Oberheim ob-1

[sam]

There is no output...I think it's a problem with the memory section.
Even in manual mode nothing works..as the memory has stopped everything.

Any thoughts...There is loads of logic chips but where do i start..?

[Steve Jones]

Start with the power supply. Always the first step in every repair like this. No point spending days looking for a computer fault if one of the supply rails is missing.

[sam]

Cheers steve...The power supply is fine, It had a leaky battery but i cleaned it up and replaced it but it still locked up.

[Steve Jones]

Have you checked very carefully that battery electrolyte has not gotten under anything and eaten it away? It tends to track along traces and eat the copper away from under the laquer. Also check the power supply rails for ripple if you have a scope.

If you don't have a cheap oscilloscope and you want to do this repair yourself it would be a good idea to get one. If you have a multimeter, see if the pots on the top panel have 10V on one side and 0V on the other relative to ground (or between the 2 outer pins on each pot.

[sam]

thank you steve...I will give that a try...

[sam]

Update.....I have replaced all the caps on the power supply and this should rectify any unwanted ripple and after 20 plus years it needed it anyway.

All the readings to the boards are correct inc the programmer.

The synth powers up with the second memory light on..

It won't change if i choose another inc the manual mode.

I only get a low rumble on the output.

Next (thanks steve) i check the pots ...

[sam]

The problem is on the programmer board..I tried another board from a ob-1 here and it works.

It's full of those 4XXX chips..

I am just wondering how to test these chips...?

No scope but a very good multimeter.

[sam]

I have found a possible cause...A CD4050 chip is not reading correctly and it's under the leaky battery.

I will replace it and see what happens..

Fingers crossed..

[sam]

Here is something i am trying to work out...

The saw wave can be swept to a sine....Which part of circuit does this...?

http://www.electrongate.com/obfiles/OB-1-1342a.jpg

[sam]

This programmer board is reading higher values than it should.

I have been looking at the LM301 ...Pin 8 gives a different voltage between 0 to 13v every time it's switched off and on again...I don't think this is normal..?

Here is the board....http://www.electrongate.com/obfiles/OB-1-1341.jpg

Any thoughts on this..

[sam]

The opamp is ok...The power supply is ok..

It's the programmer board...When the synth is switched on light 2 comes on.

If i touch a sensor and switch on it will light on that one inc the manual mode.

In manual mode it plays but no controls work.

So if anyone has experience of these problems...Please help...

[sam]

There is another strange thing.....On the schematics cem 3310's are used for the ADSR...I cannot find them on my ob here....I need to know if there are different versions and find the right schematics...?

Please anything might help...

[sam]

I just found out mine has discrete envelopes...So i need to work out what components do this..

[sam]

I have replaced the 3086 which has solved the osc fault..The cable that supplies the main board with power was causing problems with the tuning because of bad contacts..Tried to clean it up but still not 100%.

Need to find a replacement...Anyway i need to find the part of the circuit that changes the saw wave to triangle..?

If you know...?

[sam]

I have now solved the problem with the saw to triangle wave...It was a faulty 3080 on the osc section.

The memory problem was a 4069 on the prog board..


SOUNDS GREAT NOW....Thanks for following and if you have any questions regarding the ob-1 and X ..

Just ask...