Old Japanese Boss TW-1 in need of repair

[AM]

Hello,
This is my second attempt to fix an old Boss TW-1 auto wah. Japanese, silver screw, etc....so it's valuable and I'm trying my best to fix it and not completely kill it.

The problem:
The pedal doesn't do proper wah or envelope filtering when engaged. When bypassed the signal goes through just fine. When engaged, there is volume drop around 4 dB and a thin, fizzy sound coming out.

When I first got the pedal there was a high-pitched squeal instead of a wah/filter sound. I changed the caps in case there was something wrong with them. Now I get the fuzzy sound I described above.

Any ideas about how/where to troubleshoot and how I can possibly fix it? Schematic attached. There are a couple of differences between the pedal I have and what I see on the schematic but for the most part it's pretty much the same.

Thank you so much in advance!

[bartimaeus]

have you tried using an audio probe to follow the signal through the circuit?

if the caps are good, then perhaps it's a bad opamp(s) or bad transistor(s)

[AM]

No I haven't and I wish I had rigged up a device like that before I changed caps. The circuit board is old and fragile. The pedal was left in a basement for years, possibly a couple of decades...

[AM]

Update:
I made and used an audio probe today. There is no signal in various areas of the circuit, definitely on every place that's connected to the IC. There is one IC and not two as in the schematic (I'm attaching a picture). The pedal is probably an earlier version. The codes RC3403A DB and RAYC8012 are written on the IC. As I mentioned before, I need to be careful with any attempt to remove any part because when I was changing the caps I realized that the circuit board is quite fragile at this point.

[AM]

The only chip I can find readily available though is the JRC3403AD. I was reading on Analogman's website that he used the JRC3403D to replace the same chip I have when he was moding Boss OD-1 overdrives. Are those two (JRC3403D and JRC3403AD) the same?

[Kevin Mitchell]

Are those two (JRC3403D and JRC3403AD) the same?

Couldn't dig up the datasheets, but this is listed on the Smallbear product page for the JRC3403AD;
"Quad op-amp for clones of the BOSS OD-1
Name change reflects new production by an authorized source."

I'd say that's all I'd need to give it a try.

[AM]

Thank you! I think I should buy a socket too so if there is any problem I won't have to unsolder the chip. I've heard that sockets add noise but the times I've used them I didn't notice any extra noise.

[Slowpoke101]

A TL074 should be a suitable replacement for a RC3403 OP Amp. No modifications required.
Also don't worry about IC sockets adding noise, be more concerned about quality. Don't buy junk that will fail or will not hold the IC tightly - that can add noise actually.
Since this effect has spent a lot of time being stored in a basement which is not a very good environment for electronics to be in - corrosion can occur easily. I suggest checking that the potentiometers are still working correctly and that the inductor has not gone open circuit.

Forgot to ask: Can you also measure the voltages on the IC's pins and post them here. That can help a lot.

[AM]

Thanks for the suggestions. I measured the voltages between every IC pin and ground. Please check the values on the attached image. I hope the pin labelling is correct. I just copied it from a datasheet file I found on the internet.

There is variable resistance as a turn the pots and I also got a resistance measurement between every pin of the inductor and ground.

I forgot to say, when I was audio probing yesterday at the places where I wasn't getting any proper audio signal I was getting a loud popping sound when I was placing the probe on them.

[anotherjim]

The voltage provided by Zener D3 (in the 2 chip scheme) seems wrong to me. Looks like it should be a 5.1v Zener diode. This would also prevent the bypass switching from working.

[AM]

Let's ignore the schematic because the pedal I have has one 14-pin chip and and not two small ones. Btw, the bypass switching is working.

[PRR]

> Let's ignore the schematic because the pedal I have has one 14-pin chip and and not two small ones.

So they replaced the two 2-packs with a 4-pack. We still expect 4 or 5 volts at the Zener and at many IC pins.

