Re-calibrating a AVO Model 8 Mk5 - Page 1
Forum section: electronics / repair

author: Specmaster
author_rank: Super Contributor
timestamp: June 03, 2020, 12:08:51 pm
content: Hello I'm trying to recalibrate another meter for my collection, its An Avo Model 8 Mk5 and the voltage ranges are reading which means that I'm looking for a source of replacement resistors for it. The manual just lists the values and the Avo part numbers and thats it,I suspect that these would be wire wound, maybe something like 5w each and 1% to 0.3% accuracy. They are the ones located on the top board in the photo, blue in colour and the only current info I have is taken from the service manual.Resistor Value and circuit ref. Avo Part number8M (R23) 6120-2816M (R22) 6120-280400K (R18) 6120-2754M (R21) 6120-279800K (R20) 6120-278Does anyone know where I can obtain these from or have any that they are willing to part with?

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: June 03, 2020, 05:50:44 pm
content: These are metal film resistors not wire wound and of various wattages. Have you tried to measure them as it is rather unlikely that all of them have gone bad (usually some signs of over heating for example). Most of them are not off the shelf values but could be built up from available values. That appears to be a 40M in the photo (unless the decimal point is not visible). Check around eBay and surplus parts on line, you might be able to find similar parts. I would eliminate other possibilities of failure before replacing these resistors, as dividers they are in series with the input, put the meter on the lowest voltage input range, if there is no voltage reading on the meter, look for a component in series with the input that has failed. You can check with another VOM for voltage and follow it through towards the meter circuit. The biggest resistor (physically) is probably 4 to 5 watts, the smaller ones progressively less wattage, such as 3, 2 or 1 watt for the smallest.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: June 03, 2020, 06:20:08 pm
content: Hi,Have you checked the 50uA and 100mV ranges?These will tell you the condition of the meter movement.From the service manual:On the other ranges there should be a voltage proportional to the meter deflection on the 100mV test points:For example if you put in 5V on the 10V range, there should be 50mV on these test points.I would check this before changing resistors.Regards,Jay_Diddy_B

author: Specmaster
author_rank: Super Contributor
timestamp: June 03, 2020, 10:16:47 pm
content: This is only applicable to the later version which is what I have, but I cannot see where the test point is located, I presume that this is accessible somewhere on the circuit board without having to dismantle anything?

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: June 04, 2020, 01:42:45 am
content: Hi,I would start by checking the AVO 8 on the 50uA DC range. This is pretty much a direct connection to the meter movement. If the 50uA range is bad it is could be the movement.Jay_Diddy_B

author: tautech
author_rank: Super Contributor
timestamp: June 05, 2020, 05:33:09 am
content: Have you checked the trimmer on switch position 12 as all the R values you've listed are series trimmed with it.Depending on meter version the trimmer is 15 or 22K.Manual attached.

author: Specmaster
author_rank: Super Contributor
timestamp: June 05, 2020, 08:01:24 am
content: Quote from: tautech on June 05, 2020, 05:33:09 amThe only trimmer on switch position 12 is the zero ohms adjuster pot for the x1 resistance range, the other 2 22K trimmers are the low and high resistance ranges which all measure on known value test resistors just fine.

author: tautech
author_rank: Super Contributor
timestamp: June 05, 2020, 08:26:08 am
content: Quote from: Specmaster on June 05, 2020, 08:01:24 amYeah OK sorry I got that way wrong.About the resistor ratings, there's a key top left on the schematic page with value/tolerance/Watts and how to interpret the values next to each resistor in the schematic.Eg R24 40M 0.5% 2WI have a AVO 8 MkII with a nackered scale that you can have the guts out of if they're any good to you.Dunno until I open it if it has the same values but somehow I doubt it as they only went to 2.5KV not 3KV like the Mk5.

author: Specmaster
author_rank: Super Contributor
timestamp: June 05, 2020, 09:20:10 pm
content: Quote from: Jay_Diddy_B on June 04, 2020, 01:42:45 am

author: Specmaster
author_rank: Super Contributor
timestamp: June 05, 2020, 09:25:58 pm
content: Quote from: tautech on June 05, 2020, 08:26:08 amThanks for the kind offer if I need it but the shipping costs would be more than the meters really worth TBH. I just concocted myself a quick 50uA supply and the meter reads OK so that clears the movement.

author: Jay_Diddy_B
author_rank: Super Contributor
timestamp: June 05, 2020, 11:02:54 pm
content: Hi,Since the meter movement is okay, how far off are the readings on the ranges that are bad?Have you checked the values the resistors with a DMM?The AVO is 20kper volt. If you start on the 3V range and measure the input resistance with a DMM it should read 60k.The 10V range should read 200k30V range 600k.You should be able to identify the bad resistor or resistors using this method.Regards,Jay_Diddy_B

author: Specmaster
author_rank: Super Contributor
timestamp: June 06, 2020, 06:39:01 pm
content: I had already done this and came up with some odd values, these values also changed depending on if you were switching up the range or down. I also compared them against another Avo with the same ranges but was a Model 9 Mk2 whose values were as predicted and the meter reads correctly.I have now gone and pulled the resistor board from the meter, I don't like those ribbon cables, horrid things, and just hoping that though go back again with no problemsAll the resistors tested and all seem to be in spec, the only one to was out was the 800K which was 810K so I'll replace that one.

