Cossor 494AC. My first proper restoration - UK Vintage Radio Repair and Restoration Discussion Forum

ChrisBU · 8th May 2009, 10:27 pm

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?

Sideband · 8th May 2009, 10:56 pm

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?

Rich.

ChrisBU · 8th May 2009, 11:24 pm

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.

PJL · 9th May 2009, 1:01 am

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?

Peter.N. · 9th May 2009, 9:47 am

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.

kalee20 · 9th May 2009, 6:31 pm

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.

Tim · 9th May 2009, 7:42 pm

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?

ChrisBU · 9th May 2009, 11:57 pm

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.

Sideband · 10th May 2009, 3:03 pm

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.

Rich.

8th May 2009, 10:27 pm	#1
ChrisBU Retired Dormant Member Join Date: Apr 2009 Location: Fakenham Posts: 10	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?

8th May 2009, 10:56 pm	#2
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,571	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? Rich. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

8th May 2009, 11:24 pm	#3
ChrisBU Retired Dormant Member Join Date: Apr 2009 Location: Fakenham Posts: 10	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.

9th May 2009, 1:01 am	#4
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	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?

9th May 2009, 9:47 am	#5
Peter.N. Dekatron Join Date: Jul 2008 Location: Charmouth, Dorset, UK. Posts: 3,601	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.

9th May 2009, 6:31 pm	#6
kalee20 Dekatron Join Date: Feb 2007 Location: Lynton, N. Devon, UK. Posts: 7,081	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.

9th May 2009, 7:42 pm	#7
Tim Dekatron Join Date: Jan 2003 Location: Bradford on Avon, Wiltshire, UK. Posts: 3,310	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? __________________ "Nothing is as dangerous as being too modern;one is apt to grow old fashioned quite suddenly." Last edited by Tim; 9th May 2009 at 7:51 pm.

9th May 2009, 11:57 pm	#8
ChrisBU Retired Dormant Member Join Date: Apr 2009 Location: Fakenham Posts: 10	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.

10th May 2009, 3:03 pm	#9
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,571	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. Rich. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....