Boss CS-3 ACA to PSA mod

Updating an old boss CS-3 pedal to work with a normal PSA adapter.

ACA to PSA mod
Boss CS-3
ACA to PSA mod
Labeled for ACA adapter

Now, it’s a simple mod, just remove and jumper a resistor and a diode.

ACA to PSA mod
Original setup with D10 and R32 in place
ACA to PSA mod
R32 and D10 jumpered.

For info about ACA and PSA adapters visit


Image gallery issue

Zeta Sound web page is having an issue with the image galleries. We are working no the problem. So far the Tube pedals and Amps pages have been updated and working.


Marshall origin 50 and power scaling

The wreck, a Marshall Origin 50 on the workbench.

Marshall Origin50

Found that the EL34 power tubes had given up for some reason.

Marshall Origin50 EL34
Original Marshall Labeled EL34

Had the tubes replaced, but the amp did not act as it should. Only produces about 2W in any of the power settings. Low, mid and high.

Setting the power scaling switch in low and mid position gives the bias circuit for the output tubes fixed voltages, in the high pos it’s adjusted with a bias trimmer.

Marshall Origin50 bias trimmer
Bias trimmer and the LTV826 optocoupler

Measuring the voltage over the 1 ohm bias resistor indicated it hardly pass any current. Voltage reading on the screens gave a hint of what’s wrong. At the high setting the screen voltage was about 140V lower than the plate voltage.

Now both the bias voltage and the screen voltage are modulated by a LTV826 Optocoupler, the screen voltage also uses some power transistors. Checking the 1M resistor feeding B+ to pin 6 of of the LTV826 solved the problem, it had an infinity reading.

Marshall Origin50 Screens
The LTV826 and the broken 1M resistor above the power transistor

Replacing the 1M resistor made all the difference. The new tubes are now rebiased and all three settings produce the wattage they’re meant to. The amp is fully functional again.

Chart for the voltage readings when everything is fine, bias is messured via the joint 1 ohm resitor for the cathodes. Hopefully it could help someone.

Marshall Origin50 voltage readings
Marshall Origin 50 Voltage readings


MXR Micro Amp

Got a MXR micro amp in for service and modification.

MXR-micro amp, repair and mod
MXR micro amp

The fried original TL061 opamp is now replaced with an OPA134 and the old tantalum capacitors was replaced. Rewired the bypass with a new 3PDT switch for true bypass. Mounted a 2,1mm negative center DC jack.

MXR-micro amp, repair and mod
MXR microamp

View of the DC jack.

MXR-micro amp, repair and mod
MXR micro amp


Custom Rat pedal

Zeta Sounds take on the proco Rat.

Zeta Sound Rat

Circuit board.

Rat circuit board

Pretty straight forward though. A three way switch for clipping options. No electrolytics in signal path. Now it’s time to tune the circuit.

Gut shot.

Zeta Sound Rat guts

And an another image of the pedal.

Zeta Sound Rat


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