author: drjazz
author_rank: Newbie
timestamp: June 06, 2020, 09:14:41 pm
content: If all DC voltages are reading high , on 10V range 5vdc measures about 7v then one of the current shunt resistors are open circuit.

author: Specmaster
author_rank: Super Contributor
timestamp: June 06, 2020, 09:32:10 pm
content: DC volts on 3, 10 and 30V ranges are OK, 100v and above are affected and also AC on same ranges. I have ordered a NOS identical resistor, even the same red colour as a replacement for the 800K so will see what happens when that is fitted.

author: Specmaster
author_rank: Super Contributor
timestamp: June 09, 2020, 05:25:45 pm
content: OK then, my NOS 800K identical resistors to the original ones arrived today, marked as 0.3%, looked as if they had just left the factory TBH. Tested them before installing only to be disappointed, 1 was @ 3.2% and the other was 1.2%, far worse the one already fitted.So now I still need the following resistors in order to bring the meter back into specification.6Meg 0.3% 0.5W,8Meg, 0.3% 0.5W,4Meg 0.3% 0.5W140K 0.3% 0.3W800K 0.3% 0.5WOr of course better % would be great, so if anyone knows where these can be obtained from, or has any of them, please let me know.

author: Specmaster
author_rank: Super Contributor
timestamp: June 21, 2020, 09:41:23 am
content: Quick update, traced the original manufacturers of these resistors and had a chat with them about replacements. Nothing doing, some were not being made any more and the 800K was a complete special and made in lots of 1,000 to order, err no thanks.They however suggest a stockist that may have some other required values and the only one that they could with was the 140K but with a MOQ of 5 and after the P&P and VAT was added it would have come to £42, so thanks but no thanks FFS, I only paid £15 for meterYesterday I revisited the project and did some calculations and the worst 2 resistors, 140K and 800K were also still in play on the other 3 upper voltage ranges so any errors on those first 2 would be multiplied on the higher voltage readings. These 2 were mission critical in that case, so set about trying to resolve them and, seeing as the values had drifted upwards, it meant that I could tack on some smaller wattage resistors in parallel to "trim them back into spec" and it works out that I need a 20M .25W 1% resistor to be shunted across the 141.2K to pull it back to 140K and likewise a 50M .25W 1% resistor shunted across the 811K to trim it to 800K again. These have been ordered and, will hopefully be here this week then we can fit them and see if the results are acceptable. Watch this space for further updates.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: June 21, 2020, 12:04:55 pm
content: HiI don't want to rain on you, but, the resistor rift with time and voltage will continue, always upward. Please take note of the maximum applied voltage rating for any resistors that you use.George G6HIG in Dover

author: Specmaster
author_rank: Super Contributor
timestamp: June 21, 2020, 12:14:48 pm
content: Yes, I understand this will continue now that it has started, but I hopeful of finding another meter with a stuffed movement etc in the future to become a potential donor of those resistors, fingers crossed. I'm aware that they can be found on eBay but currently they are going for stupid money, even as parts only. So for in a year I have gathered 3 Avo's for approx £15 each, a Model 8 MK4 and a Nato one that are perfect but have always wanted a MKV, so, when I got this one I was disappointed to note that the values on certain resistors had drifted that much.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: June 21, 2020, 12:52:58 pm
content: HiI have a scrap Mk5, don't know just what condition it is in. It is ex BT and it rattles with the case halves taped together. I know a number ex AVO/Megger employees, they would not give a Mk5 house room.George G6HIG in Dover

author: Specmaster
author_rank: Super Contributor
timestamp: June 21, 2020, 01:14:13 pm
content: Quote from: George Edmonds on June 21, 2020, 12:52:58 pmGeorge, I can fully understand their feelings. It is in real terms a step backwards for Avo but people were calling out for a redesign, something a bit modern and this was the result, it does look more upto date than previous models, but quality took a heavy hit in my view. It is both lighter overall and you see and feel where cost-cutting has taken place. Just the feel of the switches alone give the game away, in some positions it is possible to leave the switch midway between ranges which leaves both ranges connected at the same time and this might be a possible cause of the resistors drifting?How much would you be looking for to part with your Mk5, the resistors on it may be to spec?

author: George Edmonds
author_rank: Frequent Contributor
timestamp: June 22, 2020, 07:51:10 am
content: HiI 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.

