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Old 28th Nov 2008, 5:12 pm   #47
Electrical
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Join Date: Jul 2008
Location: Gateshead, Tyne & Wear, UK.
Posts: 332
Default Re: Mullard Valve Tester Card Project

Hello everyone, I have read all the posts on this topic with interest, as I too have been collecting sets of cards in order to build up a complete set of cards. I originally found GMB’s Mullard Tester Card Project by searching on Google about 18 months ago and sent off all the copies of the missing cards I had via email.
In one of the posts it was mentioned about rouge cards and I wondered if anyone had come across cards that have been miss punched as In the example of card Diode Test 1B, (AC/HLDD) where the holes are miss registered by approximately, 5mm, as shown in attached picture. In this case, the tester did nothing, as all the contacts in the gate switch were, held open, with the tester indicating an open circuit heater. Since I found this card, I always check to see if the holes are all in the correct place. I think you could imagine what would happen if the holes had been another 3mm miss registered, the set HT holes row ‘L’ would select the mains transformer secondary taps and would short together, with out any secondary fuses the mains transformer would have little protection and would probably burn out before you realised what had happened.
With reference to the card material, the original cards appear to be, made from 0.6mm (25thou) ‘Tufnol’, which in the ‘Kite Brand’ form has a very high electric strength. It is a Phenolic paper laminate, SRBP (Synthetic Resin Bonded Paper) and is available to order in 4 foot square sheets 0.6mm thick, or 0.8mm thick, off the shelf. http://www.tufnol.com. I have made blank cards from 0.8mm Kite Brand Tufnol and punched them on my jig, and have all worked very well. I cut the cards to an approximate size and a friend of mine milled the cards to the finished size of 8.25” X 5”, with the 1.5” notch in the side. At present, I have built up a supply of hundreds of blank cards, made from acquired scraps of 0.8mm Tufnol sheet, all I need now is copies of my missing cards to punch new ones to complete the set. My punch jig has a slide fitted on the top to allow me to copy a card by using the sample card as a template to reduce errors when punching.
One member suggested fitting a digital relay card to a tester and controlling it from a PC, well this may be another one of my follies but I have already built an interface consisting of 130 single pole relays, in 13 rows of 10 columns. Each relay coil is, switched via a transistor with a 15k resistor in its base and a suppression diode across the relay, operating coil. Please see picture. The interface is built, up from 13 separate PCB’s, with links fitted across the rows from A to M and the columns are linked across the top terminal blocks. As you can see it is not finished, yet and at present I am working on a diode matrix card consisting of 130 X 3.5mm jack sockets to program and test the relays. The matrix is fitted with a jig to slide a standard card in and by inserting a 3.5mm plug with a diode wired across the terminals, through each hole in the card will energise the appropriate relay and I hope will give the correct settings for the valve tester. Using this method of programming, a card for an unlisted type of valve could be set up by varying the positions of the diode plugs in the anode voltage set row ‘L’ and the grid bias row ‘I’ until the correct conditions have been achieved. To check these conditions I have built yet another jig, which consists of an octal plug and socket joined, together with 1 ohm resistor in series with the anode, screen and cathode pins, with pairs of wires across each resistor, which will indicate the current on a mili-volt range of a digital voltmeter. The anode and screen voltages are, measured with respect to the cathode, which on the tester is usually earthy. I did bring the grid connection out as well, but connecting a 10 mega ohm impedance digital voltmeter between the grid and the earthy cathode causes the spot on the tester to drop down to just about half way and there are signs of instability in the spot. I did intend to build in three small digital voltmeters into the tester to measure the parameters as each type of valve is tested, both voltage and current, but I will have to increase the input impedance of the grid circuit voltmeter first.
I do hope this post in not too long or not applicable to the original.

Regards Stan.
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