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Fisher 400 - No Sound after mishap

healyc139

healyc139

Member
Hey everybody,

So I just did some routine cleaning of my Fisher 400, which included pulling each tube one at a time and dusting them off. Upon reinstalling one of the power tubes; one of the pins got bent so it was not inserted.. and I had not noticed, so I hooked the receiver back up and turned it on thinking everything was fine. The receiver started smoking a little, so I turned it off and unplugged it immediately. I found the bent pin and carefully fixed it, but now I have no sound out of either channel for any of the audio inputs.
This 400 was rebuilt in 2017, so the internals are pretty up to date. Please, if you have an idea of where I should start to troubleshoot, please let me know. I'm thinking it could mean replacing the power tubes, or replacing a capacitor or two.. I hope it's that easy anyway.
I can post photos of the pin and the underside of the receiver if that helps.
 
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Have you checked the fuse? Post pictures of the affected tube. Was the pin bent and touched another pin? original 7868 tube pins are soft and can bend easily. Without knowing which pin was bent not really possible to know what happened.
 
I have not checked the fuse, but the receiver seemed to fully turn on after I found the bent pin.
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That’s the plate connection. It looks like it shorted to next pin #8 that shows a little burn mark on it. That pin has internal tube connection. Looks like your amp needs voltages checked. Hopefully it just took out screen resistor somehow and tubes are not conducting.
 
Thanks for the help on that, that sounds like a good place to start. I have a voltage meter, can you tell me what I should be looking for?
 
What is your experience and comfort level checking 400v+ voltages?
Do you plan to do repairs yourself?
 
Please see the attached photos of the underside. There is also a photo of the tube section in 2018 before I swapped out the power tube capacitors. The orange component looks like it's been fried.

Yes I plan to do the repairs myself, I am very comfortable with meter reading and soldering. I used to rebuild Magnavox 9303 9304 tube amps, and when I got this Fisher, I did some improvements on that as well. That being said; I've always been fortunate enough to get pieces that had little to no issues during my rebuilding, so I have not had a lot of experience troubleshooting vintage pieces like these.
 

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  • Fisher 400 - May 2018.jpeg
    Fisher 400 - May 2018.jpeg
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[QUOTE="healyc139, post: 15589391, member: 185215"…..The orange component looks like it's been fried…...[/QUOTE]

Yup, that bugger took the hit!! Your screens and cathode resistors look like they’re OK.
 
Yep, no screen voltage at power tubes = no current flow through the tubes = no sound.
Once you replace it, I'd start the amp with variac, DBT, or both, just to be absolutely sure nothing else got affected by the mishap.
 
grindfix said:
Yep, no screen voltage at power tubes = no current flow through the tubes = no sound.
Once you replace it, I'd start the amp with variac, DBT, or both, just to be absolutely sure nothing else got affected by the mishap.
Click to expand...

Good idea. I've been meaning to get a variac for a while anyway. Maybe I'll just build a DBT and go that route
 
The percentage is simply a tolerance of how close to 1.2 Kohms the device is. 5% is closer than 10% so, it's better. Also, the physical size is about the same even though it can handle more wattage because of newer technologies.

I hope it fixes it.
 
I got a 5 amp variac from eBay couple of months ago for $39 shipped FedEx express. Received in 3 days straight from PRC. Can’t beat that!
 
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