EM80 not doing much - UK Vintage Radio Repair and Restoration Discussion Forum

TimS · 21st Sep 2006, 9:56 pm

I've replaced the EM80 (and most of the other valves for that matter ) on my now working Orion AR612.

The old EM80 feebly emitted some light and didn't respond when a station was tuned in (after fitting the new valves elsewhere). The new EM80 brightly does nothing when a station is tuned in.

Pin 9 anode shows 250V (as it should) but Pin 7 shows a mere 26V while the Orion circuit diagram says 60V - how much leeway is there here? The resistor between these two pins shows 494K when the set is cold which is only 5% over its alleged 470K.

The grid shows -660mV(ish) with respect to chassis and tuning in a station doesn't have any noticeable effect - should I see any. Do magic eyes work by responding to a DC input or an AC signal? How do I troubleshoot?

Cheers,
Tim.

paulsherwin · 21st Sep 2006, 10:32 pm

Magic eyes are just voltage indicators. They show the AGC voltage. If you're not seeing any change of voltage on the grid you won't see the eye doing anything

Investigate the AGC. The most likely explanation is a leaky decoupling cap pulling it down.

Something may also be pulling the triode anode down (pin 7), maybe another leaky decoupler.

Good luck, Paul

PJL · 21st Sep 2006, 10:35 pm

They respond to DC at the grid and there should be a 470K or thereabouts from pin 7 to pin 9. The constant -660mV on the grid is not right as I think it needs about -5V for a decent swing. Not sure of the circuit but it is often connected to the AGC. Probably a capacitor problem.

peter_sol · 22nd Sep 2006, 8:12 am

Hi. To move the magic eye you need a strong signal. Are you using an aerial?

Pumpernickel · 22nd Sep 2006, 9:47 am

Tim,

an EM80 needs about a DC voltage from 0 ... -4 V to bend the fan angle from 0...30°. If you don't measure those voltage changes by tuning, anything else is wrong.

Dieter

Nickthedentist · 23rd Sep 2006, 11:21 am

Quote:

Originally Posted by Pumpernickel

If you don't measure those voltage changes by tuning, anything else is wrong.

I had a Hacker Mayflower which worked OKish but whose eye didn't move at all.

Of course, the magic eye was fine, it was just that the rest of the radio was performing so poorly that there wasn't a lot for the eye to respond to!

General re-capping and replacing many out-of-spec resistors solved the problem - and made the set perform much better too.

Nick.

21st Sep 2006, 9:56 pm	#1
TimS Pentode Join Date: Jun 2005 Location: Forres, Moray, UK. Posts: 194	EM80 not doing much I've replaced the EM80 (and most of the other valves for that matter ) on my now working Orion AR612. The old EM80 feebly emitted some light and didn't respond when a station was tuned in (after fitting the new valves elsewhere). The new EM80 brightly does nothing when a station is tuned in. Pin 9 anode shows 250V (as it should) but Pin 7 shows a mere 26V while the Orion circuit diagram says 60V - how much leeway is there here? The resistor between these two pins shows 494K when the set is cold which is only 5% over its alleged 470K. The grid shows -660mV(ish) with respect to chassis and tuning in a station doesn't have any noticeable effect - should I see any. Do magic eyes work by responding to a DC input or an AC signal? How do I troubleshoot? Cheers, Tim. Last edited by Darren-UK; 31st Jul 2007 at 12:38 pm. Reason: Excessive use of emoticons.

21st Sep 2006, 10:32 pm	#2
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,962	Re: EM80 not doing much Magic eyes are just voltage indicators. They show the AGC voltage. If you're not seeing any change of voltage on the grid you won't see the eye doing anything Investigate the AGC. The most likely explanation is a leaky decoupling cap pulling it down. Something may also be pulling the triode anode down (pin 7), maybe another leaky decoupler. Good luck, Paul Last edited by paulsherwin; 21st Sep 2006 at 10:38 pm.

21st Sep 2006, 10:35 pm	#3
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: EM80 not doing much They respond to DC at the grid and there should be a 470K or thereabouts from pin 7 to pin 9. The constant -660mV on the grid is not right as I think it needs about -5V for a decent swing. Not sure of the circuit but it is often connected to the AGC. Probably a capacitor problem.

22nd Sep 2006, 8:12 am	#4
peter_sol Octode Join Date: Aug 2003 Location: Welwyn Garden City, Herts. UK. Posts: 1,906	Re: EM80 not doing much Hi. To move the magic eye you need a strong signal. Are you using an aerial?

22nd Sep 2006, 9:47 am	#5
Pumpernickel Retired Dormant Member Join Date: Nov 2005 Location: Germany (in the south) Posts: 6	Re: EM80 not doing much Tim, an EM80 needs about a DC voltage from 0 ... -4 V to bend the fan angle from 0...30°. If you don't measure those voltage changes by tuning, anything else is wrong. Dieter