Mullard High Speed Valve Tester Schematic

[Electronpusher0]

I have just bought an E7600/3 tester and 3 boxes of cards at a Car Boot, they came from a house clearance.
While I am waiting for the replacement capacitors I ordered to arrive I have been looking at the Schematics.
It seems to me that the /3 schematic is not very well laid out which makes understanding how it works very difficult. The /4 Schematic is far clearer.
It is also apparent that the differences between the /3 and /4 are minimal, principally a change of V4 from EF50 to EF91 and the addition of a thermistor, X1, in the cathode circuit of V6. There are also some nomenclature changes eg numbering of variable resistors.
I decided therefore to amend the /4 schematic to become the /3 schematic and add component values and notes on some of the functionality.
I did this primarily for myself but others may find it useful so I am posting a pdf of the new Schematic.
I hope I am not violating copyright.

[ukcol]

For the best description of how these testers work refer to the CT80 model which is the military version.

This can be found on GMB's web site - just scroll to the bottom of this page:

http://air-ministry.uk/

[Electronpusher0]

Thanks Colin, I have found the circuit description in the military version but still found the /4 schematic easier, hence the re-draw.

[theredhouseinn]

Hi there.
I have also just bought an E1700/3 and 4 boxes of cards. It took me 3 days to sort and clean them, quite a few duplicates, in all about 2000+.
I checked every component in the tester and found that every capacitor, both paper and electrolytic had various degrees of resistance and every 10% tolerance res was way out of tolerance. the salmon pink 2% res were all ok, just about spot on.
I have replaced all capacitors and have almost finished replacing the resistors.
Your re-draw of the cct diag. is brilliant, very helpful. I will now do a full circuit check befor running it up on the variac.
Here's hoping.
John.

[Ed_Dinning]

Hi John, have you looked at the GMB threads on this tester in the forum?
He had a full list of cards produced and was looking for details of some he had not got.
If you have these in your collection it would be useful for the members to access the hole pattern.

Ed

[theredhouseinn]

Hi ed.
I have printed off the list of his missing cards. Not had time to check yet.
I have looked at several threads and I have found only one reference to the CRT cards, and he had 1 to 6.
I have 1 to 12 in one box of cards. If anyone is interested I could email pics.
The CRT cards are in very good condition.
I have many cards with all or parts of the ledgends missing due to wear, so I have got a long way to go yet.
I will keep posting as I progress.
John.

[Ed_Dinning]

Hi John, I also have 13 CRT cards.
Note that you will need an extender cable and some adaptor sockets to use the CRT test facility. These were supplied with some versions of the tester or were available as later upgrades. I suspect that they saw little use.

Ed

[Electronpusher0]

I have now started testing the Car boot purchase. I reformed the 16uF capacitors successfully but could not get the leakage current below 6mA on the 4uF ones so have replaced them.
This seems to have damaged the positive EY51 rectifier so I have temporarily replaced it with a BY127 silicon rectifier while I order a replacement so I can continue testing.
However replacing both EY51s with silicon would be more reliable in the long term.
What do others think, have I just uttered sacrilege or is it a good idea?

[Station X]

One post moved to a new thread here:-

https://www.vintage-radio.net/forum/...d.php?t=148988

[theredhouseinn]

I have finished refurbishing my E1700/3 and all is well and functioning OK.

I changed all electrolytics, papers, 10% resistors, 2 new EF91's and a new 6X4, cleaned switches, pots and gate contacts.

Don't waste time reforming electrolytics, if they do blow you will be picking bits of silver paper out of the electronics for weeks. I remember them blowing in the 50's TV's.

Don't replace valve rectifiers with silicon ones as you will get a large increase in DC across the caps. Silicon recs are much more efficient than valve ones and they give out almost the peak value from the transformer.

Can anyone help me with the setting data for the pre-set marked -do not adjust set at the factory? This is called mains balance on the cct, it is also painted red.

John.