author: Specmaster
author_rank: Super Contributor
timestamp: June 22, 2020, 09:30:00 am
content: HiYour 800K is worse than mine then which is 811K and also measured on DMM @ 6.5 digits and also cross-checked with 6 others to make sure I was not being lied to by the meter, they all agreed at 811K.I have checked all the resistors in the meter and the others, not already identified in the photo are within their specs. When in the 100V range, which is when the 800K is switched, it is in series with the 600K (600Vac), 400K (30Vdc), 140K (10Vdc) and 58K (3Vdc) making 1.998M which is switched into the meter circuit when in the 100VDC position.I have pulled apart the rest of the meter as far as I can get it, without risking breaking it and as far as I can see everything seems to be fine. I have double-checked all solder joints and reflowed them, given all the switch contacts a good cleaning with switch cleaner etc and so far these resistors are the only smoking gun I've come across.There has been no excessive heat within the meter as this new photo of the PCB shows, any discolouration you saw on the previous photo is purely down to the light reflections and shadows on the board. The board will be given a good scrub all over with IPA before being put back in the meter to remove any grease and other contaminates.The 3V and 10VDC ranges read OK, as does the resistance range, the manual also says something about errors on particular voltage ranges are caused by the preceding resistors.

author: Specmaster
author_rank: Super Contributor
timestamp: June 24, 2020, 02:37:46 pm
content: Update time, trimmed the first resistors (800K and 140K), still reading too high. If I give it a 100Vdc it goes off the scale and with a DMM (Hi-Z) across the test point and 100mV selected manually, a 100V input gives 111mV. If I use 100V on the 300Vdc range it gives me around 42mVSo the problem is definitely within the voltage divider chain somewhere. It's as if resistors are altering their values dependent on the voltage applied.The meter is perfectly linear I have checked it using my decade box on resistance and on each range the meter responds 100% correctly, 10's, 100's, 1,000's, 10,000's and Megohms, as I add another on the decade box, the meter reads it 100% dead on.Stripped the switching PCB out and checked the switch contacts, on both sides, not an issue there, checked the bottom switch contacts, on the largest of the 2 ribbon cables, all OK, checked the ribbon cable, all OK. I have to go out for a while and when I get back I'm going to give the ribbon cable a good clean with IPA.The printed shunt resistor does not read as per drawing but its with in an ohm. The printing does have a few cuts in it here and there but I think they are intentional where they fine tune it on final calibration. Apart from that I'm stumped

author: Specmaster
author_rank: Super Contributor
timestamp: June 24, 2020, 11:01:11 pm
content: Ok then, after more playing around I have come to the conclusion that all the resistors on the resistor board need to be evicted and replaced. I'm not too bothered about the 4M for the 3,000Vac or the 40M for 3,000Vdc as I'm not very likely to need to measure anything that high with it anyway. So its either going to be a donor meter or new resistors, which has already been discovered is a no-go.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: June 25, 2020, 10:12:20 am
content: HiNow we have some real numbers to work from what I can tell you is that you appear to be barking up the wrong tree with the series resistors.If for example the 800K resistor goes high the AVO 8 will read low not high on the 100VDC range. In my opinion the problem is NOT in the voltage divider chain. Yes resistors do change resistance with applied voltage across them, but. in some sixty years I have only ever seen an increase in resistance, never a decrease, an increase in resistance will have exactly the opposite effect to what you are seeing.All of what I am saying is based on the assumption that this AVO 8 Mk5 has not been contaminated internally with a conductive substance.Given that the AVO 8 is 20K Ohm per volt its meter movement must be 50uA full scale, no ifs no buts. The basic meter movement is 37.5uA full scale in addition 12.5uA is passed in the parallel printed shunt resistor array making a total of 50uA. This is may not very clear from the circuit diagram, possibly there is something wrong with either the basic meter movement sensitivity (unlikely as moving coil meters lose sensitivity with time not gain it as in your case) or the printed shunt resistors and its switching has a problem.I believe that you have a known good AVO, put both in series on their 50uA ranges and pass 50uA through them both. Simply done with a 1.5VDC battery and some appropriate resistors totalling around approximately 33K or slightly more for a new battery.My guess is that when the reference AVO shows 50uA full scale your faulty one will be off scale.Now 0.75 of the 50uA passes through the moving coil meter and 0.25 of the 50uA passes though the printed shunt. The meter is trimmed to be exactly 2667R by R27 (a small white object) which is mounted on the Reverse Polarity switch (the moving coil meter and R27 MUST be kept together as a calibrated pair). The total resistance of the printed shunt is 8001R which allows 0.25 of the 50uA to pass through it. The entire printed shunt is put in parallel with the moving coil meter via switch contacts 1 plus 4 and 2 plus 3.Yes the printed shunt resistor is trimmed by what appear to be cuts in it. If you do try to make resistance measurements around the moving coil meter plus printed shunt do preferably not do it with an auto ranging meter as this can cause the moving coil meter to pulse and negate the readings. Just to help you I measured the DC resistance between the plus and minus terminals on my AVO 8 Mk5 set for 50uA DC full scale. I got a measured 2012.5 Ohms against a calculated 2002 Ohms, this measurement was done using a 2 wire resistance setup without any great care being taken to null out the lead and contact resistance.I also checked it on the 30VDC range and found it to be reading about 3% low, not surprised at this, like wise it was about 3% low on the 100VDC range, again not surprisedUNTIL YOU HAVE THE AVO 8 Mk5 50uA DC BASIC RANGE CORRECT YOU ARE WASTING YOUR TIME TRYING TO CORRECT ANY OTHER POSSIBLE FAULT AND POSSIBLY DOING DAMAGE TO THE REST OF THE METER IN THE PROCESS.Good luck with it. Use a lump hammer only as a last resort.George G6HIG Dover in the UK.

