[Billy]

Try using an adaptor that puts out 9v or a 9v battery to power it, 1.5v over voltage may be enough to affect the TL072 biasing and cause it to oscillate it shouldn’t but that could be the cause with 10.52v going in, good news is you know your voltage dividers R10, 11, 12 and 13 for VA and VB are doing what they’re supposed to do halfing the supply with your TL072 inputs and outputs being 5 odd volts

Use your finger to tap the TL072 to see if it affects it

Also take the readings with the TL072 out of the socket

 

 

[Billy]

Those voltages look way off Jeff

You say you get the sounds in the demo and squealing is the main problem but with the voltages given it wouldn’t work properly pin 4 should be ground 0v pin 8 power supply around 9v the other pins should all be around 4.5v

Are your readings 0.5v pin 1 and 0.1v etc aside from pin 8 which jumps from 5.7 back to 0.1v?

Did you take the readings the normal way by putting your black probe on a circuit ground point and touching your pins with the red probe with your meter set to 20v dc

I’m just confused that it worked aside from the squealing with the voltages you’ve given and suspect your meter may be set to 20v ac

[Billy]

What are the voltages on all the TL072 pins Jeff with it in and out of it’s socket

List them like this obviously indicating which is with it in the socket and with it out

1. 4.5v

2.

3.

4.

5.

6.

7.

8.

And just in case you don’t know pin on the top left at the dot / notch is pin 1

I’ve checked all of your component values and orientation which all look correct, can’t check pot values so I’d double check the correct values are in the correct place

Once you’ve posted voltages for the TL072 it should give us an indication of whether it is the cause of the oscillation i.e incorrect voltages causing mis-biasing etc

If you can post a picture of the rear of the pcb also

[Billy]

It could be several things causing the op amp to oscillate Jeff what are your the voltages on the TL072 pins

Upload some nice in focus images of both sides of the pcb

 

 

 

[Billy]

Definitely mate that’ll do nicely

Cheers

[Billy]

You primed and ready to go like a coiled spring Ray?

I’m primed and ready to try and beat you and Mark to it

[Billy]

Looks great Mark

I’m gonna give it a try cheers Ray

I just managed to wangle myself 8 MN3007 tested them all today in a clone theory build and they all work!

Totally unexpected with all the fakes kicking around

[Billy]

Nice it’s always interesting to read about other people’s methods and pick up and learn useful information

My methodology isn’t as complete but I know what you mean about ending up with a load of pcbs haha

[Billy]

Yes just to illustrate what you can do looks like

1. Germanium (1N34A)

2. LEDs

3 Schottky (1N4001, BAT41 etc)

4 Silicon (1N4148)

All different forward voltages

[Billy]

First I’d  check what you get compared to the demo

Your LED wouldn’t affect the circuit and looking at your wiring it looks correct should light in effects mode with S4 and 5 connecting the LED to ground

I’ve checked all the values I can see and they all look correct

The bass pot and treble pot should increase and decrease bass and treble double check you have the correct value potentiometers where they should be

Anyway let us know if it sounds similar to the demo and we can take it from there

[Billy]

As far as I can remember that was a manufacturing error where IN and OUT were mislabelled

On the pcb you will see 2 solder pads at IN and OUT one with a small T for tip or signal (+) the other for ground (-)

To be absolutely sure continuity check R1 with the T pads whichever one has continuity will be the IN side

With regard to the jack sockets if they are open style you can insert a jack and follow them round to see what they connect to like this ( this is a stereo jack a mono jack would not have the ring connection used only if it is wired for a battery)

For future reference the ground lug on an open style is always connected directly to the metal sleeve with a closed covered style it would usually be the lug at the angled side of the cover

 

 

[Billy]

I know it’s of no use now but I’ve been using a great tip from Cybercow Mark when using board mounted pots just solder the very tip of the lugs in initially to test the circuit, making them much easier to remove for debugging etc

As I said I’ve had a few problems recently with Alpha pots bought from reputable suppliers usually intermittent ie check ok then go to infinate when rotated resistance readings changing and so on

Bunching up in my experience usually means a duff pot or incorrect taper or resistance somewhere I don’t know if mixing up R23 47K and R25 470K would do that but easy to miss when you’ve checked values a few times

[Billy]

If you haven’t already check the resistance throughout the pots rotation see if you get any strange readings

I’ve had a few duff alpha pots lately

I’d also check the caps that work in conjunction with the pot C20 and 21 in particular

[Billy]

Yes you can wire a pot as a variable resistor and  measure it as you say Matt

Either sockets or a pot would do the job, I’d use sockets myself because you wouldn’t have to desolder them and start at 33K as suggested

I think Brako likes the idea of a varying squeal looking at what he says!

I personally wouldn’t use a roto tone you’re unlikely to get much of a varying squeal and it’s a bit of a waste if for some reason you want it I’d suggest a DPDT with resistance on one throw and none on the other

[Billy]

Very nice Bill looks great

[Billy]

Q4 the original was a low gain 2N5133 which didn’t require resistance at R25 having a low minimum hFE of around 60

Because you are using a higher hFE BC549C you will need some resistance at R25 to stop the transistor being amplified too much causing it to oscillate and make a squealing sound when all the controls are at maximum

The hFE value is the factor by which the base current is amplified so for example if your base current is 1mA and you are using a 2N5133 with a hFE of 100 your current at the collector (output) would be 1 x 100 or 100mA

If you use a higher gain BC549C with an hFE of 470 for example then it would be 470mA over 4 times as much and this may cause oscillation causing it to squeal so you add resistance to stop that happening

Put in SIL sockets and experiment for learning, start with a jumper and see if it does indeed squeal then add resistance 10K at a time until it stops

 

 

[Billy]

Use SIL single inline sockets

Just cut the number you need (2) and solder them into the pads the idea is to stop the transistor oscillating (squealing) I’d start with 33K as advised in the build document place the resistor in the sockets turn all the controls to maximum and see if it squeals if it still squeals try a higher value if it doesn’t you can try lower values if you want

as an example I’ve used them here to test different transistors in a PUFF build what I do is rather than remove them once I decide what component I’m using I simply solder one component leg into a socket to hold it in place

 

[Author]

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