Its easy to buy an Avometer 8 on ebay for not much money. Avometer 8 s probably best for general use. It has a sensitivity of 20,000 ohms/volt DC so it wont draw too much current for general use. An Avometer 7 is similar but draws more current hece is more suitable for electricians work than electronics. It ought to be accurate to 1 to 2% or better. I have two Avometer 8 mark III s and the difference between my Fluke 87 is very small :
Avometer 8 - the first one with no model number and terminals black on all four
Avometer 8 MK II - some updates but still used copper oxide rectifier
Avometer 8 MK iii - updated with germanium diodes and resistance range fuses and some wire wound resistors replaced by more modern types
Avometer 8 Mk iV - looks like previous models but printed circuit board based
Avometer 8 Mk V - complete redesign of movement plus flexible printed circuit
Avometer 8 Mk Vi - internal re-design of V to remove printed circuit weakness
Avometer 9 - versions of 8 models ii and iii and iv with international scale and dial markings
Pan climatic - rugged versions to withstand high humidity
It is rare to find an Avometer entirely fault free. Also you need the 15V special battery in place for the times 100 ohms range . Most sellers are selling untested but its worth asking for at least a basic test with a battery to see if it works at all. Most are repairable with a little (well quite a lot actually) time and patience. You will need to put aside several hours to do any of these repairs. You need good mechanical and soldering skills and a clean and organised place to work with good light. Most of the advice here applies the Avometer 8 models I to III since I don't have experience of the later models..
Reads too high - possible open circuit on shunt boards. Use a DMM to check the resistance between the probes on every setting and compare with specs. Will identify which resistor is blown which needs repair.
Reads too low: Possible magnetisation loss which can be corrected by opening the meter and, loosening the magnetic shunt on top of the movement block and moving it. Be careful if using a standard screwdriver that it doesn't get drawn into the magnet
Cutout too sensitive or not sensitive enough - its vital this works so as a start test with 9v battery shorted to 100ma DC meter setting first. The meter should trip instantly. If not Adjustment is by rotating the cut-out mechanism which is a tricky job and I suggest you study the manual before doing it (models I,II and III). I don't think that the cut out is adjustable on the model V onwards
Movement sticking - you going to have to take the movement out to see if there are iron particles or other debris fouling the magnet gap. Do not allow metallic screwdriver to touch the magnet (it will demagnetise) and instead using some cardboard to stop the screwdriver slipping onto the magnet. Its very easy to destroy the movement once out of the meter. Suggest research internet before doing this. One problem is that the movement doesn't return to zero if you connect a reverse voltage. Again you have to take the movement out and look at the tiny levers that are part of the trip mechanism. If these are not free to move they can cause this problem and also difficulty setting the trip. Its really for advanced repairers only but you can remove the small metal piece on the front of the meter by undoing the small screw with a plastic screwdriver. Lift this away and carefully remove the two tiny levers of their pivots. Note that the tiny levers are handed and look slightly different. The one that has a more curved profile along the edge must be on the left (i.e. to limit the reverse travel to the left of zero). If this is on the wrong side the movement can stick on reverse as described above. On my Avometer 9 there was contamination by oil or grease which interfered with these levers so clean everything up and polish the removed parts. Don't use liquids because if you contaminate the hairspring (which is very close) you will ruin the meter. That cleared up a sticking movement on my otherwise A1 Avometer 9 and also resulted in more consistent operation of the trip
Lettering Worn- amply small amount of household white paint with fine brush onto each symbol being careful not to get it onto the crackle finish area. Let dry for 1 hour then with clean rag systematically wipe away excess which will miraculously leave nice while lettering behind.
