Mullard valve tester E7600/3

[humphrey]

I have just replaced all the caps and the high tolerance resistors except R47 and R54. They both read very high and am waiting for replacements.
The sliders seem to have had some damp or flooding as the rivets holding the lower connections had burst. I found some 2.5mm tubular rivets at the local brico and they now work properly.The red slider looks like it has been interfered with.
With the mains card inserted, I am having probs with the green dot (more of a bean shape). It moves about and will not settle down.

[GMB]

Have you checked/replaced all the capacitors?

A poor dot is often caused either by bad capacitors, or more worryingly by leakage between primary and secondary of the mains transformer - not an uncommon problem if it has been stored in damp conditions.

[humphrey]

All the caps are new. I think that the unit got flooded at some time. The large tranny has traces of waxy spray around it and on the cover and cables. I will leave it powered up and see what happens. The fact that the sliders had broken rivets confirms the suspicion that it got wet. What does the red slider do?

[humphrey]

I now think that it is due to mains fluctuations it goes up during the ironing. The house is at the end of the road and the supply stops here. I am still not sure about the dot. It is more of a kidney shape.

[Colin]

I found duff caps to be responsible for distorting the shape and size of the spot.

Replace C5 which is a 0.1uf 500v component situated behind the mains fuse on the slider control panel. This cap decouples the negative supply line for the CRT and so prevents hum appearing on the tube which shows up as a distorted spot. This is the main one to check but I ended up replacing most of them when I did mine.

Can't remember off the top of my head what the Red slider does but I do know I adjusted it when setting up the tester.

Do you have any documentation for the E7600?

[humphrey]

Hi Colin, I was more concerned about the mains test, but after adjusting the sliders and moving on to the HT test, a round spot settled exactly on the dotted C line. It looks like all is OK.
Could someone tell me if the mains test can show a dodgy supply. ps; I changed ALL the caps.

[PJL]

The HT test checks the accuracy of the stabalised PSU. The mains test is simply for setting up the transformer tappings so the LT volts are correct.

Peter

[Electrical]

Hi Humphry
I am pleased to see you have your tester up and running.
With reference to the shape of the spot, I wondered if you have set the focus pot (POT ‘E’) on the front of the instrument, it could; affect the astigmatism of the spot if it is not correctly set. The ‘Test Mains’ card is used to adjust the testers mains transformer to the supply voltage to the nearest 2V tapping on the ‘Mains Adjust’ selector switch and the spot on the CRT screen will move up and down the screen with mains voltage fluctuations. The spot moves about 5 to 6mm for a 2V change in mains input and is adjusted to be as near as possible to the dotted ‘A’ line.
Where I live the mains voltage varies all the time, from a low of 218V to a high of 252V, which is well within the nominal 230V + or – 10% supply voltage. To overcome the supply variation I now feed my tester via a Claude Lyons AC Voltage Stabiliser. The output is set at 230V so the ‘Mains Adjust’ selector switch is in the middle of its range, this particular stabiliser is a very sophisticated servo controlled saturable reactor type and can maintain the selected output voltage over a + or - 16% mains input variation. However I have been told that a simpler version this type of regulator is still used on the continent.

Regards Stan.

[humphrey]

Thanks for the tip. My first test is with two W42 valves, one of which is new. Pos 2 electrode ins (ht off) the spot is in the red and the buttons make no difference. Ditto in pos 4 (ht ins on)
Pos 5 Emission (anode current) the spot is in the red the buttons cause slight lowering.
Does this indicate a problem with the tester?

[Electrical]

Hi Humphrey. Have you made any other cards to do some other comparison tests? If not put 563 card in the tester and try the test, with the selector switch in position 2 and 4, without the valve fitted to the tester, if the tester is OK the spot should remain around the ‘A’ dotted line. If you get the same, results there may be a fault on the selector switch or in the –200V supply used for the test. First using a digital voltmeter 10M, impedance measure the voltages at the respective pins of the valve base. On my tester, I measured -68V at the anode, screen and grid with the appropriate button pressed this caused the spot on the CRT to drop from the ‘A’ line to about 5mm above the ‘B’ line.
In this test mode the anode, screen and grid are, connected to the test pushbuttons via the selector switch SW E position 2 to earth. When a push button is pressed the electrode is connected to the testers –200V supply via a 10M resistor which forms part of a potential divider to the indicator, the only difference in position 4 is the anode is connected to HT+ and the screen is connected to screen positive supply.
If you do not get the same results, the first check is the –200V supply and if this is OK, the next step is the check the, 10M and 6M8 resistors and then check the continuity to the selector switch and push buttons. If these checks are OK, try insulation checks around the switches.
I hope this helps.
Regards Stan.

[humphrey]

On a quick check, on 2&4 the spot remains in the low red position. The resistors were changed for new.

[PJL]

A couple of years ago GMB and I started an incomplete project to provide technical information on the MHVST. I produced an emulator which works out the test conditions given a card number. No guarantees but I use it quite a bit with my MHSVT and it has proven very reliable so far.

Attached is the printout for card 563. It shows the pin connections on the top right and the test conditions for each of the available tests. You can measure the socket voltages without the valve to confirn the tester is doing the right thing.

[humphrey]

I think that it is due to a simple omission on my part. The 6m8 resistor goes to the rear wafer switch (next to the pillar) I think i left out a resistor here as it has been snipped off and I cant see where it went or what it was. The schematic is not clear at this point. Any ideas?

[humphrey]

Thanks for the pointers. I had connected the 10m resistor on rear wafer tag 2 to tag 3 instead of tag 1 from the 6m8 resistor. It seems to have solved the problem.

[humphrey]

Just performed the test card routine. The mains is ok. The reject switch 1 setting with slider B is ok on line b but the switch at 6 on line c is barely obtainable with slider c at its lowest limit.
The HT card now gives the crt spot at the top of the screen.

[Paul Stenning]

I don't know if it's relevant to this problem but I worked out how to set up "Preset A" (the red one) a while ago. See attached.

[Electrical]

Reject Limit Adjustment. In test selector switch, position 6 (Emission & Electrode O/C), you should adjust control ‘B’ (Set Zero) until the spot rests on line ‘D’. Then select Position 1 (Heater Continuity & Mains Adjust) and note the position of the spot with reference to line ‘C’, then reselect position 6 and then adjust control ‘C’ (Reject Limit) to position the spot approximately twice the distance above or below line ‘D’ as the distance the spot was above or below line ’C’. Then readjust control ‘B’ (Set Zero) to position the spot on line ‘D’. It may be necessary to do these adjustments over again to get the best results. If you can set these limits first the HT may be OK, keep checking the mains adjust to make sure it has not drifted away.
Before you start it may be best to set the controls 'B' and 'C' to approximatly the mid position, mark the position with a pencil so you will have a starting reference point.
Regards Stan.

[humphrey]

That is better. I must have been using the wrong lines. Even the HT test works properly now.
I wonder what the next problem is going to be?

[PJL]

Hi Humphrey, put card 563 in, set to position 6, and close the gate without a valve in.

Measure volts on B7 socket Pin 6 to pin 2 should be 101.6V and pin 6 to topcap connector should be -3.04V and Pin 6 to pin 7 should be 101.6V. These are theoretical values so a few % either way would be OK.

Remember to always switch the power off when your not using it as it leaves the heaters on with no HT.

[humphrey]

No such luck.
CARD 563 (no revisions)
On setting 6.
6-2=140v unsteady
6-t=20.1v steady
6-7 =140v unsteady