Old Teisco Synth repair advice - Page 1
Forum section: electronics / repair

author: aliendelon
author_rank: Newbie
timestamp: March 18, 2016, 08:09:11 pm
content: Hey guys!I'm not an expert in electronics, only basic knowledge - but i want to fix my lovely Teisco SX210 from 1983 - or die trying anywayMaybe some of you can forward me to the right direction - because at this point i'm stuck in troubleshooting this thingThing is - this synth has 8 identical voices and 16 identical Envelope Generators based on some Opamps, 4066 and 4081One of the envelope generators doesn't work - i already installed new opamps, 4066, 4081 and some caps - but it still doesn't workThere is schematics in attachment - i tracked the problem down to the one output of TL082 - it's constantly gives 15v - and supposed to be at 0v, and output 15v only when key is pressed, it's point "1" on the schematicsThere's even a circuit description - but i'm so dumb - still don't get why this opamp gives 15v constantly (second output work as expected though, so it's not opamp itself)Can you please help me analyse this circuit properly and kick me to the right path?))I'll really appreciate any help!Cheers!

author: cvanc
author_rank: Frequent Contributor
timestamp: March 18, 2016, 08:23:31 pm
content: What are the voltages on both power pins of the opamp, relative to circuit common?What is the voltage on the +input pin of the opamp, relative to circuit common?Is there any of that terrible brown glue on the PC board? This stuff degrades over time and can get conductive and/or corrosive, causing all kinds of trouble.Good luck with your repair. Post a photo of the synth, maybe?

author: aliendelon
author_rank: Newbie
timestamp: March 18, 2016, 08:32:45 pm
content: Wow, thanks for reply!Power pins are ok - +15 and -15 respectively+pin measuring +6v if opamp is removed from the socket, if not - 0vNo brown glue on the boardPhoto in attachment!)Thanks!!!

author: Shock
author_rank: Super Contributor
timestamp: March 19, 2016, 05:08:14 am
content: Check input versus output and any circuit that feeds into the opamp in the same way. Do this on both channels and then compare the problem channel to a working channel.So that means check capacitors for high resistance (ESR) and value pulling the signal down or open or shorted resistors and if signals/voltage present from ICs feeding the input. Check if another component is pulling down the output.If you test with them out of circuit check resistance to ground as well it might show a glaring difference.Test all traces make good connection, ground points show good and if the circuit hasn't suffered any leakage or contamination as mentioned above. If it's an old DUT suspect caps to be causing all sorts of problems and keep in mind if it's old you should check that you have clean voltage and supply.All components can partially fail so don't expect it to be good if one side appears ok, especially if you're stuck there for a buck component.Lastly don't be afraid to touch up the joints if it looks crusty just keep track of everything you have done so you can go back over it and check before and after.

author: cvanc
author_rank: Frequent Contributor
timestamp: March 19, 2016, 06:54:09 pm
content: OK thanks for the additional info.We're talking about opamp "1/2G" in the drawings, right?On one of your posted drawings the output resistor of 1/2G is 10 ohms; on the other drawing it is 100K ohms. Please report what the actual value is.With the opamp installed the output voltage is +15 at the same time that the + input pin is at 0, do I have that correct? If yes...1- Change opamp2- Resolder all opamp socket pins3- Replace opamp socketHope this helps.