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 Post subject: Canadian Marconi Model 81 - Snapping sounds
PostPosted: Feb Wed 21, 2018 4:43 pm 
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Joined: Oct Wed 25, 2017 2:38 pm
Posts: 6
Hello All,

I have recapped a Canadian Marconi model 81.

When the power is turned on, while waiting for the tubes to warm up, I can hear occasional snapping sounds sometimes accompanied by a flicker of the dial lights.

Once the tubes warm up and sound is heard the radio works extremely well, and is very stable. I can listen for long periods of time, and there are no further snaps.

Can anyone tell me what's going on here and how to troubleshoot it?

Thanks for any ideas.
Kevin


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 Post subject: Re: Canadian Marconi Model 81 - Snapping sounds
PostPosted: Feb Wed 21, 2018 5:08 pm 
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Joined: Jan Tue 31, 2012 1:55 am
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Location: Mountains of Mourne. Ireland.
click image to enlarge...
schematic: http://radio.codegods.ca/sites/default/ ... coni45.jpg

homepage: http://radio.codegods.ca/
Greg.


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 Post subject: Re: Canadian Marconi Model 81 - Snapping sounds
PostPosted: Feb Wed 21, 2018 5:10 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 4882
Location: Near Brandon, Iowa
You probably have something on the B+ arcing to ground as the radio warms up. Your radio uses a type 80 direct-emitting rectifier tube, which produces output voltage almost immediately after power-up since there is no cathode sleeve to warm up. However, the other tubes in the set do have cathode sleeves, so they take a while to warm up.

While the tubes' sleeves are warming up, the tubes are not drawing much (if any) plate current, so B+ can shoot up quite high during this time. If conditions are right at a particular spot (close air gap) an arc will jump, creating a sharp "pop" sound.

As the tubes heat up, conduct, and load down the B+ bus, B+ settles down to its normal voltage and arc-over no longer occurs.

The arc-over, while disconcerting, doesn't really damage the radio. I suppose it could be an ignition source if the set were to be operated in, say, a garage, where gasoline fumes might be present.

You can pull the chassis and carefully examine the B+ bus for points where an air gap is formed with a ground point. It might be easiest to just put the upended chassis (with speaker) in a very dark room, turn it on, and observe where the arc flash happens. That will let you pinpoint the problem area without much wasted effort.

Correcting the problem amounts to removing the air gap by bending component leads to ensure sufficient separation.


Last edited by lorenz200w on Feb Wed 21, 2018 9:52 pm, edited 1 time in total.

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 Post subject: Re: Canadian Marconi Model 81 - Snapping sounds
PostPosted: Feb Wed 21, 2018 5:50 pm 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 2657
Location: Lexington, KY USA
Sometimes the arc-over described above is at the band switch. This can eventually ruin the switch. ( Does this radio have such a switch?)

Anyhow, you want to fix this before you use the radio.

Ted


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 Post subject: Re: Canadian Marconi Model 81 - Snapping sounds
PostPosted: Feb Wed 21, 2018 7:05 pm 
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Joined: Oct Wed 25, 2017 2:38 pm
Posts: 6
Ok, thank you for your advice. Makes sense. I'll go looking a possible arc over point.

I will see if there are possible arc locations around the band switch also.

In the process of recapping the radio I have clearly changed some of the wiring paths. I'm going to start with the assumption that my wiring changes introduced the problem, and then move out from there.

Maybe I will try viewing the underside of the chassis while applying power in a dark room.

It may not be helpful that we have a high line voltage here.

Thanks again!

Best regards,
Kevin


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 Post subject: Re: Canadian Marconi Model 81 - Snapping sounds
PostPosted: Feb Wed 21, 2018 7:19 pm 
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Joined: Jan Tue 31, 2012 1:55 am
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Location: Mountains of Mourne. Ireland.
CL-90 
NTC INRUSH CURRENT LIMITER, 120 OHM
viewtopic.php?p=783016#p783016

Greg.


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 Post subject: Re: Canadian Marconi Model 81 - Snapping sounds
PostPosted: Feb Wed 21, 2018 7:25 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 4882
Location: Near Brandon, Iowa
VA3RCA wrote:

It may not be helpful that we have a high line voltage here.


Definitely so; although not necessarily for the arc-over problem. High line voltages can cause power transformer overheating and shortened tube life.


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 Post subject: Re: Canadian Marconi Model 81 - Snapping sounds
PostPosted: Feb Wed 21, 2018 7:36 pm 
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Joined: Oct Wed 25, 2017 2:38 pm
Posts: 6
I suspect it won't help with the HV arcing, but it helps a bit with the line voltage, and generally sounds like a very good idea for vintage electronics.

I have ordered a couple of them. Thanks for the suggeston!

Best regards,
Kevin


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 Post subject: Re: Canadian Marconi Model 81 - Snapping sounds
PostPosted: Feb Wed 21, 2018 9:44 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 7963
Location: Cleona, PA
If you monitor B+ from turnon, you may see voltage rise to around 425 volts or so just before the other tubes start conducting and bringing the voltage down. I use 500 volt rated electrolytics on radios like this, where 450 volt are specified. Just an extra margin of paranoia--oops, I mean "safety." :mrgreen:

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 Post subject: Re: Canadian Marconi Model 81 - Snapping sounds
PostPosted: Feb Thu 22, 2018 3:44 am 
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Joined: Oct Wed 25, 2017 2:38 pm
Posts: 6
It had occured to me that 450v caps left very little headroom on a high line voltage (123v) especially before the load is applied... thanks for the suggestion.

One more question just to confirm I understand.. The CL-90 is an A/C device that is placed in circuit with the line cord, correct?

Thank you!
Best regards,
Kevin


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 Post subject: Re: Canadian Marconi Model 81 - Snapping sounds
PostPosted: Feb Thu 22, 2018 3:54 am 
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Joined: May Sun 13, 2012 8:12 pm
Posts: 11411
Location: Central PA 16801
I have had arc-over "snap" in new 450 volt capacitors that saw the current inrush surge well over their 450 volt rating.

I have had arc-over in the band switch from some "tecknishiunn" who sprayed it with WD-40. the arc-over burnt quite a hole and rendered the band switch useless.

http://www.antiqueradios.com/forums/vie ... 6&t=259314

http://www.antiqueradios.com/forums/vie ... 6&t=282134

steve

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I am not responsible for any damage, injury, or death from any content / advice in this thread.


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 Post subject: Re: Canadian Marconi Model 81 - Snapping sounds
PostPosted: Feb Thu 22, 2018 3:59 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 7963
Location: Cleona, PA
Yes, the CL-90 inrush current limiter goes in the hot leg of the mains line. It looks like a fat ceramic disk cap. It has a high resistance when cold: at turn-on, the resistance drops over a few seconds to about 4 ohms, as I recall, giving a soft start ending with slightly less than line voltage operation, a bonus for older equipment. In operation it gets hot, so it should be mounted under chassis not touching other components and in the open where air can circulate around it. I think they are most useful in that they give tube heaters a soft start. Since they slow down the start of both heaters and B+, don't know if they would help the B+ initial surge. Maybe others have checked that surge voltage with and without a CL-90.

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Reece


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 Post subject: Re: Canadian Marconi Model 81 - Snapping sounds
PostPosted: Feb Thu 22, 2018 4:03 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 7963
Location: Cleona, PA
Arc over in a bandswitch can burn the wafer to carbon and cause a short. If B+ is routed through the bandswitch, it may be possible, depending on the circuit, to make a simple change to get high voltage out of the switch.

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Reece


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