Dial glass cleaning:
Receiver dial glass can get dusty over time. Additionally, the heat generated by the cooling air towards drawn through the unit by convection can leave deposits on the dial glass of any tuner or receiver. More true in a non-smoke free home.
In the case of a Fisher, it's best to remove the dial glass while the front panel is off. If the unit has not been touched since new, there is insulating foam surrounding three sides of the dial glass and the dial glass is held on by four screws at the corners that hold retaining arms.
To remove the dial glass, there are typically two things you have to do:
1. Break the seal of the insulating foam surrounds. The foam has been compressed and heated repeatedly and often disintegrates or rolls off completely. All we need to do is to break the seals where they hold the four dial glass retaining arms in place.
2. Unscrew the screws holding the retaining arms, but without letting the dial glass drop on the workbench.
Step 1: Use a razor knife or exacto blade to break the seals around the retaining arms at all four corners.
Step 2: Unscrew all four screws that hold the retaining arms, with one screw holding one retaining arm in place at each corner of the dial glass. The bottom ones do not need to be completely removed. They may help keep the dial glass from falling out if left in loosely.
Step 3: Remove the dial glass. This can take some time. My experience is that it's best to get the top of the dial glass to swing out and then lift it up and out. It may be stuck. Take enough time to loosen it without cracking or chipping it. Avoid using a screwdriver. Use something non-metal right at the top right and left corners to wedge it out.
Work on it only under daylight or very bright light. Do not hurry or you may repent at leisure. You've been warned.
Some words of warning here to people who want to clean dial glasses that aren't Fisher dial glasses: The artwork is likely to be on the back of the glass, not the side facing out to the world. Some artwork is very delicate, is likely to be some decades old and cannot tolerate any rubbing.
***It is better to err on the side of caution and clean around lettering with a q-tip dipped in something than to discover after the fact that you've been too aggressive and cleaned off or moved your lettering.***
For that, I'd only wipe the back of the dial glass on the parts where there's no lettering with a glass-cleaner dampened paper towel or rag. I wouldn't even let the rest of the rag trail over the artwork if it's delicate. If you must test, pick an area of the artwork that you can live without if it gets wiped off and try wiping that. In general, if the glass is clean in the front and around the lettering in the back, that's a heck of a lot better than not cleaned at all.
Edited 4/10/2013:
Here's a thread about cleaning dial glass that is worth the time to read and consider:
http://www.audiokarma.org/forums/showthread.php?p=6685578#post6685578
For Fishers, where the lettering is painted on.
Before going any further, repeat after me, "clean the glass and blot the art . . . clean the glass and blot the art".
Catchy; isn't it?
Fisher artwork is on the back of the dial glass. Nicotine seems to weaken the lettering's hold on the glass.
What I do is spray glass cleaner on the front and wipe off with a couple of paper towels. Then wipe again with a dry towel. For the back, I spray both sides where there is no lettering. I do not spray the center where the artwork is. Then I blot the center with the already dampened paper towel where the artwork is with slight, and I do mean slight, lateral movement in the areas where there isn't a lot of artwork. Others use a Qtip and avoid the lettering altogether. Then wipe the sides. Rewipe the sides with a dry towel. The artwork isn't super fragile, but you do have to be careful.
Next, wipe the backplate with an unused paint brush, being careful not to dislodge or bend the tuning indicator on the tuning string.
After the dial glass is fully dry, go over the remaining compressed foam with a razor knife or something comparable. The point is not to remove the compressed foam fully, though you can remove it with no consequences. The remaining foam actually acts as a pretty good template for where to put replacement light blocking stripping.
What I do depends on the condition of the dial glass. Just be sure that there's a surface there that a self-adhesive strip can attach to long enough for you to put the front panel back on.
To put the dial glass back in place, lightly fasten the two screws for the bottom with the retaining arms, unless you did not remove them when you lifted the dial glass out. Tension them just enough so they won't fall out and you can slip the dial glass back in and have them hold it in place.
Next attach the top screws and retaining arms. Tension all four arms reasonably tight, but remember that too tight could crack the glass.
For your final step, you can wait until putting the front panel on.
The Fishers have two dial lamps on the sides of the dial glass called festoons. If you know yours are Ok and the lights work, don't touch them. The glue holding the metal to the glass dried out years ago and if you touch them, roll them, or move them, it's extremely likely that your next step will be replacing them.
There are many differing versions of festoons for different receivers and tuners, or different lengths and diameters. 500B and 800B likely both had thinner diameter festoons than the 500C, 800C and 400. Tuner festoons are sometimes shorter.
I've successfully used LED replacements and have been happy with how evenly they disburse light.
For festoon light sources, see this thread:
http://www.audiokarma.org/forums/showthread.php?t=409667
When you are ready to put the faceplate on, line three sides of the dial glass where the compressed foam was with new self-adhesive stripping. I've successfully used D-Profile rubber tape, open cell narrow gap tape (what you see in the second figure) and closed cell sponge tape. You just don't want it so thick that it won't compress enough to push the faceplate back and tighten the two nuts at each end of the faceplate.