Cossor 500 I.F transformer cores

[Sideband]

Hi guys. I'm working on a Cossor 500 at the moment which seems a bit flat and not very sensitive. The I.F transformers seem to be very broad/not tuning very well and I'm wondering, before I remove them, if they are the type where the ferrite slug can become detached from the adjusting screw . I've attatched a picture to this posting to show the type fitted.

I think the performance of the radio should be much better than it is although I've not worked on one of these before. There is a distinct musty smell about the radio and there was a fair degree of surface rust indicating storage in damp conditions. However the wound components seem to have survived well. It's the version with Loctal valves. Running these through the Taylor 45C shows them to be OK.

I didn't want to remove the transformers unless they are the suspect type.

Rich.

[Steve_P]

Well if it's been moist, give it time to thoroughly dry out.

Also, are the caps across the coils OK. They could have succumbed to the moisture. If you do have a problem with cores, a small spring in between them can work sometimes.

Cheers,

Steve P.

[PJL]

If all IF's behave the same way then it is unlikely to be a fault other than damp as suggested by Steve.

If the IF does not peak but improves the further you screw the core in then most likely the capacitors have gone low in value.

If turning the screw has no effect then the core may have dropped off. Try turning the set upside down and giving it a gentle shake and if the gain changes it pretty much confirms the core is floating around.

It's also worth checking the AGC...

[Sideband]

It's actually number two that doesn't seem to peak although number 1 seems very broad. Anyway a little job over the weekend to see what's wrong.



Rich.

[Sideband]

Hmmm. Anyone dismantled these before? I've taken number two out and removed the can. There appears to be a bakelite 'frame' made in two halves that encloses the coils, with a retaining clip across the top and bottom through which the brass adjusting screws pass. It seems I will have to remove the two capacitors to get the frame appart. What I don't want is for the whole thing to fall to bits and be impossible to get it together as I don't have any replacements!

Maybe I should just check it on the inductance bridge.....!



Rich.

[Steve_P]

You could do, yes. And check those two caps while you are at it. Always do it the easy way wherever possible. Also shake the can and have a listen....

Cheers,

Steve P.

[Sideband]

No rattle when shaken but I suppose the cores could be stuck. I'll have to carefully unsolder the capacitors and then see what gives I suppose ! Both coils read exactly the same resistance on the digital meter so I doubt there is a problem there so it's going to be cores or capacitors and as it seems unlikely that both capacitors would fail, my bet is the cores have come adrift.....



Rich.

[geofy]

In the picture the caps are on top of what looks like paper, the caps of this type tend to be reliable, is it pressing down on the paper and altering the value, try making an air gap between the cap and paper. The other problem could be if it is the ferrite slug coming adrift from the brass adjuster.

Geof

[matthewhouse]

I also have a Cossor with the same chassis as this at the moment, also not very sensitive, I could not see how to open the IF units further than you have, without damaging the copper caps, looks like the copper bits should pull off, complete with the slug and adjuster, but that didn't work.

[Sideband]

Actually I have taken the transformer apart and re-assembled it...fiddly and not for the fainthearted! I'll post some pictures later. However it turned out that the cores were not the problem but it was a good exercise in any case. Measuring the parallel capacitors on the Marconi bridge, it seems that one of the 100pf mica capacitors is waaay out of value....the tollerance is 10% and one of the capacitors measures close to 110pf....just within spec and the other reads 160pf which is well out...in this sort of circuit anyway and both capacitors are quite lossy. Tomorrow, I'll unearth some new 1% 100pf mica capacitors that I know I have somewhere.....if only I could remember where I put them....!


Rich.

[Sideband]

Well the transformer is back in place and tuning nicely now. Not core problems as originally suspected but capacitor problems and despite my comment about it being unlikely both capacitors being faulty....they were!

It seems that damp has played a major part in the downfall of this radio. I still have a problem with sensitivity and I've discovered that the aerial trimmers won't trim.....literally they are doing nothing. However that will have to be for another thread but I visualise having to remove the aerial coil as I am suspicious that this has a duff winding.... Trouble is, it goes like a bomb on shortwaves.....

Are we mad playing around with these old sets......?


Rich.

[Lee de Forest]

Is the aerial trimmer one of those small two plate leaf spring type capacitors that you adjust with a screwdiver. I have found a couple of these that do not trim, do they fail with age?
I've also wondered why I play around with these olds sets but for me I just enjoy doing it and I don't think we need to care if it matters or not. If you like doing it and it does not harm anyone then do it, you've worked hard for it so why not.

[AlanBeckett]

Rich,
Certainly entirely mad. But then there are a lot of other hobbies that are worse. At least this one produces some tangible results, and the sets will be there for future generations.
Alan

[Tim]

Trouble is, it goes like a bomb on shortwaves.....

That's interesting. I have a 494 which is just the same, goes like a bomb on SW but LW and MW deaf as a post. I have been looking at the oscillator,the MW/LW osc coil has been subject to some repairs, by someone with a knife and fork by the looks of it! I might check out the IF coils although ISTR the response was Ok on the sig gen.
The capacitors inside IF cans can go "off" sometimes, and can nearly always be proved by connecting 100pF or so in parallel, and see if this improves things. If it does, the caps probably need attention, if worse or no change, they're probably OK.

[Sideband]

OK as I said earlier I would post some pictures of my repair on the I.F transformer. I'll have to split this over several posts.

The first three pictures of the transformer with the can removed are at the beginning of the thread so we'll start here with the next stage of dismantling. You only need to go this far if there is a problem with the cores or the windings. If it's just the capacitors that are faulty, that's all you need to change!

If you need to go further then these instructions may help. Before even removing the transformer from the radio, it is very wise to mark what way round it is mounted on the chassis. Having got as far as removing the can, make sure you also mark the various parts as you dismantle them so you can put it back the right way. This is particularly so with the coils so you don't get them out-of-phase. Firstly remove the brass retainers at each end by screwing the cores inwards and gently lifting the retainers. The cores won't come out at this stage because they are still held by the bakelite casing...at least they didn't on mine! Next, remove the capacitors and the paper insulator underneath them. This will reveal the ferrite rings and thin wires from the windings.

Next you need to unsolder the thin wires VERY CAREFULLY. At this point you can remove one half of the bakelite casing and the cores. See the pictures.

Carried on in next post.

[Sideband]

Once the cores are removed, you can remove the winding and it's ferrite ring. Make a note of what way round everything goes!

When reassembling, just reverse the procedure. In my case I fitted new 100pF capacitors. Dont forget the paper insulators under the capacitors and the paxolin insulators that go over the top of them. Finally refit the transformers and re-align. Hopefully everything will now be OK. If the ferrite cores have come adrift from the brass adjusters, they can be reattached with some superglue SPARINGLY!!


Hope this helps.

Rich

[matthewhouse]

Thanks Richard, that will save me some thinking time! Excellent description.

Cheers,
Matty