Author Topic: Small Clone - need help with debugging - no modulation  (Read 946 times)

fentom_lord

Small Clone - need help with debugging - no modulation
« on: November 13, 2019, 03:46:33 PM »
Hello, folks.
So I've built the Small Clone chorus using a kit avaliable here
http://efekty-diy.pl/small%20clone.html
The schematic and layout can be downloaded from there upon clicking the British flag. I made no part substitutions.
The problem is that although the signal does go through the unit, there is no modulation whatsoever. I moved the trim pot in both directions, but I've had no luck there. I inspected the board in search of solder bridges, cold joints, shorts and so on, but failed to find anything. I even heated the joints with a soldering iron, just in case, but to no avail. I suppose the problem must be lying elsewhere.
I did do some research on my problem and found some posts on this very forum, but, unfortunately, no one has had the same readings as I have.
I tested the circuit with an audio probe and found that the signal reaches pin 3 of the the MN3007 through the 33k resistor and that's as far as it can get into the modulation circuit. After hours of thinking, reading, testing etc., I finally decided to turn to those smarter and more experienced than me. I am more than certain to get some help here.
The voltage readings are as follows:

Wall wart: 8.96

4558
1. 4.38
2. 4.39
3. 4.28
4. 0
5. 4.34
6. 4. 35
7. 4.32
8. 8.7

MN3007
1. 8.1
2. 0
3. 4.34
4. 0
5. 0
6. 8.1
7. 8.01
8. 8.01

LM358
1. Oscillates from less than 1 V to about 2 V
2. 3.95
3. 3.91
4. 0
5. 0.7-2.9
6. 2.15
7. 0.5-7
8. 8.7

CD4047
1. 0
2. 8.1
3. 0.5-1.6
4. 8.1
5. 8.1
6. 8.1
7. 0
8. 0
9. 0
10. 8.1
11. 0
12. 0
13. 0
14. 8.1

Q1 (2N5087)
C 0
B 3.18
E 3.8

Q2 (2N5088)
C 8.7
B 8.68
E 8.1

Q3 (2N5088)
C 8.7
B 7.4
E 6.9

D1 (1N914)
A 8.12
K 1.25 (at first the voltage goes down to about 0.7 V and then rises to settle at 1.25 V)

D2 (1N914)
A 6.12
K 5.59

Compared to the readings I've seen in other posts the MN3007 and CD4047 voltages are very much off, so I guess they must be the culprits. I did change those chips for new ones, but it didn't help either.

Any help and/or suggestions welcome.
Thanks in advance,
Maciek

Mark Hammer

Re: Small Clone - need help with debugging - no modulation
« Reply #1 on: November 13, 2019, 04:50:52 PM »
Correct biasing (see the 100k trimmer) is crucial to functioning BBDs.  Though adjusting bias by o-scope is best, you can adjust it by ear reasonably well.  As you approach the optimal bias voltage it will go from not passing any signal at all to passing a distorted signal of low level and finally a cleaner and louder signal.  As you pass through the optimal bias voltage, it will go back to softer and distorted and then disappear.

If you are going to adjust be ear, I would recommend unsoldering/lifting one end of the resistor that mixes the dry/clean signal in with the delayed signal.  This should be a 22k resistor.  Leaving the clean signal out will make it easier to hear when the delayed signal is adjusted best. Once you find the optimal bias voltage, just resolder the free end of the resistor to the board.

fentom_lord

Re: Small Clone - need help with debugging - no modulation
« Reply #2 on: November 13, 2019, 05:36:28 PM »
Thank you for your time, Mr. Hammer. It turned out that the diode on pin 2 of the CD4047 was faulty. I replaced it with a new one and it worked like a charm. The Small Clone is operational. At least for the time being, that is.

jimladladlooklike

Re: Small Clone - need help with debugging - no modulation
« Reply #3 on: November 18, 2019, 08:00:58 AM »
Where'd you get the MN3007 from? I made the mistake of buying cheap off of eBay. After weeks of debugging and then months of leaving the pedal to one side from frustration i decided to buy another just in case and wadayaknow? The circuit was fine all along and i had been sold a fake IC. Obviously best to go through all of the debug process first but just a heads up, theres some sketchy sellers out the, eslecially when it comes to obsolete ICs.

Yazoo

Re: Small Clone - need help with debugging - no modulation
« Reply #4 on: November 19, 2019, 05:31:32 PM »
I've had exactly the same problem with fake BBDs, both from Chinese and UK suppliers. Basically, if they are cheap, they are most likely fakes. I have fortunately managed to get some genune ones.

The Small Clone is a very nice chorus and the stereo version is well worth building. Tonepad's layout includes a stereo option.

fentom_lord

Re: Small Clone - need help with debugging - no modulation
« Reply #5 on: November 19, 2019, 05:42:06 PM »
As for the MN3007, it was included in the kit I bought. I have no idea where they got it from. It doesn't matter now because the Small Clone chorus I built works awesome.
As I have already posted above, a faulty 1N914 was to blame. I replaced it and it started working. Thanks for the heads up.

jimladladlooklike

Re: Small Clone - need help with debugging - no modulation
« Reply #6 on: November 20, 2019, 07:53:41 AM »
Oops! Sorry about that, i should have read properly. Glad you got it working!