author: Specmaster
author_rank: Super Contributor
timestamp: June 25, 2020, 11:54:01 am
content: Hi George,Thanks for your observations and great help and advice. Let me flesh out my results a bit more.I soldered two pin headers into the holes on the switch PCB next to the test point for the meter (marked on the drawing as 100mV test point) and then connected a 6.5 digit meter (manual switcher so no auto ranging errors) and I applied 3v from a bench power supply to the 3Vdc position. I also connected a very high Gohm impedance DMM (Fluke 8842A) to prevent any impact on the OPV of the Avo, The fluke verified the 3V input and the Avo displayed 3V FSD, and the DMM on the test point displayed 100mA. That gave me confidence in the meter, and also confirmed my earlier reports that the 3V range did read correctly.Page 10 of the service manual also states that if the resistors R16 to R24 are low, the instrument will read high, but if the R's are high, the instrument will read low and I must have had a brain fart or something that day and took it the other way roundSo I removed the switch PCB to gain access to the bottom switch contacts and the ribbon cable terminations which were slightly oxidised. Cleaned then and reflowed the terminations and then checked the readings, via the switch terminations and ribbon cable back to the various range drop resistors, and they are low as follows:-R16 58K 58.2KR17 140K 139KR18 400K 398KR19 600K 565KR20 800K 795KR21 4M 3.62MR22 6M 5.1MR23 8M 6.4MR24 40M 41MI can only assume that when I first read the values of the resistors on the resistor PCB, the leads were a bit contaminated but now they have been cleaned up and hence the lower readings, which is now entirely in keeping with page 10 of the manual, i.e. resistors low results in the instrument reading higher., which is perfectly logical.Now as these values are specifically made up against large orders, have effectively 3 options open to me:-1- Source a donor meter and hope that the resistors are OK.2- Try to make up the equivalent values if space permits, from available values.3- Unhook 1 leg of each resistor and connect a series resistor to restore them to near enough their originals. There is just enough space to allow a maximum of 1 series resistor per value. The original resistors may well continue to drift slightly as they age even further, and then again maybe they won't. As long as the meter provides a pretty good indication of the voltage it will be fine. Where accuracy is required, I use DMM's.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: June 26, 2020, 12:31:36 pm
content: HiNow having read your last posting there are more things that I do not understand, for instance in sixty years I have never cleaned some resistors and their mounting PCB and had the resistance go down, up yes, as the unwanted effectively in parallel resistors resulting from the contamination are cleaned away.The resistor value table that you provided also does not make sense, for instance R22 (600VDC) which should be 6M you say is 5.1M and R23 (1000VDC) which should be 8M you say is 6.4M, in my experience this is simply too large a change to be correct AND its simply in the wrong direction. Have you lifted one end of each resistor and then independently measured it? It may take some time for the high value resistors to fully recover from the thermal effects being lifted.I have taken my AVO 8 Mk5 which is of 1973 vintage, with still has intact factory seals, and measured with a Keithley 2000 the resistance between the plus and minus meter terminals on the DC ranges. Again these are just quick measurements and subject to some small Keithley 2000 lack of nulling errors.I got the following resistances:-3VDC 60.2K. (58K of R16 plus 2K of movement resistance, within my measuring error)10VDC 200K.30VDC 604K.100VDC 2.04M. (again within the 2% error I am seeing at full scale)300VDC 6.3M.600VDC 11.97M.1000VDC 19.7M.3000VDC 60.9M.This is entirely consistent with the observed full scale DC Voltage errors of my AVO 8 Mk5. For example on the 100VDC range the terminal to terminal resistance should be 2.00M, it is actually 2.04M which is about 2% high which will give a 2% low full scale error reading, which it does.Just a thought, has your AVO 8 Mk5 ever had a leaking D cell in it? The gold “long lasting” ones that were sold as none leaking actually leak very badly with time, when they do you get an Alkaline corrosive and conductive film covering deposited which is almost impossible to remove unless done chemically, washing with IPA will not remove it. Is it just a coincidence that the most effected resistors are located very close the D cell holder, any sign of corrosion on the D cell contacts? .I still have a scrap AVO 8 Mk5, maybe time to pull the series resistor board and check all of the resistors.I still keep a number of moving coil meters just to prevent having to do constant mental maths on measured voltages when repairing late valve and early transistor kit. My go to meter is a Simpson 260, not an AVO.Keep up the fight. Keep in mind in my experience, on their own resistors never go down in resistance with age, only up.George G6HIG Dover in the UK.