[ukcol]

You'll find what you want here:

https://www.vintage-radio.net/forum/...d.php?t=106331

[theredhouseinn]

Thanks Colin.
Mains balance set. It was a long way out.Tester now working ok.
John.

[ukcol]

You're welcome.

I overhauled mine some years ago now and I find it a useful, if not essential, piece of kit.

The Mullard testers are "Go - No Go" testers in the sense that they don't give measurement figures of a valves performance. If you are someone selling valves for example, and need to quote emission and mutual conductance figures, then the AVO characteristic testers or something similar are required.

[PJL]

The Mullard testers are "Go - No Go" testers in the sense that they don't give measurement figures of a valves performance.

I have done the maths that calculates the test conditions for any card. I also tested the CRT deflection and it is pretty linear. It is therefore possible to get some meaningful results but without major surgery it is not going to be as flexible as the AVO tester. That said, the test conditions are regulated DC for sensible max working conditions and there are multiple LT tappings for each voltage to test near the manufacturers recommended minimum.

Any valve that has low gm is highly unlikely to pass a DC test but I did start investigating an add-on to inject variable amplitude AC into the grid and into the deflection amplifier so that that a null in the spot height could be used to determine a 'GM value'. The 'GM value' would then need to be keyed into the calculator to convert to GM.

I would say the test performed by the Mullard Tester is more realistic of working conditions and more accurate than the AVO. As an example an AVO will never get a valve to true working temperature. But the AVO is designed for engineers and the Mullard for valve retailers.

[Ed_Dinning]

Hi PJL, I like the idea of your adaption, have you any details you would care to publish?

Cheers, Ed

[ukcol]

Yes, I'd like to see that too.

[PJL]

It did not get beyond a concept and a few sketches. I have created a tester simulator and it will need this to understand the test conditions so it can calculate the GM.

[Electronpusher0]

Thanks for the tip about reforming the 16uF caps, looks like I will have to change them after all.
I have now got the machine working and passing the three set up cards. I have put small "dymo" type labels on the mains selector switch so I know which tap I am selecting - these can be peeled off if I ever decide to pass the machine on.
I have now fitted new EY51s, the BY127 was a temporary measure while I waited for the new EY51s to arrive.
John is right, the silicon rectifier gave a significantly higher voltage.
I have replaced the 4uF caps and a lot of out of tolerance resistors. I have cleaned all the gate switches, they are in very good condition.
The previous owner had replaced all the waxies so that saved me a job.
The rotary switches were quite cruddy but are all clean now.
The HT regulator was not working, turned out the EF50 had zero emission when I tested it on my AVO 2 panel tester, the heater was OK though. Fortunately I had a spare.

I am getting an anomalous result though.
A dual triode 12AU7 tests fine, the emission comes in the middle of the green on the dotted line.
However I cannot get an EL84 to pass. I have tried several known good valves including a brand new one but they all only indicate amber whereas the AVO shows a gm of around 10 or 11.
Does anyone have any ideas please?
Peter

[theredhouseinn]

Hi Peter.
I also have a similar probllem with the EL84 test.
A known good one from Watford Valve co. gives me 1cm above the bottom line of the green section on emmission test.
I have just measured the voltages on the B9 base as follows:-
with respect to the cathode pin 3,
Grid -7v
Heater 7.8v ac
Anode 260v dc
Screen 260v dc
I also get a normal reading for a 12ax7 emmission, green center line.
It may be that we have a fault here that the refurbishment process did not cure.
I will be interested to see what voltages you get
I used my Avo 8 for the measurements.
John.

[PJL]

The results from my calculator are attached.

Your heater volts look wrong and suggests the mains adjustment is wildly out. From memory (so maybe not accurate), the mains card compares the -200V with the reference voltage and the 'red' adjuster is the factory adjustment for this.

I re-calibrated by measuring the mains using a true RMS meter, set the dials to that voltage, inserted the MAINS card, and set to the target using the red adjuster.