I could use some brain power, as mine apparently isn't up to the task. Here's the situation: I'm working on a Technics SL-1100. When the power switch is moved to On, the platter begins spinning. Of course, it isn't supposed to do that, but rather instead wait for Start to be pressed. Once the platter is spinning, pressing Stop will often (but not always) stop it, but usually only for a few seconds, and then it will often begin spinning again. During those times when the platter stops and stays stopped, pressing Start will have no effect.
I've replaced both mini snap switches used for Start/Stop. I've also tested the following transistors in circuit, all were ok (not shorted), though I'll pull any that folks think warrant out-of-circuit testing.
TR9
TR11
TR12
TR13
TR14
All electrolytic caps tested with my Peak ESR70 Plus--nothing out of the ordinary. I checked all of the solder joints and touched up a few. In terms of voltages. I've measured around TR11, TR12, TR13, and TR14, and everything is close to what's shown on the schematic. One odd thing is that all voltages are showing negative with my common lead connected to pin 7, per the instructions in the SM.
I feel like I'm missing an obvious culprit here. . . .
I've replaced both mini snap switches used for Start/Stop. I've also tested the following transistors in circuit, all were ok (not shorted), though I'll pull any that folks think warrant out-of-circuit testing.
TR9
TR11
TR12
TR13
TR14
All electrolytic caps tested with my Peak ESR70 Plus--nothing out of the ordinary. I checked all of the solder joints and touched up a few. In terms of voltages. I've measured around TR11, TR12, TR13, and TR14, and everything is close to what's shown on the schematic. One odd thing is that all voltages are showing negative with my common lead connected to pin 7, per the instructions in the SM.
I feel like I'm missing an obvious culprit here. . . .
I have a very similar problem, turntable turns by itself when on, stop button will stop the platter but then it gradually starts by itself. Just like you i tested all components, changed the micro switches , cleaned all pots ,sub pots and contacts. My suspicion is that the 6.2 volts zener is leaky. It' s the only component i am not able to test. I have tried another motor so i know it' s from the main board. I will swap components from another board ,beginning with the zener, and i' ll keep you posted on what i find.
In my case, it turned out to be bad solder joints at the board side plug-in connector from the switches. I'd also polish the pins, as there was corrosion on one.
Do you measure 6.2 ± 0.1 volts across it when the circuit is powered? If so the zener is fine and is doing what id needs to do. BTW a zener works by leaking, or at least breaking down at the specified voltage when reverse biased.
I have swapped tr11 from another working board and it solved the problem. Testing the transistor with diode check and gain checker gives nothing wrong. I suspect the tranny is noisy or causes oscillation in the circuit.