Old Japanese 6 band eq pedal repair

[hamandles]

Dear all,
I wonder if anyone can help. I've got an old Solec Graphic EQ pedal that I'm trying to repair. It outputs a non-eq'd signal whether the switch is engaged or not (SPST). The switch works, and the board is getting voltage. The led is not lighting (but it works when tested using a continuity tester). I'm happy using a soldering iron and a multimeter but not sure where my troubleshooting/testing should go next. Is there someone who is able to point me in the right direction? Many thanks!

[tubegeek]

In general, EQs and tone controls do nothing if their ground connection is broken - they work by sending some signals to ground and the output is whatever's left. So that may be a clue?

[idy]

So the LED does not go on and the effect remains in bypass when you hit the switch, right? This points to either the switch itself, probably a momentary contact, or the circuit that uses FETs to switch the circuit in and out of the signal path. It's called a "flip flop." We may not find a schematic of this particular pedal. I would:

test the switch. It probably should be normally open and when it is being pressed should, momentarily, show continuity. Test this with pedal unpowered.

Trace the signal, at least to locate the buffer(s) and the FETs, and the transistors that "flip" them off and on. You will need the power on now.

If you get this serious you will need to read up on "boss style switching" (or ibanez style or dod style or...) Find schematics of the original circuit, not the DIY true bypass version.

There are unfortunately several things that can go wrong with that flip-flop. Bad transitor, bad solder connection, bad capacitor....

[willienillie]

If you get this serious you will need to read up on "boss style switching" (or ibanez style or dod style or...)

http://www.geofex.com/Article_Folders/bosstech.pdf

We may not find a schematic of this particular pedal.

Lots of TS-9 parts in there, but the PCB doesn't look very "Maxon" to me, so maybe less chance of this being the same circuit as something more well-known.

[idy]

One more easy thing to check before all else:  are the wires to the switch soldered to the board or did they come loose? This happens.

[DIY Bass]

Take a piece of wire and short together the 2 switch contacts on the board.  Does that change the bypass to on and then off again?

[willienillie]

Seems this is a Solec EQ-771



Which it seems was also painted a different color and sold as an Electra 603G



I can't find a schematic under either name, but it sure looks like it would have FET bypass switching.

[Rob Strand]

I can't find a schematic under either name, but it sure looks like it would have FET bypass switching.

You would think so.  I suppose if it's got 2xJFETs on the board it might be a good place to start.
At the top right corner I can see two diodes and two maybe 47nF caps.  So that might be the signals to the JFETs in a "Boss like" JFET switching ckt.



See if the output of the collectors change from 0 to 9V when you press the switch.
When one collector is 0 the other should be 9V.

[hamandles]

 Thank you, all. I've got some stuff to try and will report back! Much appreciated.

[duck_arse]

top left of your board pic is a TA7555P, which to me would be a cmos 555 timer, like might be used as a bypass debounce flipping flopping thing. I'm sure it's been used there in something ....

and cause no-one else has said it yet, welcome to the forum, hamandles!

[edit :] gah! my eyes. " 8 " .

[willienillie]

top left of your board pic is a TA7555P, which to me would be a cmos 555 timer

No it's a dual op-amp, the one people don't seem to like in some of the later TS-9s.  Personally, I don't think they sound so bad.

[PRR]

>> TA7555P, which to me would be a cmos 555 timer
> No it's a dual op-amp...

All the hits I get say TA7555P is a timer.

[willienillie]

Well, it's actually TA75558P.  I didn't notice the missing 8 in Duck's post earlier.  But it's a dual opamp.

[hamandles]

Thanks again for all the replies, and for your welcome, duck_arse - appreciated. I'm actually a lapsed and returned member! The excellent folks and resources on this forum guided me through making one of the Matsumin valvecaster pedal variants probably the best part of 8-9 years ago, which I am still enjoying using. So not quite a prodigal son story, but it's nice to be back!

A couple of things: willienillie, you are absolutely correct - it is the eq-771 with the fetching purple paint job.

Idy: that's right - the effect remains in bypass whether the switch is depressed or not. The switch is definitely a toggle, not a momentary switch, SPST (or actually SPDT with one terminal unconnected). It seems to connect the ground to the board. Interestingly, there is a 5.6v pd across the two connected terminals when the switch is off (the battery tests at 8.6v).

