setting the cutout on an old avo8 mk9 - Page 1
Forum section: electronics / beginners

author: m3vuv
author_rank: Super Contributor
timestamp: January 27, 2022, 04:18:50 am
content: Hi all,i have an old avo8 that i need to refix the meter glass on,this means removing the movement,but this will screw up the overload cutout,grom memory this needs to trip at approx 12x whatever range its set on,so if i have a psu with 10v at 30 amp output am i right in thinking i need a 12 ohm resistor to limit the current to 12 amps?,just to set the trip up.tia

author: m3vuv
author_rank: Super Contributor
timestamp: January 27, 2022, 06:02:35 am
content: am i correct in thinking a 12 ohm resistor will allow 12 amps of current to flow at 12v?,or am i having a brain fart?

author: MK14
author_rank: Super Contributor
timestamp: January 27, 2022, 06:23:33 am
content: Incorrect.https://ohmslawcalculator.com/ohms-law-calculator

author: m3vuv
author_rank: Super Contributor
timestamp: January 27, 2022, 06:33:05 am
content: so after looking at that i need 1 ohm for 12a at 12v?,bear in mind this it a test to accelerate the meter needle for a split second ,the calcs say 144 watts power so i guess i can get away with my 30a psu set on 12v thru a 1 ohm 100 watt resistor with the meter on the 1 amp range should work?

author: MK14
author_rank: Super Contributor
timestamp: January 27, 2022, 06:44:22 am
content: Yes, it should be ok for a very short period. But, to dissipate 100 watts, most modern resistors, need heatsinking via their metal tabs, otherwise they have much lower ratings, such as 25 watts (see datasheet).Unless it is a huge type of resistor, such as a very old type, which usually didn't have metal tabs.

author: m3vuv
author_rank: Super Contributor
timestamp: January 27, 2022, 06:51:03 am
content: It should take no longer than 0.25 seconds to test,the resistor i have is an aluminium wirewound one thats gold anodised and ribbed,i just want to set it without instantly having the movement coil going chernobyle!.

author: MK14
author_rank: Super Contributor
timestamp: January 27, 2022, 06:59:10 am
content: Yes, that was what I meant by metal tab (my description wasn't very good at all), but I didn't know what type you had.A datasheet (not necessarily any connection to yours), seems to say it can take 5 seconds at 10x overload, so 0.25 sec doesn't sound too bad.If it is a very cheap Chinese ebay sourced or similar, I wouldn't trust the advertised ratings. I think I've heard the Chinese ones can explode (I think it was mentioned in an electronics video, I can't remember which video), if overloaded (maybe it didn't even have to be overloaded), so please take care.

author: Gyro
author_rank: Super Contributor
timestamp: January 27, 2022, 10:13:39 am
content: Err... it's a mechanical trip to protect the movement itself - you can use a voltage range, no reason to use a high current one.I found that you can set the trip finer than x10 (not x12 btw). It's possible to get it down to x3 - x5 in the forward direction. It will be significantly higher for reverse polarity as you don't have the advantage of the needle inertial as it hits the trip lever. The setting is a compromise between nuisance trips (when you put the meter down heavily for example) and best protection.P.S. Don't let steel screwdrivers touch the magnet (the screwdriver will be strongly attracted) as you will reduce it's flux, possibly to the point where it can't be adjusted. Also be careful not to bend the trip pin as you reinstall the movement - make sure that the mechanism is already in the tripped state. Also make sure the jewel on the trip lever in intact.EDIT: I'm confused, on meters as late as the Mk9, the cutout is integral to the meter movement and comes out as one piece - Actually I don't think they even went up to Mk9 (?).

author: MK14
author_rank: Super Contributor
timestamp: February 03, 2022, 05:45:28 pm
content: Quote from: Gyro on January 27, 2022, 10:13:39 amI didn't see this, because it was probably a late edit, so I got no notifications.https://en.wikipedia.org/wiki/AvometerSeems to mention Avo 9's. Also see here:EDIT: My turn to edit, now. I recognize that icon, I think the youtuber, is/was a member of this forum.

author: Gyro
author_rank: Super Contributor
timestamp: February 03, 2022, 06:22:54 pm
content: Oh sure, there is an AVO 9 - I've got one! The OP's thread title refers to an AVO 8 Mk9 though.I think the last AVO 8 was the Mk5, at which stage it was a cheap stand-alone movement in a plastic case with the cutout integrated. A shadow of its former self.

author: MK14
author_rank: Super Contributor
timestamp: February 03, 2022, 06:56:02 pm
content: Quote from: Gyro on February 03, 2022, 06:22:54 pmSorry, it was READING comprehension failure on my part.I know, for quite a long time, RS sold a (very expensive), AVO, I'm not sure of the exact model number or mk number, but it was approximately an AVO 8, and had a rather shiny, very smart and modern appearance. Despite being moving coil, at a time when digital meters, were common place. They seemed to be for these 'old fuddy duddies', at the time. Who were still holding out, and wanted to use a real meter, not those digital nonsense ones, as Dave would possibly say.At the time those meters (AVO) were current, they were very, very good meters. I suspect/understand. I've used them, and been impressed. In my perception, AVO were the Flukes of the earlier decades. Especially when valves(tubes) and/or Germanium transistors were common-place.

