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Old 11th Aug 2017, 5:17 pm   #1
G4_Pete
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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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Old 11th Aug 2017, 8:41 pm   #2
Tim
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Default Re: Belmont 700 finally up and working

Blimey! Sounds as though you got your money's worth as regards entertainment on that one.
Well done, I don't think I would have found half of those.
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Old 11th Aug 2017, 9:07 pm   #3
G4_Pete
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Default Re: Belmont 700 finally up and working

Tim,
The summary does not illustrate several weeks of picking it up and putting back down again!
The worst was the oscillator problem which confused me because on a simple capacitance bridge the grid capacitor was giving 57 pF against a marked 50 pF so I discounted it. It was only after I broke the feedback loop and started to measure the stage amplification of injected RF that I found the 50 pF coupling capacitor was attenuating the signal. One new mica capacitor and away it went.
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Old 12th Aug 2017, 8:59 pm   #4
Phil G4SPZ
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Default Re: Belmont 700 finally up and working

Pete, I must congratulate you on your tenacity and fault diagnosis skills, not to mention a great write-up. I've encountered many of those faults myself, but not all together in the same set! What a horror. I wonder which fault was actually the one that caused the set to be 'retired'? Probably the lightning strike silenced it.

It's worth saying that silvered mica capacitors, once considered above suspicion, are now cropping up more and more often. The fault is usually either loss of capacitance or high internal resistance. I believe the silvering migrates away from the riveted connections after many decades.
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Last edited by Phil G4SPZ; 12th Aug 2017 at 9:02 pm. Reason: Typo
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