Measuring the voltage from the FETs as labelled on your diagram (thank you for doing a diagram - really helpful!) gives the following voltages compared with ground

FET 1 (top left in picture)
E 0.57
B 0.06
C 0.0

FET 2 (to the right of FET 1)
E 5.78
B 3.71
C 0.0

These values are the same whether the switch is on or off. But here's the interesting thing. When I was testing I accidentally shorted the FET 2 collector leg with the terminal next to it (capacitor leg) and the led flickered on. I connected up a guitar and the EQ and level controls were working when this short was active. It was a bit noisy but the faders all worked! Short marked in orange on the diagram.

What's your thoughts?!

Thanks, guys.

[Rob Strand]

The switch is definitely a toggle, not a momentary switch, SPST (or actually SPDT with one terminal unconnected). It seems to connect the ground to the board. Interestingly, there is a 5.6v pd across the two connected terminals when the switch is off (the battery tests at 8.6v).

OK it's pretty clear your pedal does have Boss-like JFET switching and the switch *is* momentary.  There's no big deal about the 5.6V.   It could be your multimeter is loading down the circuit and making a 8.6V measurement look like 5.6V.  Or it could be the Ibanez variant of the Boss circuit which drops the voltage across the switch.   It's nothing to be concerned about at this point.

Measuring the voltage from the FETs as labelled on your diagram (thank you for doing a diagram - really helpful!) gives the following voltages compared with ground

Just to be clear:
The circuit has two *transistors* (BJT's) which I marked Q1 and Q2 on my pic.    The circuit I drew is only one of those transistors.  There's two of these circuits.     The JFETs are not the parts you measured the JFETs are actually somewhere else on the board (where ever the tracks of those two diodes go).

So here's how the things should read,

FET 1 (top left in picture)       --->  Q1
E 0.57                                  ---> B 0.57V
B 0.06                                  ---> C 0.06V
C 0.0                                   ---> E 0.0V

FET 2 (to the right of FET 1) ---> Q2
E 5.78                                ---> B 5.78V
B 3.71                                ---> C 3.71V
C 0.0                                  ---> E 0.0V

The way I see it is Q2's Base "B" measurement looks like a real problem, it should be 0V or 0.6V or so.  Since the other transistor's base (Q1) is 0.6 I would expect Q2's base to be at 0V.  Another thing is, in the normal Boss circuit Q2's base is driven by Q1's C collector.  However your measurement show Q1's collector at 0.06V.  So to me that means something is shorted to Q2's base to make it sit at a high voltage.  So look on the PCB track on Q2's base for a short.  (Because of the short, Q2 may be damaged, depending what it is shorted to).

On this pic you can see, normally, the base can only be pulled up by the other transistor's collector through the cross-connected 100k resistor. Your circuit has something "extra" pulling it up, likely to be a short.

These values are the same whether the switch is on or off. But here's the interesting thing. When I was testing I accidentally shorted the FET 2 collector leg with the terminal next to it (capacitor leg) and the led flickered on. I connected up a guitar and the EQ and level controls were working when this short was active. It was a bit noisy but the faders all worked! Short marked in orange on the diagram.

That makes a lot of sense.   The way your circuit has failed keeps the effect off.   The aim is not to force that signal like you have done here but to fix the problem with Q2 which is stopping the circuit toggling with the foot switch.

[hamandles]

Success! Thank you Rob and all, especially for putting me right in my transistor identification (every day a school day!). I checked for shorts and couldn't find any on the track. I wondered about the transistor having failed, and found another C1815 in my scrapped components store (actually pulled it off an old tape deck board). When I removed the transistor from the pedal (Q2) I saw that one of the legs (base as you've probably guessed) was hanging off. I wasn't sure whether I'd done this in the process of removing it, but when I replaced it with the "new" one, the LED lit when the switch was selected and the whole thing worked when I plugged a guitar in. Happy days!

