The 7805 output is low but not low enough that the system shouldn't work. It may be drawing too much current on the +5Volt. Got a soldering iron, a pair is small diagonal cutters, and a multimeter that measures up to two Amps of current? Yes, then cut the +5 volt pin on the 7805, bend legs in different directions, attach the ammeter to the different legs (check if you have to put the DMM probe into a different hole) and measure the current at 4.85 volts. My guess that a good system will be between 300 and 800 milliAmps. That's normal for a microprocessor-based 'light' synthesizer of the mid 1980s. Are any of the ICs physically hotter than the others?
From: xjnx0x
To: vintagesynthrepair@yahoogroups.com
Sent: Sunday, April 29, 2012 12:51 PM
Subject: [vintagesynthrepair] Re: Simmons SDE
To: vintagesynthrepair@yahoogroups.com
Sent: Sunday, April 29, 2012 12:51 PM
Subject: [vintagesynthrepair] Re: Simmons SDE
Thank you all so much for tips so far.
I took some measurements of the ICs in the power supply, the rest of the device was connected during the measurements. I used the center leg of the 7805 as the ground reference. I'm making up board numbers here, so if a schematic exists and happens to turn up (please please please), don't take these numbers as reference... IC 1 may not be IC 1 on the schematic, I'm just numbering them right to left.
IC 1: 7805
in 10.6v ;
g 0v
out 4.86v
IC 2: 7806
in 16v
g 0v
out 5.98v
IC 3: 8535 (7906)
in -15.9v
g 0v
out -6v
the last part was a little hard to find.. 8535 apparently refers to an AVR microcontroller now, but I found this cross reference:
http://www.datasheetarchive.com/5962-01-068-8535-datasheet.html
which lead me to this:
http://www.datasheetarchive.com/KA79M12-datasheet.html
It also says 7906c on it, which was much easier to find:
http://www.alldatasheet.com/datasheet-pdf/pdf/91828/MOTOROLA/MC7906CD2T.html
The input and output of IC 1 seem out of line... should I also measure this with the rest of the device disconnected to see if it changes?
What next? Replace IC 1? Test the diodes? Measure resistance of each diode in both directions with the power off and look for one that conducts in both directions?
Thanks for your help, any pointers would be appreciated!
Christian
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