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Sansui Amplifier Service
 

 

Japanese Amplifier Service

We service many vintage Japanese stereo amplifiers with a conventional metal chassis assembly up to 60watts per channel from 150.00.

Our service includes
;  fault diagnosis and rectification, replacement of out of tolerance components, and a full performance test followed by a 48 hour soak test and PAT*.  

Contact us on 01905 821737 and state your model number for a quotation.  

The general features in this service example will also apply to a greater of lesser degree to other amplifiers of this size and complexity.  

This is a Sansui G30X. Unlike older vintage Sansui amps there is no solid metal chassis. Instead the case panels are snapped together. 
In this particular repair example there was no output on one channel and the balance pot was extremely noisy causing both channels to randomly increase and decrease in volume.  A typical repair proceeds as follows: 

Hooking the amplifier up to an oscilloscope and signal generator  quickly revealed the symptoms;  no output on the RH channel and a jittery signal on the LH side.  A check on the dc voltages at the RH output stage revealed an open circuit  4.7 ohm fusible resistor and O/C emitter resistor. The upper output transistor had a collector/emitter S/C.  The dc voltages around the rest of the circuit  were correct suggesting that the original fault was just a blown output transistor.  The current surge during failure would have stressed the rest of the output stage including the pre-driver, driver and output pair. So there is no alternative but to replace the full complement.

In order to gain access to the front panel controls on this model , it is necessary to dismantle the snap panels and uncouple the input selector switch cable and several push on connectors. The result, is a pile of  electronic boards and wiring. See opposite. The PCBs are now easily accessible and the faulty balance pot and the full pre-driver, driver and /output transistor chain are easily replaced along with a new fusible base resistor and emitter resistor pair. This is also a good time to clean the input selector slide switch contacts and rotary control tracks. A  thorough visual inspection of the circuit boards under a magnifying lamp reveals potential dry joints and any frail looking components.  

The panels are now reassembled and the amplifier re-hooked up to the test rig. After the recommended warm-up period, the dc output offset voltage is set to zero volts and bias pre-sets are carefully adjusted to the levels recommended in the service manual.  In our repair example, both channels now show a solid unwavering output signal.


A pair of 8 ohm dummy loads are now connected to both outputs and a 1kHz sine wave is applied of sufficient amplitude to cause slight clipping. The signal is then backed of slightly below clipping level and a reading taken to ascertain the r.m.s output.  The  r.m.s output voltage squared, divided by the load resistance (8 ohms) gives the power output in watts at 1Khz.  This is then checked against the manufacturer's stated output power.    A similar test at 10kHz and 100Hz is carried out.

 A spectrum  analyser is then attached to the aux input and LS output of each channel to check the frequency response and harmonic distortion level at low and medium output levels.  A reverse RIAA filter is then added to the phono inputs and the amplifier is checked for a flat, low noise, low distortion output level commensurate with the manufacturer's specified levels.   After confirming  correct performance, the lid of the amplifier is re-fitted and the rear I/O contacts are cleaned. The amp is then moved to the soak bench and hooked up to a pair of monitor speakers. After a short listening test the amplifier is connected to  a dummy load and run at a moderate output level for several hours to check reliability and heat levels. Finally after passing the soak test a PAT is performed. 

*PAT Portable Appliance Test. A conformal test of the electrical safety of the appliance. This includes insulation test and earth bonding test.
 A PASS sticker is attached to the underside of the Quad 303 with the date of the test and the engineers signature.
In the event of a fail (very unlikely), the owner will be informed before further work is carried out.

O/C = open circuit. S/C = short circuit

 

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Last modified: Monday, 27. April 2015 12:57