I don't fix these myself, but I have seen it done as I helped Hirsch to install the Fisher Doctor kit in his first Fisher 400 using my modded receiver as a reference model.
Don't be put off by a little rust. Generally on the 400 rust will form on the covers of the power supply transformer and output transformer. It can usually be removed with a rag and some metal preservative/rust remover such as BreakFree CLP (used on gun metal and sold in sporting goods departments), perhaps with some really fine 0000 steel wool if it is really bad. An old tooth brush works well for getting into small spaces with a drop or two of the BreakFree.
The chassis might not shine, but it is unlikely to rust itself unless the unit was really neglected for a long time. If the transformers are intact and not shorted or otherwise damaged, the unit is worth fixing.
I don't think the Fisher receivers or amplifiers are it is a good first time project, only because there are qute a few parts that need to be replaced (if you install all of the parts provided in the Fisher Doctor's Resoration Pak) and their location is not always obvious or easy to get to.
Don't attempt to plug in the unit unless you have a variac unit and are prepared to slowly (over a period of 12 or 24 hours) bring the unit up to operating voltage.
To do this:
Pull the output tubes (assuming you have all four) but label them as to which socket location they came from. Connect some loading resistors to the speaker terminals or set the speaker switch on the front to the OFF position (this engages internal resistors).
With the unit unplugged, turn it over and remove the bottom plate. Look for obvious signs of trouble like a resistor that is cracked or a coupling cap that looks like that guy's chest in Alien. If everything looks undamaged, proceed. If you see any signs of distress on any of these parts, stop.
Reinstall the bottom plate and turn the unit right side up again. Check the line fuse and if it is not blown, proceed. If the fuse is blown replace it only with the correct value 3 amp rated slow-blow fuse.
Set your variac to about 20 volts and plug in the unit (turn the power switch on with the volume control and leave the volume control at the lowest point and all other controls centered and switches in the off position, including the speaker switch unless you have connected a pair of speakers to the output terminals. Select the FM stereo selector switch position, as this will send power to the FM circuits as well as the amplifier.
Bring the unit up slowly from about 20 volts to just over 50 volts over several hours. If the unit does not emit smoke or blow the line fuse, continue. If the fuse blows or you see smoke or smell something buring, stop.
If the unit passes the first test, power it off, install the output tubes, and continue to slowly raise the voltage from 50 volts up to 85 - 90 volts. Watch for malfunctioning in the form of overheating output tubes (glowing cherry red (not normal) while their sibling tubes are just barely glowing (normal) as the tube starts to operate (somewhere around 90 volts). Although you should generally proceed slowly with the variac, between about 85 volts and 110 volts pick up the pace. You risk blowing the line fuse with the variac voltage set in this range when the voltage is below about 90 volts and the output tubes are warmed up and starting to emit. The tubes will start to operate at about 100-105 volts. Continue to raise the voltage to 110 volts and stop, go no higher.
Ideally you should test the voltages on the pins of the output tubes but of course you probably won't have the correct tool , a vacuum tube volt meter (VTVM) to do this safely.
If the unit makes it this far, plug in a source you don't care about frying (the chassis may have stray DC voltages on it that could damage a solid state piece like a CD player, so use a cassette walkman or a cheap PCDP, or try the FM radio (you'll need to hook up an antenna.
You might get a hum (loud or soft, or sound from only one channel, but at least you will know whether you are starting with a unit that is basically working or not.
In order of importance:
Replace the selenium rectifier with a silicon diode full-wave bridge rectifier
Replace the coupling caps on each of the output tubes
Install the (4) 1/4 watt resistors between the cathode connection at pin 3 and the chassis ground on each output tube socket.
Here is a link to some photos of this modification:
http://www.antiqueradio.org/fish01.htm
(Note: This site shows some underside photos of the Fisher 400 and Fisher 800 rceievers and also some close ups of the installation of the above mentioned resistors and coupling caps (Note: The closeup photos in the link are for a Fisher 800 which uses 7591 tubes not the 7868 tubes used in the 400. The pin configuration is different, but the installation is essentially the same.)
Replace the other high-voltage caps specified and/or provided in the Fisher Doctor kit.
Replace the resistors in the FM tuner circuit, and the FM coupling capacitor (If you intend to use the FM radio)
Always operate your Fisher 400 at 110-115 volts, for coolest operating temperature and longest life of the tubes and other components.
Also, install a small fan at the rear of the unit to cool the output tubes and ventilate the rest of the chassis. (Note: The 7868 tubes used in the Fisher 400 are expensive, out of production, and while not impossible to obtain now, replacements won't always be readily available. If you have a Fisher 400, be sure you have at least one spare set of (4) 7868 tubes.) The fan will significantly lower the operating temperature of these tubes and and the entire unit, the tubes life will be doubled or even tripled.
BTW, my Fisher 400, purchased from an Ebay seller in Ann Arbor Michigan, was modded by a local techician in Michigan who was supplied with the Fisher Doctor's Restoration Pak and instructions (this includes the schematic diagram). Armed with the schematic and the instructions and the parts list, any good technician should be able to install the Fisher Doctor kits in under 4 hours. Depending on what the hourly labor rate is where you live, this could cost perhaps $300, well worth it to know it was done properly and safely.
I believe that Chizzler had his Fisher 400 modded with the help of his brother (an electronics engineer) and his brother successfully installed all of the mods in the Fisher Doc kit, excluding the FM radio mods, in just a few hours.
It's not a good project to learn (or relearn) how to solder components, but it is not difficult and with patience and a steady hand you could do it yourself provided you can read a schematic diagram and can solder.
