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Old 8th May 2009, 10:27 pm   #1
ChrisBU
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Default Cossor 494AC. My first proper restoration

I bought this as not working and I think I should have scrapped it. I didn't try it out before I started and just got stuck into it, changing the wax caps, filter caps and a cathode cap, replaced a resistor that was way out, new tuner drive cord, opened and lubed the pots. I cleaned up the cabinet and put it all back together. Finally it was time to switch on - nothing. I took it out again and found part of the output transformer primary was open circuit. I took one from a scrap set and put it in - still nothing just a faint hum from the speaker. When I came to check the audio output valve there was no cathode voltage. I had a spare 7C5 and tried that - still no cathode voltage. Would it be possible to change the socket and use a 6V6 here?
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Old 8th May 2009, 10:56 pm   #2
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Default Re: Cossor 494AC. My first proper restoration

If there is no cathode voltage, the valve isn't drawing any current. Have you got voltage anywhere else? It seems unlikely that you would have two faulty valves giving exactly the same results. Check that you have anode and screen-grid volts. If there is a cathode bypass capacitor, make sure it's not short-circuit.

Let us have a list of voltages that you have and it should help in locating the fault.

The 7C5 is basically a 6V6 with a different base. You could change the valve socket to octal and rewire for a 6V6 but why?


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Old 8th May 2009, 11:24 pm   #3
ChrisBU
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Default Re: Cossor 494AC. My first proper restoration

I do have screen and anode voltages and the bypass cap is new. My valve tester reads bad on both the valves I have, but I have plenty of 6V6's. A new 7C5 is more than I want to pay, especially as I don't think this radio is worth what I've spent on it.
I can't give the voltages at the mo as it's at my workshop.
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Old 9th May 2009, 1:01 am   #4
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Default Re: Cossor 494AC. My first proper restoration

It's unusual for valves to end up with zero emissions. Do the heaters light up? Have you checked the value of the cathode resistor? should be 270 ohms. Cleaned the socket carefully?
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Old 9th May 2009, 9:47 am   #5
Peter.N.
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Default Re: Cossor 494AC. My first proper restoration

If you have volts on the anode and screen, the only other cause of no cathode volts would be a s/c by pass cap or a hefty negative voltage on the grid (pin 5) but I can't see what could cause that - ohterwise I think you must have two dud valves.
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Old 9th May 2009, 6:31 pm   #6
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Default Re: Cossor 494AC. My first proper restoration

I have a 7C5 you can have to try. It's suffered due to a leaky grid coupling capacitor putting the grid positive and vastly over-running it, so emission is not what it should be. But, it's useable. Came from a Cossor 500, so it'll nearly be going back home!

Send me a PM with your address. I don't want anything for it and it won't cost much to post.
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Old 9th May 2009, 7:42 pm   #7
Tim
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Default Re: Cossor 494AC. My first proper restoration

Valveholder?
I have often found pins on this series of valves need a good clean. If you pull the valve slightly can you measure 270 ohms to chassis fom the cathode pin of the actual valve?
If you have a good 6V6 knocking around you could always connect one in temporarily on the end of some long wires soldered to the back of the 7C5 socket, just to prove the set.



If you do scrap it, can I bagsi the Mw/LW oscilator coil please?
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Last edited by Tim; 9th May 2009 at 7:51 pm.
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Old 9th May 2009, 11:57 pm   #8
ChrisBU
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Default Re: Cossor 494AC. My first proper restoration

Thanks to all for your replies, and thanks to Kalee20 for your kind offer but I won't be needing it now. I got it working and despite a worn out 7C5 it seems pretty good. I think it was dirty valve pins that were the problem. I cleaned the pins on all the valves - they were black and needed scraping. After that it worked, a tweak of the oscillator caps brought in plenty of stations on all bands. I'll do a proper alignment later and fit a good 7C5 when I get one.
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Old 10th May 2009, 3:03 pm   #9
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Default Re: Cossor 494AC. My first proper restoration

Quite common to have blackened valve pins on this range of valves. A good brush with a metal suede brush will have them shining and a drop of switch cleaner will help to prevent further tarnishing. You'll probably need to repeat the operation some time in the future though.



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