author: Specmaster
author_rank: Super Contributor
timestamp: June 26, 2020, 03:10:18 pm
content: Hi GeorgeAfter I posted last time, in response to your post I began to doubt my findings, so I stripped out resistor board and checked the readings again, I still got the same sort of results. So I lifted one end of each and low and behold, they were spot on. I then removed the AC transformer and popped the board into a ultrasonic cleaner and gave it a couple of goes. Once it had all dried out nicely, I resoldered the lifted ends back on the board and checked values, again spot on. Then I reattached the transformer and ribbon cables, checked values again, and they were back to the original valuesPrior to the above I had stripped the meter down and disconnected everything as in the manual and removed the top switch board. Ribbon cables all look to be in good condition, the top switch board seems to be OK, contacts look nice and clean, no pitting etc and good contact is made with the moving contacts. Bottom contacts also inspected and the ribbon cable connections checked and all are good, a real head scratcher this one.Today, following your post, I rechecked the total resistance per range via the plus and minus terminals on the DC ranges and using my Brymen BM867 (goes upto 50M) so I had to use a cheap manual switcher to read the 3,000V reading as it goes upto just over 60M. Any way I obtained the following readings.DC Volts Resistance.3 60.3K10 201.5K30 666K100 2.06M300 5.03M600 7.78M1000 10.8M3000 58.8MI did notice that the readings were creeping upwards if I left the meters connected, just like a capacitor being slowly charged?I then got out my other 2 Model 8's, not to sure which marks they are but both have the 3,000 range as opposed to more normal 2,500v One is the normal domestic specification and the other is a NATO issue one and weighs a bleeding ton. These are the reading obtained from them in the same fashion as the Mk5.Normal one3 30.1K10 200.8K30 603K100 2M300 6M600 11.99M1000 19.88M3000 40.5MNATO one3 60.3K10 200.3K30 603K100 2M300 5.9M600 Not supported on this meter1000 19.4M3000 39.98MNeither of these two meters exhibit the slow upwards creepage that the Mk5 does and the capacitors on the Mk5 are minimal values and should not be in circuit as the AC side is switched out of circuit? Could the ribbon cable be acting as a capacitor I wonder?To answer your other question about has a D cell leaked at some point, I think the answer to that is yes some evidence of slight corrosion on the plus terminal but zero sign of anything inside the meter or the battery compartment itself.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: June 27, 2020, 10:13:23 am
content: HiFollowing your latest posting I am now almost 100% convinced that you have a chemically contaminated AVO 8 Mk5. The only other possibility is that excessive applied voltage has caused some localised tracking damage. It is of course virtually impossible to be absolutely certain without seeing it and doing some high voltage insulation tests on it.The final clincher is when you said “the readings were creeping upwards if the meter was left connected, just like a capacitor being slowly charged”. There are no capacitors in a AVO 8 Mk5 large enough to show this behaviour and as you correctly say they are on the AC side and not in circuit when making DC measurements.The inter track capacitance of the ribbon cables will be in the order of pF’s and can be totally discounted but their insulation resistance cannot. If you can see the above creeping effect using a DMM with an applied voltage of around one volt it must be serious. If push comes to shove all of the printed ribbon cables can be replaced with simple point to point wiring.My best guess at this time is that the 1.5V D cell has possibly caused this problem. If a NiCad had been used in place of a normal dry cell these can produce a highly conductive/corrosive out gassing vapour when they reach the end of their life. The damage/contamination caused by this vapour can be extremely difficult to see visibly. My suspicion is that you are not seeing a capacitor charging but an electrochemical reaction taking place, don’t forget that with your AVO 8 Mk3 set for over 30VDC full scale the resistances involved are over 1M Ohm so a very little contamination can/will have large consequences.Had I have been working on the AVO myself and once seen the series resistors changed downward in value I would have first done a Litmus paper test for the presence of an acidic or alkali contamination and then with an AVO BM403 started doing a though series of insulation tests on it with the moving coil meter isolated and finally with the printed ribbon cables removed so as to isolate various sections of the meter. The AVO BM403 can do insulation tests at three different voltages up to 1KV.There may well be other small problems with your AVO 8 Mk5, BUT, the main problem as shown by your resistance measurements must be addressed and resolved first. It may be worth considering that the “contamination” has been absorbed and cannot be totally by the PCB material of the series resistor deck and the range switches.George G6HIG in Dover UK.

author: Specmaster
author_rank: Super Contributor
timestamp: June 27, 2020, 12:35:31 pm
content: HiYes, I'm inclined to agree with your thoughts as I've exhausted all the other possibilities.As an educated guess, where do you think the problem is, the PCB board the resistors are located on, the ribbon cables or the switch board?I suspect the reason why the resistance measurements are working OK is that the range selector and most of the control is done with the right switch and the DC volts is on the left switch, meaning the battery voltage, although it maybe 15V on the higher range, but that the signal path of that is away from the contaminated area?What would you recommend cleaning everything with now to try to effect a cure?

