This was how this story starts, as far as I am concerned. I was aware of the Maestrovox because of prior research into vintage tube keyboards, so when I saw this one linked from Matrixsynth I had to investigate. I bought it. Shipping cost more than the unit did!

And no, it did not work.

In the keyboard itself, this is the oscillator sub-chassis that carries the tubes V1 and V2. Part numbers correspond to the circuit diagram on the Maestrovox Official Website.


Again, all components and sleeved wiring needed replacing.


This sub-chassis is very hard to access, as the bottom rear corner has a hole through it, through which a shielded cable passes. I filed away the sub-chassis so I could free the cable from it. Along the rear edge, the sub-chassis has been hacked away, at the factory, to allow it to fit without hitting the keyboard resistor string.


These are  the Low and Medium octave oscillator capacitors. The rectangles are 1% capacitors, while the cylinders are compression trimmers. There is also one of this sort of capacitor directly on the rear of the octave select rotary switch. All three of these capacitors will need to be replaced. Two of them have already failed, reading around 5k6 on the resistance range of a multimeter.

In the keyboard itself, this is the tagstrip with the majority of components on it. Part numbers correspond to the circuit diagram on the Maestrovox Official Website.

Most of these components will need to be replaced due to them failing or going well out of specification with age. Some of the wiring is ratty, and in a few places, old rubber power flex has been used. Again, the sleeving used around tinned copper wire links had degraded to a sticky mess and will need replacing.

The sub-chassis to which the two tubes, V1 and V2, are mounted is bolted down. Circuit earth is through a lug sandwiched between it and the case, rather than by a wire to an appropriate tag.

The schematic for the amp as received. I have added a bleed resistor for the HT supply (not shown on this diagram), something that was sorely missing from the original, especially as the HT supply terminates in an exposed pin plug!

 Wiring is finished for the amplifier. The original hole for the power cord now has a jewel in it, back-lit by a LED. The three electrolytic capacitors take the place of both the chassis mounted dual capacitor, and the tag mounted capacitor. 5W wire wound resistors replace the original higher wattage carbon composition resistors and will be a lot less noisy. Metal film resistors have been used in place of the lower wattage carbon composition resistors. The HT fuse is now in a safe fuse holder mounted where the original power switch went.

Above the chassis you can see an insulating sheet has been put over the terminals of the HT filter choke. The HT wiring to the speaker transformer (right) has been heat-shrinked. In the former location of the main filter capacitor is a small die-cast box that contains a modern IEC power socket, switch and fuse combination.

 All of the capacitors and resistors in the unit need replacing. This is the main filter capacitor. As you can see, its insides are attempting to break for freedom. Both of the high voltage electrolytic capacitors were rated at 350V peak, which is concerning because that is very close to what voltage they were operating at. This one is a dual, with 16uF and 32uF sections. I have some modern 32uF 450V axial capacitors I will be using in all places. The difference does not matter. These old capacitors had a tolerance of something like -20%/+100%, so it may well have been operating as 32uF and 64uF.

 To correct all the issues with the mains connection (no fuse, no cable clamp, etc.) I am using a modern IEC connector with integral fuse holder and power switch. They are mounted in a die-cast box attached to the chassis with an aluminium bracket. The bracket is attached to the chassis in the place formerly occupied by the main filter capacitor. Earth wire lugs now have star washers between them and the metal work, giving more reliable connections.

Rewiring has commenced. A tag strip has been added to support the axial electrolytics and the current limiting resistors. The replacement wire is double insulated, salvaged from an old microwave oven. The location formerly occupied by the power switch now has the HT fuse holder.

 Replacement capacitor across the speaker transformer primary.

The wiring underneath is mostly done with solid copper wire run through woven insulation. The insulation appears to have been varnished, but this is breaking down and turning into a sticky mess. All wires run directly between the transformer and the valve/tube sockets. I need the space, so they will be replaced with flex.

The original cathode bias network, and its replacement. Electrolytics have become somewhat smaller in the 60 years since this was made.