[sdiy] Datasheet Toshiba TA7505M / modern replacement

[Oren Leavitt]

Happy New Year!

I wonder if that buffer op amp is seeing a capacitive load and is 
oscillating at high freq?
Try a 100 ohm resistor between it and the VCO1. If that cures the issue, 
place the 100 ohm R "in-loop" with the buffer - there are example of 
this for capacitive decoupling in most op amp datasheets.

- Oren

On 1/4/25 9:27 AM, Florian Anwander wrote:
> Hello,
> 
> I built a replacement for the SSK board in a Yamaha CS15, essentially 
> using Old Crow's approach. My schematic is here:
> https://florian-anwander.de/yamaha_cs15/CS15_SSK_FA__Nano_2_sch.png
> The original schematic is here:
> https://florian-anwander.de/yamaha_cs15/Yamaha_CS-15_Servicemanual.pdf
> 
> Everything works perfectly. Only after switching on, VCO2 plays in an 
> extremely wide range, while VCO1 only plays over one octave across the 
> entire keyboard. I can "cure" this by briefly interrupting the CV 
> connection to VCO1.
> 
> The difference between the two VCOs is that the TA7505M OpAmp, which 
> provides the buffering for the portamento circuit, is connected directly 
> to the IG00153 of VCO1; while for VCO2 there is an additional OpAmp 
> stage before the IG00153 that provides the detune voltage. In my circuit 
> I use a TL072 instead of the Toshiba TA7505M.
> 
> Now I am looking for the datasheet of the TA7505M to find out what could 
> cause the problem. There doesn't seem to be the datasheet on the web.
> 
> Do any of you have a Toshiba Databook from the late 70s? Or does anyone 
> have an idea how I could solve the problem in a simple way without 
> having to make a new board (it is clear that I could also put a buffer 
> in front of the IG00153).
> 
> 
> Best regards
> Florian
> 
> --
> http://www.florian-anwander.de/
>