Fisher 500C troubleshooting

[mholz]

After hearing some surprisingly loud pops coming from my Fisher 500C, I cut the power, opened up the chassis and found that 3 of the four cathode safety resistors installed on the power tubes had blown.

Since both channels are affected, I'm assuming this isn't a case of a tube going south (the tubes are original), but something in the power supply.

Any tips on what to check first? Maybe the cap can, which is hasn't been replaced?

Thanks for any help you can offer.

 

[jackinnj]

Was this the first time you had powered it up?

Jack

 

[Tom Bavis]

My bet: bias supply cap or rectifier. And the UNtoasted cathode resistor may be open as well.

 

[mholz]

No, I've been enjoying it for the past six months or so. I got it from a guy who did some standard component updates and upgrades, including replacing the selenium rectifier. The cap can looks untouched, so I figured that might be the first thing that would go.

I'll check the bias supply cap, though. Thanks for the tip!

 

[KaDe]

What was the selenium rectifier replaced with ?
With a solid state diode bridge you will get way too high B+ :
- Too high voltage for the caps and the tubes to fail as well -

 

[TubeHead Johnny]

Also could be those 120K plate resistors located in the tone circuit. I would definitely replace those safety resistors first.

~~~John~~~

 

[mojoworkin3]

Hey guys,

I have a 500c as well and I am getting no output from one channel, but there is no visible signs of any problem. I have 7591a tubes in one channel and tried to use some hammond 7591 tubes in the other as I thought they are compatible. The hammonds on power up were hotter than the other tubes so I shut it down. Swapped the good tubes in the other channel and now no output and they seems hotter than they were in the other sockets. Any thoughts on this problem?? What to look for? This was my father's 500c and it is good shape. I hope I have no output trans problem. Please help. thanks. mojo