Bass överdriver – stuck on max dirt

[Caspar]

Hi

I’ve been trying to complete a BO kit. Yesterday it kind of worked, but the pots worked backwards. This morning I desoldered the pots and turned them around. Vol is fine, but I get a very overdriven signal, and turning the Drive pot changes nothing. I think I did some damage removing the pots, so maybe need to put in jumper wires from pin 1 or 3 (or both) to the next component in the circuit, but don’t know what those would be.

All advice much appreciated!

PS I say it “kind of worked” yesterday because I got the impression the Drive pot was acting more like a vol pot on max overdrive – I never got a subtle drive sound.

 

 

 

[Billy]

Post some nice clear pictures of both sides of the board and your offboard wiring to jacks 3PDT etc so it can be traced Caspar

Have you actually pulled solder pads off the pcb

We’d need to see it to advise

[Caspar]

Hi

Thanks, Billy.

Yup, at least one solder pad came off, and there were little brass-looking rings round the legs of the drive pot that I assume started life in the board. for that reason I replaced them with fat wires instead of just trying to solder the pots back in the other way round.

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I hope these are clear enough. I can confirm that the 3PDT switch is working, as are the jacks and power socket (when I switch the 3PDT I get a clean signal in the amp and the LED changes colour), so I think the issue is on the board or in the pot connection.

 

[Barry]

I would also list the Voltages of all 3 transistors in order.

Q1:  D   S   G

Q2:  D   S   G

Q3:  D   S   G

[Caspar]

Damn, thought I’d done this a few days ago.

Here we go:

Q1: D: 5.98   S: 0.63   G: 0

Q2: D: 4.08    S: 1.11    G: 0

Q3: D: 1.81   S: 1.21  G: 0

Hopefully measured those right and got them in the right order.

All advice appreciated.

Caspar

[Billy]

You want to set the drain pins to 5v initially using their trimmers

With regard to pulling pads off you can solder on either side of the pcb if it’s drive pot lug 1 with the little 1 silkscreened next to it that goes to ground so you can directly wire the pot lug to any ground point

As you can see it’s wired as a voltage divider – circuit in to lug 3 out lug 2 and 1 to ground so like a volume pot it’ll control how much signal goes to the output and how much gets dumped to ground depending on the pot rotation if lug 1 has a poor connection it would act weird easy enough to continuity check with your meter placing one probe to the actual pot lug ( not the wire) the other to ground

The schematic shows connections with the drive pot lug 3 is connected to C4 lug 2 to R7 and 1 to ground

So before you change anything I’d check continuity on them first to see if theres a problem following the changes you made

 

[Author]

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