High voltage diodes

[JosephfromItaly]

Hi
please I have a problem with my D83 Telequipment oscope, the brightness changes itself like turning the knob fastly up and down, I suspect the HV rectifier diodes (6Kv 10 mA), the telequip. (Tek) part no is:152-0515-00.
does someone knows a equivalent diode to replace them? or where to buy them?
thanks Giuseppe.

 

[Nigel Goodwin]

I wouldn't suspect the HV diodes, if the HV is varying then the display size will change far more than the brightness.

 

[JosephfromItaly]

Hi Nigel,
thanks for replying,
changes only the trace brightness, and the cathode voltage (from the schematic) should be 2.5 Kv but is nearly 3 Kv! (2850 Volts) even with the R301 (used to set cathode voltage) fully turned. Unfortunately this HV is obtained from a class C oscillator working at 28 khz, an oscillating transistor TR304 (2N5191) on a trasformer T301.
can I insert a image here? how?

 

[mvs sarma]

Hi
why not suspect some capacitor (filter type decoupling to ground )misbeheviour befor landing on HV diode

 

[JosephfromItaly]

Hi Sarma,
yes I'm checking these HV capacitors (that have 3000V insulation) I hope to find one bad! but the cathode voltage is higher than what should be!! I think if a filter cap is leaky the voltage will be lower...
bye Giuseppe

 

[Hero999]

If it's higher then it's probably because the load isn't connected. The cathode might be bad but I hope not.

 

[JosephfromItaly]

Hi friends,
finally I fixed it! now the cathode voltage is settable to -2.5 Kv and almost all voltages on the schematic are right! the problem was a 6.8 Mohm fixed resistor gone (open), it is on the feedback network from 2.5 Kv to the amlpifier that drives the oscillator power transistor. this resistor works with 366Vdc on it!

 

[Nigel Goodwin]

I never thought to mention! - ALWAYS check high value resistors, they very commonly go high or O/C.