Old Japanese Boss TW-1 in need of repair

[AM]

Here is a picture of the soldered side of the circuit board. The red wire only connects the negative leg of C7 and R10. I just haven't cut it yet.
Can you please tell me which pins are 12, 13 and 14 on the IC drawing I attached a couple of posts above? Sorry for the silly question but I couldn't find a certain numbering on the data sheet I have.

[Slowpoke101]

Here you go.



I'll now see if I can find anything obvious with your board image.

[AM]

R27 and R29 (this must be the one above the trim pot? It's hard to read) read 0.87 on all legs. Btw, R29 reads around 67K and not 100K.

IC pins: 12=2.06, 13=0.87 and 0.87 on both down and up on drive switch.

[Slowpoke101]

A quick look at the board image has shown a few areas of concern. One of which would have disabled the whole thing. I have numbered what I have found so far.



The board is in fairly good condition considering its age. Also repairing what I have found is quite simple. The areas of concern are numbered as follows;

1. Break in track. This goes off to the sensitivity pot. Would have disabled the effect. Bridge with wire and solder.

2. Possible corrosion under green solder mask. Scrap off mask in area and apply solder to copper track. Be careful not damage track.

3. Damage to track under wire lead cut-off. This is why there is no power to IC1 pin 4. Remove solder mask in immediate area and bridge with wire and solder. Make certain that 9V has been restored to pin 4 of IC1.

4. Possible corrosion under green solder mask. Same solution as point 2. This track supplies Vref to pin 10 of IC1 which reads low in voltage. This track may be of high resistance and this could be causing it.

[Slowpoke101]

R27 and R29 (this must be the one above the trim pot? It's hard to read) read 0.87 on all legs. Btw, R29 reads around 67K and not 100K.

IC pins: 12=2.06, 13=0.87 and 0.87 on both down and up on drive switch.

R29 is indeed the one next to (well, above the trim pot). The comic (service note) is hard to read. Also trying to measure resistors accurately in circuit almost never works. They will almost always read a lower resistance that what they are marked due to everything else in the circuit that connects to them. It is to be expected.

Now 2.06V on pin 12. Good. This starting to make some sense - maybe.

A few more voltage measurements to go. Don't forget to have power for pin 4 of IC1 correct. Black probe to ground. Three voltage measurements are needed from the flying lead PCB pin connections at the top of the board. Place the "Drive" switch in the DOWN position and leave it there. What voltages are on PCB pins 6, 7 and 13. None of these voltages are affected by the problems that I have noted with the circuit board and can be done before repairs are done.

[AM]

I'm sorry, I made a mistake before. Here are the correct measurements:
Drive Down:
Pin6=0.31
Pin7=0.31
Pin12=2.36
Pin13=1.19
Pin14=1.49

Drive Up:
Pins 12,13,14=0.87

The marks 1,2,4 on your photo don't seem to be cuts or corrosion but rather a bit of a bubble on the mask. Mark 3 seems to also be fine but I need to inspect even closer. I'm wondering if I should just bite the bullet and change the IC.

[Slowpoke101]

Interesting on the voltages. Perhaps just changing the IC may be the best idea. It would eliminate quite a few variables (and introduce some more). I would just do it and see what happens.

OK on the PCB. But with point 3, we know there is a problem there because that is where the volts for pin 4 of IC1 stop dead. That pad does not connect to that track. Anyway, do check it out and do closely check point 1. It really does look bad at high magnification. But that may just be a JPG artefact (erroneous image) too. Note: It also may be a tiny splash of solder.

[AM]

I will check again all 4 points on your photo and pay extra attention to 1 and 3.
I'll fix whatever needs to be fixed and if the problem persists I'll swap the IC. I'll report back probably Friday because tomorrow is going to be a busy day and I won't have time to work on it. Once again, thank you sir! You're a legend!

[AM]

I touched up the four spots on your photo. Here are voltage measurements from IC pins to ground:
1=3.94 / 2,3=3.63 / 4=9.79 / 5=4.17 / 6=1.22 / 7=1.16
8,9=0.12 / 10=4.17 / 11=-0 / 12=4.16 / 13=4.18 / 14=4.91

Sound got much louder when pedal engaged, pretty much unity with bypassed sound. No wah though. Pedal sounds like a treble booster with more high frequency content with the drive switch up and less with it down.

[Slowpoke101]

Dealing with those points on the PCB seems to have helped a bit. This is good and has now raised some more questions - as usual, but it is progress. At least the pedal is sounding better......

The voltages on pins 1, 2, 3, 12, 13 and 14 look good. These are the sections of IC1 that form the envelope follower section of the circuit. This translates to IC2 pins 1, 2 and 3 then IC2 pins 5, 6 and 7 as shown on the service notes. That part of the circuit appears to be OK. Of course pin 4 and 11 are also OK. But now we come to the bad news....Pins 5 and 10 are OK so it's not all bad but pins 6, 7, 8 and 9 all should be about 4 to 4.5V. This part of the circuit is what controls the resonance of the inductor. It's what controls the wah system. It's weird that changing the Drive direction switch can alter the tonal range slightly with this section being faulty but weird seems to be a normal state of affairs so.....next. The most likely cause would be IC1 is unwell but check for any solder splashes or poor solder joints. Capacitors C6 and C7 are also of interest. Measuring the voltages on the pins of Q2 may also be helpful too.

A modern LM324 could also be a usable replacement for IC1 as well but I would try a TL074 first to see how it behaves.

[AM]

Voltage measurements on Q2 as you look at it from above on the diagram are (from left to right: 0.70 / 4.50 / 0.10

Is there a benefit in trying to find a TL074 instead of the JRC3403AD I've seen being used when fixing OD-1 pedals with the same chip? The only reason why I'm asking is because I will try to go with whatever I find easily available.

[Slowpoke101]

The voltages on Q2 look good, so that does not give any clue as to possible reasons for IC1's odd voltage readings on pins 6, 7, 8 and 9. So it still seems that IC1 is the most likely culprit.

The only real benefit to using a TL074 is that you may actually be able to find one. Finding a real JRC3403AD could be difficult but if you can find one, use it. You may know of some electronics retailers that may have some available.

[AM]

I think I can find one. I'll try next week and see what I'll come up with.

[AM]

I fixed it! I found both JRC3403AD and TL074 chips. The first was more than three times the price of the latter but I bought it because for that kind of money it doesn't matter.
In the beginning I wasn't happy with how the filter swept when the drive knob was in the up position but I started tuning it by ear by adjusting the internal trim pot and I found the sweet spot.

Ian thank you so much for your contribution!
A big thank you to everyone else who also read my post and contributed.

[Slowpoke101]

That is good news 
And adjusting the trim-pot by ear was the method that I was going to suggest as well.
Well done and enjoy playing with the pedal. Auto wahs are lots of fun.