Re: HMV 2152 Restoration
You should measure 2V DC across the speaker terminals with this amplifier when the speaker is disconnected. The amplifier uses a bootstrap arrangement via R79 and R114/the loudspeaker, which will result in some DC appearing on the terminals. With the speaker connected, the voltage will drop to more like 200mV.
The important voltage to measure is the midpoint voltage at the junction of R80, R81 and C83. That should be close to 10.7v, using the chassis (positive supply) as the reference point. If it is, the DC conditions in the amplifier are correct. If not, check that the heatsink isn't getting hot before doing anything else. If you can, check the voltage on the emitter of VT9 as well, it should be 14.75 volts.
With DC coupled amplifiers, a fault anywhere in the circuit will throw the voltages out everywhere which makes them difficult to repair. The safest approach, is to check resistances with the power off. Compare them with the values in the good channel. That should allow you to narrow down on the area of trouble.
Paula
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