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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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20th May 2017, 4:08 pm | #1 |
Octode
Join Date: Jan 2007
Location: Dorridge, West Midlands, UK.
Posts: 1,474
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GEC Selectest early model.
GEC Selectest
Some assistance needed please, I recently acquired an early model Selectest which has some problems. There are several models of Selectest, the early heavy Bakelite seems to have 2 models the visible difference seems to be the swapping of the GEC logo and position of the cut-out. The one I have has the black not red centre portion with the cut-out at the top and the Logo between the main selection knobs. I noticed several requests for info and circuits on these models but without success. What I am seeking is the value of several of the resistors in the voltage ranges. The resistors are the old type bobbins and several seem to be open circuit and one has a short. In the absence of a circuit could anyone please assist in looking at the attached photo which shows the values of the known units and a < ? > against the faulty ones. Some have the values marked on the bobbin others look like they have been fried and the numbers are unreadable. If anyone has a working unit if they could let me know the missing values next time they get chance. For info the tests I have done to date indicate the meter movement is OK as are both the AC and DC current ranges along with the basic initial DC voltage range. All the switches show good continuity so I am hopeful that we can get it going again probably using modern 1% resistors hidden under the bobbins. It’s not that I need another meter I just don’t like seeing them broken! Thanks in anticipation Chris |
20th May 2017, 5:39 pm | #2 |
Dekatron
Join Date: Feb 2009
Location: Chesterfield, Derbyshire, UK.
Posts: 3,737
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Re: GEC Selectest early model.
Hi Chris could you select a position that doesn't work find the coil that is at fault , connect a pot across it start with high value and adjust to read the voltage you have fed in , worth a try . Mick.
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20th May 2017, 5:43 pm | #3 |
Nonode
Join Date: Mar 2008
Location: Aberdeen, UK.
Posts: 2,838
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Re: GEC Selectest early model.
Hi Chris,
I had a post back on the 11 Aug 2016 - " GEC SELECTEST MULTIMETER Mk1". Its u/s on a couple of ranges. You can have it bucksheesh for postage only(Or FCS). PM me if you're interested. Regards, David |
20th May 2017, 6:12 pm | #4 |
Octode
Join Date: Jan 2007
Location: Dorridge, West Midlands, UK.
Posts: 1,474
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Re: GEC Selectest early model.
Mick thanks I had not thought of that, I will try it but it may be a few days to get round to it.
David thank you for your offer I will PM you. Regards Chris |
21st May 2017, 8:30 am | #5 |
Dekatron
Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
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Re: GEC Selectest early model.
Mick's suggestion in post #2 is a good one. I've done it myself.
Alternatively, if you know the ohms-per-volt rating of the meter, and hence the FSD of the movement, the multiplier resistor values can be calculated quite easily. First, use a DMM to measure the resistance across the terminals with the Selectest set to its lowest DC voltage range. This will give you the basic resistance of the movement and any shunts, including the multiplier for the lowest range. Take a hypothetical example. If the meter has 20,000 ohms per volt sensitivity (50uA FSD) and ranges of (say) 1,10 and 100 volts, the multiplier for the ten volt range would be (10-1) x 20,000, or 180k. Similarly the 100 volt multiplier would be (100-10) x 20,000, or 1.8 Megs. Double-check that on each voltage range the total resistance of the multiplier chain adds up to the correct value, including the lowest range. You might calculate some odd resistance values, but these can be made up from several standard resistors in series, selected using a DMM to achieve the right value.
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Phil Optimist [n]: One who is not in possession of the full facts Last edited by Phil G4SPZ; 21st May 2017 at 8:34 am. Reason: Acknowledgement |
21st May 2017, 11:59 am | #6 |
Octode
Join Date: Oct 2007
Location: Exeter, Devon, UK.
Posts: 1,553
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Re: GEC Selectest early model.
GEC Selectest. I attach the circuit diagram of the later "super50" model. This uses a 50ua meter, and directly competed with the Avo8.
Earlier Selectest models I think used a 1ma meter, so comparable with Avo7. The catch is that while Avo used a more sensitive meter with a universal shunt across it, to give damping and to avoid errors caused by switch contact resistance on current ranges, GEC seems to have used magnetic damping from the coil construction, and hoped for the best over contact resistance. The lowest current range usually indicates the meter full scale current, so you can calculate the resistances required. Because you have to allow for the meter resistance, the actual value resistor will be some odd value which you will not be able to get off the shelf. So you have to fit the nearest, and calibrate. For voltage ranges, you can initially t ignore the meter resistance. For current ranges, you will have to allow for it, perhaps 1000ohm for a 1ma movement. Then calibrate by comparison with a know voltage source, and put high value resistors in parallel with the range resistor until it comes right. Rather as "vinrads" suggests. wme_bill. |
4th Jun 2017, 12:32 pm | #7 |
Octode
Join Date: Jan 2007
Location: Dorridge, West Midlands, UK.
