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Old 15th May 2017, 5:22 pm   #42
FERNSEH
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Join Date: Mar 2005
Location: Gateshead, Tyne and Wear, UK.
Posts: 7,444
Default Re: HMV 1804 Television.

Hi Robin,
The restoration of the set is getting nearer to completion.
This morning it was the sound circuits that came under scrutiny.
The sound IF amplifier, demodulator and noise limiter circuit were modified by the unknown engineer.
The first attachment shows the original EMI sound circuits. The noise limiter operates by means of a cancelling circuit. The out phase noise spikes detected by the second section of the D63 double-diode are compared with those from the demodulator.
The second attachment shows the modified circuits. The "B" section of the D63/6H6 double diode functions as series type noise limiter, the type that was common use in 405 line TVs. The interference limiter diode is normally conducting. However, when a when a positive going spike comes along the time constant of the resistor and capacitor at the anode is long enough prevent the valve conducting.
An added refinement is AVC. One of the diodes in the DH63/6Q7G is used an AGC detector to control the gain of the sound IF amplifier valve. A degree of AVC delay is provided by the cathode bias volts of the DH63.
Once again, it's not clear why all these modifications were carried out, after all there wasn't anything wrong with the original EMI circuits.

DFWB.
Attached Thumbnails
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Last edited by FERNSEH; 15th May 2017 at 5:31 pm.
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