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Old 8th Nov 2020, 4:32 pm   #128
Pinörkel
Pentode
 
Join Date: Jun 2019
Location: Dortmund, Germany
Posts: 111
Default Re: Telequipment D75 scope.

I'm still waiting for the replacement resistors for my D75 to arrive.

Debugging my D755 has gotten even more complicated, since it decided to catch yet another illness. Regarding the zooming-in issue described earlier(see first two images), I checked most of the resistors and transistors on PC128, but I did not find anything suspicious yet. TR354 and TR351 are, however, not socketed, so I will have to desolder those for testing. At the moment, I am not sure, if testing the circuit components in a non-powered state will show any errors, because the zooming issue only occurs after a 10 minute warm-up phase. Also, I still have not figured out how exactly to measure the 40V output voltage swing as described by wme_bill. According to the manual, there should be a 6V square wave.

In addition to that, the unblanking circuit now stops working after a short while of operation. When that happens, the brightness ramps up significantly (see images three and four) and cannot be turned down again via the power knob. It can, however, be turned up even further. One can clearly see that the flyback of the trace is then visible on screen. Depending on the shown signal, this can be in form of a simple line or two line segments, connected via a steep drop. Keeping in mind the Tektronix oscilloscope troubleshooting guide, I am kind of suspecting that the DC-restorer HV diode D303 at the HV-transformer may have failed. If I remember correctly, HV-diodes are simply several diodes in serial. So a normal DMM-diode test will probably not work here. Instead, I could remove the diode, apply a larger voltage over a suitable resistor(e.g. 5kohms, 5W) in series and then measure the voltage drop over the diode. At the moment, I am kind of hesitating desoldering the diode between the HV transformer and PC148, because I do not know how the transformer soldering posts react to heat and it is kind of difficult reaching that place.

Moreover, I have not been able to find a source for suitable replacement HV-diodes (6kV, 10mA) yet. Can I pick ones with larger voltage and current rating? Is it maybe important to pick one that consists of the same amount of serialized diodes, since that would have a large influence over the voltage drop. On the other hand, a few volts are small compared to the maybe 2k5? volts present here.
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