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Old 28th Nov 2022, 9:45 am   #1
kestrelmusic
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Default Cossor 501 alignment

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?
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Old 28th Nov 2022, 10:05 am   #2
ms660
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Default Re: Cossor 501 alignment

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Originally Posted by kestrelmusic View Post
C9 and C13 are presumably just coupling caps
C13 is a padder capacitor and forms part of the oscillator's tuned circuit.

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.

Last edited by ms660; 28th Nov 2022 at 10:18 am. Reason: extra info
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Old 28th Nov 2022, 10:56 am   #3
Cobaltblue
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Default 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
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Old 28th Nov 2022, 11:03 am   #4
Herald1360
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Default Re: Cossor 501 alignment

Check that C12 is still a capacitor?

Is the oscillator offset high or low?
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Old 28th Nov 2022, 2:14 pm   #5
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Default Re: Cossor 501 alignment

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Originally Posted by Cobaltblue View Post
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.
That was my conclusion as well. I used to have a 501 and spent ages trying to improve the alignment. I even tried putting the coils in a warm oven for several hours and also trying the trick of passing a small current through them to warm up the windings and drive moisture out (similar to drying out TV line transformers). It only made a marginal improvement and I managed to peak the aerial coil....just. I decided it was probably the former and passed the set on.
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Old 28th Nov 2022, 3:18 pm   #6
ms660
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Default 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.
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Old 28th Nov 2022, 3:33 pm   #7
Heatercathodeshort
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Default Re: Cossor 501 alignment

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Originally Posted by Cobaltblue View Post
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
Totally agree Mike. I have service a number of these Cossor 501 series and without exception all of them have suffered with low Q coils. I have two under the bench that are both poor examples of a 4+1 superhet. Usually you need a long aerial to stimulate a few stations. LW in my case is particularly poor [More wire on the coil?] It should pick up with 10ft of wire but I have also suffered the same with the oscillator and I.F. transformers. John.
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Old 28th Nov 2022, 6:24 pm   #8
kestrelmusic
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Default Re: Cossor 501 alignment

Quote:
Originally Posted by Cobaltblue View Post
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
Ah. This set has been very damp (as well as very dirty) at some stage - I've had to remove no end of rust from the chassis. I'd assumed the coil was damp-proofed, though of course this is only the outer surface. Maybe the former is some mildly hygroscopic material.

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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