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Old 26th Oct 2020, 12:04 pm   #126
Join Date: Oct 2007
Location: Exeter, Devon, UK.
Posts: 1,360
Default Re: Telequipment D75 scope.

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.

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.

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.

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.

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.

You can make a high voltage probe easily with a string of standard 10M carbon and put your DVM or Avo8 across a 1M at the bottom of the chain. Calibrate it against a existing supply, such as the X amplifier feed. Those resistors will not fail under voltage stress in the 10 minutes that you use it. Just recalibrate each time you start using it again. See previous notes by me and others. Do a search here, may be under D83.
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