IC Question (TA7505M)

[argonrepublic]

I have little experience but like to learn and recycle. I am working on an old analog synthesizer. Key voltage is high when disconnected. Synth will not function when connected. Key voltage should be between .25V and 4V(depending on key played) and is put out of pin 6. Presently, pin 6 is puting out 14V even with keyboard disconnected. I have the diagram and will try to attach.
BTW: In diagram it is IC6.
Thanks
I can't attach file because it is 187KB

 

[argonrepublic]

TA7505

Attachment

 

[argonrepublic]

TA7505M

Here are pin voltages when connected:
1 -17v
2 +13v
3 50mv(fluctuating)
4 -17v
5 -17v
6 +14v
7 +17v
Thanks again

 

[Brion55]

Does the voltage change on the input (PK) when different keys are depressed? If so, it sounds like the op amp (IC) is fried and "locked up" to the positive rail.

 

[argonrepublic]

IC

Thanks. I manually grounded and opened pin 3. Output changed.
It seems my problem is somwhere between the YM24800 IC and the opamp. I added a schematic. I don't know best way to check this IC.
Thanks

 

[Brion55]

I worked for a company that made analog synthesizers back in the 70's, so I may be able to help you with this. I can't tell from the two schematics you attached where they join. Would it be possible to post a more complete schematic of the circuit in question? Or you could also email me.