I am only getting the chorus hiss and an extremely faint and muffled sound when I press a key. No change when I change patches, other than the chorus his goes away if the patch I call up does not have chorus.
I pulled the header on the Power Supply board which feeds the CPU board and I get the following voltages:
if there is no load on the power supply the voltages will all read inaccurately.
plug it back in then retest under a load.
However it sounds like an op amp is dead. If you have an oscilloscope, start with the sound generator chip and look at the signal on the oscilloscope. Using the schematic follow the signal on the opamps to the output. Wherever the signal should be and isn’t, is the problem IC.
Pin 2 of IC3 should be more like 5 V, even unloaded it should be somewhere around there. There is also IC2 M5230L which along with TR2 and TR1 provide the +15 and -15 V, respectively. Check the voltage at their collectors (or the cathode/striped side of D2 and anode of D1 to avoid shorting transistor legs together).
Have you recapped the power supply? Usually best to start there if it hasn’t been done in the past 30 years then start checking ICs and transistors.
I am feeling like the 5v power supply is not happening. I will replace the 7805 and the M5231L. I will also recap the power supply, as they are definitely long in the tooth.
The 106 has two secondary windings. That 7805 is providing a regulated +5v for the digital side.The +9v is coming straight from your bridge rectifier. Both of those voltages look okay.
The other secondary winding is used to provide +5v, +15v, and -15v for the analog side.
According to the schematic I’m looking at, pin 30 should be a ground, so .002v looks okay. Not sure why you were expecting +9v. I’m also assuming your meter is attached to pin 27/28? Since the two windings do not share a common ground on the power supply board, you’ll need to reference pin 30 when reading the analog side - especially when disconnected from the rest of the 106.
Travis is right. Pin 1 of IC3 is connected to the +15v line, so if you’re readings are right, I’d say your problem is much earlier than IC3. That should be closer to +15v.
Looking at the schematic for the module board, +5v is used in the switching circuitry, and the 15v rails are used by amplifiers. That would be consistent with the symptoms you are describing.
This might fall under technician error, as I reseated a ribbon connector while powered up and didn’t realize the shielding underneath the Module board had any possibility to short out against the board. Ugh.
It seems that the D7811G (TC29) shorted out when I reseated the ribbon next to it. I know this because there are indentations from the pins in the metal shielding now that I have taken the Module board out.
This is making sense why I get 5v on the power supply without the connector for the Module board connected and it goes down to 1.3v when I connect it.
Gonna assume that TC29 is toast but I need to check further.
Anyone who has been down this road before, I would appreciate any advice you may have.