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Old 7th Feb 2021, 3:19 pm   #11
WME_bill
Octode
 
Join Date: Oct 2007
Location: Exeter, Devon, UK.
Posts: 1,381
Default Re: Telequipment D67 Fault

Telequipment D67.
I suggest that the thermistors (two in the Y amplifiers, 1 in each of the timebases) are very much second order effects, and not vital. Just temperature compensation.
On my D67 and D67A, the thermistors appear sound.

The common problem is with the pnp transistor MPS6518. Even Telequipment-Tektronix began replacing them with 2N3906. I have them failed in the bright-up and the B timebase.

As MotorBikeLes says, the bright-up circuit is very uncomfortable to work upon, as most of it is sitting at -1.5kV. But you can do a lot measuring the voltages across each transistor with a pocket digital voltmeter in a plastic case sitting between the EHT rail (210-188) and the bright-up power rail ((210-175).
I have drawn a line across the circuit diagram separating where the high voltage lies, through the 3 capacitors which feed the pulses from the TB (C251,C252,C257), and onto the PCB layout. It gives me much more confidence.
wme_bill
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