Mullard valve tester

[Bizzie]

I've just bought a Mullard high speed valve tester off ebay it was a local pickup only and was only a couple of miles from me in S****horpe so I fetched it today. He was very honest about it in that the light dot isn't a dot but more like a streak on the screen but it does come to a point.

With it came two boxes of cards and the instruction manual, I have tried to set it up as per the manual and can calibrate the voltage and the upper and lower limit for the fail test but when I do the final test for the HT it fails the test and all the manual says is that it needs to be repaired by Mullard.

The man I bought it off says he has a friend who thinks that its one of the capacitors at fault and for something as old as this it seems like this might be the case.

What I would like to know is has anyone had any experience with the Mullard tester would it be something that I can fix myself or does it need specialist setup equipment I have the three cards that are used for setting it up and I thought about just changing all the capacitors with the same type and value.

[steve s]

I have one of these ..i also had a duff one that paul barker ended up with....

they are good testers.. have you checked all the valves.. mine has el38's, i think...i have spare valves...

if it is a cap you may be able to tell by looking at them all.. they can blow and burn the case...otherwise its quite a task..

I don't have a circuit.. but you should be able to find one on the net

if you want to see mine working.. to check/ compare voltages etc. your welcome

steve

[Bizzie]

Thanks Steve I do have the circuit diagram its inthe back of the manual, I haven't had the back off yet so I don't know if anything looks amiss inside yet I think that I will start by cleaning all the switches and see if that improves it. I'm hoping that because it is working to some extent that it isn't to far gone.

[Bizzie]

I have been looking inside the tester hoping to see an obvious cause for the faults and have had some success because while cleaning the gate switch mechanism I found a piece of paper inside it that was stopping some of the contacts from working and after this was removed I get a positive result on the HT test.

There is still a problem to be resolved and that is the spot on the CRT is still like a comet tail. I found a forum about test equipment and the same problem is described and was resolved by changing all the capacitors for new ones. I would prefer to find the offending cap and change that as there are a lot of caps inside and it would be a lot of work and expense to change them all, but I don't know where to start to look for the right one.

Any suggestions would be appreciated.

[Richard]

Can you get a clue as to which caps from the circuit diagram?

Rich

[Bizzie]

My knowledge of how circuits work is very limited, I can follow what goes where in a diagram but as yet not fully understand what each part does. I was told by the seller that he thought the problem was a power supply cap and there are two large 4uf electrolitics on the power supply, as I under stand it this type of cap is usually less reliable if unused for a while so would you think that it is a good place to start.

On a test equipment forum I've found someone said they changed the wax caps (some of these feel really waxy on the out side is this normal) first and then went on to the electrolitics. I'm not sure what type of caps to use in place of the ones already fitted apart from the electrolitics as obviously the ones originally used are not available new apart from maybe NOS which may or may not work properly would it be OK to use any that are the right physical size and correct value as its not like an amp where you may be trying to get a nice sound.

[Richard]

Yes modern caps of the correct value will be fine and generally more reliable. The tolerances are also better these days, and I can't think of anywhere in that machine that might depend on a tightly toleranced cap, so just replace ones you feel may be the problem with modern equivs. Electrolytics and wax/paper types can go leaky. However some caps can recover if they've not leaked and it may be a good idea to leave the beast powered up for a day or two and see if it improves. Switch it on and off a few times during this period to cycle the caps and see if they reform.

Rich

[Bizzie]

Thanks Richard I have been leaving it powered up while I'm at home and ther does seem to be an improvement in the stability of the mains test if I leave it powered up for about 5 minutes it is stable and passes the test without adjustment the same applies to the upper and lower fail test setting limits the only problem that still remains is the comet tail on the test spot.

This morning I have performed a test on known working valves and all pass all tests but the comet tail seems to get worse when doing the emissions test which I would think is a high current draw to power the valve.

[Richard]

s****horpe birmingham huh? so this bb takes the **** out of s****horpe but not the ming out of birmingham I had never thought of such things before - where would we be without someone protecting our innocence...

anyway, where were we, yes that would seem a fair assumption so perhaps look to the ps caps first but is there anything you can identify in the circuit that focusses the spot - sorry never used this tester - could be a resistor gone high for example. What about the rectifier valves, have they been replaced?

Rich

[Bizzie]

The list of valves are EF91 x3, EL37 x1, GZ33 x1, EZ90 x1, 85A2 x1, EY51 x2 the only one that I know is a rectifier is the GZ33 and the nearest that I have to that is GZ34 can that be used just to try and see if its causing any problems? the rest I don't know what they do but the EF91 are connected via the anodes to the CRT so I would think that they control the X+Y axis.