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Blog posts tagged with 'mullard high speed valve tester'

NEW FOR 1954 - FACILITIES FOR THE MULLARD HIGH SPEED VALVE TESTER

As careless consumers were forever popping the HT fuse due to sputtering rectifiers and valve overloads, Mullard hit upon the idea of introducing an HT Overload cutout for all 1954 production models of the MHSVT.   It was based on a polarised relay which would trip at overload condition but could be reset at the press of a button.  The cut out trip current was 200mA.

Another handy wheeze was what Mullard termed "the MHSVT pre-heat device" where a toggle switch was added which allowed the instrument circuitry valves to have heaters lit without a valve setting card inserted and the gate handle in the "Off" position.  To avoid CRT wear, no EHT - just LT was applied.  On inserting a card and valve, the CRT indicating spot appeared virtually instantaneously which was considered a boon for the busy commercial workshop.

These enhancements could be retrofitted to earlier instruments on their return for routine overhaul - Mullard were very specific in that kits of parts could not be sent to users for self installation.

The cost of upgrade was as follows: - 

Pre heat device: £2 10/-

Overload cut-out: £2

Both pre-heat & cutout: £4

The diagram below shows the circuit amendments that were made - does your MHSVT have them?:-

 

 

WHAT CAN THE MULLARD HIGH SPEED VALVE TESTER DO

Today, I have had an e-mail from Veaceslav in Moldova, asking what tests the MHSVT can do.   This valve tester can do seven main tests sequentially and these are: - 

Test 1: check heater filament continuity.

Test 2: check insulation between electrodes - cold.

Test 3: check heater - cathode insulation.

Test 4: check insulation between electrodes - hot.

Test 5: check reverse grid current is acceptable.

Test 6: measures emission indicating 'goodness' on a CRT screen.

Test 7: checks correct connection of each electrode to each valve pin.

Parameters for each test are set using the test card for a particular valve which is slotted into the rear of the tester.

A useful tester but for me, I prefer the AVO VCM series and this is what we use at Mullard Magic HQ.

 

SOMEWHERE TO KEEP YOUR MULLARD HIGH SPEED VALVE TESTER CARDS

In early 1951, Mullard introduced a handy steel container finished in 'Dimenso' - that's posh for silver hammer finish paint. The container was designed to hold 600 cards in six separate compartments.   This number of cards would test 750 different valve types.    The container featured rubber feet and was sized to be equivalent in length to the depth of a tester so it could sit alongside - provided enough room was left to allow the clamshell top to open!   And the price for this marvel of modern engineering in March 1951 was 45s - a snip!

EXOTIC COMPONENTRY IN THE MULLARD HIGH SPEED VALVE TESTER

At the time of introduction, two 'hollow state' devices the MHSVT contained were considered exotic.   The first, the 85A1 voltage regulator is a neon filled reference two electrode device which serves to provide the MHSVT with it's reference voltage stable in normal useage to 0.17% variance and over it's entire life. stable to within 0.5% of nominal value.     The second is the DG7/5 cathode ray tube which is an electrostatic medium persistance tube .    What is interesting is that this CRT is used in this application purely as a voltage indicator, measuring various currents by means of voltage drops across resistances brought into circuit by the punched card valve selectors.  No need for a timebase here you see.  

With the passage of 60 years or so, opinions change and now we consider a CRT and a neon regulator rather passe and instead become animated if not rather excited by the EL37 beam tetrode that a MHSVT also contains - more on this desirable audio valve in a future blog entry.

BUILDING THE MULLARD HIGH SPEED VALVE TESTER

These were selling like hot cakes in 1951 and details of their build and performance featured in much of the Mullard official literature of the time. Here we see Arnold Polkinhorne doing something with a Philips valve voltmeter.  Just look at that fat bottle EL37 lurking on the chassis as well as a stash of more MHSVT in the background.

 

The fun cotinues as here is Dorothy Prinkle nimbly constructing and lacing a wiring loom for a MHSVT - looks like she's using the silicon covered wire display from Phil Marrison's BVWS stall - haha!

THE MULLARD HIGH SPEED VALVE TESTER - AT THE DEALER NEAR YOU - NOW!

Resuming our Mullard story in 1951, we see that Mullard is still vigorously pushing the MHSVT and that it is being received with praise by the radio & TV trade.  Here we see Chris Evan's grandad posing with his machine, in his letter to Mullard he went on to explain how he is clawing back his purchase capital outlay by charging customers "a bob" to have their valves tested - he thought about doing it for free but thought that customers wouldn't appreciate a service they didn't pay for(!) -  that's the spirit!

 

And yes, the MHSVT really was at the dealer near you - NOW as this shop sign from sunny Norway shows.

