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2015 Sunking Gain pot not working properly / not making any gain

Started by Lurktastic, April 21, 2021, 09:28:12 AM

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Lurktastic

Alright let me re-assess something here: I am getting 4.55v (VR) on ALL lugs of the Gain pot regardless of it's orientation... is this normal? It appears to connect to all the points it should connect to, without any shorts - I measured continuity to be sure.
I'm getting signal on pin 7 of IC1_B, but it's very weak and not very gainy at all (regardless of orientation of the Gain pot).
When turning the Gain pot the tone goes from bright to dark, around the midpoint there's a slight volume drop. I'm guessing something is awfully messed up with the pot (yet when desoldered it measures properly to it's intended value). Also on Gain B I measure 1.5ish K between the wiper and either 1 or 3, when pot is turned to either extreme. I guess this is normal because there's a 1.5k (R4) in parallel over lugs 1 and 3 of the pot.

I just did another hour of checking and testing and I'm stumped. Maybe I'll just order a pedalpcb kliché and rebuild it, I'm getting tired of chasing a ghost :(

Zerro

Moving Gain pot at the min. position will almost wipe away signal at the input of IC 1B. It is normal.

4.5V potentials at IC1B is not a problem. Maybe I would wait slightly less voltage at pin 6 of IC 1B. But finally it will be some mechanical issue, I guess. Crackle of pcb track around IC1, bad socket....

If straight connect of resistors with VB doesn't make gain jump, something is wrong here. Maybe shortage at output? Here is capacitor C10. Measure it, if it is not kicked thru. After it, controll, if switch bypass BYP_B is not bad, to ground output signal.
"Nudíte se? Kupte si našeho cvičeného ježka! Pobaví vás svými veselýmí kousky!"

Lurktastic

Quote from: Zerro on July 01, 2021, 05:07:36 PM
Moving Gain pot at the min. position will almost wipe away signal at the input of IC 1B. It is normal.

4.5V potentials at IC1B is not a problem. Maybe I would wait slightly less voltage at pin 6 of IC 1B. But finally it will be some mechanical issue, I guess. Crackle of pcb track around IC1, bad socket....

If straight connect of resistors with VB doesn't make gain jump, something is wrong here. Maybe shortage at output? Here is capacitor C10. Measure it, if it is not kicked thru. After it, controll, if switch bypass BYP_B is not bad, to ground output signal.

I have checked the socket, socket is good because I measure directly from the IC pins to the corresponding resistor pins (skipping the traces, to test continuity). Also checked on high ohm setting to check if the resistors are the correct values and making good contact, for example measuring between IC1_B pin 7 and R7/R8/C7 node. I have checked everything around IC1_B this way for shorts, continuity, values, etc.
C10 is not broken, it measures a good 1.1uF. I also checked BYP_B to check if it's not switching to ground when the effect is engaged. I measure no connection between R10/C11 node and ground when the pedal is "ON".

EDIT:

I even checked if the signal is reaching IC1_B, which is the case. Checked with audioprobe and the signal arrives at IC1_B pin 5. Continuity between IC1_A pin 1 and all components until IC1_B pin 5 is good, too. No shorts, correct values etc... I don't know where to look anymore :(

Zerro

Last thing I can offer is to measure if pins put in socket have the same values as nodes, where they are connected around. Continuity in circuit with socket. Now I am really wonder, what shit will be there engaged.
"Nudíte se? Kupte si našeho cvičeného ježka! Pobaví vás svými veselýmí kousky!"

Lurktastic

Quote from: Zerro on July 04, 2021, 10:10:18 AM
Last thing I can offer is to measure if pins put in socket have the same values as nodes, where they are connected around. Continuity in circuit with socket. Now I am really wonder, what shit will be there engaged.

What do you mean exactly? Should I measure from the pins of the socket without the IC in it directly to the nodes to check if the values are correct and if there are no shorts? Or should I check with the IC in the socket? I measured from the pins of the IC, while the IC was in the socket... so I would think that if I measure it that way the socket is tested for faults too...

Zerro

This way sometimes helped to me - to measure what is at pins in socket, and what is at nodes around, connected with those pins. Even if pin is hanging in air, something can induce at it from circuit in opamp. So, you think you have contact, but it is not.

This opamp connection is nothing tricky, so, I wonder, why this issue? That's standard connection of opamp with negative feedback. The more ratio of both resistors in feedback, the more gain. Nothing more. It is really strange.
"Nudíte se? Kupte si našeho cvičeného ježka! Pobaví vás svými veselýmí kousky!"

Lurktastic

You have a good point! It is indeed really strange.

I measured the pins while the opamp was out of the socket, and I see slightly lower voltages on pin 6 and 7 than when it is in the socket! This is strange as well to me  :o
These are the measurements I made:
Gain pot fully CCW
IC1_B pin 6: 4,50 volts
IC1_B pin 7: 4,31 volts

Gain pot fully CW
IC1_B pin 6: 4,54 volts
IC1_B pin 7: 4,36 volts

Still no shorts or links to ground where they shouldn't be though...

Zerro

Personally, I hate sockets in pcb. I always solder detail straight to pcb. Here is only problem with sockets, espacially when time goes on, because or transitional resistances between pins and socket. In time, they are rising - for it is just mechanical contact. Try to solder it in, slightly, and then try it again. It is only way how to make it clear.
"Nudíte se? Kupte si našeho cvičeného ježka! Pobaví vás svými veselýmí kousky!"

Lurktastic

First off, I'm sorry for taking so long to reply. Life got in the way haha.

Alas, that solution did not help either. I don't know what's up with it, maybe there's an issue with the PCB somewhere which I can't find... very strange. I think I'll just order a different PCB and try again. Thanks a lot for trying to help me :)


Zerro

It's pitty, that it doesn't work. I guess, that bad pcb is now most suspectable. Try to measure cunductivity of it. Use multimeter at this function (diode glyph), and measure all tracks from point to point.
"Nudíte se? Kupte si našeho cvičeného ježka! Pobaví vás svými veselýmí kousky!"

Lurktastic

Quote from: Zerro on August 01, 2021, 03:51:12 PM
It's pitty, that it doesn't work. I guess, that bad pcb is now most suspectable. Try to measure cunductivity of it. Use multimeter at this function (diode glyph), and measure all tracks from point to point.

First and foremost, I'm sorry for taking three months to reply to you...
I tried measuring the traces/tracks way back when I (we) tried to find the problem with the circuit, but alas, no luck. I decided to replace the PCB with a PCB Guitar Mania Kloned Centaur PCB. Even though it only has a voltage swing of 18v (+9v and -9v) I really like the sound of it :) I'll salvage the old Sunking PCB for parts probably someday...

Thank you for all the help and patience!