There's one thing I'm puzzling about. In all the (helpful) messages the switch is talked about as a momentary switch, and I understand why this would be to operate the flip flop switching on the board. I understand the term momentary switch to mean a push to make (or push to break) kind of a switch, whereas the one on the pedal is definitely a latching/toggle affair. I mean, I'm not complaining, but I'd like to understand this if you've got some wisdom to share.

Thanks for your help, team!

[Rob Strand]

Success!

Excellent.

There's one thing I'm puzzling about. In all the (helpful) messages the switch is talked about as a momentary switch, and I understand why this would be to operate the flip flop switching on the board. I understand the term momentary switch to mean a push to make (or push to break) kind of a switch, whereas the one on the pedal is definitely a latching/toggle affair. I mean, I'm not complaining, but I'd like to understand this if you've got some wisdom to share.

The difference in the switches is of course as you say.   The confusing thing is the circuit.   Normally with JFET switching you would use a Flip-Flop.   The reason is since the switching is electronic via JFETs you might as well save some money on the switch by using a simple momentary switch.   The other advantage of the electronic switching is you can switch many things all over the board with that simple switch.  With a latching switch you would need heaps of contacts and it quickly gets out of hand. That's what is normally done and  why.

So your pedal uses an odd-ball design.  It has JFET switch and transistors but the switch is still the latching type.  In theory JFET switching is still quieter than a real switch so there is a small advantage to this design.  Nonetheless there's plenty of pedals which use latching footswitches and no one complains about the any switching noise.  So I can only speculate why they use this intermediate approach.

Early on in the thread I tried to find some info on Solec pedals to get an example of their switching circuit but I couldn't find any.   There was a loose link that the Solec pedals might be similar to this Locobox pedal but I couldn't confirm it and didn't want to confuse the thread by putting it up!

[Click to Enlarge]


Your circuit is slightly different in that it has on and off JFETs coming off different transistors.

As you can see this pedal has JFETs and transistors but uses a latching switch and no flip-flip.   So maybe there are some similarities.  I still don't know.   I would have to trace out your pedal from the pic.  The pic has a few details missing so it's possible I'd get stuck and end up not knowing the circuit in your unit anyway!

[hamandles]

Makes sense. I had a look at some of those Locobox pedals - the 80s versions certainly share some of the enclosure components so I wouldn't be surprised if the circuitry was the same.

The pedal is now reassembled and working....almost! The 800K control doesn't seem to be working - doesn't seem to affect the sound like all the others obviously do! I will dissassemble, check for dry solder/try to test that slider against the others to see what I can find out. So near...and yet so far!

Thanks, folks.

[Rob Strand]

The pedal is now reassembled and working....almost! The 800K control doesn't seem to be working - doesn't seem to affect the sound like all the others obviously do! I will dissassemble, check for dry solder/try to test that slider against the others to see what I can find out. So near...and yet so far!

I've seen a lot of faulty sliders in my time.   The wiper loses contact.

If you want to temporarily test the rest of the circuit for the 800Hz band, first set the 800Hz pot to the center position, then short the pot wiper to the each of the ends to see if you can hear boost and cut.   If you don't get boost and cut there's something wrong with the circuit for that band.   

The circuit for each band is just one transistor.  The bases on the transistors for all the band circuits should sit just below half the supply voltage, and the emitters will be 0.6V below that.   Collectors sit at the supply so be careful not to short to the collector when testing.  So if you measure the DC voltages on the 800Hz transistor you should be able to work out if the transistor is OK.

[hamandles]

OK,  took me a little while to do this: had to remove a slider and a layer of insulating cardboard before I could get to the transistor. Re soldered the slider with jump leads in order to test the transistor.  Scores on the doors:
E: 3.68v
C: 9.31v (fresh battery!)
B: 3.91v

Doesn't quite match Rob's predicted voltages but seems plausible.

Tested it again with some white noise and I could definitely hear the 800hz control making a difference now. I wonder if a slightly wonky looking capacitor could have been to blame - I reflowed solder there when I was connecting the temporary jump leads (they shared a pad) which I reckon made the difference.

Anyway, with your help I now have a working EQ pedal with which I am very pleased and which will get regularly used: I have a one knob valve amp which is lovely but lacking in tone shaping functionality! Vintage gear brought back to life by the power of diystompboxes! Thank you Rob and all.