My philosophy of restoration is to produce an instrument as it might have been found in a lab some time after purchase, but at the peak of its "career". That would mean in excellent cosmetic condition, working correctly and in calibration, but not devoid of the minor wear, scratches and dings that equipment picks up in normal use.
My goal is to remove accumulated grime, stickers, ID plates, pen and pencil marks, etc., leaving the instrument in "as delivered" condition. Internally, most instruments are in pretty good shape, but electrical repairs must be made as needed, loose hardware tightened, and a proper calibration performed.
The first step is an overall inspection and evaluation. Note any cosmetic problems and make a plan before beginning any work. It helps to state your goals clearly at the beginning. Decide if major electrical repairs are needed, or just an external cosmetic tune-up and calibration.
Very early GR equipment used hard rubber panels and construction techniques of the time. My interests are in the later equipment, so I don't have much information pertaining to the very early instruments. Antique wireless organizations are a better source of general techniques from this era. Early GR equipment was painted with a special crackle finish, then the markings were engraved through the paint, exposing the lighter aluminum underneath. The method was durable, but relies on the integrity of the painted finish. The painting process relied on multiple coats of paint with special additives, probably followed by curing in carbon monoxide rich gas ovens. Though we can't duplicate the original process today, there are modern materials that can give the same results. That would be an avenue for making a completely new panel, but I'm not aware of any useful repair method for the original finish.
The paint recipe appears to have changed slightly over the years, and the instruments were first produced in black, then dark gray, then a smooth light gray finish. Note that the smooth light gray finish is not engraved, but has the markings silk-screened just like modern instruments. I've also seen a few dark gray instruments with a smooth non-crackle finish and silk-screening.
It's debatable which versions are most desirable. I prefer the later electrical revisions, so am partial to the dark gray instruments as opposed to the black. Being newer, they tend to be in a bit better condition. On the other hand a black faced instrument in good condition can be a thing of beauty. The "A" or "B" suffix version are almost always preferable to the first release. The modern light gray instruments, some of which are still in production, command a higher price, and are certainly desirable. They don't however, elicit the same feeling of tradition for me that the early instruments do.
The paint, regardless of color, is subject to damage by almost all solvents, and will dull or dissolve if cleaned with alcohol, xylene, toluol, or other aggressive means. DO NOT USE SOLVENTS. Even detergents and spray cleaners can have a detrimental effect, so test on an inconspicuous area first. Cleaners with ammonia typically do cause damage.
Many stickers can be removed by soaking in hot water. I usually apply a wet piece of paper towel repeatedly until the sticker softens. This can take overnight, so don't be impatient. I generally remove all stickers with the exception of original GR stickers applied at the time of manufacture. All calibration stickers, including the GR ones are removed, though I typically reapply those to a piece of paper and keep them with the instrument file. Make note of the dates and company data on any stickers you remove. Do not remove any internal stickers or markings, as these were usually applied by GR personnel.
Some instruments may have ID plates riveted to the panel. If you're a true historian, you'll probably keep them. I find them distracting and typically remove them. The screws or push-in rivets can be reinserted and painted, or the holes left as they are. If you have trouble with the rivets, grind the heads off, remove the plate, then pull out the rivet with locking pliers.
The next level of attack is the use of WD-40. This stuff is excellent at removing goo that hasn't hardened, and doesn't seem to damage the finish in any way.
Some stickers may have to be sliced off, and that takes just the right tool and a special knack. The trick is to use a perfectly flat sharp blade, that's been honed to have no burrs. A single edged razor blade works, but the utmost care is required to avoid scratching or surfacing the finish. I prefer a large square ended X-acto knife blade in a large style handle. Test the blade on a piece of Plexiglas. If it scratches, continue stoning the edge. If you can get a corner of the sticker to lift, try to peel it the rest of the way. Apply heat from a hot air gun to get things moving. Heat is very effective and the panels can take a substantial amount if you stay away from the plastic parts.
In spite of your best efforts there will be stickers that leave a hard brown residue that's almost impossible to remove. It's a judgement call, but I generally violate my own advice above and resort to xylene. It attacks the paint slowly, so it can be used in brief and local efforts if removed quickly. Use a Q-tip. If you see color on the Q-tip, stop immediately. It will dull the finish, but that can be dealt with by a coat of wax or polishing with Armor-All.