THE MULLARD ONE ARM BANDIT FROM DECEMBER 1950

Many users found the depression of the RHS handle to operate the Mullard High Speed Valve Tester  quite wearing -  and this at a time before  repetetive strain injuries were invented - stiff upper lip, grin and bear it!    The boffins at Salfords listened and voila, the tester was modified to have a one arm bandit style operating arm, not only that, if you pulled it and your valve tested for good emission, three cherries in a line appeared on the CRT.......................................  sorry couldn't resist that one.

Mullard advertised the change as ergonomics with  an added security measure, to stop customers from interfering with the valve tester, the knob and arm could be unscrewed to prevent meddlesome usage by  shifty spiv types and young schoolboys with caps, short trousers and sticky fingers. 

 

 

 

THE EVOLUTION OF THE MULLARD HIGH SPEED VALVE TESTER
I have been corresponding with my new pal George Bichard, some of you may know George through his work to build a database  of all cards and issues of cards produced for the Mullard High Speed Valve Tester (MHSVT) , the Mullard Master Test Board (MMTB) and the Philips Service Tube Tester (PSTT)  – if you can help, fill gaps then George would love to hear from you – see what he needs and shout up if you are able to help, so please, look him up at  http://home.btconnect.com/gmb/mullard.htm
 
George and myself have been discussing the origins of the MHSVT between ourselves, in particular whether or not the MHSVT owes it’s lineage to the MMTB or the PSTT.   In the course of this discussion, we jointly realised that not many people have experience of the MMTB and even less people will know about the PSTT so I think I need to describe both of these instruments further in my next blog entry. 
MORE FACTS ABOUT THE MULLARD HIGH SPEED VALVE TESTER

Here we have a few more words on the MH-SVT from Mullard from 1951 along with a few of my own added in blue text:-

This valve testing apparatus is capable of carrying out tests as similar as possible to those standard production tests to which all Mullard valves are subject. The tests are not of course as exhaustive as those carried out during manufacture but they do ensure that the valve under test is satisfactory in all respects insofar as it is within the approved tolerances of operation.

Use of Punched Cards:  arrangement of the test circuit and selection of test parameters is performed using a  perforated card corrresponding to the valve under test.

This card is inserted into a gate switch contained within the valve tester.  Three such cards are supplied with each instrument for use when setting up and adjusting the tests.  Numbered and punched cards for the actual tests are available together with a printed list giving the valve types and the reference numbers of the corresponding cards.

Here's what the complex and slightly unreliable gate switch internals look like.

Three Test Limits:  in place of a moving coil meter, a cathode ray tube (CRT)  and vertical coloured scale are employed to indicate the results of various tests.  If the valve is within specified limits, the spot on the screen of the CRT is defelected to the green part of the scale.  If, on the other hand, the valve fails to meet the test requirement, the spot is deflected to the red part of the scale.  a reading in the intermediate part of the scale indicates that the test valve will soon need replacement.

But the real pity was that the  CRT based 'snog-marry-avoidometer'   style of indication allowed no quantitative estimation of valve performance to be made - the MH-SVT  was useless for valve matching.  


Construction:  the test apparatus is mounted in a metal case with a sloping front.  All internal components are readily accessible for servicing by removing one or more sections of the case. 

In the later 50s Mullard described the hammertone finish of the MH-SVT as being a 'densito finish' - can't think why though - perhaps it pre dated Hammerite!   The gate switch contacts were an inspired idea, however, their complexity made them liable to corrosion, arcing and consequent unreliability.

At the top right hand side of the front panel is situated the switch which selects the tests in sequence and below it is a rotary switch by means of which fine adjustment of the operating voltage can be made in accordance with the prevailing voltage of the supply mains.

To the left of the CRT are four buttons by means of which individual electrodes may be tested for short circuits, insulation and open circuits.

The front panel carries a removeable plate behind which are situated the adjustments for zero setting, reject limit setting, brilliance and focus and for lateral shifts of the spot in case phosphor burn becomes a problem, a fresh part of the screen may be used.

It is said that some dubious dealerships encouraged customer valve testing, often at the cost of 6d to 1/- on a MH-SVT whose reject limit settings has been adjusted very high so that a good valve would appear marginal.   A West Yorkshire ex TV Engineer pal told me that his boss bragged that  he achieved a 50% on test replacement valve sale rate by using this deplorable tactic. 

Mains Adjustment:   the instrument has been designed to operate on 50 c/s AC from 180 to 260 volts. Adjustment of the various voltages is by means of 20V tappings on the mains transformer located on the control panel in conjunction with an eleven position rotary switch operating in 2V steps situated on the indicator panel and marked MAINS ADJUST.

Initial & Periodical Check Routine:  to ensure correct function, special check cards must be used to check mains voltage against an internal standard.  Meter sensitivity and HT stabiliser may also be checked.