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Old 26th Oct 2020, 9:56 pm   #127
Join Date: Jun 2019
Location: Dortmund, Germany
Posts: 111
Default Re: Telequipment D75 scope.

Originally Posted by WME_bill View Post
Telequipment D75 and D83.
Those resistors in the EHT chain always fail. It was a false economy by the manufacturers (Tektronix owned by then) to use standard carbon film or metal film, which are rated at about 250v. All these resistors will go high under stress over a period of perhaps a year or more, depending how much the instrument is kept switched on.
The proper solution is to change to metal glaze, as Philips / Vishay VR37, which are rated at 2500V. Lovely light blue body, you can often find several suitable in a old TV chassis.
However, the exact value down the EHT chain is not critical, as long as the ratio each side of the focus control is about right, and the grid chain is many megohms.
You can make up a chain for these scopes with 5 of 10M for the grid chain, and 10M+5M and 5M for the focus/cathode chain. Adjust the focus chain if necessary by inserting a 1watt carbon which are rated at 750V.
After all the scope you have was made between 1974 to 1980, so to fail now after 40 years or so is not too bad. Early versions of the PCB have space for lots of resistors in the chain, later versions use just one or two resistors.
Thank you for the reply. My D75 is an early version with carbon resistors, my D755 already has the fewer high voltage ones. I will try to understand how the EHT chains work based on your description.
Originally Posted by WME_bill View Post
The only resistor from the results you have obtained which needs replacing, I suggest, is the 80M R322 in the grid chain. Should be 20M. Put a couple of 10M carbon film in as a short term repair, and look out for some nice light blue ones. The others on the focus chain R323-4-5 are close enough in ratio each side of the focus control and it should operate correctly.
I will probably try to replace all carbon resistors in the MOhms range in the EHT to ensure reduce necessity of future repairs. Unfortunately I do not have even one ~10 MOhms resistor lying around for a quick test.

Originally Posted by WME_bill View Post
On the D75, you can get at the back of EHT power oscillator pcb148 quite easily without unsoldering everything by removing the securing screws and undoing the oscillator transistor on its heat sink.
Hmmm, I will definitely check on that. My first impression was that even after unscrewing PC 148 it cannot be moved because it is connected to short wires in nearly every direction.

Originally Posted by WME_bill View Post
One other tip. Most of the PCB are earthed to chassis by the fixing screws. An oxide film can build up between the aluminium chassis and the back of a PCB, giving a most mysterious fault. So check the securing screws are firm and the chassis clean underneath, particularly for the Y Amplifier and the Bright-up.
Thanks, that sounds like a very valuable tip. I would definitely not have come up with that.
Originally Posted by WME_bill View Post
Your other trouble could be the Bright-up circuit itself. You can check that without a High Voltage probe. The transistors there often seem to fail. The output voltage swing is 40v or so, when you switch from ExtX to TB, not triggered.
Not sure if I understand this correctly. I will see if I can locate that part of the circuit on the diagrams.
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