Warning- the light gray finish is very durable, but the silk-screened lettering is not. Solvents will remove the lettering completely! DO NOT CLEAN LIGHT GRAY INSTRUMENTS WITH ANY SOLVENTS! WD-40 appears to be OK.
I do very little touchup painting, but will fill chips and major scratches with an appropriate mixture of Testors enamel for plastic models.
Remove knobs to clean underneath them. Clean the knobs with a toothbrush and mild soap. Note that the round gray knobs on the newer instruments simply pull off. There is an aluminum insert underneath that fastens to the shaft with a set screw. Use Armor-All on clear plastic covers and windows, but be sure to polish all residue off with a soft cloth to avoid a gummy mess. Meters are subject to movement from static charges, so those windows may need to be treated with an anti static compound if the zero is unstable.
Some instruments, particularly the acoustic measuring systems and the Digibridges, used a gray foam for padding in various locations. It invariably turns to a sticky gray goo. Scrape it out and replace it with a similar looking modern foam.
Once the instrument is clean, I begin electrical checkout and calibration. When a piece of GR equipment, especially bridges, fails to work, the problem is usually obvious. Try to get a manual, as the documentation is excellent and some of the range and function switching can be hard to follow. I have to admit that I own instruments where I paid more for the hard-to-get manual than I did for the instrument itself. The situation today is much different; almost all manuals are available for free download from BAMA or IET. Though I cannot provide copies, feel free to email any questions on most GR bridges and equipment. I can usually provide at least some advice. Almost all problems I've had have been mechanical and visible with a careful inspection. The only true "electrical" failures I've had have been capacitors (not surprising considering their age) and burned out resistors caused by users applying voltages outside the acceptable ranges. I did have one particularly horrendous problem with an impedance comparator having a bad solder joint, but that's the exception to the rule.
Hopefully you have a working bridge to test components from the non-working one. The capacitors in GR equipment hold up remarkable well. I strongly recommend leaving all working capacitors alone. Check all electrolytics and replace any that are more than 20% below their marked value. Also replace any with an excessive dissipation factor, though that's somewhat of a judgement call. Check manufacturers specs for similar modern parts to get an idea of what's reasonable. There is one other "gotcha", especially with tube equipment. Any leakage into the grid of a tube will shift the operating point and cause mysterious problems. The paper capacitor s often used in this location are famous for leakage and have to be checked carefully. You will not see this problem with a capacitance and dissipation measurement. You may have to actually put the cap under its normal working voltage and measure the current, though often a simple DVM resistance measurement will reveal a problem. Tube operating voltages will also tell the tale if you know what the correct values should be. Another clue is excessive distortion in an amplifier stage. I replace these with high quality polyester caps of suitable voltage rating if necessary. The tubes themselves are run very conservatively and I've rarely had a bad tube.
One component unique to GR is the famous compensated potentiometer. As the wiper rotates around the pot, a cam follows an adjustable ramp to lead or lag the wiper slightly. In this way the linearity can be adjusted to a fine degree. These usually don't need adjustment unless someone has fooled with it. If you have a range of good standards, it's not too difficult to bring the unit back into specification. Be sure the cam follower is lubricated with a light grease and works correctly. There are conflicting recommendations about lubrication of the windings on these pots. If perfectly clean and dry, there will be some wear. If lubricated with a bit of petroleum jelly, they work much better. The problem is that when the lubricant gets dirty, the wear is probably worse than if the pot were dry. A bit of tuner cleaner with silicone may be the lesser evil. I do lubricate the large brass rotary switch contacts found in most bridges with petroleum jelly after cleaning out the hardened remains of the original grease.
You may find that the calibration instructions for some of the instruments are a bit cryptic. I sometimes wonder if this was intentional- if you weren't smart enough to read between the lines, you were more apt to take advantage of the GR calibration services. The 716-C bridge is a good example. To really do this right you need to fabricate an aluminum cover that includes holes to insert the adjustment tools. The procedure is also sufficiently cryptic as to leave some of the high frequency adjustments in doubt. The 1608-A is a similar case in that a certain amount of padding may be necessary, but whether or not and where to do so is a bit vague. Most of the other instruments are much clearer in this regard.
I might add that most of my GR equipment is in use on a weekly if not daily basis. Though old, the results from these bridges are extremely accurate and often provide insight that modern handheld testers don't. Certainly you can buy a modern digital LCR meter that will do everything these bridges do, and far more, but not at a price the hobbyist can typically afford.
C. Hoffman
last edit October 11, 2011