Please consider donating to the web site: use the Donate button.

Sinclair Neoteric 60 amplifier service notes

These are Sinclair Radionics original service notes for the Mark 2 Neoteric 60 amplifier. Originally published as Sinclair Technical Information Sheet No. MG.1.49.1. August 1973. The paper has been transcribed without modification.

Although I wrote this, on re-reading it I find the style not very pleasing - but it was clearly written from experience in the service department, bearing in mind that many service returns were under guarantee, repairing problems which had not been caught in production. It also incorporates tests which we did in the service department.

I have altered a few points to clarify things. Sorry but the technical Information sheets referred to are not available.

Attempting to service the Neoteric 60 should not be taken lightly - it is a fairly complicated piece of equipment and only skilled persons should accept responsibility for repair.

The following service procedures are only offered as a general guidance with typical faults: more difficult faults will require the assistance of an experienced engineer.

Reference to the attached diagrams should also be made.

1: Dismantling

Lay the amplifier upside down on a soft surface and remove the four plastic buttons (new ones will usually be required on re-assembly).

Turn the amplifier the correct way round with its rear panel towards you. Grip the black cover with one hand on each side and slide it off (easiest done by pulling the centre of the back of the amplifier against your knee).

Do not use excessive force in pulling the amplifier cover off and be sure to pull the cover evenly.

It may be necessary to ease the back off by sliding a thin knife gently between the cover and the amplifier body at the rear.

A piece of double sided sticky tape is used to fix the front of the cover to the rosewood: take care that this does not foul the inner cardboard insulator. A knife may ne used to break the seal.

With the amplifier the correct way up, front towards you remove the cardboard insulating sheet and retain this.

Reference should now be made to sheet MG.1.42.1. where the position of the four printed circuit board retaining screws are indicated. NO OTHER PCB SCREWS SHOULD BE REMOVED.

Turn the amplifier upside down and remove the silver bottom cover. Remove the two brass screws holding the rosewood front panel in place. Also remove the heatsink fixing screws - 2 located near the centre of the chassis bottom, and 2 located 4" and 5" from the front of the rosewood panel on the left hand side. These may be pozidrive type screws but are generally replaced in service by standard slotted screws.

The amplifier can now be rested on its right hand side and the P.C.B. hinged out. If it does not hinge out far enough the brown earth wire connected to the P.C.B. near S1 (refer to sheet MG 1.41.1.) may be temporarily disconnected.

2: Initial tests

Access to usual service equipment is assumed. (See also Technical Sheet No. AF 1.49.1.)

The power supply used for service should be connected in the first instance to the points marked H.T. and S (refer to MG 1.42.1.) It should, ideally, be set to 52v with a current limit of 100mA preventing further damage.

VR9R and VR9L (refer to MG 1.41.1)should be turned to their minimum settings, i.e. in the opposite direction to the arrows marked on the P.C.B. and VR8R should be turned fully anti-clockwise and VR8L should be turned fully clockwise. The current under no-signal conditions should be approximately 40mA.

Check that the speaker outlet switch is set to the outlet you have connected to and that S10, the speaker system switch is set to 'moving coil type'. A sine wave should be applied to the 'RAD' input, with the input level control at the rear of the amplifier turned fully clockwise.

An output should be seen, unless the initial current was high, in which case, the power transistors should be disconnected one by one (by removing the wires to emitter and base) until the current sinks from the full limiting value. he NPN transistor (NKT0033 or 2N3054) is more likely to fail than the PNP one. If both channels now work check operation of controls with square wave input and replace faulty power transistors.

3: General fault diagnosis

Possibly all the power transistors may be disconnected and the current is still too high and one or both channels are not operating correctly. If one channel is operating correctly, short between the preamplifier output (refer to MG 1.41.1) or between volume control sliders to test for correct operation of the preamplifier. If the fault is in the main amplifier, test the power transistors (most conveniently done with an ohm-meter) and replace and reconnect as necessary.

The most likely faults in the preamplifier are faulty transistors, and these are easily tested in situ., e.g. with a diotester. another common fault is intermittent controls, which can be cured by replacement, if resoldering joints and application of cleaning fluid does not effect a cure. c24 sometimes goes leaky and this produces low output and with square wave signal, a lack of bass reponse.

Faults in the power amplifier are more common and if it is not just power transistor failure then the driver transistors TR9 and TR10 should be checked. These can be tested in circuit with the power transistors disconnected, e.g. with a Diotester, as can all the other transistors in this part of the amplifier except TR8. TR6 rarely fails, and if an output is obtainable but the current drawn is too high, about 90mA, TR8 is probably O/C.

Other faults will only be found by systematic testing and the backing of experience, but one peculiarity should be noted. Some amplifiers, when TR12 is disconnected, do not give a full supply voltage swing. If TR12 is replaced full swing should now be observed. This is normal and assumes a standard 1K load on the output. If full voltage swing cannot be obtained with a 1K load, but can with no load, then TR12 is of low gain and should be replaced again. With a current limit of 2A this effect will also be observed when a 15 ohm load is applied, when the output swing will drop by 10v - 20v on both channels showing that one of the NPN output transistors should be replaced.