author: Specmaster
author_rank: Super Contributor
timestamp: June 27, 2020, 01:00:46 pm
content: I could remove certain resistors totally from the board, say R21 through to R25, leaving just 3,10,30 and 100V connected and see if there is any différances

author: Specmaster
author_rank: Super Contributor
timestamp: June 27, 2020, 05:41:39 pm
content: OK then, I removed completely from the board, all the resistors from 100v tap upwards, but there was still a high resistance connection on the 300V and above positions. This did not rule out the ribbon cable, so I traced the connections in the ribbon and isolated the 300 and above connections to the resistor board and the readings went open. This proves then your theory that there had a chemical contamination. Now can I scrub the board with some solution or other, or do I need another donor board?The readings for 3, 10, 30, and 100V ranges now reads as3V 66K10V 207K30V 635K100V 2M

author: George Edmonds
author_rank: Frequent Contributor
timestamp: June 27, 2020, 08:26:48 pm
content: HiMy own gut feeling would be to remove the shunt and series resistor boards, this will effectively cut the problem in half, then do insulation tests on the range switches and printed ribbon cables. With the switches and ribbon cables first do insulation tests between adjacent switch contacts.At this point it would also be a simple matter to measure the overall resistance of the series resistance chain between the cold end of R16 and the hot end of R23 and check to overall resistance for stability. Given that R25 and R24 effectively have their hot ends floating I would not include them in any initial tests so as to allow your better quality DMM to be used. The total resistance you should be seeing is 19,998M which if you add the 2K moving coil meter resistance will give you 20M which is exactly what a an AVO 8 Mk5 should be on the 1KVDC range. Not the 10.8M you reported when you made the terminal to terminal resistance measurements.At this point I saw your more recent posting, fully agree that a spare board is probably the only way forward as I doubt if the contamination can ever be fully removed from the original board.I have removed the series resistance chain board from the scrap AVO 8 Mk5 that I have, washed it quickly with IPA. It now needs some time to recover fully, but my Keithley 2000 at turn on reads 20.043M from the cold end of R16 to the hot end of R24. Most importantly the resistance is slowly drifting slightly upwards, not downwards, which is what I would expect from a newly washed in IPA board with almost forty year old resistors. At these resistance values even a fingerprint can produce serious errors.What I don’t wish you to do is to start off with yet another unknown board. I will come back to you later on Sunday with what I find.George G6HIG in Dover UK.

author: Specmaster
author_rank: Super Contributor
timestamp: June 27, 2020, 10:10:19 pm
content: Hi,That sounds like it could be good news, I'll check on the total chain value tomorrow and post the details here, but it will only be a summation of the total values as I have removed some of them and I have put the meter away for the night now.Edit:I just had to measure the entire chain from end to end, thats R16 cold end to the hot end of R24 and it is 20.076M, plus the 2k would 20.078M, just 0.37% error which would be a good result?

author: George Edmonds
author_rank: Frequent Contributor
timestamp: June 28, 2020, 02:20:53 pm
content: HiOvernight test, R16 to R24 is 20.06MGeorge G6HIG

author: Specmaster
author_rank: Super Contributor
timestamp: June 28, 2020, 04:10:26 pm
content: Hi,There is something very odd going on here, when I measured the chain last night, I measured from R16 to R21 and then measured R21, R22 and R23 and added them onto the R16 to R21 because I had removed those last 3 resistors from the board. This morning I was unable to find any evidence that there was a problem with the board, no matter what or where I probed, I could not any get any sign of leakage, zero.So I resoldered those resistors in place and measured it again and from the cold end of R16 to the hot end of R23 (8M) I get 20.17M. Add on the 2K meter circuit, totals 20.19M near enough to be within the quoted tolerance of 1%The ribbon cable is just about shot to pieces now with the desoldering and resoldering taking its toll on it, so I have ordered 1 metre of 28awg multicolour ribbon cable to replace the plastic rubbish, and wire this point to point, doubling up on the same colour where the blue plastic ribbon has a couple of thicker tracks.This cable will hopefully be with me on Wednesday, as should my insulation tester, so I'll know more then.I'm wondering if the old ribbon cable was to blame all along and if so, the damage only seemed to manifest itself when the meter was all reassembled at which time the ribbon cable is pinched quite tightly between the switch board and the meter reverse and cutout switches.

author: 2N3055
author_rank: Super Contributor
timestamp: June 28, 2020, 04:46:49 pm
content: Just a note, old phenolic paper boards can absorb acids/bases/salts and they then become very humidity dependent...I'm not saying that is problem, but something to keep in mind. Also on one device I found a flat cable that was basically strands of wire caught between two parts of PAPER adhesive tape.. Battery leak made it a defacto resistor pack...

