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FWIW I did try soldering a 100k resistor in parallel with R12 and if anything that made it worse

OK. Another question but still on the Drebbel thumping. I have 4 of these that don't thump when switched, but 2 that do. The difference is that the 2 that thump have a DI out in the box as well. I am assuming that this is the problem - just looking for ideas on solutions. The DI has a...

Fingers crossed it stays that way

Those voltages do look fine

The ICs should have 0V on pin 4, 9V on pin 8, and something close to 4.5V on the other pins.

What voltage readings are you getting on your ICs? Also, I can see you have used pot dust covers on the single gang pots. If you have spares I have successfully cut one down to go on a dual gang pot before.

I don't have time to work artwork for someone else, but Inkscape is free. You may be able to re-jig your design yourself if need be.

On the ICs with the arch, that is the pin one indicator. The other marks are mould ejector pin marks

The board also has the option to solder SMT transistors directly to the main PCB rather than using the adapter board. Either will work fine though, and what you have proposed looks right

There might be, but I can't think of any off hand. There are various ways I would think about removing that chip. One it to use small cutters to cut each pin, and then once all the pins are cut and the IC if off (but rubbish) then you can remove each pin individually. That method is...

Bummer. The only other thing connected to that net is the op amp. If it was socketed it would be simple to pull it out and test if that sorted out the voltages. It's not though so it would need to be desoldered. That is not a simple task with 8 pins - especially if you want to avoid breaking...

With that cap gone what voltages do you have on the op amp pins?

OK, I think this. If you look at the track side there is a brown wire soldered across it. In your photo the left hand end of that brown wire I am pretty sure connects to the capacitor you need. I would take some measurements of voltage on both sides of that capacitor just to make sure there...

You are going to need a multimeter to get this one working. That trim pot needs to be set so that the voltage on the output (pin 2) is 4.5V. Until you have done that there is no point in further troubleshooting.

Almost certainly not. It will be a cylinder with a metal label wrapped around it. You probably won't be able to identify the correct one without removing the board and looking at the traces. One leg of the capacitor that you are looking for will be directly connected to one end of the...

Yes, I think I would be pulling out the cap in parallel with the resistor that you have marked as 8.9 and measuring the voltages again. I am 90% sure that will ring that 0.26V back up to 4.5v or so

I think those are the 2 resistors you are looking for alright, and that 0.26V corresponds to the bias voltage you are getting at the op amps. That 0.26V should be (with 9.29V as the input voltage) about 4.6V. It looks as if something is shorting that voltage nearly to ground. The resistor...

In the seabed delay linked to above, the resistors in question are 10K (brown, black, orange) which is fairly standard. I can see you have a few on the board in the right place, so use a continuity measure on your multimeter to find the right ones. One will have one end on 9V and the other...

You can do it from the top as long as you can work out which resistors are giving you the 4.5V. Essentially there would normally be 2 resistors between the ground and 9V, and the point where they both join would be halfway voltage, so 4.5V for a 9V power supply. That 4.5V is used to bias the...

That looks like a problem. I would be expecting 9V on 8, 0 on 4, and about 4.5 or so on the rest. You will need to get those voltages sorted before moving on. Somewhere near where the power comes into the board I would normally expect a network of components to derive that 4.5V and some...