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Old 2nd Sep 2007, 5:28 pm   #21
Ed_Dinning
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Default Re: Cossor 500 Short wave weak

Hi Kaylee, sorry to dissapoint you but this is the classic winding arrangement for SW coils, with the two windings interleaved. When you have the former apart look for leakage and then bake/varnish it.
If you PM me I can send you some 0.07mm wire if required.

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Old 4th Sep 2007, 1:00 pm   #22
kalee20
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Default Re: Cossor 500 Short wave weak

Ed, thanks for offer of wire. By an amazing coincidence, the wire used might be the same as one of 6 types of resistance wire that I have here.

I'm now going to display my ignorance and inexperience with HF oscillators.

The coil was originally 7.5 turns 0.48mm tinned copper wire, with interleaved feedback winding of 5.5 turns 0.07mm enamelled wire, resistance 94 ohms/metre (which is consistent with figures for 0.08mm Eureka wire). The former is a well designed, moulded part. See photo.

I'm thinking that tinned copper wire was chosen, as it saves the operation of stripping enamel. With the former having a helical groove, insulation is not an issue. But, enamelled wire will give better protection against corrosion and so I'm going to rewind with Grade 2 enamelled wire. Futher, being a totally copper conductor, skin effect losses will be less than the original wire, which has a tin coating just where good conductivity is needed. I'll beef up the gauge to 0.5mm from 0.48mm. (I'm lucky enough to have access to many different gauges). Can anyone predict if the enamel will add significant extra dielectric losses?

I'm also thinking, now I've discovered the feedback winding is only 5.5 turns which are well spaced, self-resonance will not be an issue - thus having sufficient distributed resistance to damp its Q will not be important. So, why was resistance wire used at all, rather than the less-expensive copper of a more substantial (and easier to handle) gauge? Did the Cossor designers know something that I don't?

I intend to have a play with the coil now it is stripped, trying different wires for the feedback winding as a learning exercise. (I have access to 6 different reels of resistance wire). But, I would like to know why it was designed as it was!

Finally, I don't want to depart too far from the Service Sheet resistance values. That's not for reasons of originality, but because I don't want the next person to be confused, when this radio needs its next service in 50 years time...
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Old 4th Sep 2007, 1:56 pm   #23
M0TAW Tony
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Default Re: Cossor 500 Short wave weak

In my case (Cossor thread) the resistance wire had a break where it passed thro' the former. After sweating out the wax I removed the winding and replaced it with 40 swg enamel covered wire and put a 22R resistor in series. Worked fine and continues to do so with no instability. BTW the resistance wire had no insulation - it didnt need it as it rested in the groove moulded into the former.
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Old 6th Sep 2007, 12:49 pm   #24
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Default Re: Cossor 500 Short wave weak

Does the former have a two start thread ie two grooves to allow interleaving?
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Old 8th Sep 2007, 10:57 am   #25
kalee20
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Default Re: Cossor 500 Short wave weak

Hi Graham, yes it is a two-start thread. Two interleaved helical grooves. One groove is deeper than he other, to allow for thicker wire gauge - it really is a well designed former.

Zooming in on the middle photo (my 4th Sept post) may show this.

I'm away at present. Progress keeps getting interrupted! However, I do intend to try a feedback winding using much more substantial wire, just to see what happens... 40swg and finer is undesirably fragile!

Tony - did you try any experiments such as bypassing your 22R resistor?
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Old 8th Sep 2007, 12:11 pm   #26
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Default Re: Cossor 500 Short wave weak

Yes - it didn't seem to make any difference without the 22R - but I put it in (just a small 1/8 watt one) anyway, hidden away.
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Old 22nd Sep 2007, 12:18 am   #27
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Default Re: Cossor 500 Short wave weak

Hi Tony, thanks for letting me know this! That's what I thought: the resistance of this particular feedback winding isn't a necessity - either distributed, or lumped externally.

I rewound the oscillator coil on the original former using 0.5mm wire (main) and 0.2mm wire (feedback), the different gauges so that I could easily tell them apart. The former was well cleaned in white spirit, IPA, and baked for several hours.

Guess what? No difference - oscillation still dies around 9MHz. (Wavechange switch out of circuit; coil connected directly to valve coupling capacitors and tuning capacitor).

Next, I tried eliminating the tuning capacitor and used fixed low-loss polystyrene capacitors in place. With 235pF (two 470pF in series), oscillation: with 470pF, nowt! This would correspond to near the low frequency end of SW.

Next, I eliminated the coil former, using a tube of two layers rolled-up 3 thou' Nomex, same diameter as original former. Windings were 7 1/2 turns 0.56mm (main) and 5 1/2 turns 0.25mm (feedback), held in place with little dabs of epoxy (not too much, so as not to introduce losses). I wired this coil in circuit (no core slug). Same result! The photo shows the two coils.

