Quad 405 Amplifier Restoration & Upgrade Using MJ15022 As Final Transistors (Part 1)

My friend ask me to restore his Quad 405 amplifier and make some modification to improve the sound. This amplifier is the early generation Qad 405 built in 1976, I am not even born yet at that year. So let's begin this project.

I quite surprise this little amplifier is possible to running on class A. The overall condition is quite good and only several dirty spot found.

Inside this amplifier, many components are being replaced. Notice the big electrolytic caps are changed to the low quality capacitors. This is the first things I need to replace to the better one.

I dismantle all the components, then I also clean them up so it will looks like a amplifier. 

Old cables tends to wear and oxidize due the time, replace them with new cables will offer more tidy look and result more reliable performance of the amplifier.

This dual transformer will also re-wiring to make dual mono power supply configuration. It means each channel get their own power supply compare to the standard wiring is combine them all with single supply. The benefit will impact on better channel separation.

How messy my table now.

I am not sure why previous owner using sticky gum in the transformer. They are melted everywhere. I have to clean them up and this is a time consume job! Or this is what Quad manufacture did?

See how clean after all the dirt are washed away!

I attach all together and the first job to do is re-wiring the AC line input to the transformer.

I redesign the board for capable of 4 electrolytic caps and 2 bridge diode. This will lead the amplifier to dual mono setup.

I also inspect the amplifier board and determine what components should be replace to it's original states.

Is it the counterfeit OPA627? well, you bet. I better change them to the original as describe on the service manual.

Back to the power supply electrolytic capacitor. I will using four electrolytic caps from Marantz. They are actually produced by Nippon-chemicon. Those capacitors are expensive and gives the big contribution for all the cost of this restoration. They are main filter for this amplifier and the high quality capacitors will give best impact on sound improvement.

Besides the modification on the gain stage as describe here, I also will replace the MJ15003 to the MJ15022. This transistor has bigger headroom on maximum voltage handling.  The HFE for this new transistors are pre-checked to be match one with others.

I think it is enough for the part 1. The next part will be show this amplifier modification process. Stay tuned:)

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