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I recently purchased a TR-707 and am trying to bring it back to life.
I would really appreciate some assistance with identifying the suspected issue with the power supply circuit.
I have the original wall wart power supply (negative tip) 12volt 200ma which is giving a reading of 17.8 volt without load, so I'm assuming this is working correctly.
I have already replaced the power jack on my 707 because the connection was originally loose, however unfortunately it wasn't that simple to get the 707 running properly again.
From studying the schematic I believe I should be getting a reading of -10v at the anode of D2 and +5v at the output of IC1, however I'm getting a +2.6v at both these points?
Furthermore if I take a measurement of the voltage directly from the power input jack. When the device is switched off it gives a reading of 17.8 volts, which is the same as the reading from the tip of the wallwart psu. However when I switch the 707 on, the voltage drops to around 5.5v.
One last bit of information that might help you identify what going on is, the TR 707 has temporarily turned on for approximately 30 seconds on a couple separate occasions and then it just switches off. Today I managed to monitor the voltage whilst it was temporarily on, the voltage dropped from 17.8v when switch off, to 11.7v for 5 seconds when the device was on, then to 5.5v where it stays and the device is obviously off.
Hope this makes sense to someone and you can help me make some progress on my repair
I'm really enjoying learning about repairing vintage synthesisers and drum machines, but please bare with me as my knowledge is fairly limited. However I did an introduction to electronics during my sound engineering course, so I do have some basic knowledge on circuits and electronic components.
Thanks in advance.
Jay