author: Gyro
author_rank: Super Contributor
timestamp: February 03, 2022, 07:31:31 pm
content: Easily done!The ones that RS sold were almost certainly AVO 8 Mk5s. We had one per engineer at Racal in the early '80s, I don't particularly remember any DMMs around the place, but it was a mainly RF lab. Yes, they were really expensive. One of the main useful features, even today is the low burden on AC current ranges (the DC aren't bad either) due to the use of an internal current transformer. [Edit: The 3kV range too, with much lower circuit loading than a 10M input DMM]The Mk5s didn't seem particularly robust compared to the earlier models, I remember quite a few experienced engineers wrapping the needle without the cutout saving them. The centre magnets had a habit of coming loose too! They were flex-print, PCB current shunts and PCB switches inside too, rather than being hand wired. An easy way to visually remember them is the horizontal row of 4 oval buttons below the dial (Rev MC, Trip reset etc), usually with the pink rubber dust sleeves split and hanging out underneath them!I am by no means an early AVO fanboy though - yes, they were very robust and repairable (you could even get individual coils, pointers, pivots etc for the movements) but they're a bugger to work on if you need to get to anything in the depths, loads of identically coloured exact length wires to unsolder!The front panel on an AVO 9 tends to look more modern than its age due to the use of international symbols rather than text - that didn't happen on the AVO 8 until the Mk5 iirc.Peter Vis's site is an excellent reference...https://www.petervis.com/avo-meters/avo-meters.htmlRichards Radio too...http://www.richardsradios.co.uk/avometersdata.html

author: MK14
author_rank: Super Contributor
timestamp: February 03, 2022, 08:14:31 pm
content: Thanks for the links. That 1923 AVO (from your links), shown here:Looks cute, very vintage, and (not trying to be cruel here, honest!), I'm convinced, it would look like an old valve (tube) radio, to many totally-non-electronics people.Even when digital multimeters, were around. I think I remember, sometimes, with some types of signals. Such as a 0V to 10V one, somewhat randomly changing, 2 or 3 times a second. Old, non-digital or non-storage scopes, wouldn't show the signal too well. Neither would early digital handheld multimeters.But the analogue movement meters, such as most AVO models, would allow one to see the patterns, every 2 or 3 seconds, within the 0V to 10V signal, of interest.To me, the small handheld AVOs, are more collectible/cute, compared to the big boxy, AVO 8 (and similar) models. EDITS: Resized too big picture, and removed 2nd one.

author: Gyro
author_rank: Super Contributor
timestamp: February 03, 2022, 08:51:38 pm
content: The AVO 7 and 8 Brail models struck me as a very nice touch for blind or partially sighted engineers, but the thought of working on 1950s era electrics (the AVO 7s had 1mA movements) and electronics would probably be seen as a health and safety nightmare today. Presumably many would have been used in more controlled environments like factory equipment alignment. I don't know.The GPO / British Telecom AVOs are a nice size and relatively robust if you want something to use on the bench. The 12D is probably the nicest (several revisions) but there are others too. Prices on ebay seem to vary wildly.Ouch, sorry. Those links came out big.

author: MK14
author_rank: Super Contributor
timestamp: February 03, 2022, 09:33:27 pm
content: Quote from: Gyro on February 03, 2022, 08:51:38 pmWOW!A Brail model, introduced in 1951. That's amazing!These days, many (me included), would never think an old mechanical analogue movement meter, from 1951 onwards, would be able to be read in Brail. Someone may think it would need a significant development team, powerful (MCU) computing abilities. Especially to handle the reading to spoken voice conversion. And/or complicated electro-mechanical workings, to create the (what looks like) complicated bump patterns, depending on the voltage level. Also, doing it using just a milliamp, or so.Very good point about the safety aspects. I think (I'm not 100% sure, by any means), there were a lot of people, involved in electronics, in the 1950s. Repair shop round every street corner. Home users changing their own worn out valves (tubes), in Radios and TVs. People going round with carts, full of type A and type B (rusty memories, of what I was told about the old days) batteries. Which were lead acid and had been recharged. One for the high voltage (high tension), the other (B ?), for the 6.3V filaments. I could easily be wrong about the technical terms, A & B etc, as I said, rusty memories of what I was told.Possibly, one type (H.T. ??), didn't need so frequent replacement and was a large dry battery, perhaps 90V, whereas the filament batteries needed weekly replacement with charged ones).As you said. It was probably to give work to partially sighted or blind people, on a low voltage production line. Checking for open/closed filaments in valves/bulbs or checking the value of resistors, and marking them. Which might not of been automated, a very long time ago. Reportedly when they had 20% tolerance (scarlet or salmon coloured tolerance band ??), carbon resistors, as the values were virtually random, after production. So, 20% +/-, allowed all resistors to be classed as some kind of useable value, rather than choosing the values, to a fair extent, like today's typical product lines.Thanks for the suggestions on possible cute ones. I'm amazed at the pictures inside those old AVO meters. They seem amazingly complicated inside. I was impressed with the cut-out versions, which is nice to protect the meter. It's a nice action, when it (I believe) magnetically pops out and disconnects, if it is overloaded.[Start of sarcasm] Things areSOmuch better these days. We just have to replace a cheaphigh rupture current fuse, rather than have to go to the physical effort of having to push back in a nice cut out. How horrible that must have been, the fuses are only £12 (price varies) a pop, and I'm so pleased when that happens.

author: Paul B
author_rank: Contributor
timestamp: February 04, 2022, 04:54:50 pm
content: hiI think Gyro has the best answer as to use an over voltage to set trip personally if altering the trip i would go for a X5 protectionplease note the difference between model and MarkI believe the last version for the model 8 had the 3 KV terminals removedthe AVO model 8 Mk 5 has the 3 Kv terminals and the AVO model 8 Mk 4 has the 2.5 Kv terminals and 5 resistant ranges (LR & INS )also note there can be a significant difference in the reading accuracy depending on whether the meter moment is vertical or horizontalPaul B