author: Specmaster
author_rank: Super Contributor
timestamp: June 28, 2020, 05:29:05 pm
content: Yeh, thanks, that is the problem with those old type of boards, but I would have thought that if the paper had absorbed something that the board itself might have started to delaminate, warp or bubble but it shows none of these signs. It has been a complete mystery from the start. I got from a retired electrical engineer who said it had in a drawer for years and that was all he could tell me about other than it worked last time he used it, but that might not have been on DCV. He said that he was only selling it because his Son was joking with him saying that when the time came, he would not have a clue about what the various bits of his tools were or how they worked etc. So that prompted him to dispose of them now while they may of some value to someone.The larger of the two ribbon cables is all connected to the range switches is mounted in such a fashion that it makes it impossible to really inspect the contacts 100% unless the complete ribbon is removed which would completely destroy the cable anyway. Maybe when the new ribbon cable arrives, I might discover the reason for the strange happenings. This is my third Avo 8 meter, the other 2 are properly built to a specification rather than a price.The mark 5 meter onwards was not Avo's finest period of history coming along as it did, after Thorn acquired them and set about as most companies do, milking the maximum profit from the products

author: Specmaster
author_rank: Super Contributor
timestamp: June 29, 2020, 11:41:43 am
content: Update time, as you will have learnt from previous posts, I am going to rip out this god awful plastic ribbon cable and instead wiring it with some flexible multicoloured ribbon cable which will be more suitable I feel as it will not tend to crack and will generally make it far easier to remove boards for inspection etc should it ever become necessary in the future.To make this process easy, it is advisable to trace out just where the connections on the existing ribbon actually go to reduce the chances of cross wiring as not all the terminations are clearly identified on the various switches and boards.Here are some preparations I have I done so far towards this goal. Later I will remove the top switch board to gain access to the bottom layer of switches and the old ribbon, which is not numbered or identified as to what each terminal is doing and I will post these pics, both before I remove the cable and after for clarity to aid the next person who decides that they want to this.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: June 29, 2020, 12:01:59 pm
content: HiThe complete layout for the main printed ribbon cable is in the AVO 8 Mk5 manual.George G6HIG in Dover

author: Specmaster
author_rank: Super Contributor
timestamp: June 29, 2020, 01:23:15 pm
content: HiThis is absolutely correct but only the switch positions are marked (only on the layout drawing, and nothing for the resistor board end) the actual printed ribbon does not carry any printing on that I can see. I aim to take a detailed photo of the chassis, showing all the switch terminals and then label them and post that as belt and braces for anyone to use if they so wish.I just know that I'm going to have to destroy that printed ribbon when it comes to getting off the switch terminals as my desoldering gun cannot handle the spade terminals on those switches.

author: Specmaster
author_rank: Super Contributor
timestamp: June 29, 2020, 04:51:05 pm
content: Eureka, I'm pretty sure I've found the smoking gun on this son of a bitch, conductive gunk, hidden and trapped between the range switches and the ghastly printed ribbon cable, the latter was totally fucked in the process of unsoldering it from the switches, just I predicted, so it will be point to point wiring now on this meter.Now to bathe and scrub these switches to banish this gunk once and for all, fingers crossed the rest works OK after this.

author: Specmaster
author_rank: Super Contributor
timestamp: June 30, 2020, 12:24:39 am
content: Update on Avo time. Switch contact rings popped into ultrasonic cleaner on a long cycle and then left to air dry, switch mounting wells also given a thorough cleaning with IPA and air dried. Tested contact ring and there is no sign of any residue of the conductive gunk and no matter where I pop a can find nothing conductive that isn't meant to be.Bottom section of switches reassembled and now labelled ready for the new ribbon cable to be delivered and installed.

author: Specmaster
author_rank: Super Contributor
timestamp: June 30, 2020, 10:11:19 am
content: Gahthis is evil, did a quick check this morning and there are still a couple of contact positions that actually have a 30M connection between themBack into the ultrasonic cleaner they go again. If the meter thinks its going to beat me, it'd better think again, I'm going to whip its candy arse into line yet

author: George Edmonds
author_rank: Frequent Contributor
timestamp: June 30, 2020, 10:45:58 am
content: HiI finally took my scrap AVO 8 Mk5 apart, the already damaged printed ribbon cable was completely destroyed in the process, as expected. The switches were very clean and showed no sign of any contamination.I cannot see how the contamination got into yours unless it was done during manufacture.George G6HIG in Dover UK

author: Specmaster
author_rank: Super Contributor
timestamp: June 30, 2020, 11:11:22 am
content: Yes, with care you can remove the printed ribbon from the resistor board a number of times, but the switches, no, that is certainly not possible. A piece of poor design IMO. The manual makes me laugh, it very clearly states that soldering the ribbon, to use only just enough heat and minimal solder to make the connection. On the switch there was a massive amount of solder. This meter still had the makers seals over the screws so if the gunk was the residue flux from that solder, then yes it was done at manufacture and has taken this long to become conductive. On the other hand, if a D cell had leaked, then the switch with the problem is directly below the D cells location, but no sign of leakage inside the case.We will never know just how it came to be there, but it is a real PITA.

author: Specmaster
author_rank: Super Contributor
timestamp: June 30, 2020, 04:43:41 pm
content: @George EdmondsHave you stripped your scrap AVO 8 Mk5 right down to the contact rings that the printed ribbon connected to? If yes, could you measure the resistance between the contacts on the DC range switch for me? Thats the switch ring on the right when the meter is on its face with the switches nearest you, starting position 1 as on the pic below (switch indicates AC) and progress round in (from the back clockwise), 10A, 1A etc and see what resistance figures you get on your contacts?I think my contacts have been buggered internally and are tracking in the moulding and the resistance readings I have are dramatically affected by temperature, the higher the temp, the lower the resistance.Contacts Value1 & 2 30M2 & 3 44M3 & 4 O/L4 & 5 41M5 & 6 O/L6 & 7 O/L7 & 8 37M8 & 9 24M9 & 10 23M10 & 11 34M11 & 12 O/L12 & 13 50M13 & 14 O/L14 & 15 44M15 & 16 50M16 & 17 O/L17 & 18 O/L18 & 1 O/LConsidering that contact 11 is 3V, 12 is 10V, 13 is 30V, 14 is 100V, 15 is 300V, 16 is 600V and 17 is 1KV then those resistances on mine are not very helpful.

