High voltage diodes

J

JosephfromItaly

New Member

• Nov 14, 2006

• #1

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

Super Moderator

Most Helpful Member

• Nov 14, 2006

• #2

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

 

J

JosephfromItaly

New Member

• Nov 14, 2006

• #3

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

Well-Known Member

• Nov 15, 2006

• #4

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

 

J

JosephfromItaly

New Member

• Nov 16, 2006

• #5

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

Banned

• Nov 16, 2006

• #6

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

 

J

JosephfromItaly

New Member

• Nov 22, 2006

• #7

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

Super Moderator

Most Helpful Member

• Nov 22, 2006

• #8

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