TS808 repair

[Big John Studd]

Somebody gave me a broken TS808 over the weekend and said if I can fix it I can have it. Something heavy fell on it on the stage. The shaft on the level knob is sheared clean off, and he said the circuit board inside is cracked. Hopefully I can get this thing running again. My summer schedule looks like work, work, work, cousin's wedding, work, work, niece's baptism, work, work, etc., so progress may be slow. I'll post some pictures again when I get it opened up, and maybe some people here have worked on these and can share advice. Hopefully this will culminate with a youtube video of mediocre guitar playing through a functioning stomp box!!

[lefty73]

Good luck! And is that the formula for the meaning of life on the Post-It note? ;) I was told there would be no math... :)

[Big John Studd]

I couldn't resist cracking it open over lunch. Clearly the back side of that pot is broken, but if that is the only damage then it should be an easy fix...assuming I can find the right replacement part. I should have a potentiometer laying around I can wire up temporarily to see if that is the only problem.

[Big John Studd]

And it looks like some place called Supersonic Mods will send the part to me for $6.50. Cool!

[11 Gauge]

Originally Posted by Big John Studd

Clearly the back side of that pot is broken, but if that is the only damage then it should be an easy fix...

That's the great thing about the pots in the Ibanez and Boss stuff, much of it anyways.

Pots in a TS are:

drive - 500K log (audio)

level - 100K lin

tone - 20K "W" taper - the only goofy one

...many people just use a linear taper pot in their TS builds/repairs, or even a 25K lin pot.

Most drive boxes seem to use either 100K, 500K, or 1Meg drive/gain/dist pots. And most/many seem to use either 10K or 100K for the level pot. I'm talking Ibanez/Maxon/Boss stuff...

[Big John Studd]

That's awesome. I'm sure I have a 10K or a 100K in the junk drawer I can wire up just to see if that makes it go and then buy the real one that fits in the box. It may now be that biggest obstacle to this project is just sorting through my junk drawer...LOL.

[11 Gauge]

Originally Posted by Big John Studd

That's awesome. I'm sure have a 10K or a 100K in the junk drawer I can wire up just to see if that makes it go and then buy the real one that fits in the box. It may now be that biggest obstacle to this project is just sorting through my junk drawer...LOL.

Also cool is that you can usually tweak a pedal around whatever pots you do have.

Say for instance that the 500K log drive pot on your TS is bad, and you only have a 100K log pot...

You could replace the 4.7K resistor/.047uF capacitor high pass filter combo with 1K/.22uF and essentially have a very similar gain sweep and range, and an identical high pass filter (~720Hz).

The TS also has a 51K "minimum drive" resistor that should be dropped to 10K or less with a 100K pot substituted.

The coolest part about that is there is only a single .047uF cap, 4.7K resistor, and 51K resistor in the actual TS circuit (IIRC, there may be a pair of .047's in the bypass circuit).

Same thing basically holds for the level pot, with the biggest difference being that if you go with a smaller pot, you may want to increase the value of the output cap. But a TS already has a rather large 10uF output cap. In comparison, the Boss SD-1 has a 10K level pot, but uses a 1uF output cap. IMO, 1uF is sufficient for the output cap in most instances.

...Something like Paul C.'s Timmy has a 10K level pot w/1uF output cap, too.

[Big John Studd]

I did find an Ohmite 100K pot (that is probably older than I am) laying around, but after further consideration I figured I'd do more damage soldering this one up just for testing then desoldering it and replacing it with the actual part later. I guess I could have jumpered it in there, but then I'd have to find the jumpers, etc...LOL. Anyway I just put 1 kHz signal on the input and measured the output that goes to the Level knob. Here's what I got. First picture is with Drive knob all the way off, second is with Drive knob all the way on. It looks like it is "working". There is noticeable distortion with the Drive off (first picture). I was surprised by that, but perhaps this is the way the circuit is supposed to operate. I did a visual inspection on the rest of the board while I was in there, and everything looks OK. I noticed it has the famed JRC4558 chip...ooo-la-la.

[11 Gauge]

Originally Posted by Big John Studd

Anyway I just put 1 kHz signal on the input and measured the output that goes to the Level knob. Here's what I got. First picture is with Drive knob all the way off, second is with Drive knob all the way on. It looks like it is "working". There is noticeable distortion with the Drive off (first picture). I was surprised by that...

So there's no attenuation from a level control?

That would explain the distortion even with drive @ zero. Also, there is the 51K "minimum gain" resistor in a TS, So it's the same thing as having a 50K pot dimed, always. So min gain is (51K + 4.7K)/4.7K, or "12" - you should have clipping with that.

[Big John Studd]

That's right, no attenuation. The Level pot was just totally destroyed on the inside...phenolic all cracked, wipers bent. So I just removed it and measured at the wire that was connected to the top end of the Level knob.

[Big John Studd]

I think it's going to live! I'm not sure whether or not I like their "minimum gain" concept. I may fiddle with that at some point.

[11 Gauge]

Originally Posted by Big John Studd

I think it's going to live! I'm not sure whether or not I like their "minimum gain" concept.

The SD-1 uses a 33K resistor in place of the TS's 51K. IMO, it's a little better, and going even lower than that may be better yet.

[Big John Studd]

I finally got back to working on this thing. I got a new volume pot from Supersonic Mods and popped it in...a perfect fit! I should have also ordered a knob, but I didn't think about it at the time so I used some random knob from the junk drawer. I figured it would be a good idea to check it out on the scope before plugging it into an amplifier. How could you possibly screw up connecting a volume pot? Well, I have a track record of not paying close attention to what I'm doing and replacing the wrong part, putting a part in backwards, etc. Anyway, it looks like I did a good job this time and the Tube Screamer seems to be functioning properly. Next up: live fire testing at a gig next week.

[ludashoeless]

Wow that's awesome I'd gladly take a broken ts

[Big John Studd]

I tried it at the gig last night. It worked just fine! I'm gonna give it back to the guy that gave it to me next time I see him...he told me it was his favorite pedal.