click to expand...
When you changed belts, etc. did you remove the Sensor Circuit Board shown above?
Also, when you changed the belts did you put in the correct ones. If your belts are slacking the tape, then the auto stop will stop the transport to prevent damage.
What are the numbers for the belts and where did you buy them?
 
I got this belt kit right here:

www.ebay.com

YAMAHA KX-1200 CASSETTE CAPSTAN & BACK TENSION BELTS WITH IDLER TIRE | eBay

If the situation improves in the future we may reconsider that position.
www.ebay.com

I only removed the mechanism from the deck, nothing else. Had to unhook a bunch of connectors which I all put back after servicing. The tape does not appear to be slacking at all when the deck is operating for said 2 seconds. Even when holding the play button, nothing of note is happening to the tape. All looks normal, no noises, nothing.
 
Thanks. I'm not sure how you can troubleshoot any more without a way to check the operation of the take-up & supply reel sensors. You might be able to put a volt meter on the two terminals and rotate the hub by hand and see if you can measure the voltage going up/down as you turn it slowly.
 
You must log in or register to reply here.
Back
Top Bottom