author: George Edmonds
author_rank: Frequent Contributor
timestamp: June 30, 2020, 07:18:58 pm
content: HiWill get back to you as soon as I am able to make the measurementsGeorge G6HIG in Dover UK.

author: Specmaster
author_rank: Super Contributor
timestamp: July 01, 2020, 08:30:14 am
content: Measured the same contact ring this morning and the results are as follows:-Yesterday TodayContacts Value1 & 2 30m 55M2 & 3 44M 111M3 & 4 O/L 142M4 & 5 41M 62M5 & 6 O/L 500M6 & 7 O/L 500M7 & 8 37M 55M8 & 9 24M 45M9 & 10 23M 38M10 & 11 34M 125M11 & 12 O/L 250M12 & 13 50M 500M13 & 14 O/L 125M14 & 15 44M 77M15 & 16 50M 111M16 & 17 O/L 67M17 & 18 O/L 200M18 & 1 O/L 200MBoth sets of readings were taken using a Brymen BM867s, yesterdays were taken in straight ohms range, which tops out at approx 51M. Today I noticed many more readings were O/L therefore in excess of 51M so I switched into nanosiemens and measured again and then converted the readings into Mohms.Now this has set me thinking, does this mean that these contact rings have become sort of porous, taking in moisture when the humidity is a certain point, therefore meaning that almost any meter reading taken on this AVO will never be trustworthy. Or is this something that I may have caused, unwittingly by using a switch cleaning lubricant, in this case Servisol Super 10, which I have used for years with no problems. Has this one occasion been one where the cleaner may have reacted with excess flux/and/or the printed ribbon and made the cleaner conductive evenIn which case I have fffed up my own meter?In view of the above results, I think I now need to repeat the same test on the AC switch contacts with the meter set nS mode. I do that and post the results shortly.

author: Specmaster
author_rank: Super Contributor
timestamp: July 01, 2020, 11:00:46 am
content: Here are the measurements for the AC switch ringContacts Value1 & 2 125M2 & 3 1250M3 & 4 52M4 & 5 S/C5 & 6 62M6 & 7 270M7 & 8 500M8 & 9 1250M9 & 10 200M10 & 11 200M11 & 12 1250M12 & 13 530M13 & 14 263M14 & 15 500M15 & 16 750M16 & 17 200M17 & 18 17M18 & 1 66M

author: George Edmonds
author_rank: Frequent Contributor
timestamp: July 01, 2020, 06:00:45 pm
content: HIi have strapped every other switch contacts together and then done an insulation resistance test at 1Kv between the two strapped contacts and the contact in the centre. Insulation resistance found to be >999M at 1KV on all occasions.Looks like your switches are well past their sell by date.George G6HIG in Dover UK

author: Specmaster
author_rank: Super Contributor
timestamp: July 03, 2020, 02:22:38 pm
content: Update on the meter...., all done and working.Transpired I believe to be entirely my own fault, when I acquire something new I always tear it down for inspection and on this occasion, I took it upon myself to clean the zero ohm pots and switches with contact cleaner while in there with some Servisol Super 10 cleaner, which I have used for years with no problems. This it turned to be a big mistake, I think maybe the formula for it might have changed, but now it seems to have adopted the characteristics of WD40, in as much as, it will penetrate into any microscopic hole or crack what ever, and it will remain there, regardless how many times you scrub, immerse and even ultrasonic clean it, the Servisol remains and is conductive. Hence why my DC volts were not being read correctly because of this conductivity between some switch sections on the contact rings was lowering the effective resistance and because this contact cleaner was reading into the Mega Ohms, it made the DC Volts read progressively higher as I increased the range. This was due to the parallel resistance that was added, lowering the effective range resistance and therefore volts read higher.The cure was to remove all wiring from the contact rings and pop them into an oven set at 250 degrees on a layer of cooking foil, contacts down, for a few minutes to dry out the cleaner, and now I have resistance of 3,000M ohms or better between all contacts.Printed plastic ribbon cable replaced with conventional ribbon, switches all rewired and connected up and the ranges tested and all seems to be within the factory prescribed specs for this meter, all ranges tested except for the 3KV as I don't have anything that high to test it with.Many thanks to the various forum members, in particular George Edmonds, for all advice and help given on this project (George Edmonds, I sent you a PM, did you receive it?)Here are some shots of its progress, I discovered after doing the voltage tests, the needle zero position was a tad to the left of the correct position, so the accuracy of the value shown is actually higher than it looks.