Re: troubleshooting Tek453 Mod 210H

Fabio Trevisan
 

Hello Chuck,

On your finding of D940 opened, there is a screaming question... How did you tested it open?
H.V. diodes usually have a very large forward voltage drop (circa 20 volts), and they won't test right on a regular diode test.
Still, assuming your diagnostic is correct and D940 is indeed open, it's a Tek part number 152-0192-00, and the commercial part number is manufacturer specific, it's a VARO 7701-5X.
You can find its specs here:
http://w140.com/Tektronix_Xref_sm.pdf, page 12-2, line 8.
It's a 5kV diode, 50mA rated average forward current. and it has 15V forward voltage drop at the rated current.

Back in 2016, When I needed to diagnose a H.V. problem on my former 464, one of my first suspects were the H.V. diodes, and they're of this same vintage (not the same specs, but are also from VARO).
Back then, I sourced from a local electronics store, a few modern ESJA53-12. They are 12kV, low recovery time H.V. diodes, for currents up to 5mA and they worked fine on the 464, but the original parts on the 464 were 12kV 5mA (so the ESJA53-12 were a perfect match).

The ones on the 453, however, are 50mA, so I guess you will need to source different ones:
Either original parts (new old stock), and two reputable sources that users from Tekscopes group often resort to are:
Sphere: (http://www.sphere.bc.ca/) or...
QService: (https://www.qservice.tv/)...
(No affiliation though... just a happy customer)
Or you can try these folks here: http://hvstuff.com/
Never dealt with them, but they're almost the only source you can find on the net for H.V. oriented stuff.

Krgrds,

Fabio

On Wed, Oct 10, 2018 at 05:16 PM, chuck wrote:


hi folks,
Just confirmed this is a 453 pre A version.
Found D940 was open and when I looked at the two versions of this circuit
there is considerable differences.
D940 is the biggest diode I've ever seen so I guess a 1n911 is not going to do
the job. I haven't had a chance to look at the BOM but if any one knows the
commercial P/N would be appreciated. Also stray thought what blew the diode?
Everything else in the grid bias ckt seems to be alive and well. Any thoughts?
Thanks for your help been a great learning experience for me.
Chuck

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