Posts: 1,474
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Re: GEC Selectest early model.
Here is a quick update so far, I have got the Selectest from David and compared it with mine and there are two models with similar cases but substantially different inside, with a different meter movement, safety cut out and bobbins so this looks like it is going to take a bit longer than I thought. I'm going to have to trace out the circuits and understand the interactions on the switches so this will have to wait until I get some other things finished first.
I will keep you posted in the meantime if anyone finds a circuit diagram for either of the early Bakelite versions it would be most helpful. Chris |
18th Oct 2017, 10:37 am | #8 |
Octode
Join Date: Jan 2007
Location: Dorridge, West Midlands, UK.
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Re: GEC Selectest early model.
Finally some success whilst at the last Harpenden meeting I spotted a box of old meters AVOs etc that Mike knocked out at £5, what I was after was hidden in the bottom was an Old Selectest so I thought I’d take a chance.
Most of the items were pretty far gone including the Selectest with a bent meter needle and only working on a couple of low ranges. However compared with the one my earlier post the resistor bobbins had readable values on them and more of them were in working order. This allowed me to take out some of the bobbins and calculate the remaining 2 using selected 2W metal film 1% resistors. To assist others I have completed the chart of resistor values first photo. The unit needed a good clean up of the contacts and the removal of a lot of sticky grease around the knobs, a removal of some swarf around the magnets to get things working. I cleaned up and sorted a battery holder to take a D cell and got it all working as one would expect. Calibration is like early AVOs done by adjusting a magnetic shunt on the lowest range 150mV and the rest of the ranges if working properly drop into place which they did. So I now have another 1K per volt meter to add to my ever increasing collection! A couple of observations, Salford instruments dropped a real clanger when designing the reset mechanism as the reset plunger passes through the arc of the needle so if it needs a reset and you press the plunger without correcting the setting the needle wraps itself round it. This is what had happened with the damaged one from Harpenden. I wonder how many apprentices got a rollicking from the foreman for not thinking before pressing the reset? See last picture, also shown in the picture is the roughly cut Bakelite to allow adjustment of the reset contacts, clearly an afterthought as both meters had the same hacksaw cut chunk out of the Bakelite holder. The Bakelite casings seem a bit rough compared with other Bakelite casings on all 3 Selectests that I have seen, The nice bit of them though is the very clear meter with a large swing which make reading easy. Hopefully this will inspire others with old Selectests to get them working! Chris |
18th Oct 2017, 11:09 am | #9 |
Nonode
Join Date: Mar 2008
Location: Aberdeen, UK.
Posts: 2,838
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Re: GEC Selectest early model.
Fine project, Chris. The Selectest certainly is a lovely looking old vintage multimeter. You've done a great job.
Regards, David |
18th Oct 2017, 11:17 am | #10 |
Dekatron
Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
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Re: GEC Selectest early model.
Lovely meter. Well done, Chris!
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Phil Optimist [n]: One who is not in possession of the full facts |
18th Oct 2017, 10:36 pm | #11 |
Dekatron
Join Date: Aug 2016
Location: Colchester, Essex, UK.
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Re: GEC Selectest early model.
Notice that the ranges and functionality closely follow the Avo Model D, but it has a somewhat higher ohms/DC volt sensitivity.
There don't seem to be a huge number of them about, i wonder if the design shortcoming in the reset button has killed quite a lot of them. |
20th Oct 2017, 3:26 pm | #12 |
Octode
Join Date: Jan 2007
Location: Dorridge, West Midlands, UK.
Posts: 1,474
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Re: GEC Selectest early model.
Thank you for the kind words about the repair.
I suspect that there are 2 reasons not many seem to be about, one clearly is the design error the other is they never seemed to have the market profile of AVO so perhaps struggled in the market to get the traction. I noticed when you look through Wireless World of the 50s much more advertising by AVO compared with Salford instruments. The slightly later Selectests with a red raised section in the centre have a different meter movement and have moved the reset position to avoid the clash, I do have one of those as well but lack the time to analyse and repair it. Chris |
20th Oct 2017, 6:10 pm | #13 |
Dekatron
Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
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Re: GEC Selectest early model.
The only Selectest I ever used was where I once worked, and I seem to recall it was only branded GEC. No mention of Salford Instruments. I presume they were acquired by GEC.
GEC was a huge company and could have surely afforded to compete with AVO in advertising. Perhaps GEC went after different markets, the GPO and the universities perhaps, leaving the military and industrial market to AVO? Perhaps GEC also realised that their meters lacked the ruggedness and long pedigree of AVO.
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