What breaks on a Fisher 500C (and 800C), and what to check first
The 500C and 800C are the tube receivers most customers actually own, and their 7591 output stage runs closer to the tube's published limit than any other mainstream design of the era. That is why they arrive at a bench blowing tubes, and it is also why the fix is documented and repeatable rather than a mystery. The rows below come from rebuild threads and technician write-ups; each carries the confidence the knowledge base gives it, and none of them pretends to be factory documentation.
The big one: it eats 7591 output tubes
The pattern is an output tube red-plating and failing, bias that refuses to set, and volume that dies after a few minutes. The cause is arithmetic: the 330 K control-grid leak resistors on the 7591s exceed the 300 K fixed-bias maximum in the RCA tube manual, the stock carbon-composition parts drift upward with age (over 360 K has been measured), line voltage is higher now than when the design was drawn, and current-production 7591s tolerate all of this less than the originals did. One forum tech calls the stock value right on the ragged edge.
The documented fix: drop the grid leaks to 220 K (one rebuild used 200 K for Electro-Harmonix tubes), double the coupling capacitors from 0.047 uF to 0.1 uF so the RC time constant stays put, and add 10 ohm cathode resistors at pin 5 so current can be read per tube. Roughly 38 mA idle is the target.
And if a channel still eats 7591s after a retube and the grid resistor change: the stock design has no per-tube bias adjustment, so add one. A documented 800-B rebuild replaced R126, the 5.6 K bias feed, with a 5 K trim pot to reach minus 20 to minus 22 V on pin 6, and added 100 ohm screen stability resistors.
The smaller, certain failures
The power switch dies
The switch is under-designed for turn-on inrush, and the failure repeats across rebuild threads. The documented fixes are a CL-70 inrush limiter or a relay.
The bias supply drifts or dies
Selenium rectifier aging plus tired 1000 uF bias electrolytics. The documented fix replaces the selenium stack with a modern bridge and rebuilds the bias electrolytics. This one is not optional on a set you intend to keep: an uncontrolled bias supply is how the output tube story in the first section starts.
FM is deaf and AM selectivity is poor
The IF and discriminator cans were made for Fisher by Automatic, and some contain a failed mica sandwich inside the can. Single documented rebuild, so confidence is low: the fix is replacing or rebuilding the affected can.
No power at all, and it is not the fuse
A burned power transformer was the presenting fault on one documented rebuild. The same rebuild found 3 W 470 ohm resistors running about 130 degrees Fahrenheit off the board and relocated them. Confirm the transformer before quoting the job.
FM still sounds wrong after a recap
Out-of-spec ceramic capacitors in the signal path, C35, C36, C38 and C40, plus badly misaligned RF slugs. Replace the ceramics, then align. On some 800C units the service manual swaps the V14 and V15 designations relative to the chassis, so do not assume the manual matches your serial number.
Questions people ask
Why does my 500C keep blowing output tubes?
Grid resistors over the tube manual's limit, drifting carbon comps, and today's line voltage, all pressing on a 7591 that was already near its published limit. The resistor, coupling cap and cathode resistor fix above is the documented cure.
Why does the power switch fail?
Turn-on inrush through an under-designed switch. An inrush limiter or a relay is the documented fix, and either is fitted while the set is open anyway.
Is a 500C worth rebuilding?
The failure list above is long but bounded, and every documented fix is a known quantity. That is the honest case for the 500C: the problems are on the record.
Ask about yours
AudiophileGPT answers from this same knowledge base, with your own room and gear list in front of it. Three questions right now, no account.
Sources
Audiokarma rebuild threads (fisher-500c-with-expanded-ibba-complete, fisher-500c-rebuild-advice-needed, the 800C 7591 thread), an Audio Asylum tube thread, a philcoradio.com rebuild log, and a fmtuners.groups.io discussion. Confidence on each row is carried in the knowledge base; the 7591 grid-leak finding is repeated across more than one independent source.