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Old 18th Aug 2018, 5:10 pm   #1
Andrewdsutton
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Default Mullard HSVT. E7600/4 transformer information required.

Mullard E7600/4 transformer information required.

Just purchased a HSVT and after replacing the damaged mains lead and socket.
Plugged tester in to soak. All OK for 2 hours then R93 and R42 started to smoke. Also the anode on V10 glowed intensely.
Measured voltages from transformer to board and found 750v on a calibrated Fluke 77.
Then checked each end to earth.
750v to cable where R93 is attached and 22v to cable where end of capacitor is connected at opposite end of winding. Don't know if this is correct.
Put mains card in and dot on crt will not alter. Reject card and nothing alters. HT card and thermal trips instantly.

Assistance would be greatly appreciated.
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Old 20th Aug 2018, 6:50 am   #2
Electronpusher0
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Default Re: Mullard HSVT. E7600/4 transformer information required.

Hi, just a quick thought. Be careful making measurements around V9 & V10 and the circuit they supply which is the differential measuring circuit V2 & V3. This whole circuit is floating with respect to 0V. Measurements should be make with respect to the floating 0V which is the junction of C3/C4. There is a more convenient point to pick this up on the right hand side at the junction of C14 / C5 (both 100nF waxies which probably need to be changed).
The voltage across C3 should be about 475V and across C4 about 485V.
V9 and V10 form a voltage doubler circuit so the voltage on C3 is positive wrt floating 0V and on C4 negative wrt floating 0V.
I would check C3, 4uF. I could not get either C3 or C4 to reform on mine and had to replace them, I replaced them with non-electrolytic types. I left the old ones in place and used their tags to support a tag board on which I mounted the new ones and their shunt resistors R102/R103
Peter
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Old 20th Aug 2018, 9:41 am   #3
ukcol
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Default Re: Mullard HSVT. E7600/4 transformer information required.

As Peter says those components are in the positive half of the voltage doubler that supplies EHT to the CRT. R42 (4.7k) is rated at 1 watt which would allow it to pass some 15mA. It would require somewhat more than that to make it smoke. To pass that sort of current the EY51 (which is rated for a maximum anode current of 0.5mA) will be damaged.

The components mentioned by Peter will need checking/replacing as necessary but the mains transformer is most unlikely to be the cause of the fault symptom although it could conceivably have been damaged by the fault.

As you have just found out the hard way, it is always a bad idea to power up a piece of equipment like this before you have made a thorough cold check of certain known vulnerable components like wax/paper capacitors and electrolytic ones.

This advice applies particularly to equipment like the E7600/4 HSVT because if a fault damages the mains transformer you are talking a lot of expense to have it rewound.
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Old 20th Aug 2018, 2:36 pm   #4
Andrewdsutton
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Default Re: Mullard HSVT. E7600/4 transformer information required.

Many thanks gents for the info.

Have got caps already ordered.

Will keep you informed once recapped and resistors fully checked and replaced.
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Old 21st Aug 2018, 6:36 pm   #5
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Default Re: Mullard HSVT. E7600/4 transformer information required.

Here is a picture of how I mounted the replacement 4uF capacitors. I left the original ones on place and used their tags to mount a tag board which carries the replacement caps and shunt resistors.
Peter
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