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Old 17th Jan 2021, 5:43 pm   #1
C & G Course 48
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Default Mullard 7600-4 Valve Tester C3 & C4 Voltage imbalance

Having over the years inherited 3 at Mullard 7600-4 Valve testers I have decided about time get all 3 working before I look at selling them but not sure why after doing much identical work on all 3 now all 3 have the same fault.

I have in all 3 replaced every capacitor, also replaced or checked if out of tolerance every resistor listed below
R7, 8,19, 20, 25, 26, 27, 28, 29, 30, 33, 34, 35, 36, 37, 38, 42, 43, 44, 45, 46, 47, 48, 49, 51, 53, 54, 55, 56, 60, 61, 68, 71, 85, 87, 88, 89, 90, 91, 93, 94, 96, 97, 98, 99, 100, 101, 102, 103

I now have a strange fault on all 3 when checking them out with the "Reject" test card the voltage directly across C3 & C4 that being on system No 1=C3 395v C4 595v.... No 2=C3 400v C4 630v ..... No3=C395v C4 590v therefore with such an in balance to set up the beam it is impossible like on the "Reject" card I am unable to get the beam high enough, and that is with RV4 wound to zero so both V2 and V3 anodes are the same potential. (worried at 630v on C4 as spot on its max working voltage)

I am not sure what I am missing, and why directly checking across C3, & C4 such a voltage imbalance, which I think is the cause of not being able to set up the unit correctly.

On all 3 the spot is perfect and very stable also shape is in focus and brilliance working ok as is position x shift, it is just "set zero" and "reject limit" not working as they should.

To end with not sure again why but on all 3 the "mains adjustment switch" with the mains test card in makes no difference but I think that is because of above ?

Any help advice would be appreciated as this is crazy as I am not sure why I have this fault, also new EY51's fitted and R42, 93, 94,102, 103 replaced twice just in case !!
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Old 18th Jan 2021, 8:58 am   #2
Electronpusher0
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Default Re: Mullard 7600-4 Valve Tester C3 & C4 Voltage imbalance

Hello and welcome to the forum.

I would not worry too much about the imbalance in the voltage on the capacitors, the total voltage across the two contributes to the beam current and focus.
The spot position is governed by the voltage difference across the X or Y plates.

When testing in this area its important to note that the whole circuit surrounding the CRT is floating with respect to chassis and the normal 0V, when making measurements in this area the “0v” reference is the junction of C3 and C4.

The spot position in the vertical position “high enough” is determined by the voltage difference between the two anodes of the EF91s. It’s worth measuring these voltages (with respect to C3/C4 junction)

When the voltage on the two anodes is exactly matched then the spot should be dead centre.

A couple of points to bear in mind.
1 There is a design issue in that if RV4 is set to the end where it is minimum resistance (I can’t remember which end this is) then the amplifier has zero gain and the spot will not move with signal.
2 R33 and R34 add an offset to the amplifier to move the spot to centre position when the test selector switch is set to position 4. If the selector switch is leaky on these contacts (due to crud) then the spot will have a permanent upwards offset.

Peter
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Old 20th Jan 2021, 7:17 pm   #3
C & G Course 48
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Default Re: Mullard 7600-4 Valve Tester C3 & C4 Voltage imbalance

Thank you Peter all the information is of great help much appreciated, what I have found is that on all 3 with the "mains test card" inserted the spot is 10mm below (a) the dotted line, and cant be got any higher, it does not move even after 2 hours left like this. Checking the reject limit RV4 it is at minimum resistance fully anti clock wise so therefore both V2 and V3 anodes are at the same potential. That being 180vdc measured from C3-C4 junction. What I have discovered with no mains connected and the "mains test card" in and the gate lever closed and checking for as you say leaky switch contacts on the test selector switch testing I have found on all 3 that with my meter on ohms across R33 the switch does not short R33 in position 4 but in position 5 not position 4. I only have 1 "mains test card" and F2 is the only hole punched outon that card. I am just wondering if this home made "mains test card" has the hole in the wrong place ?. Thanks for help,, Graham
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Old 21st Jan 2021, 11:47 am   #4
GMB
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Default Re: Mullard 7600-4 Valve Tester C3 & C4 Voltage imbalance

Quote:
I only have 1 "mains test card" and F2 is the only hole punched outon that card.
That is it - so long as it is properly in the right place.

The test cards can be found on my web site at http://air-ministry.uk
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Old 21st Jan 2021, 9:01 pm   #5
Ed_Dinning
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Default Re: Mullard 7600-4 Valve Tester C3 & C4 Voltage imbalance

Hi on my working Mains card the hole centre is 24mm in from the LHS and 90mm up from the bottom of the card

Ed
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Old 29th Jan 2021, 2:39 pm   #6
C & G Course 48
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Default Re: Mullard 7600-4 Valve Tester C3 & C4 Voltage imbalance

Thank you and I have checked and my mains card like the reject card is correct. What I have found on all 3 is the"Set Mains Adjustment" makes totally no difference, the other thing is that control "B" adjustment done correctly is totally affected when "C" is adjusted so keeping it simple both are doing exactly the same in tandem as if one control.

I have tried doing the set up so many times with no joy. (The problem is all 3 had been played with got at before I got my hands on them so all 3 did not work also odd capacitors and resistors had been fitted,now as above all replaced to correct components as well as a few missing valves fitted) any thoughts or ideas will be much appreciated. Thanks in advance Graham
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