TS9 no signal when engaged

[acobberson]

I know, I know... there are TONS of other threads on this or related subjects, but I promise this is different.

I have a 1982 TS9 that I am trying to bring back to life. I get signal when it is engaged, but nothing when engaged. I figured the flip-flop circuit was failing, so I replaced the transistors in the flip-flop circuit. Still nothing. I understand that a failure in the actual overdrive circuit of the pedal is pretty rare, so I went ahead and bypassed the flip-flop circuit as well as the input and output buffers. Still nothing. I basically wired the input jack directly to C2 in the drive circuit and then pin 2 of the level pot directly to the output jack, but to no avail.

Am I wrong in assuming that this means there is a failure in the actual overdrive circuit? I took readings of the Opamp, and all the voltages look good. I can take the readings again and post them.

Power is getting to the board, the protection diode looks good... next I guess I`ll have to get out the probe and start tracing backwards to see where the signal is getting lost.

If anyone has any other ideas, let me know!

[GibsonGM]

"Going forward", can you post the TS-9 schematic?  To make us speak a common language and all.

As long as you haven't boned the signal path (didn't try to look up what you've done...), you should be able to audio probe where the signal ends, within the circuit.

If I got one non-working, I'd have gone in where the switching FETs connect to signal path and started there, determine if it's in the switching or something else like a component failure or something.   

If you have signal when 'disengaged' (I guess?), can you flip from 'signal' to 'dead' using the switch?  Working disengaged suggests at least partial switch function + output crap (as well as the input buffer), with problem at the other end.

[acobberson]

Hi and thanks for the response.

Heres the schematic I am using. Being that mine is an older TS9, the switching JFETs are 2sk30. Those are the ones that I replaced when I assumed the flip-flop switching circuit was simply failing.

Yes, I am able to swtich from "disengaged" signal to "dead" with the switch. That's what is leading me to believe the issue lies somewhere within the drive/tone section as the signal goes through input and output buffers even when the pedal is disengaged.

I'll fish out the audio probe this weekend and see if I can find the problem.

[antonis]

Start checking from C2 & C7..
(an open cap mutes everything..)
Then proceed to Level pot lug 3 & wiper..
Finally, check C5 for short..

[GibsonGM]

^ Yes, the electrolytic caps may have dried up and gone bad, or the level pot may be bad, something on that order is pretty likely.  Let us know!

[anotherjim]

My first suspect with an old product would be the output Level pot. The wiper isn't connecting anymore =  silence from the FX. This can happen if the knob has been pushed hard from above and driven the spindle down.

[acobberson]

Ok, here is my update...

All Pots are good and function perfectly. Input/Output buffers are working. Flip-flop circuit is working.

Voltages on Opamp as follows:

Pin 1, 2, 3, 5, 6, 7 = 4.5V
Pin 8 = 9V
Pin 4 = 0V

I started probing the drive/tone circuit and it seems as though the signal is being lost at pin 2 of the Opamp. The signal at pin 1 is pretty weak, but it is noticable. It does not however change when I turn the Distortion pot up and down. I assumed this was due to something between pin 2 and pin 1 either needing to be reflowed or a cap being bad. I desoldered the 51p cap and tested it. It's good. I resoldered it, then tested the 47n cap. It is also good. I reflowed all points between pin 2 and pin 1. Still nothing at pin 2. Also nothing at pin 3, which makes sense.

Next step is to test the diodes. They look fine, and there is no noticeable burn marks or anything, but better safe than sorry.

I did notice that the 4k7 resistor R4, which according to the schematic I posted should be going to ground, is going to the 4.5V output of C16 in the power circuit. But after looking at a couple other schematics of the TS9 it shows the 4k7 going to VR. So I guess it can be soldered either way?

I am starting to think that the Opamp itself is faulty, which would be a bummer because it is the original 4558 chip from 1982. I personally don't think there is anything special about an older chip, but for resale purposes most collectors want the original chip. 

IF the diodes check out as good I wanted to see if anyone had any other ideas before I desolder the chip and test out a new one in its place.

[GibsonGM]

I think you're approaching it right, cob.  Is the signal level about the same before & after the input buffer?  If the diodes are ok, then I'd also suspect the chip and just go ahead an replace it.  Use an IC socket if you have one.

The 4k7 is often shown going to ground, but VR is the AC ground - it 'fits in there' better.  C16 just sends any AC right to ground (while blocking DC).  If you think about how the signal is 'jacked up' by adding 4.5V bias to it, it will start to make sense...center point...

[acobberson]

Update:
I removed the diodes and tested them. They checked out but since I had them out I figured I'd just replace them. Checked the pedal. Same problem.

At this point all that was left was the opamp. I was pretty bummed because, like I said, a replaced opamp on these pedals in particular significantly lower its value.

I desoldered the opamp and started digging through my opamps... then I remembered that I had worked on an old EHX Pedal years ago... I think it was a Microsynth. And I remembered that I had salvaged a handful of 4558 chips from it. So as I'm going through my bags of opamps, there it was. 3 perfectly good 4558 chips from.... yep, 1982.

I popped that bad boy in and now I'm peeling the paint off the walls! Pedal works perfectly, and while it's technically not completely original it has all the correctly dated important parts.

Sometimes things just work out