Cossor 523 7 valve radio - Tuning Indicator

[davemead1972]

Hi all,

Its my first post here, I'd like to say its a brilliant site!

I've recently purchased a 1957 Cossor 523 7 valve radio. My interest in old electronics, plus the fact that my late Uncle was a TV engineer for Telefusion in the 70s, has prompted me to take some time to learn a little more about old valve technology.

The radio is in excellent condition, considering I purchased it for around £20! It receives well on all frequencies, both AM and FM, with a small wire aerial hung indoors. The reception is very strong, giving good volume at a modest volume setting, and is very clear. There is no hum.

One electrolytic looks like it needs changing, whilst the rest of the components seem original. However, there is some evidence of repair in the past, and only 2 of the valves, including the EM81 tuning indicator, is original Cossor.

I wondered whether anyone has experience in this set, whether there is anything I should look out for?

Specifically, the EM81 is dim, and while there is an image, the shape does not change, no matter how strong the signal is. I was wondering what would cause this, should I check all the resistor values?

Your help and advice would be much appreciated.

Many thanks,

Dave.

[Steve_P]

First thing you must change is what is known as 'that cap'. This is C45 in this set. Start here and have a read...

http://www.vintage-radio.com/repair-...ion/index.html

You can get the manufacturers data from the link at the top right. Click here:

http://www.service-data.com/section.php/1384/1

Nice set that. Do a cap change and try a new EM81.

It's more capacitors than resistors usually. It works though so you won't have much to do.

Cheers,

Steve P.

[yestertech]

Hi Dave and a warm welcome to our formidable, friendly fabulous forum !
( best not said whilst eating crisps ! )
I restored one of these for a friend. Nice enough sets , even if the styling is a bit drab.(?)

The EM81 will be dim 'cos it's knackered ! These valves had a relatively short life and once they failed most people would tune by ear, never bothering to get them replaced.
They can still be obtained, although may cost you half of what you paid for the radio.

Although the set is playing OK , you WILL need to change what we call "that cap"
It couples the EABC80 ( ?) stage to the output valve ( probably EL84 by now...) The value should be 0.01 microF.
As it ages, it will leak and pass DC - This can be checked by measuring the voltage at pin 2 of said valve. If it is more than about 0.5v positive ( with volume turned down ) then change the cap and recheck.
Left as it is, the output stage can draw excessive current and wreck a number of other parts. Alas, you may only see the effect of this when the set stops working.

I also seem to recall that when switching to FM from AM, there was quite a drop in level. This simply needed correcting with the volume control, although switching back could be a bit disconcerting !

Enjoy your new radio ! It WON'T be your only one !!


Andy

[G8UWM-MildMartin]

And the usual leaky Moldseal capacitors on the AGC line (and elsewhere) don't help the tuning indicator's operation either.
If it is dim and looks dark and sooty on the target (phosphorescent screen), then the Russian 6e1p or 6E1N (depending on how you transliterate the Cyrillic) is a suitable cheap replacement.
It's always worth checking the 470K resistor to pin 7 and the 0.003-uF capacitor from pin 1 to chassis, as well.
(Note: the original mains lead is a weedy, brittle PVC fig-8 effort, but I'll refrain from further comment & just mention that this subject has been discussed...)
I have 3 of these and a 524 and find they are not that good on FM, after replacement of suspect caps. & with good valves.
Does anyone have a good one?
(Also, the smaller knobs are either missing or fallen to bits...)
I would like to know whether I should do any more work on them, or if it's just that they were among the 1st AM/FM sets here?

[davemead1972]

Hi again,

I've checked the circuit more closely now, and the capacitor you mention on the amplifier valve has been replaced, its an rs replacement, and the voltages are fine.
I.ve checked the EM81 more closely too, and its definately responding well, its just very dim indeed. I'll order a replacement.

Interestingly, I noticed there was no tone control, regardless of the setting of the tone potentiometer. On inspection, I noticed it was disconnected from a wire connecting it to the EABC80 AF amplifier valve d2 at pin2.
On attempting to connect this wire, the radio went completely dead apart from a hum, on disconnecting again, the radio worked as before.

Could anyone advise why this is? Does it indicate an internal short in the Tone potentiometer?

Any help would be again much appreciated.

Kind regards,

Dave.

[davemead1972]

Hi, me again!

I've another quick question - On the circuit diagram, there is a component shown as M1, which isn't on the component list. It appears with a semiconductor diode symbol, which I can't believe - is this for a metal rectifier? I cant see a metal rectifier in the radio, and it has an EZ80 ht rectifier valve - can anyone advise what this might be?

thanks again,

Dave

[Nickthedentist]

Hello Dave,

Yes, it is a solid-state diode.

The Trader sheet (No. 1200) says it's to do with delayed Automatic Gain Control.

Apparently, it "operates as a clamping diode and short circuits the AGC line to chassis until the AGC voltage is large enough to overcome the delay potential applied to its anode via R15, R16, R17".

Nick.

[Steve_P]

The tone control is R36, 500K. Tone is changed with this and a cap, C50, 0.01uF. Change the cap.

Cheers,

Steve P.

[davemead1972]

Hi all,

Thank you very much for your help and replies, I'll try your suggestions.

Thanks again,

Dave