Hi
I wanted to have time to take a look at my scrap AVO 8 MK5 before replying to you. It is ex PO TEL, it has a burn out track on one of the printed ribbon cables and has been partial dismantled (got at) internally, it appears to be all there apart from the battery compartment cover. Movement appears good.
I have measured only the 800K resistor with a Fluke 2000 DMM (6.5 Digits) and found it to be some 813K, some of the other resistors I cannot get easy access to so as to measure them. It looks like the “special” 800K resistors made for AVO all have a problem with age related drift, even when not used. If you really want 800K look on eBay, no shortage of 400K at 0.1% and in a voltage divider it is better to use two not one as this reduces the applied voltage across them, one of the main reasons resistors go high.
Now if only the 800K is in error by +13K the meter should on the 100VDC range read 0.65% low at full scale, you will be hard pushed to see this given that the best accuracy for a brand new AVO 8 Mk5 is 1% at full scale. Calculated using +13K as a percentage of 2Meg, the total divider chain resistance on the 100VDC range.
Your postings leave a number of unanswered questions, for example what is the accuracy of your applied test voltages, and what are/were the actual meter readings obtained from them?
In one of your postings you say that when measuring the total external resistance of the AVO 8 Mk5, on say the 100VDC range when it should be 2Meg, the resistance varies by which direction you rotate the switches, this points to a problem with the switches or the internal wiring, certainly not the fixed resistors.
The 100mv test points are on the polarity reverse switch, checking the accuracy of the meter movement is the first step, before anything else, all moving coil meters lose sensitivity with time due to the degradation of their magnetic field.
One of your photographs appears to show that the resistor PCB has been subjected to long term heat around the area of the 800K resistor, caused by the AC current transformer?
George G6HIG Dover in the UK.