Wanted. Service manual for Selectest Super 50 meter - Page 1
Forum section: electronics / general

author: Specmaster
author_rank: Super Contributor
timestamp: August 25, 2022, 07:09:11 pm
content: On the bench I have the first of 2 of these Selectest Super 50 multimeters made by Salford Electrical Instruments Ltd.I want to strip them down in order to repair the resistance ranges and also to service them. I have the owners manual and the schematic but need the service manual and was wondering anyone has a copy that they could share with me or for sale?

author: coromonadalix
author_rank: Super Contributor
timestamp: August 26, 2022, 06:45:31 am
content: something like this? :https://davidjwatts.com/youtube/Selectest-Super-50.pdfit seems to have only 3 trimmers, i would take the center pins and side pins values and not them on a sheet, and fiddle their adjustment to see how can you adjust it ??With this schematic, i should not be a problem to repair itAdjustment / calibration can or could be a small guessing at start, and you compare it with a good recent meter say 20000 counts and more ... since in dc an ac you start at 1% up to 2.5 % deviationAnd dc is relatively easy to calibrate, you need some good precision resistors i should say in the 1% or less range, Ac voltages could be a little but tricky, since you need to cover some rangesI would use at first use deoxit cleaner to help with the rotary selectors before doing anything else in itI would be careful with the plastic if it can or could became brittle over time ...The fact it goes at max 2500 volts is surprising, never saw that in theses needle meters, i hope your's is fine and clean

author: theHWcave
author_rank: Regular Contributor
timestamp: August 26, 2022, 09:26:34 am
content: Hey, I got one of these ... re-branded but the same thing ... and in need of a calibration as you can see, if I ever get to it.The size of this meter is just enormous. That's a Fluke 187 next to it which is by itself quite a big meterI don't have any service manual either, I think the schematic in the user manual is all there isQuote from: Specmaster on August 25, 2022, 07:09:11 pmI wonder what your plans are about the BLR121 15V battery for the higher Ohms range? Are you trying to source these? They look quite expensive and who uses analog meters for measuring ohms these days? I thought I might try to see if a 9V battery would work reasonably with its clip wired to the 15V terminals ... or a small DC-2-DC converter? But it would not be a true restoration. I guess I need to make up my mind keep it for myself or sell it

author: Specmaster
author_rank: Super Contributor
timestamp: August 26, 2022, 09:36:13 am
content: Thanks for the pointers, good tip to note the values of the trimmers from centre pin to the outers, so that if need be I can put them back as they were.Yes that is the manual that I downloaded from the same source, but I was looking for more than that, ie, the magnetic damper, the correct way to remove / dismantle various bits and bobs rather than trial and error and possibly break something in the process.I'm surprised that you haven't seen such high voltages on analogue meter before, there are plenty of examples around, some AVO's will read upto 3,000V, I have 4 of those, other earlier AVO's are generally capable of reading 2,500V and I have a couple of those, but those are still way lower than another couple of meters I have, the TMK 700 and the Kyoritsu Model 1400 (also sold as an Eagle meter) both of which go to 5,000V.

author: Specmaster
author_rank: Super Contributor
timestamp: August 26, 2022, 09:55:57 am
content: Quote from: theHWcave on August 26, 2022, 09:26:34 amThere are many options for the 15V battery, I've seen 5 button cells in a heat shrink tube used quite successfully, I think that they were LR44 cells from memory, but it is easy to work out which ones they need to be by adding their lengths together and if required washers etc could be inserted make up length.There must be someone around with the service manuals, they had to have them in the service dept at least, and I cannot believe that they simply got dumped, surely not?Yours is one that was made for the Post Office who also placed orders with AVO for similar meters and there are plenty of examples of those around as well.

author: Gyro
author_rank: Super Contributor
timestamp: August 26, 2022, 07:20:34 pm
content: Quote from: Specmaster on August 26, 2022, 09:36:13 amThis a big and unnecessary cop-out on modern (and not so modern) DMMs - setting the input resistance at 10Megs rather than staying with the Ohms per Volt route on higher ranges. A 10Meg input DMM draws 100uA at 1kV, while the humble old 20k Ohms per Volt moving coil meter draws a mere 50uA. On its 3kV range*it still pulls a maximum of... 50uA (against what would be 300uA). I think that'sonereason why you don't see higher ranges on DMMs, the circuit loading becomes even more silly.Edit:*Thinking of my AVO 9.

author: Gyro
author_rank: Super Contributor
timestamp: August 26, 2022, 07:32:32 pm
content: Quote from: Specmaster on August 26, 2022, 09:55:57 amBLR121 'replacement' batteries, in the right rectangular form factor are less than £6 on ebay. Once you have the casing, I'm sure it would be easy to re-stuff them - assuming that you ever need to.

author: coromonadalix
author_rank: Super Contributor
timestamp: August 26, 2022, 11:53:26 pm
content: Nice meters, i had one from my past away father, black kinda bakelite casing with an 48 volts mercury battery at the time, sadly i dropped it, and the case went in many partsnice to see well preserved meters like thesesmaybe 3d print a battery casing