Cossor 501 alignment
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I've nearly finished restoring a Cossor Melody Maker 501. It is pulling in signals nicely, but the alignment is way off and I can't get it anywhere near right.
The setting of the pointer is correct for the position of the gang but the oscillator frequency is out by about 50kc/s. Diagram of the tuned circuits attached. I don't think the coil can have gone seriously out of value as it's wax encapsulated. It can't be C16 or C18 as they are shorted out in the MW position. C9 and C13 are presumably just coupling caps and so if they have gone out of value it shouldn't affect the tuning. C15 is out of circuit in the MW position. C12 is screwed down as hard as I dare without risking damage. Any thoughts on this? |
Re: Cossor 501 alignment
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C9 along with R3 provide the oscillator's grid bias voltage, that capacitor along with the valve's internal grid capacitance and associated stray capacitance can affect the oscillator frequency but usually less so if it's value has drifted. Lawrence. |
Re: Cossor 501 alignment
The coils on these seem to get effected by damp making them hard to align, deaf or both. The wax doesn't seem to help.
I never seem to get to the bottom of whether it's the coil itself or the former becoming lossy. Cheers Mike T |
Re: Cossor 501 alignment
Check that C12 is still a capacitor?
Is the oscillator offset high or low? |
Re: Cossor 501 alignment
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Re: Cossor 501 alignment
The oscillator frequency when tuned to the LF end of the MW band is primarily governed by the tuning capacitor, the padder capacitor and the coil, at the HF end it's governed by the same plus the trimmer.
With alignment it's always good to know that the signal generator is within spec. and that a dummy antenna is being used if specified. Sometimes when aligning shortwave, aligning to the wrong oscillator peak catches folks out. Lawrence. |
Re: Cossor 501 alignment
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Re: Cossor 501 alignment
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The set pulls in plenty on LW and MW ranges; it's just that what is tuned and what the dial reads are two very different things. I'm quite pleased to have got the thing working at all, given the state it was in when I started. Rust everywhere, tuning gang seized, several knackered caps, filthy inside from being used in a room with a coal fire and by the look of it several pipe smokers... but it's a nice set and it's been fun to work on. |
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