Ibanez SD9 switching issue

Keefy

I have had a 1980s MIJ Ibanez SD9 since the early 90s and have done loads of gigs with it, although it's been relegated to the spare pedals drawer for a while now. Someone on here posted a WTB ad (ha!) and I dug it out with a view to selling it. I quickly changed my mind when I plugged a Les Paul into it but I found it does a weird thing when switching from dirty to bypass, in that the volume drops for about a second before coming back up to normal. These things are well known for leaking distorted sound through their switching FETs, and this example is no exception, but the volume dip thing is a new one on me. I was using it on a true bypass loop switch for the years preceding its retirement.

Does anyone understand FET switching well enough to suggest what might be happening here? I have a copy of the schematic (see below) and I'm not afraid to use a soldering iron!

thermionic

Could it be a capacitor not charging/discharging as it should?

Keefy

thermionic said:

Could it be a capacitor not charging/discharging as it should?

If so, those three 1nF caps at the bottom would be prime candidates. (I expect the electrolytics are well overdue for replacement too but I don’t think they would affect the switch operation.)

ICBM

Yes, it sounds like a leaking cap in the switching circuit. It's less usual for non-electrolytic caps to die, but it's the most likely cause for that exact problem. A resistor - or more likely a solder joint on one - in the switching circuit could also do it I think.

Keefy

ICBM said:

Yes, it sounds like a leaking cap in the switching circuit. It's less usual for non-electrolytic caps to die, but it's the most likely cause for that exact problem. A resistor - or more likely a solder joint on one - in the switching circuit could also do it I think.

Cheers - well there's a project for me!

Keefy

I replaced all 4 electrolytics to see if that helped. 3 of them tested ok but the 47uF cap running between the bias voltage (1/2Vcc) and earth was well out of spec, and had an ESR of 14R.

The pedal now works exactly as it should - yay!

ICBM

Keefy said:

I replaced all 4 electrolytics to see if that helped. 3 of them tested ok but the 47uF cap running between the bias voltage (1/2Vcc) and earth was well out of spec, and had an ESR of 14R.

The pedal now works exactly as it should - yay!

Interesting - I would have suggested that as it seems to be a very common failure in older pedals now, but I didn't think it would affect the switching speed in one direction only... it's got nothing to do with the flip-flop circuit, and the bias voltage is applied equally to both signal transistors.

Just goes to show you can't always assume what some faults will do!

Keefy

ICBM said:

Keefy said:

I replaced all 4 electrolytics to see if that helped. 3 of them tested ok but the 47uF cap running between the bias voltage (1/2Vcc) and earth was well out of spec, and had an ESR of 14R.

The pedal now works exactly as it should - yay!

Interesting - I would have suggested that as it seems to be a very common failure in older pedals now, but I didn't think it would affect the switching speed in one direction only... it's got nothing to do with the flip-flop circuit, and the bias voltage is applied equally to both signal transistors.

Just goes to show you can't always assume what some faults will do!

Yeah I don’t understand how that helped but I’m not complaining!