The QUAD 50 Amplifier

The QUAD 50 is one of my favourite old bangers. It was manufactured until the 1980's, but has many features reminiscent of a bygone age. The topology is very like a typical valve amplifier. This one is a fairly early example and has a serial number in the 2000's.
The board is rather crusty, but nothing has exploded. These components are all original.
Internally looks fairly OK. Interesting that the capacitor is in great shape, very low ESR, no leakage, and plenty of microfarads.
The first problem found was a complete lack of direct volts. Two of these diodes in the bridge were open.
Fortunately the main transformer is working well, and no insulation problems were found.
A top view, the output transformer is connected to the front panel socket by ribbon.
Access to the octal socket for an input balun transformer.
Good quality transformer construction, the windings are well protected by this gunk.
Two new diodes in the bridge.
The first stage of board repair. Here I have replaced two very obviously faulty caps and tidied up the signal input connection.
A look under the back cover. The transistors and thermistor test OK, but a visual inspection is always a good idea.
Board repairs completed. Not as simple as I first thought. All the electrolytics and five of the transistors were replaced.

I am usually very conservative with repairs, I don't just shotgun the whole thing. The objective is to get it working properly and in spec, and be electrically safe. If it passes a long soak test at high power, I find it very likely that it will be a few years before anything else goes bad. I don't think the shotgun approach would do very much better, certainly not enough to justify the time and expense.
Tested and adjusted, ready to go back together for a soak.
RT-Q50E pcb image
Price £12.50 with free p&p in UK.
£3.50 p&p for ROW.
I have designed a replacement PCB. Please get in touch if you have any interest.
Click for schematic pdf.

This is a redrawing of the original Quad diagram.
Click for schematic pdf.

This is a redrawing of the PCB part of the diagram with the output transformer connections.