Finally, I eliminated the valveholder, using the triode of an ECH81. With this, I just get oscillation down to 6.5MHz but not to the extreme low-frequency end (checking actual frequency it is nearer 7.5MHz). The circuit now looked like a birdsnest - all leads short and soldered, but far away from the original layout at this stage. The stray inductances will be markedly different, but I'm near convinced that the original Cossor circuit was not good. I'm seeing a massive variation in amplitude across the band, which is a bad characteristic of superhet oscillators.

So, I'll be assembling the original coil (rewound), with near original windings, and accepting I'll have a partly-dead short wave. Until I come across another Cossor 500 / 501 for comparison, that's got to be the way!
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Old 22nd Sep 2007, 12:34 am   #28
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Default Re: Cossor 500 Short wave weak

Assemble it with new components and newly wound coils. And put everything back where it was when you started. This will take out any stray inductances. Then check and Align IF and RF. Make sure the IF is dead on where it should be, and the RF is right on all bands. Short Wave as far as possible.

Then maybe we can start again here.

Cheers,

Steve P
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Old 22nd Sep 2007, 12:06 pm   #29
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Default Re: Cossor 500 Short wave weak

You don't mention it but I assume the osc. anode load resistance has been replaced? You are confident the new valve is good? Whats the osc. anode volts when it has stopped oscilating? I think you have already done it but have you tried disconnecting the SW trimming cap?

A problem with the core material?
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Old 22nd Sep 2007, 3:46 pm   #30
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Default Re: Cossor 500 Short wave weak

Hi PGL - yes the osc anode load (and grid leak) have been replaced, as have the associated capacitors. The original 7S7 has been replaced with a new one. (And when I tried the ECH81, that was using separate R's and C's anyway).

HT is up and fine.

But, no I've not disconnected the SW trimming capacitor! Thanks for suggestion!! (Although, whatever it does, I'd expect to be swamped by the full value of the variable capacitor at the low frequency end).

Steve P - the IF is dead right (set up with modern sig gen, with current calibration certificate). Everything is now going back as it was. But, I'll try disconnecting the trimmer - and I'll take a close look at the coil core, which does look as though it's composed of only part of the length it should be. This could be a clue! Inadequate inductance, compensated by extra capacitance, lowers the L/C ratio. (Though when running, frequency can be set to agree with the tuning scale...

Thank you both for your interest!
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Old 23rd Sep 2007, 10:57 am   #31
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Default Re: Cossor 500 Short wave weak

Kalee,

Been watching this thread with interest - I have to admire your tenacity!

What type of capacitors did you replace the various decouplers with? The problem smacks of increasing reactance with decreasing frequency - the problem you would get if inappropraite capacitors were used. Disc ceramics with short leads would probably be best.

One other possibility that occurs to me. Have you ensured that the capacitor moving vanes are making good rf contact with the chassis. If poor this could introduce losses which could stop oscillation in a marginal feedback situation.

Good luck - I shall be interested to hear the eventual fix!

Regards,

Keith
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Old 23rd Sep 2007, 7:15 pm   #32
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Default Re: Cossor 500 Short wave weak

Quote:
Originally Posted by Keith View Post
Kalee,

One other possibility that occurs to me. Have you ensured that the capacitor moving vanes are making good rf contact with the chassis. If poor this could introduce losses which could stop oscillation in a marginal feedback situation.
I know this sounds silly and that you've probably checked, but there is a solidly-connected braid from the variable capacitor to earth, isn't there?
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Old 6th Oct 2007, 6:32 pm   #33
kalee20
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Default Re: Cossor 500 Short wave weak

OK, the coil is rewound, baked, and given a thin layer of varnish, the circuit is reassembled, and there's no improvement!

PJL - it's not the SW trimmer. (Left disconnected [you can just see this in the photo] - oscillator still gives up).

Keith - decoupling capacitors are polyester, connected as originals (albeit with shorter leads because the components are smaller than the original paper types). I agree with your ideas to use disc ceramic, but I've been clinging to the (naive?) belief that it worked originally! And the oscillator is fed via the 33k anode resistor, so low HT impedance should not be an issue here. Also, yes I checked the variable capacitor, it's solidly earthed with braid, and there's a springy piece of piano wire forcing a good connction between the rotor shaft and the frame. But, I'd also tried a separate short lead clipped between chassis and a moving plate. And I'd also tried disconnecting the variable capacitor and substituting a fixed 470pF polystyrene with short leads (hopefully similar to the variable at maximum. Guess what? No oscillation either way!

Russell also - see above paragraph. Thanks for interest!

Interestingly, prior to refitting, I measured the inductance of the SW oscillator coil and it's 0.5uH. (This agrees with the inductance calculated by Wheeler's formula, for the turns and dimensions.) However, the calculated inductance to resonate at 6.5MHz with 500pF tuning capacitance is 1.3uH. So, the layout stray inductances obviously contribute the extra 0.7uH, meaning the feedback winding is not coupled to over 50% of the inductance! It's not surprising that amplitude varies widely, and I'm now going to assume the SW never worked properly and call it a day, until I come across another Cossor 500/501.

Thanks for all interest, folks!
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