Other faults are:-

Loose phono input sockets which can be repaired by using tinned copper wire (22 swg is suitable) to ensure good electrical contact, and then gluing the socket to the P.C.B. with a hard setting glue, e.g. I.S.12, ot a small amount of Araldite. Loose inductors can also be glued similarly.

Input level controls can easily be destroyed by misuse, or by mistakes when dismantling, causing intermittent operation. This is easily detected by rotating each potentiometer in turn until the offending one is located, it will also probably be the easiest one to turn.

Intermittency or complete loss of one input is usually caused by one or more of the valve pin connectors to the switch bank breaking. The easiest cure is to remove all of these connectors and solder the input P.C.B. directly to the switch bank.

C31 sometimes breaks loose giving very little intermittent output, and this should be picked up in the initial tests.

Breaks around the output switches sometimes occur and this can also repaired with t.c.w.

4: Testing Trip circuitry and Adjustment of Presets

The amplifier should now be functioning correctly into an 8 ohm load (3 ohm to 4 ohm loads should not be connected without the power coming through the trip circuit) and the presets may now be adjusted.

VR8R and VR8L.

First check the voltage, with respect to chassis, on point 'HT'. This is nominally 52v. Check voltages on points 'e' and adjust these by means of VR8R and VR8L to 26v. The settings are interdependent - increasing one voltage will somewhat reduce the corresponding voltage on the other channel so over compensation of one side to start with may be helpful.

These controls can also be adjusted for symmetrical clipping, using a 15ohm load, off the internal power supply.

VR9R and VR9L

There should be no signal when adjusting these presets. The quiescent current with both controls at minimum should be about 40ma. Each control should be turned in the direction of th arrows on the P.C.B. SLOWLY until the current reading increases by 2.5mA for each control. Care must be taken when adjusting these controls as sudden increase in current will occur at about 1/3 rotation. adjustments of these potentiometers can be done with mains input and the red wire on the P.C.B. replaced with a milliammeter, e.g. 10mA f.s.d. which will not read the preamplifier current.

The positive power input should now be connected to the point marked 'T' (see sheet no. MG 1.42.1.). The supply should bw set at about 55v and limited to 6A, With maximum signal swing at no load, i.e. approximately 50v peak to peak the 3ohm/4ohm load should be applied, when the trip should operate instantaneously. If the trip does not work then TR13, TR14, TR15 and TR16 should be checked. TR15 is the one most likely to have failed. When the trip circuit is working the printed resistor may need adjusting. The trip current should be about 4A and this can be seen by turning the signal level up with a 3ohm/4ohm load, if it is too low, i.e. the trip is too sensitive, part of the resistor should be shorted out. Insensitivity can be cured by cutting part of the resistor, thus increasing its value, until the required sensitivity is obtained. This resistor can also be adjusted by setting the amplifier to draw a constant 1A (with sine or square wave) and measuring the voltage across R56, i.e. between point 'T' and the earth end of R55. This should be 0.08v.

If the trip operates but will not latch on, TR16 is probably leaky and should be replaced.

Soak testing, if desired, should be done at this stage before casing.

5: Final testing and casing

All inputs should be tested individually and operation of filters and mono button checked. The rumble filter (high pass) is only operative on pick-up inputs and should be checked with a low frequency square wave imput.

If headphone socket will not take the standard parallel 'A' type plug, a soldering iron with a ¼@quot; bit will be sufficient to ease the socket by pushing it in firmly and removing it after 1 or 2 seconds.

Mains operation can now be tried, replacing the brown earth wire and wires to the mains switch if they were removed. If the transformer hums, it should be tightened up on its 2 BA fixing screws. The rectifier diodes will rarely require replacing, but this will be obvious by burning to the P.C.B. immediately round them.

The amplifier should be screwed up, taking care not to trap wires under the heatsink mounting. The production test pins for the preamplifier output should be cut short - they can touch the smoothing capacitor clip causing intermittency.

With the volume control set to mid-position, mains should be applied with a 3ohm/4ohm load and the output should be seen; increasing the level should not cause tripping, but if the load is removed then reconnected the amplifier should immediately trip. This is another test for trip sensitivity. Of course the available swing is limited by the transformer regulation and the output will be approximately 20 watts maximum with the mains transformer in the amplifier.

The hum-bucking coils are set in position and should not be moved unless the transformer is replaced.

The insulating sheet should now be replaced and the top cover slid onto the amplifier, checking that the bottom cover is retained by the top cover. A new piece of double sided sticky tape should be put in position before finally pulling up the cover completely. Four new buttons may now be inserted and the amplifier given a final test.

Page Information

valid-html401-blue Document URI:
Page first published: 24th October 2015
Last modified: Wed, 15 Jul 2020 21:21:25 BST
Written by and © Richard Torrens.