Sinclair Neoteric 60 amplifier. service hints and tips

I have a copy of the original service instructions for the Neoteric. This is in process of being transcribed to the www site so contact me if you need assistance.

Contents, this page

Failed electrolytic capacitors

Components that have almost certainly failed will be the electrolytic capacitors. These dry out with time and the Neoteric dates from around 1970. So if in doubt it's best to replace all electrolytics.

This includes C44, the 2500µ 63v main reservoir capacitor, and C41L/R - the speaker capacitors, 100µ 25v.

C43 can also cause problems: if this has failed the amplifier may not switch on reliably. C43 is the power supply trip reset and needs to be 22µF 63v or higher,

In general, there is no problem with replacing electrolytic capacitors with ones of higher capacitance and voltage.

Failed transistors

The transistors that are stressed most are the power transistors. The PNP power transistors were Germanium ones, type AL102, made by ATES who later combined with SGS. These are still available. The two NPN Power transistors were silicon, NKT0033, manufactured by Newmarket Transistors. The nearest readily available replacement is likely to be 2N3054, which is the same TO66 case and a near enough equivalent.

Broken circuit board

The board was of SRBP (simple resin bonded paper) so is fragile. Inserting a plug into the board-mounted phono sockets could cause the tracks to break away from the board. The photographs of the Neoteric show these tracks, where they have been repaired,

Mains Hum

Just behind the push switches, visible in the photos, are two red plastic pieces, with crosses cut into them. These are the hum-bucking coils. They should not normally require adjustment - unless the transformer has moved (could happen if the amplifier has been dropped) or has been changed.

The mains transformer is in relatively close proximity to the inputs and, although it is separated as much as space allows, some mains hum is induced in input wiring. These two items hold coils which are rotated with the plastic piece, to minimise mains hum. In practise this works well, though the pedantic, using an oscilloscope, may notice that, because the magnetic field from the transformer is distorted, the coils can be set to minimise either 100Hz hum or 50Hz hum, but not both at the same time. So the adjustment is best done by listening closely to the loudspeakers and tuning for least audible hum.

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Page first published: Tuesday the 4th of August, 2015
Last modified: Tue, 14 Nov 2017 20:24:25 GMT
Written by and © Richard Torrens.