Doctors don't do well diagnosing themselves, Lawyers don't do well representing themselves, Architects do poorly at construction and Engineers do not troubleshoot well. I too, was talking to the screen while going down this bizarre expedition. When we started measuring film capacitors rated >100 V I started smiling (a wry, condescending smile, I'm afraid). Toward the end of nearly an hour, I was expecting an analysis of the fiber "orientation" of the fiberglass PCB.
If engineers actually spent several years fixing the the things they design, they might find the weak points affecting durability and long life. It took over 40 years for people to figure out that if they did not use thick gold for low current switches & contacts, they must use sealed relays or electronic cross-points.
The problems encountered with old test equipment (& other commercial / medical electronics): 1)
CONTACTS (connectors,
switches, relays, potentiometers) 2)
SOLDER CONNECTIONS 3)
LOOSE GROUNDS, i.e "grounding screws" from terminals or PCB to ground 4)
Electrolytic Capacitors: the higher the voltage and larger the capacitance, the more suspect. This is based on 100s of thousands of repairs. No troubleshooting manual or schematic is needed for checking these. Years of experience is. Common sense of what is the most likely cause also helps.
The switches and relays used in this type of equipment can be several different types, but all have some factors common. Low current contacts (aka "signal" / "dry" / "logic level") have either brass, alloy, silver or gold plating on the contact points. Most are silver. If the visible parts of the contact are gray or black, the silver is oxidized. Silver Oxide is an insulator. Certain contact cleaners will clean these well. The "grease" you may find is not there primarily as a mechanical lubricant, but to keep oxygen away from the silver. After cleaning with a spray liquid cleaner, the "grease" is washed away, ensuring future contact problems. To make a lasting repair, use an electric grade of silicone grease on both the moving and stationary contacts. If there are still issues, disassemble the contacts, clean them (IPA works) and look at them under well lit magnification. If it has the silver or gold worn away by mechanical friction from use, your only option is to replace the switch or relay (or even pot). This is usually impossible. Another alternative is to add and wire-in relays or semiconductor switches.
This is the major issue with used test equipment from eBay. Even if it works flawlessly, or is "not right", "out of calibration", or the dreaded "intermittent",
clean all contacts (switches, relays, connectors, pots)
first! CAIG makes some great products for this
http://store.caig.com/s.nl/sc.2/category.292/.f http://store.caig.com/s.nl/sc.2/category.188/.f. Don't use any cleaner which is flammable, because these may harm / swell the plastics holding the contacts. When buying test equipment without being able to have it inspected by someone who knows what to look for prior to purchase, assume that it will need 'fixin' and that parts may not be available and that whatever you pay for the item and shipping will be lost.
OFAE