geezin' Posted February 9, 2022 Share Posted February 9, 2022 Sitting at the computer last night listening to music. All of a sudden a loud pop and bright light on the floor next to my foot. Unplugged the receiver right away. Apparently the (original 1965) power cord broke internally and shorted. There was music as I unplugged it so I don't think it damaged the unit. So I need to replace the power cord. Everything I find is polarized but the original is not. Is this something I should be concerned about? Quote Link to comment Share on other sites More sharing options...
captainbeefheart Posted February 9, 2022 Share Posted February 9, 2022 What is the exact model of the Fisher? Quote Link to comment Share on other sites More sharing options...
geezin' Posted February 9, 2022 Author Share Posted February 9, 2022 440T with 500T power unit. A couple photos if that'll help. Quote Link to comment Share on other sites More sharing options...
Peter P. Posted February 9, 2022 Share Posted February 9, 2022 If the original plug is not polarized, then it's not an issue. You can still get non-polarized plugs from your hardware store. Quote Link to comment Share on other sites More sharing options...
captainbeefheart Posted February 9, 2022 Share Posted February 9, 2022 Looking at the schematic the power switch also energizes the accessory receptacles on the unit and also connects a .01uF capacitor to ground. The other side is fused and has a 820k resistor going to ground before the fuse. I would wire it up with the hot on switch side and the neutral on the fuse side. Before committing to this maybe check with a scope to see which connection has the least noise, my professional opinion would put be like I said, hot on the switch side which is the little electrode and the neutral on the fuse side which is the large electrode on a polarized plug. Replace the .01uF capacitor with a safety rated Y capacitor. 1 Quote Link to comment Share on other sites More sharing options...
captainbeefheart Posted February 9, 2022 Share Posted February 9, 2022 2 hours ago, Peter P. said: If the original plug is not polarized, then it's not an issue. You can still get non-polarized plugs from your hardware store. Yes since they used a non-polarized plug it will not cause any damage either way but one should exhibit less noise which is the way you want to wire it up. So the best way to do it is connect it whichever way has less noise. I typically see the neutral with a resistor to chassis and the hot with a capacitor to chassis. If it was wired in reverse you would have 120v on the chassis if you left it plugged in with the power switch off. The 820k resistor will limit the current to 146uA which is not enough current to kill you but what if that resistor failed short? Then you would have 120v with no current limiting ability on the chassis before the fuse which is bad. The branch circuit feeding the receptacle the amp is plugged into is a 15A circuit breaker which is typically the smallest you'll see, well 15A is plenty enough current to kill you. The only way it would be safe would be relying on a GFI receptacle. 1 Quote Link to comment Share on other sites More sharing options...
captainbeefheart Posted February 9, 2022 Share Posted February 9, 2022 It's hard to see in the picture but it looks like that .01uF capacitor on the mains to chassis is a one of those yellow print Sprague paper capacitors. They do not hold up to time very well and many that I test are very leaky, it's the last thing I would want connecting the hot to the chassis as eventually it will be able to leak enough current to kill you. For inquiring minds the red paint 'difilm' Sprague capacitors are very high quality and almost all I test are still functioning fine and do not leak current. Pyramid paper capacitors were black molded with yellow paint also and look like that, they too are usually not in good condition and not safe for that application especially at it's age. I highly recommend removing it and replacing it with a safety Y capacitor. https://www.digikey.com/en/products/detail/kemet/PME271YB5100MR19T0/3790120 EDIT: Looking even closer at the picture I can clearly see the 820k resistor connected to the left side electrode of the extra power plugs on the back of the amp, that's the neutral side. The right side is hot, which should be switch side and connected to the capacitor. Quote Link to comment Share on other sites More sharing options...
geezin' Posted February 9, 2022 Author Share Posted February 9, 2022 OK sounds like I should pay someone to do this. Any takers? Quote Link to comment Share on other sites More sharing options...
geezin' Posted February 9, 2022 Author Share Posted February 9, 2022 Does this help any Captain? Quote Link to comment Share on other sites More sharing options...
Deang Posted February 10, 2022 Share Posted February 10, 2022 @NOS Valves Quote Link to comment Share on other sites More sharing options...
captainbeefheart Posted February 10, 2022 Share Posted February 10, 2022 20 hours ago, geezin' said: Does this help any Captain? Yes thanks. If you can solder it should be a pretty easy repair. With a polarized plug wire the hot which is the smaller prong of the two to where I circled in red. Then solder the neutral which is the larger/wider prong of the plug to where I circled in green. That's it simple as that. Quote Link to comment Share on other sites More sharing options...
geezin' Posted February 10, 2022 Author Share Posted February 10, 2022 Cool. What made me wonder is your comment about the capacitor being hazardous. Quote Link to comment Share on other sites More sharing options...
tigerwoodKhorns Posted February 11, 2022 Share Posted February 11, 2022 2 hours ago, geezin' said: Cool. What made me wonder is your comment about the capacitor being hazardous. Maybe this? Why is it called the death capacitor? The "death cap" got its name because if it fails as a short it can electrify the chassis with full mains power, which is 15 amps at 125 volts in the USA and plenty of power to kill you. Quote Link to comment Share on other sites More sharing options...
geezin' Posted February 11, 2022 Author Share Posted February 11, 2022 Soldering a new power cord on is within my ability. Beyond that I'm out of my league. So explain that to me please. Quote Link to comment Share on other sites More sharing options...
geezin' Posted February 11, 2022 Author Share Posted February 11, 2022 Guess I could clarify that a bit. Which capacitor are you referring to? Quote Link to comment Share on other sites More sharing options...
captainbeefheart Posted February 11, 2022 Share Posted February 11, 2022 11 hours ago, geezin' said: Guess I could clarify that a bit. Which capacitor are you referring to? It's the paper capacitor I circled in this picture. You can probably just snip one lead and if it doesn't cause any noise issues just leave it. Or go to the link I posted above and purchase one of those Y safety capacitors and replace it. Y capacitors fail open and are designed for this specific application. They differ from X capacitors which go across the line to neutral and fail short. So make sure you get a Y rated safety capacitor and you can sleep at night knowing your amp won't kill you. 1 Quote Link to comment Share on other sites More sharing options...
geezin' Posted February 12, 2022 Author Share Posted February 12, 2022 Check your inbox Captain. Just out of curiosity why is the capacitor there? It's a slow blow fuse where it's attached. Quote Link to comment Share on other sites More sharing options...
geezin' Posted February 12, 2022 Author Share Posted February 12, 2022 How difficult would it be to install a 3 prong cord? Would that eliminate the need for the capacitor? Quote Link to comment Share on other sites More sharing options...
tigerwoodKhorns Posted February 12, 2022 Share Posted February 12, 2022 2 hours ago, geezin' said: Check your inbox Captain. Just out of curiosity why is the capacitor there? It's a slow blow fuse where it's attached. 20 hours ago, captainbeefheart said: It's the paper capacitor I circled in this picture. You can probably just snip one lead and if it doesn't cause any noise issues just leave it. Or go to the link I posted above and purchase one of those Y safety capacitors and replace it. Captain is the pro here. He answered this above. It is a noise filter. You can google it. Here is a video, I did not watch it. Quote Link to comment Share on other sites More sharing options...
geezin' Posted February 12, 2022 Author Share Posted February 12, 2022 Thanks. That's why I come here. You guys know it and I do not. Wanna know how to build fast Harleys? I'm your man. I will repair it but am impatient and listening to music on my iMac is no longer acceptable. So I ordered a Topping PA3s for short term. In the end I'll have 2 systems in the house soon. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.