Help with '77 Oldcake Drive issue please

[John M]

I have built the '77 Oldcake a per the following Vero Vero layout: http://www.aronnelson.com/gallery/main.php/v/ulysses/ulysses_layouts/77OldCake/ulysses_77_oldcake_1_01.gif.html

I am having a problem in that the drive seems to remain almost constant somewhere in the middle of the hotcake range (based on my later "presence control" version), no matter how I adjust the drive pot.

I have measured the drive pot and it is supplying variable resistance, so I do not believe it to be the cause. I also tried the only other reverse log pot I had which was 1M and it also had the same issue.

I have however done it on Perfboard but have double checked it and believe it to be correct, but I am known for not seeing something that is right in front of my nose. I however do not have a camera so can't supply pictures.

I have made some part substitutions. They are capacitors 1,6 & 8 where I have used a value of 0.01 uF instead of 0.012 uF, capacitor 4 where I have used a value of 0.1 uF instead of 0.082 and capacitor 5 where I have used 500pF instead of 470pF. Otherwise all part values are identical.

I have also jumpered Switch 1 A & B as I wish to make it true bypass when I box it up, but at the moment it has no on/off switching.

The battery measured 9.35V and the readings on the IC were:
1    0.00V
2    5.00V
3    2.26V
4    0.00V
5    0.00V
6    9.30V
7    4.98V
8    0.00V

I am sure I must just be doing something really stupid, but sometimes when you are continually staring at something you can't see the wood for the trees. I did leave it alone for a day to see if it became obvious when I looked at it with a fresh mind, but I still can't see it. Any help in suggesting what the issue might be or some direction as to what area of the circuit I should be concentrating my  efforts on would be greatly appreciated as doing this with just a vero layout, I am not sure I am understanding the circuit (been doing this for a couple of years, but still very noobish).

Thank you

John

[George Giblet]

Check the link under the IC is going to pin 7 and not pin 6.  I suspect you have wired it to pin 6 because pin 6 is showing +9V (when it is pin 7 that should connect to the battery +ve and show 9V).

[John M]

Thanks for your help.

I have to give myself a big slap on the wrist in that I posted the voltages the wrong way round. 6 is 4.98V and 7 is 9.30. So my wiring there is correct (?), my typing skills need work.

Thanks for spotting that.

John

[George Giblet]

No problem, it's easy to do.  But it makes working out the problem a lot harder.

The thing that really puzzles me is pin 2 (in-) and pin 7 (out) both look about right indicating the feedback is working.  However, the voltage on pin 3 (in+) is way off, it should equal the voltage on pin 2, and from this perspective it seems the feedback might not be working.

All I can think of is there's something wrong with either pin 1 or pin 5 which would cause a large offset to exist between pins 2 and pin 3.  Perhaps check for shorts, solder bridges, uncut tracks, on these pins.  Failing that I'd be looking at the opamp.

[John M]

Thanks for the help.

I have checked and can't see any shorts or solder bridges on the IC pins. I tried a different IC and the same thing happened. I then decided to remove the IC and do a continuity check in the socket to see if there was any connection between the pins you mentioned. There was not. There is however between pin 4 and 7 when the positive probe is on 4 and negative on 7, but not the other way around. Is this is to be expected ( I am struggling to see why) or could this be the cause of my problem?

Thanks once again.

John

[George Giblet]

I think you are OK with the continuity.

I've also worked out why the voltage on pin 3 doesn't equal the voltage on pin 2.  It's a little harder to spot the problem without an actual schematic.  There is a voltage divider created by R2 and R3 and this feeds pin 3 via R4 then R5.  R4 is 1M and I suspect your meter has a 1M input impedance.  When you measure pin 3 your meter loads down the 1M resistor and you get about half of 5V which is 2.5V, and that's what you are seeing.  In short the whole opamp and feedback thing seems OK - which is good!

That would make me suspect there may be a problem with the way you have to drive pot connected ... but ... you have already checked the pot resistance is varying.  So I'm thinking there's something wrong with:

- R6 (120ohms), perhaps you have 120kohm?
- R7 (100kohm), perhaps you have the wrong value
- C3  not soldered properly and so effectively disconnecting the drive pot.

[John M]

Problem solved and thank you very much for your help George, you have helped me move from a frustrated builder to a happy player. 

For the record, so that if anyone is as noobish as me in the future, here is the solution to my problem. It was on R6. I have never encountered R used in place of the Ohm symbol and as all the others were K (except for the 1M one) I assumed it was a typo and put in a 120K resistor. I do not have a 120 Ohm one but put in a 100 Ohm one and all is working fine.

Thanks again for your help George.

John

[9 volts]

I built this and noticed that when I turn my guitar volume pot down i get instant feedback, wonder if yours does a similar thing?

[John M]

At full drive I did get the oscillation issues that have been mentioned regarding this pedal which resulted in some feedback type issues. This was not a problem at lower drive settings, only in about the last 10% pot's rotation. So after all my issues with R6, I did the mod that Ry recommends on another thread about this pedal and change it to 2K2. This has solved the issue, but be aware it changes the character of the pedal slightly although not necessarily in a bad way. I may yet try some values between the two to see what happens.

Some of my personal observations on this pedal are that I prefer the TL071 to the LM741 as I found it a bit "farty" and the TL071 is a bit smoother. This could also be because I am accustomed to the sound of my later model of the genuine item, which I adore. I will also probably try a presence control and I then believe I will have a pedal that will have a permanent place on my board.

Also, I am a bit embarrassed not to have thanked Ulysses previously for the layout on this, so thanks for your effort, it has allowed me to build a cracking pedal.

John