But wait. You have 1.8V at the chip supply, which says dead battery or burnt power supply parts.

[AM]

The other way round. The single chip model is the predecessor of the double chip one.
I checked the battery. It measured just shy of 9V.

[Slowpoke101]

The 2 IC schematic is so far the only one that I have found so far as well so we may have to refer to it at times. However I think I may have found some fairly high resolution images of both sides of a 14 pin board that I can refer to so that may help somewhat.

Your audio probe - what value capacitor did you use in series with the probe? It only needs to be about 0.1uF. Also be aware that when probing the IC, most of its pins will not really have "audio" on them. You will hear sound (of sorts - if they are working) on them but it will not be that recognisable.

The voltages provided are concerning. +VS is 1.81V . Really? Where did you place the multimeter's negative (black) probe? And what voltage is your battery or power supply? +VS should be about +9V. If Bypass audio passes cleanly then +VS should be at +9V unless there is something very wrong with the PCB in that area. As Jim pointed out D3 could be concern but again Bypass can't work if D3 isn't OK. Please test the voltages again. Just so you know other than pin 4 (+VS) and pin 11 (-VS), all the other pins should be about +4.5V

Edit: You and Paul (Hi Paul) type too quickly. Or I just type far too slowly - yes that's the excuse.

[AM]

I'll quote and answer one by one:
1. Your audio probe - what value capacitor did you use in series with the probe?
I used an 0.1 uF tantalum cap.

2. Where did you place the multimeter's negative (black) probe?
On the inside part of the metal case's corner.

3. I tried with a regulated 9v power supply and a fresh 9v battery (connected via a battery snap to barrel cable I put together, because the original battery snap inside the pedal is snipped). I found an older Roland ACA power supply today and I'll try to power the pedal with that too. I believe that puts out 12V which is what those pedals were to designed to take at the DC in jack.

A sincere thank you to everyone who has taken the time to help so far. I really appreciate it! As I wrote in my first post, when I got that pedal it was just oscillating. After the change of the electrolytic capacitors, the oscillation stopped. I don't know how far I am from making it work properly and if that is even possible but you guys have given me some hope.

[Slowpoke101]

For the moment keep using the 9V battery. Yes, the old PSUs do output about 12V. Try to measure the voltages again. If they are still wrong, trace the PCB track going pin 4 of the IC and see if it's damaged or if there are any faulty solder joints along it.

Your audio probe - a 0.1uF tantalum? Tantalums are polarised. Not a good choice. Any there any chance of using a film or ceramic type instead?

[PRR]

> +VS is 1.81V . Really?

That's what it says; and there is no point probing for audio since standard opamps are about stone-dead at 1.8V.

(I hope you know you need a plug in a jack to turn-on the power.)

On the wrong schematic: There's no power filtering series element to burn-up, so it is a mystery. There does seem to be a diode. Zener? Reverse protection? And a cap, which may be pad-damaged over the years or from re-cap.

[AM]

Paul:
(I hope you know you need a plug in a jack to turn-on the power.)
-Yes. I had a dummy plug inserted in the input at all times.

Ian:
Try to measure the voltages again. If they are still wrong, trace the PCB track going pin 4 of the IC and see if it's damaged or if there are any faulty solder joints along it.
-I will do that.

Any there any chance of using a film or ceramic type instead?
-I have a film cap that's 0.22 uF. Would that be OK? Everything else I have right now is either way smaller or way bigger than that.

[Slowpoke101]

0.22uF would be fine.
Also when you are checking the voltages just probe around and confirm that you are getting voltages higher than 1.81V in other areas. If you are not, move your negative probe lead to another position and try again. You may have a poor ground connection where you are connecting that probe to.

[AM]

Update:
Checked voltages again. Nothing new to report. Resoldered various cables (most of them) going from jacks to PCB. Nothing has really changed. Loud and clear bypass signal, just a weak fuzz sound when the pedal is engaged.