Noob - Troubleshooting Help Needed

[Tackleberry5692]

Hi Everyone

I am new to pedal building and this is my first post.

Am currently up to my 3rd pedal build and am comfortable using a soldering iron.

My first pedal worked straight away which was probably the worst thing to happen as I though things were going to be easy.

I couldn't get my second pedal to work no matter what. I made a conscious decision not to post about the issue and resolve it myself. In the process I learnt a lot about following schematics and even built myself a test box with built in sound probe. This helped me diagnose the troublesome part (was provided a Mosfet instead of a n-jfet) and that fixed that.

Now for my current problem.

I am building the '73 Ram's Head variant of the attached schematic http://www.diyguitarpedals.com.au/shop/boms/Siberian.pdf.

The pedal kind of works. Signal does come out and is changed by moving the dials, but it is substantially lower than unity volume.

I can trace the signal through the circuit (which is boosted through the pedal) up until the Output Booster (Volume) part of the circuit. Specifically, Q4 (2N5088) where I notice the significant drop in volume.

The following are measurements that I have taken which hopefully helps.

Input voltage of circuit is 9.20v

R19 and R20 = 1.77v
R21 = 8.49v
R22 = 1.18v

Q4 (2N5088)
Emitter = 1.18v
Base = 1.77v
Collector = 8.49v

I also measured the volume pot which ranges in voltage on the pins depending on position of pot between 0.04v and Nil (except ground which is Nil).

Any help greatly appreciated.

Regards

Greg

[Slowpoke101]

Welcome to the forum.

From your voltage readings I suspect that you have Q4 in backwards. Take it out, flip it around and resolder.

[Tackleberry5692]

Hi Slowpoke101

I tried it both ways (have it socketed so was easy to check and replace with others to ensure the part wasn't faulty) and it is not what the problem is.

I may have the voltages written against the incorrect pins (DSG). As I said, I am new to this so not sure which is which.

As Q4 is recorded on the schematic from top to bottom on the circuit board layout (flat side left and rounded side right) the voltages are as follows:

Top = 1.18v
Middle = 1.77v
Bottom = 8.49v

[Slowpoke101]

You have a JFET (J201 ) installed in the Q4 position? Q4 is meant to be a BJT (2N5088 or similar ).
What modifications did you do so that a JFET would work there?
Very curious now.

[Tackleberry5692]

Hi Slowpoke 101

Sorry for the confusion, I got this and my previous pedal mixed up a bit when explaining the problem which I have now modified.

I am using the 2N5088's.

Greg

[Slowpoke101]

Confusion is quite understandable with these things.

Generally Q4 should have voltages close to these;

Collector = 4.0V
Base      = 2.5V
Emitter   = 1.8V

Your measurements;

Collector = 8.49V
Base       = 1.77V
Emitter    = 1.18V

The transistor does not appear to be working at all as the Collector voltage is so high. I suppose that you have tried another transistor in Q4's position and no improvement was noted. So what now?

Check that R19 (100K ), R20 (470K ), R21 (10K ) and R22 (2K7 ) are the correct value. It is easy to misread these things. It would also be very helpful if you could upload some clear pictures of the board too.

[Tackleberry5692]

I swapped out a number of different Transistors and it made no difference.

Please note that the variant I am making has the BOM on page 5 of the link I posted, so the values differ from those you quoted in your response.

That said, the only one that varies to the BOM is R21 where I used a 10k as I did not have any 12k resistors (I had forgotten I had done this). Not sure if this is part of the issue.

I also measured the other Transistors as follows:

Q3 (2N5088)
Emitter = 0.44v
Base = 1.02v
Collector = 6.19v

Q2 (2N5088)
Emitter = 0.47v
Base = 1.06v
Collector = 6.47v

Q1 (2N5088)
Emitter = 0.04v
Base = 0.64v
Collector = 4.26v

[Slowpoke101]

From your picture.

Check R21. It appears to be 2K (Red, Black, Black, Brown, Brown )
and not 10K which is Brown, Black, Black, Red, Brown.

2K in R21's position would not be too good.

Edit: NOTE - R11 and R16 appear to be 2k2 and not 100R. R5 (100R )appears to be the correct value.

[Tackleberry5692]

Hi Slowpoke101

Another lesson learned.

It looks like I was given more components that were incorrectly marked (like the issue I had with my 2nd pedal build).

Everything I bought was in individually marked resealable bags which I thought was very helpful, but it seems to have been marked incorrectly.

I looked at the single remaining Resistor in the bag marked 100R and there is one 2k2 left in it, so what you said makes sense now. I will have to order more 100R's.

In future, I am going to measure each component before putting them in just to be sure.

It will probably take me a week or so to get the new one's to try them out. Hopefully this will fix everything.

Thanks for you help and patience.

Regards

Greg

[Slowpoke101]

No problem at all. Happy to help.

Testing each component (mainly resistors - they are not easy to read with aging eyes damn it ) before installing it is a good idea but time consuming. But I do so myself. I have been repairing, building and designing all sorts of equipment for years and I have determined that making sure that what you think something is, is actually what you need and not what someone else thought it was......

A good multimeter and a simple (?!!!! ) component tester (eBay has some good ones for about $18.00 Aussie Dollars ) are good investments.

[bluebunny]

When you get completely hooked by this little hobby - which you surely will - start buying your components in bulk, not as kits.  You'll find that resistors in particular are supplied on "tapes" - so you can measure one to confirm values, then mark up the bag (or tape) yourself.

Welcome to the forum.

+1

[Tackleberry5692]

Hi All

Just a little update.

I replaced R11, R16 & R21 and now it's seems right.

I ended up testing all of my (limited) stock and noted a couple of others that were incorrectly labelled on the bag (I must take the blame for some of this as I recently had to get glasses and am not yet always wearing them).

I actually managed to finish my 2nd and 3rd builds yesterday after all the help I got.

I just need to wait for my son to come home to test them properly as I can't play (I am building them all for him), but the couple of strums I did seemed to suggest they are working properly.

Thanks again.

[Marcos - Munky]

Just something to add. Those colored resistors (blue or green ones) sometimes have hard to read color codes. The color of the resistor may blend with the color of the band, and you get a totally different color. It's a nice idea to measure them.

I remember once I got a green resistor I could clearly see a orange band (3), but I didn't ordered any resistor related to that code. Instead, I ordered a 47K resistor, but the yellow band (4) blended with the green from the resistor, and the band turned out to orange.

Also, welcome to the forum, and glad you got it solved.