Glass dirty- remove movement and clean glass from inside using a sft and clean cloth with isopropyl alcohol
Intermittent readings- possible dry joint in meter needs to be located and maybe polish the leaf contacts with metal polish (not emery paper). Also check for burned out contacts on the trip which may need to be replaced. If you open the meter check for grease in early models - it was there to collect debris so replace and clean as required. There is another fault for intermittent readings but only consider if you have checked every single joint or contact elsewhere. If you tap the front of the meter around or on the Rev movement switch the reading might fluctuate. Basically it means the Rev switch contacts are not good. You need to carefully remove the movement first then you will see the two spring contacts. Each of the tiny screws need to be removed (two per side). Do each side separately so you have a reference when you refit them and remember where the wires go that attach to the movement. Bend the contact on each spring downwards so it puts pressure on the fixed contact in the meter. Its a bit fiddly getting these back in position as the spring fits in a small gap where the fixed contact is.
Trouble zeroing divide by 100 ohms range- obvious is to replace battery. Some of the D type cells do not fit well in the Avo because the negative end can be dished. A small blob of solder properly applied to the battery.will sort this out. I had problems with energiser so I do not know about other brands. Please do not not modify the meter terminals and spoil its originality. Another cause is that the fuse ant its holder are corroded so try removing the fuse and cleaning it up together with its holder. If you still cant get it to work take the meter apart and remove the movement. polish the leaf spring contacts with metal polish. Dont use abrasive and spoil the coating (probably NiCd). Next take apart he divide by 100 potentiometer,Take a note of the order and orientation of all the washers and fitting because its tricky to get them back. There is a hidden spring contact that needs cleaning that wipes the copper part of the adjustment knob assembly. This needs cleaning with fine emery paper since its the most likely cause of extra resistance. Also with fine emery clean the contact surface of the 5 ohm wire wound resistor. as a final measure re-make the solder joint at either end of the wire wound resistor. Use some flux paste to avoid adding more solder to the joint. Put the who lot together and hey presto you will easily zeo the divide by 100 range
Watch out for cracked case on Avometer iv. For avometer repair having a spare or donor meter is invaluable for both spares and testing out for dismantling on the meter you want to keep. Odd value resitors can be made up quite accurately by combinations of series and parallel resistors which you can surround in heat shrink to make a reasonably neat job.
Difficult other Problems Sometimes when there is no obvious component failure a reading is still low or intermittent. For example on AC the meter is hugely out for no obvious reason. One cause maybe the cam operated leaf switches which can be either dirty or fail to open or close completely. This is perhaps one of the most difficult to solve. If you have a known good meter you can compare the positions. Sometimes the leaf springs can be bent using a small set of long nosed pliers but they are very difficult to get out and need care. Another issue is where it becomes almost impossible to set the cut out. It is either too sensitive and pops out under the slightest disturbance or if once set you test it again after a few days it wont trip. I think this is caused by too much down pressure in the mechanism which causes the jewelled pawl mechanism to somehow seize. A possible solution is to adjust the 4 spring contacts (two inner for low current which break first and an outer higher current pair that break second. You need to bend them upwards slightly. This reduces the force holding the cut out mechanism in and makes it easier to set with more predictable operation. Be careful that the inner pair of contacts break before the outer (See the Avo service manual).
How to Avoid Causing Damage:
Things not to do include touching the magnets with a screwdriver (unless non-magnetic) since it will deplete magnet strength. Also keep dust and dirt away if you open the meter you must remove anything you find inside. Don't blow on the hairsprings other than with light force (e.g. photo blower), don't use oil anywhere near the movement which will contaminate and ruin the hairsprings. If you remove the movement failure to trip the cut-out beforehand could cause the actuating wire to be damaged. Don't leave batteries in the meter for long periods for obvious reasons. Don't over tighten screws especially those around the case and use the right sized screwdrivers to avoid damage and preserve appearance. Don't put oil or grease on the switch mechanisms since it could compromise insulation - leave it as the factory intended. If testing start with the higher ranges first. Be aware also these meters do not include the same degree of protection against mis-use as more modern devices. They don't include standard shrouded connectors.
Overall an immaculate Avometer 8 (or Avometer 9) is an rather nice and useful thing to own. The older ones (ii and III) come from an era of hand soldered and hardwired circuits but, with a little effort, can be restored to a condition close to when new over 50 years ago. Oh and BTW one you bought one and got it sorted you will want to give a home to at least one more and perhaps venture out into other models!