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Cracked HV diode in a Tenma 72-320. Y16GA? Hitachi says no such part

JasonhatesH20

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Hello. I have a Tenma 72-320 scope I bought with no trace, got a trace back but the trace is spotty. One of the "Y16GA" diodes is cracked. I contacted Hitachi and they said there was a Y10GA but not a Y16GA. The datasheets they sent indicate that Y10GA is 6 KV, and Y16LA is 10 KV. Forward voltage of 22 volts Io 5 ma and 35 volts Io 3 ma respectively. I've sourced some diodes with higher forward current and reverse voltage ratings, but the forward voltage is lower like in the 9 to 14 volt range. Will that cause an issue or do I need to try and find an exact match even on forward voltage? I intended to put the 12 KV diodes I found in because the parts list and Hitachi revelation, along with the fact that the diode on the board has a yellow band, which isn't shown in any of the Hitachi datasheets leaves me questioning what I'm actually looking at, so I wanted to err in the higher voltage and current direction to hopefully save my butt. By the way, does anyone have anything that shows that there IS a Y16GA?
 

dick_freebird

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A diode like this is probably for the HV supply and a couple
of volts on, what, 10kV is not going to amount to much. Low
Vf is likely acceptable and slightly higher not much of an impact.
Stacked diodes like this are sure meant to not break down,
so "mobetta" there (as long as it doesn't cost somehow else).

You might like to look at a broad line HV rectifier supplier
and see if any of the P/Ns look like likely smears / scrubs /
other marking damage to a likely (parameters-wise) device.
Like maybe it's Y18LA instead, or something like that and the
crack just spalled off a bit of the brand.
 

JasonhatesH20

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A diode like this is probably for the HV supply and a couple
of volts on, what, 10kV is not going to amount to much. Low
Vf is likely acceptable and slightly higher not much of an impact.
Stacked diodes like this are sure meant to not break down,
so "mobetta" there (as long as it doesn't cost somehow else).

You might like to look at a broad line HV rectifier supplier
and see if any of the P/Ns look like likely smears / scrubs /
other marking damage to a likely (parameters-wise) device.
Like maybe it's Y18LA instead, or something like that and the
crack just spalled off a bit of the brand.
I wish there was a pn on the diode but the only thing I could find was one yellow stripe on the cathode lead itself. The part number I found came off of the schematic and parts list for a Hung Chang OS-620 scope, which is for all intents and purposes identical to the Tenma. It's D6 and D18. Here's the schematic, or part of it anyway.
Screenshot_20211218-163927.png
Whoever did the pdf of the manual for the Hung Chang locked the file so you can't print or share individual pages of the manual. My understanding from reading elsewhere said that lower forward voltage can cause you to have to recalibrate the scope, which it needs it anyhow with all the messing around and accidentally frying stuff that I've done. I've accidentally zapped the 1SS83s in the grid bias dc restorer, replaced those with BAV21s. I've also recapped the power supply, naturally it was before I did the diode zap so I probably need to replace C24 again since I've noticed intensity modulation that I don't THINK is related to the HV diode lead being broke. But, I wanted to replace that cracked diode so I at least knew I had an obvious failure fixed before I went chasing the rest of the gremlins in here. There's a problem with vertical channel A, a slight AC waveform, like maybe some ripple or leakage somewhere in the input that doesn't show on B. I probably need to recap the vertical section too I guess. I don't know, I barely know what I'm doing, and have been pulling my hair out for a few weeks over this parts or repair scope I bought for a "fun" project, lol!
Here also are datasheets for the Y10GA/EA and the Y16LA/LC. Neither one mentions a yellow cathode band. So as long as I make sure it's a high speed diode with enough forward current capacity and reverse voltage is equal or higher than original, and forward voltage isn't too far out of bounds I may be okay?
 

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