Fixing up and Old boss TWAH

[maiko]

hello sirs

Ive came upon a vintage Boss TW1 twah.

Issues Encountered

1.)   tried to run it but i noticed that my 9v regulated power supply is not liting up the led bright enough.  tried a 9v battery and it seems to light up ok but (Although it bypasses) when effect on the led does not light up when i press it again the bypass it lights up and shuts off again.   

2.) if the effect is on it is not Wah wah - ing its more like an Overdrive.   The sens act like a volume and  peak does nothing.

i know that this has been discussed here by somebody sometime ago already but was never resolved. 

Some things i already did

** Checked all cap to ensure non were dried up or shorted
** Checked all transistors to see non were shorted
** checked all diodes again non were shorted

I noticed D1 is an RD11EB3 diode which is 11v  .   I understand a long long time ago in a galaxy far away hahaha boss employed ACA power which was marked 9v but actually 12v.   im running strictly regulated 9v so could this be a problem  or the problem ?



I would be Greatly appreciative to anyone who could look at the photo and give an opininon. 

Mike

[bloxstompboxes]

I suspect the regulated 9V is the problem. I recently bought a CS-3 off ebay from someone who listed this exact issue with distortion when using a 9V adapter. I knew nothing about it until after I got it and noticed no distorion when just using a battery but then had distortion with a 9V adapater. It is the ACA vs PSA adapter issue. The ACA is brought down to 9V with the diode. If I remember correctly you remove the diode and possibly add a jumper to convert it to a standard 9V adapter accepting pedal. Google should tell you for sure.

Edit: Well here you go. Couldn't remember exactly and was driving me nuts. http://stinkfoot.se/archives/1019

[maiko]

Jumpered the diode and resistor.   Now my regulated power works but still something wrong with the circuit no effect just the overdrive instead of a wah

[VintageGear]

Here's a better schematic:
http://www.diystompboxes.com/pedals/BossTW1.jpg

Is the pedal responding to the drive switch? Or no change at al?

[maiko]

yes as a matter of fact yes flipping the drive switch does somewhat change tone

What really i dont understand is where in the schematic is the inductor found

also these scheatics show the dual opamps whats on mine is the single quad opamp.   ive tried swapping that
out for 3204 but still the same luckily i lifted the old ic as gently and neatly as possible. 

[Rob Strand]

Given the LED isn't looking so good you should debug this problem first as it is likely to show something.

- Measure the voltage across D1/C23 (the 100uF cap) to see what voltage the is present on the circuit board itself.

- Measure the current draw when in battery mode.  You can do this by lifting one leg of the 9V battery snap-on and bridge the two bare battery terminals with the multimeter set to current.  (Don't forget to remove pull the leads out of the current terminals when you finish the measurement.)   Press the foot switch to see if the current changes.

- Check the DC voltages to ground on Q1, Q2, IC1 and IC2, D3.  Maybe even Q8 and Q9.

BTW: The inductor is located near the emitter of Q2 on the schematic.


It is likely to be a simple problem so don't go changing parts until you work out where the problem is!

To me, the fact the up-down switch changes the sound means a lot of stuff is working.  The LED issue is still a concern but the above measurement will pin this down.  After checking above I suspect maybe the Sensitivity pot is open.  You could set it to full and short the upper pair of terminals of VR2 then set it to min and short the low pair of terminals.  Another thing to check is to see if the voltage on the "UP terminal of the UP/down switch changes when you have signal and no signal.  The best way is to put the -ve probe of the meter on the cathode of D3 and use the +ve terminal to measure the UP signal.

[VintageGear]

Agree with Rob. The sound difference associated with the switch means it is working for the most part. Furthermore, I'd start by just simply replacing the elec. capacitors instead of "checking". If it aint dead now, it will be in ~10 years. There are only a couple of them anyways. Also measure the pots, see if resistance value changes upon turning the knob.

[maiko]

Thanks for this info will check and report back

Btw i highly appreciate the help. 

I will give all the voltages in the next post.

[maiko]

my ic is an 3404 quad not 2 4558s like in the schematic

pin 1 4.2            pin 14 3.63
pin 2 3.4            pin 13 3.6
pin 3 3.6            pin 12 3.6
pin 4 9.95         pin 11 ---
pin 5 4.29          pin 10 4.25
pin 6 4.29         pin 9 4.21
pin 7 4.29          pin 8 4.30

d3 vs ground is 4.2v

q2 vs ground is 4.2

q1 vs ground is 4.1

q9 source 2.79  drain 2.15
q8 source 4.19  drain 4.21

[Rob Strand]

The voltages all look good, except maybe q9.

q9 source 2.79  drain 2.15

The possible explanations are:

1)  Q9 on your (older) quad opamp version is not the same as Q9 on the dual opamp schematic.
     Given there are only two JFETs in that effect you can read the part values for Q8 and Q9.
     Are both of these parts JFETS on you board?
     If Q9 is a JFET then see next point:

2) Q9 is a JFET and there is a fault in this part of the circuit.
    Given that there is something weird going on with the LED,
    and the gate circuit of Q9 connection connects to the LED,
    I would not be surprised this is in fact the faulty area of the circuit!
    Check out the circuit and PCB from Q7/R51 (56k), through to LED, R43 (1M),
    C21 (0.047uF), D5 and then finally Q9.

