SERVICE TIPS REGARDING THE SL1200 RUNAWAY SPEED
Runaway Speed at Power On
Symptoms: At power on, both 33 and 45 lamps illuminate together and the platter takes off at 100 mph, without ever touching the start stop button
Make a voltage measurement on test point 15; this is the 9 volt supply
for the IC 201. In sets we tested, this voltage will show to be around
Check R202, 10K resistor and replace if faulty. Check Q201 solder joints, known to fracture and come loose, followed by replacement of Q201 if defective.
If this still does not restore the 9V reference, then replace IC201.
All Functions OK, but platter running fast.
Check for the Main 20V and the IC201 9.4V
Next, check IC201 pins 23 and 22 for 3.0V. If this voltage is low or even 0, check C205 220UF 6.3V Capacitor.[ I was suprised by this one. But in most cases, if all the functions work, but the platter is fast, something is amiss with the Frequency Generator circuit.] In this case, C205 was shorted. This removed the feedback, so the motor controller thought the platter was standing still and applied full power to it, causing it to spin out of control. You can also check the FG coil around the magnet.This should read 4.5 ohms from R203 to ground. It should not be significantly less nor should it be an open circuit.
Intermittenly OK then going fast, not responding, or back and forth like a washing machine.
Intermittent problems are the hardest to find but are most likely a bad connection, and in this one case, in the motor. Take a voltage measurement on C105, 106 and 107 for 15.5V to ground. This is the positions sensing elements and I found one to be intermittenly opening up. When open the measurement drops to 7V. A new motor assembly restored this player.
Quartz lock OK, intermittent when using pitch control , sometimes working, sometimes not responding, just standing still
First, place ohmeter on brown pitch lead and confirm smooth continuous resistance from 0 to 10K as you move the pitch control. Next, place ohmeter on pin 2 of pitch integrated circuit and confirm resistance varies when you move the pitch control. Next, using frequency counter, confirm frequency at test point #11. This should be adjustable with the VR301. In this case it was not changing. just drifting around 260Khz. What I found was if I flexed the PC board, the correct reading 262.08Khz would come and go. So this tell me we have a bad connection somewhere. Flipping the board over and inspecting the solder connections I could see that one lead of R303 was not visible through the factory solder joint. Sure enough, I was able to lift up one side of R303 from the board. It was never soldered, ever, right from the factory! Removed the old solder, pushed R303 all the way through the board and resoldered it. Problem resolved. It is interesting to note that someone had been in this deck before and there were new solder joints on every Integrated Circuit. Must have been quite a let down to do all that work and still not solve the problem.
Other things to check when the platter is mis behaving
This problem is either in the Frequency control IC302; Pitch Control Wiring , IC101 Motor Driver. You need a voltmeter and frequency counter. First, make sure pitch control cable is plugged in and quartz lamp is on. Next, using a frequency counter, confirm that you have the same measurement(262.08kHz) on pins 2 and 3 of IC201. If yes, nothing is wrong with the frequency control and pitch sections. If no, problem is related to IC302. Next check voltage on pin 21 of IC201. This should vary from 0.18(stop) and 2, 3 or 5V in start mode. I have seen all 3 different voltages now on properly working decks. I have 2 older decks that read 0.18 / 5V, I have one with the latest version of the motor driver and it reads 0.18 / 3V and recently serviced 2 decks that read 0.18 / 2V. At one time I thought this was indicative of pending failure, but now have doubts . Therefore, if the frequency control sections prove OK, I would suspect IC201 as the culprit.
The Service manual gives general guidlines for troubleshooting this as well. But older sets are going to experience unusual problems.
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