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Hi all,
Built a few of these and never had this problem. The circuit is passing signal but the Gian 1 on the drive circuit seems really ineffective. On all other SD70s builds that gain will tear your head off. Swapped IC chips and pots and no change. Confounding. Thoughts? The fuzz and boost circuit work perfectly. Thanks and have a good day.
Is it that you don’t get the gain or is it the crunch that you are missing? Assuming that you have checked all voltages and that they are as expected, make sure that you have the 10K in R5 and that the 500K pot functions properly across the whole rotation. The drive circuit should function like a basic TS. So the gain is not normally considered high. Make sure you have continuity where expected. And make sure you have selected good diodes for clipping and that they function properly. You could socket R5 and try some other resistors around that 10K value. Clear pictures might help.
Playsforfun,
Thanks for your response! I will check on the above and report back with photos. I did swap out the pot and put it on a DMM to check before I installed and it was functioning properly. I used the standard 1N914 diodes the build doc called for. Could they be bad? Are there similar or better choices? Checking on the R5 10k now. Thanks again.
AZ_Son, It looks like a 1K in the R5. Slot R4, R5 and R6 all look the same. R5 should be brown,black,black,red,brown for a 10K stock value. The rest of the that circuit looks fine. Simple swap should get you going. If you have trouble cleaning out the holes of solder, a quality pointy tooth pick comes in handy.
Playsforfun,
I checked the R5 resistor with a DMM and it reads 10K. R4 and R6 both read 1K. The rest of the drive circuit…tone and volume work just fine. I reflowed all the solder joints just to be sure and still nothing. No volume or drive increase when the pot is rotated. Pulling my hair out!
Best to supply us with voltages of the input voltage and each pin of the op amp. Also if you can provide photos of the solder side and all off board wiring we should be able to nail it down. To answer a previous question, yes, the diodes can fail, especially since they are glass. As a set for clipping the 1N914 or 1N4148 sounds amazing to most folks.
Ok! Thank you. Will do. Here are pics and I will get voltages in the morning. Thanks again for all the help!
Also….What is the best way to check voltages in your opinion? Set DMM to DC voltage and ground one probe then check pins on IC? Where do I check input voltage? Sorry…I am a novice!
You are correct on how to check the pin voltages. Here is a picture that shows the pin orientation on an 8 pin IC.
Perfect! Thanks Steve.
Input voltage 9.2v
Pin 1- 4.6v
Pin 2- 4.5v
Pin 3- 4.3v
Pin 4- 0v
Pin 5- 4.6v
Pin 6- 4.6v
Pin 7- 4.6v
Pin 8- 9.2v
Your voltages are good
To check your diodes with no power to the circuit use your meters diode check place the red probe on the anode side of D4 the black probe on the cathode banded side of D3 where you should get a forward voltage reading of around 1.2v swap the probes around and you should get OL (over limit) or a very high resistance
Do the same checks for D1 and 2 place the red probe on D1 anode and the black probe on D2 cathode again you should get around 1.2v ish and OL in the reverse direction
Also continuity check D4 anode and D2 cathode connect to IC pin 1 and D3 cathode and D1 anode connect to pin 2
Place your meter probes on the diode pins not the solder joints we’re checking to see if your diodes are OK and they are connected correctly
Strictly speaking you shouldn’t check diodes in circuit but you should get a fairly accurate reading that will let you know the diodes are conducting as they should
Billy,
thank you so much.. I am out of town currently but will check the above when I return. Thanks again
Billy,
checked diodes for voltage and continuity and here are the results
Voltages
Red prob anode D4/Black probe cathode D3 =OL
Red prob cathode D4 = .563
Red probe anode D1/Black probe cathode D2 =OL
Red probe cathode D1/Black probe anode D2 =.576
Continuity
D4 anode/D2 cathode =IC pin 1
D3 anode/D1 cathode =IC pin 2
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some readings are opposite of what you indicated they would be. Could this be the issue? Seems the diodes are oriented properly and functioning at half your stated voltages.
thank you!You may have a different pcb from me but it looks the same layout you’d want
Red on D4 anode Black D3 cathode should be just over 1v (basically the combined forward voltage of the 2 diodes) OL with the leads reversed
Red on D1 anode Black D2 cathode again just over 1v and OL reversed
if you dont get those readings try reflowing the diode solder joints
As I say you may not get exact readings with them in circuit
All solder joints have been reflowed and diodes check again. Same results as above. No power to the circuit. Gain still not working. Not sure what is going on here. Looks like I have an older version of the circuit board but I did receive a few of those and they worked with no incident. Thanks for your help!
I know you reflowed everything a dozen times but it still seems as though there is a stray wire strand touching where it ought not. Check for that and any possible bridges between pins one and two for the op amp or lugs 1 and 2/3 of the gain pot or anything in between. Seems a short of some kind possibly making the op amp more of a buffer there with no affect with the pot. Check off board real good. Even under the pots where you can’t see between the enclosure and pot. Stray component cut-offs trapped in the wrong place happens. Using your DMM, does the resistance change between pins 1 and 2 of the op amp when the gain pot is rotated while the circuit has no power applied?
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