3)  Q9 is a JFET and there is no fault.  The circuit is just like that.

    *** What you might need to do is measure the voltages on Q8 and Q9 twice:
    one with the effect on and once with the effect off.
------------
As a side note from your voltages I can see opamp pins 1 to 3 on your board correspond
to opamp IC2 pins 1 to 3 on the dual opamp version, and opamp pins 12 to 14 correspond
to opamp IC2 pins 5 to 7 on the dual opamp versions.
--------------
Is is possible for you to do this measurement:
- set the Sensitivity pot to fully clockwize
- Connect multimeter +ve terminal to pin 14 of the IC
- Connect multimeter -ve terminal to pin D3 cathode.
- Measure the DC voltage with no signal (say with you guitar volume set to zero)
- Measure the DC voltage with signal (with you guitar volume set to full)

You should be able to see a DC voltage which varies with the level of your guitar signal.

An optional test would be to see if the DC voltage with signal is reduced when the sensitivity
pot is set of minimum.

[maiko]

id like to add :

ive tried to jumper the source and drain on q9 just to have the effect "on all the time"   still no effect. 

[Rob Strand]

ive tried to jumper the source and drain on q9 just to have the effect "on all the time"   still no effect. 

That method might show-up something but you need to:
Bridge Q8, open Q9.   To open Q9 you can lift one end of C5.

[maiko]

Rob

thanks for the reply.  will try your suggestions.

Today i went ahead and replaced fit flop transistors and the fets c945 and 2ks30a respectively

to my surprise it still does not work.     the led lites up when switch is engaged but as soon as it is disengaged it shuts off

I feel like i should solve this first before dwelling into the more complicated side.  i dont understand why it still doenst work.

Also got a phaser ph1 and its exactly like this so i feel i should really solve this first

[Rob Strand]

The seems to something weird going on with the switching circuit.

If there is something wrong with the up-stream circuits (the LED and other stuff I mentioned before) it can upset the operation of the flip-flop.

At this point I would be pulling the LED or LED wire and seeing what happens.   What this does is removes the LED load from the flip-flop.

It is even possible the LED wires are shorting on something.  Check for frayed wires near where the wires enter the PCB.

[maiko]

its really driving me nuts.   changed the diodes to the fets. still the same.   It has to be some component fault.
may be an open cap.  If i have to i will remove each cap to test.   worried about the pcb trace thought
these old boards tend to lift.

[Rob Strand]

I think you need to work through the circuit in small steps.

Start by checking the flip-flop is working:
- Pull the LED
- measure the voltage on the collector of Q6
- measure the voltage on the collector of Q7
- Press the foot switch
- measure the voltage on the collector of Q6
- measure the voltage on the collector of Q7

You should see these switch from 0V to about 6V when one is 0V the other is 6V and visa versa.

If you don't see this then try temporarily replacing the switch.
You could even replace the switch with two wires and "flick" them to simulate the switch.
Switches can fail.

If you cannot get this to work there is a problem with the flip-flop or the PCB.  (Sometimes if PCB gets wet you can get all sort of weird problems which are not obvious.  Your eyes see everything normal but the board may be covered in a conductive film making it working electrically different to what you think.)

[maiko]

hmmmmm

the guy i got these from did mention that it was stored i a flooded room.    but i dismissed
that coz there was no obvious water damage.   Apart from the box that was rotten from being wet

[Rob Strand]

hmmmmm

Dirty water isn't good for the pots either!

It's probably best to debug the circuit from an electronic perspective in small steps.   If you find something totally weird then maybe it's time to clean the board.

Sometimes you can measure the weirdness, for example a 1MEG resistor might measure somewhat less.  However, it's *very hard* to make conclusions based on *in-circuit measurements* as a circuit which is working perfectly can also measure low values in-circuit.  If you have a 1MEG resistor which is totally isolated, because you have lifted all the components surrounding it, measures low then you know it must leakage on the PCB.

[maiko]

rob

thanks for the reply

during testing i unsolder/remove the component from the board to avoid said problems that you mention.

also cleaned the board but no shorts no bridges and no breaks. 

[duck_arse]

hmmmmm

Dirty water isn't good for the pots either!

"Well, I love that dirty water". sorry, couldn't resist. carry on.