SH-101 Gate stuck open

[soniccustard]

Hey, been visiting the site for a while, first time posting, but now I need some help!

I have an SH-101 that needs fixing and it seems like the gate is constantly open, it will never stop making a sound until you pull the sustain part of the envelope down. I'm sure this can't be normal activity for a 101? It works perfectly via external CV control, it seems to be just to do with the internal gate.

Can anyone help at all? I keep prodding around it but I just can't find the source of the problem. I've got the service notes with have a lot of block diagrams and schematics for it which are kind of helpful but I still can't identify the problem.

Thanks!

[HideawayStudio]

Hey, been visiting the site for a while, first time posting, but now I need some help!

I have an SH-101 that needs fixing and it seems like the gate is constantly open, it will never stop making a sound until you pull the sustain part of the envelope down. I'm sure this can't be normal activity for a 101? It works perfectly via external CV control, it seems to be just to do with the internal gate.

Can anyone help at all? I keep prodding around it but I just can't find the source of the problem. I've got the service notes with have a lot of block diagrams and schematics for it which are kind of helpful but I still can't identify the problem.

Thanks!

Hi,

Does the sound stop if you plug a jack into the gate input? (with nothing connected at the other end of the lead).

[soniccustard]

Yeah it does, the Envelope works via the LFO, too.

Edit - I've actually disconnected everything now, the buttons, the keyboard and most of the bender board, it's still going.

[HideawayStudio]

Yeah it does, the Envelope works via the LFO, too.

Edit - I've actually disconnected everything now, the buttons, the keyboard and most of the bender board, it's still going.

Is the Hold LED lit?

[soniccustard]

No, the button seems to work as well, when you press it the light comes on and off and the arpeggiator hold function works accordingly.

[HideawayStudio]

No, the button seems to work as well, when you press it the light comes on and off and the arpeggiator hold function works accordingly.

So you are definitely saying that if you insert a jack into the gate input the sound stops?

If so then it is possible TR9 has failed or there is a dry joint. It would be worth checking, if you have a scope or logic gate, whether the voltage to the 100K standoff resistor (R49) feeding its base is seeing a state change during key on/off - if not then you might have a cracked track or dry joint anywhere from pin 38 on the CPU to TR9.

[soniccustard]

Yes, an inserted jack kills the sound. I'll have a look at TR9, I thought from looking at the schematic that TR29 might be the issue, but then I've never opened a 101 before!

Thanks for the tip, I'll give it a look right now.

[HideawayStudio]

Yes, an inserted jack kills the sound. I'll have a look at TR9, I thought from looking at the schematic that TR29 might be the issue, but then I've never opened a 101 before!

Thanks for the tip, I'll give it a look right now.

This is why I got you to try plugging in a jack into the gate input - by doing this it determined whether the gate receiving circuit (TR29) was stuck or the gate generating circuit (TR9) was stuck as the gate input is normalised on the SH-101 thus disconnecting the internal gate generator.

In your case its the gate generator (TR9) that is stuck which is driven directly by the CPU.