Getting sound, but not really

Hello all,

I read as much elsewhere first, hoping to find a solution, but to no avail.

I’ve got midi, and I get sound through the direct osc out… however, I only get this while the oscilloscope is connected to the osc pin! I get nothing when connecting the osc pin directly to the audio out. Even stranger, this happens when the osc cable is not even connected to the audio board. Furthermore, the sound is VERY quiet.

To add to the confusion, while I had the scope disconnected at one point, I found I suddenly DID have audio through the SRM including the filter, however it was still very quiet.

Now this morning it’s back to the previous scenario where I only get audio while the scope is connected.

I’m not sure where this point towards a problem, but I’m all ears if anyone has any suggestions!

Bad ground connection in your audio jack?

I just redid the solder points for the jacks and no difference. I just noticed however, that (while scope is still connected to osc. lead) pulling the jack out causes the signal to become much louder i.e. normal volume, however with no filter. Pushing the jack all the way back in causes the signal to drop dramatically.

The osc. lead is now plugged back into the audio board, and I am now getting signal + filter, but still at a super quiet level…

Sicne, you have a scope, can you look at which stage you see the drop in signal level?

It actually looks to drop immediately at point 2 or before. Redoing the IC socket points did nothing. Does this point to something in between J4 and IC3?

The 3 resistors above IC3 or the C3/C4 capacitors might be involved.

The resistors all checked out ok. The 100k resistor fluctuates some, but sticks pretty close to 100. The caps I guess can’t be specifically tested without replacing. One potential issue is that I initially had the caps in the wrong place near the power (poor materials assessment on my part). I switched them before I ever powered on, but in the process of desoldering, which was quite difficult, they got very hot. I don’t know how much heat is needed to make them faultly, or if this something would actually result in the super quiet output I’m getting now?

And I am still very confused how I am able to get a nice loud signal when connecting the osc lead to the scope, with the audio jack pulled halfway out!

It doesn’t look like a problem with the capacitors.

Can you probe with the scope the signal at the left pin of the 100k resistor above IC3?

I get a strong signal there, but then it immediately becomes less strong by pin 2 of IC3. It actually is a little stronger at all other points (3-7) but is virtually non-existent at point 8 (pin 8 of IC3).

Since you have a scope, could you please tell me the amplitudes in V?

Ok - I’m not very familiar with this scope (or scopes in general!) but hopefully you’ll glean something from these images. The scope is set to the smallest V increment (5mV), and then x5. Images are Point 1, Point 2, Point 2 (up close for res.) and Point 7. I get nothing from Point 8.




Ok, so it turns out everything was fine and I just made a mistake![](:confused: Had resistors 68R and 68K swapped. Tada, now everything’s grand:) Thanks again for helping with the ultimately unnecessary trouble shooting) I at least learned a little more about the workings of the unit.

I did the same thing if it makes you feel any better…