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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

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Old 8th Jan 2003, 8:47 pm   #1
Paul Stenning
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Default Ferguson 354U, HMV 1373 etc - AGC decoupling

This information applies to the above sets and probably several other Thorn/BRC made sets using the same PCB. These are basic MW/LW AC/DC 4-valve (UCH81, UBF89, UCL83, UY85) sets from the late 50s. They are built on a PCB around the size of a postcard, with the tuning capacitor and other controls mounted remotely.

The AGC decoupling capacitor in most sets is between 0.02uF and 0.1uF (very often 0.05uF). On this set the AGC decoupling capacitor (C22 in the manufacturers service data) is only 0.005uF. This value is shown on the circuit and fitted to the PCB.

Such a small capacitor is insufficient to fully filter the audio off of the AGC line. Consequently the AGC voltage varies in sympathy with the lower frequencies in the audio.

Since this varies the gain of the IF stage, it has the result of making the lower frequencies a bit weak and distorted. The set just doesn't sound quite right, as though it is not correctly tuned in.

Increasing C22 to 0.05uF makes the set sound significantly better. I have made this change on my examples of both sets mentioned, and both showed the same improvement.

Whether this was a mistake or a deliberate attempt to "cheapen" the sound of the set is unknown.

C22 is located towards the back edge of the PCB, alongside the 2K7 carbon power resistor, R14.
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Last edited by Paul Stenning; 27th Dec 2004 at 1:10 pm. Reason: Fix link or code for vBulletin and add attachments
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