Buying a Mullard High Speed Valve Tester

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Buying a Mullard High-Speed Valve Tester (HSVT)

These machines were used for testing electronic valves (tubes) of the kind used in vintage TV, radio and audio appliances.  They use a punched card system to set the machine up to test different valves.  Since there were many hundreds of different types of valve, there are many hundreds of different cards.  Additionally, there are 3 cards which are used test the machine itself.

The machine came in 4 variants - the Marks 1, 3 and 4, plus a military version known as the CT 80.  They're all very similar in operation, but differ slightly electrically and in the layout of their controls. The machine shown is the best-looking, in my opinion, the Mark 4.

What will it do?   What this machine excels at is in testing a large number of valves quickly, as implied by its name.  It's pretty easy-to-use  for non-technical people and will give a good indication if a valve is good, has a limited life left, or is only fit for scrap.  Since it uses a CRT to display its results, it's far more robust than a machine that uses a delicate meter - the latter are very easy to damage and it's difficult (and VERY expensive) to find a replacement.  It can test for all the most common valve problems - poor inter-electrode insulation, loss of vacuum, low emission, failed heaters, etc.

What won't it do?  Well, it won't test a valve if you don't have the card or adapter for that particular valve.   In some applications (for example, high-fidelity audio amps and guitar amps) you may need to have an accurately matched pair or even a matched quad set of valves; it won't allow you to accurately match valves - you need a valve characteristic meter for that sort of work.

Some pitfalls:

 Cards - The machines were originally sold with a suite of cards, (usually around 800) for the more common valves, with the option to specify extra cards to add to your collection.  Without the cards, THE MACHINE IS USELESS at any price, however good it might appear - and vice-versa, of course.  Some unscrupulous sellers will state that the cards can be found on the Internet.  Technically this is true, since one seller (at the time of writing) is selling individual cards at £5 apiece.  This is useful if you only need one or two cards to complete your collection, but it doesn't take a genius to work out that to buy a full suite in this way would cost you thousands of pounds.  Very infrequently, a set may come up on this site.  I've only seen one set in the last year or so.

Adapters - In addition, the machine would have been supplied with a set of 10 adapters to allow a greater range of valves to be tested.  If these are absent - as they usually are - it limits the range of valves you can test.  Adapters were also supplied to allow the machine to test CRTs.

Documentation - the machine was supplied with a set of look-up cards listing which punched cards you would use for a particular valve.  If these are present and in good condition, it enhances the value of the machine, but their absence needn't be a deal-breaker, since this info CAN be found on the internet, as can the operating manuals for the different machines.

Condition - This is the real potential deal-breaker.  The external condition of the machine is usually obvious in the photographs and may give you clues to how the machine has been stored.  If the exterior has rusted through its Hammer paint, it has been stored in a damp garage or shed for perhaps 30 years or so.  The outer cabinet is not sealed and any moisture that has affected the outside will also have affected the inside.  

The main show-stoppers internally are the large and very complex mains transformer and the even larger and more complex 130-way gate switch, and, to a lesser extent, the large rotary test selector switch.   The transformer has many different voltage taps and is vulnerable to damp causing leakage between the taps.  After drying out, the machine may work after a fashion, but will never give reliable readings.  There are no spares available, unless you can find a donor machine, and it would cost many times the value of the machine to have a new transformer custom-wound, even if you could find a company prepared to do it.

The 130-way gate switch "translates" the punched cards, selecting the various voltages and circuits needed for different valves.  The 260 contacts are made of brass and can be cleaned of tarnish if the gate switch is dismantled (a fiddly task); however, the 260 springs that activate the contacts are of untreated steel and if the outside of the machine is rusty, the chances are that they will be heavily rusted as well.   To replace the springs the contacts have to be disconnected with a soldering iron - hundreds of them - and even more daunting is the prospect of putting it back together correctly in the confined space of the machine.  Personally, as an electronics technician with 50 years of experience, I wouldn't contemplate it - even if you have the skills needed, you'll never recoup the time cost involved.

Finally,  you have to bear in mind that these machines are 60-some years old.  Regardless of the external condition, unless the machine has been professionally restored and in regular use, ALL of the wax capacitors will be leaky, ALL of the electrolytic capacitors will require reforming or replacing and most of the standard 10% carbon resistors will have gone high-value and will need to be replaced.  There are around 14 capacitors and 103 resistors.  This is do-able if you can understand a circuit diagram, read the component values in the machine and are handy with a soldering iron, but you'll need to budget around £40 for the parts needed (as at Nov 15).  The Mark 4 machine is beautifully laid-out and it's quite easy to access most of the components for testing or replacement, but the others are considerably harder to work on.

I'd summarize by saying that unless you are a competent electronics technician with plenty of time to spare, OR you can actually see the machine being set up, calibrated and testing a few valves, I'd consider buying something less daunting.

If you do contemplate bidding, contact the seller and ascertain the following:
Are the cards there (around 8-900)?
Are the test cards present?
Are the adapters there?
Is the mains lead present? (some of these machines  use very expensive  3-pin plugs  at around £30-£40 on this site)
Is the machine complete? (some of the valves are very expensive, and other parts are unobtainable)

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