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Old 11th Aug 2017, 5:17 pm   #1
G4_Pete
Hexode
 
Join Date: Feb 2014
Location: Southampton, Hampshire, UK.
Posts: 419
Default Belmont 700 finally up and working

Apart from all the normal ,perished grommets , capacitors and a few high resistors, dull magic eye etc this one had a real collection of other “interesting faults” which I thought might be of interest.

1. Burnt out LW aerial coupler , open circuit MW coupler. Lightening? Rewound LW ( 42 SWG!) and fixed MW open circuit.
2. Smoothing capacitor negative wired back to the chassis which is the wrong place. The Belmont 700 uses the smoothing capacitor unconventionally as part of audio negative feedback to the bias resistors. (my interpretation)
3. 5Z3 “power the Queen Mary” power rectifier valve fitted 3 amp filaments. – plug in Type 80 from HRO spares was a better match.
4. Oscillator only worked over part of MW and LW and not at all on short wave. After some time found the 50 pF grid feed Mica capacitor had high internal resistance when put on an impedance bridge.
5. Motor boating on LW after the oscillator was fixed. LW antenna coil alignment was converging on the same frequency as the oscillator causing instability. Correct alignment fixed
6. Intermittent short on Mica compression capacitor in the oscillator section. Slackened off cleaned dust and grime , centered the compression washer which was well over to one side and all seems well.
7. Instability on tapping the chassis . Aluminium rivets on the valve holder/screening can poor connection. Drilled out and bolted down valve holder/screening can support.
8. Phono / radio switch causing audio distortion . Did seem ok on initial ohms test after a few operations but high resistance gradually came back – new switch fitted.


Now all working really well, this is a superbly performing radio with a very sensitive and stable short wave band.

Last edited by G4_Pete; 11th Aug 2017 at 5:18 pm. Reason: typo
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