Need a discontinued HV diode

Lestraveled

Joined May 19, 2014
1,946
Could you check something. There is a zener diode (D1038, I think) and a Capacitor (C1038, I think). (schematic is hard to read.) They are connected to J2 pin 1, ( second connector left of the cathode of the tube). Look up the zener diode voltage and make sure the voltage at J2-1 is correct. This voltage could impact the brightness.
 

N11778

Joined Dec 4, 2015
174
Is R1021 CRT Bias the britness control?
I would also check or replace C1021 it looks like its a 1uf 160v electrolytic.
Electrolytics tend to go bad with age before other stuff.
 

Thread Starter

adam555

Joined Aug 17, 2013
858
Is R1021 CRT Bias the britness control?
I would also check or replace C1021 it looks like its a 1uf 160v electrolytic.
Electrolytics tend to go bad with age before other stuff.
R1021 is the CRT Bias, which controls how much brightness you want on the trace. This was the first component I checked and it seems to be working well; however, no matter where you set it you can never get a bright trace as when I got the oscilloscope. You could get an extremely bright trace, as like new, and now you can barely see it in a sunny day.

I have a diagram with the voltages I took in that section:
HV measures.jpg

This are the voltages I got at R1021 when moved:
With the CRT BIAS pot in the middle:
- 45.5V with the intensity knob (front panel) at minimum
- 46.5V with the intensity knob (front panel) at maximum.​
With the CRT BIAS at minimum I get:
- 8V with the intensity knob (front panel) at minimum and at maximum; there is no change in voltage or brightness on the screen.​
And with the CRT BIAS at maximum:
- 75.6V with the intensity (front panel) at minimum and at maximum; though the brightness increases towards the maximum.​

Could you check something. There is a zener diode (D1038, I think) and a Capacitor (C1038, I think). (schematic is hard to read.) They are connected to J2 pin 1, ( second connector left of the cathode of the tube). Look up the zener diode voltage and make sure the voltage at J2-1 is correct. This voltage could impact the brightness.
I don't have the voltages for that section, just 76.7V at the cathode of the D1038 zener diode. I'll opened again this afternoon and check those components as well as the C1021 suggested by N11778.
 

Thread Starter

adam555

Joined Aug 17, 2013
858
What is the voltage at the anode of D1038?
D1038 measures 77V at the cathode and 55.7V at the anode

Also see what the voltage is at C1021 when you turn the brightness up and down
71.3V at minimum brightness and 72.4V at maximum brightness with the CRT Bias as I had it. And the with the CRT Bias set in the middle I got 55.8 and 56.9V; roughly 1.1V difference between maximum and minimum brightness.
 

N11778

Joined Dec 4, 2015
174
I posted a high voltage probe In the earlier post you may have missed it. I just edited it
It mite get you by to see the high voltages

D1038 voltage sounds ok it a 22v Zener diode I think.
And did i get this right the voltage at C1021 only went from 71.3 to 72.4 thats not much.
that could be the reason for only a small change in briteness.
 

Thread Starter

adam555

Joined Aug 17, 2013
858
Have you checked HV manuf such as http://www.deantechnology.com/pdf/ckehvcat.pdf ?
Any of the CRT rectifiers suitable?
Max.
Thanks Max, I had a look and was unable to find one that matched all the specs. I'm not sure if all the specs have to match, or how much can they deviate from the one I need, so I can't pick a suitable replacement.

Try here: http://www.utsource.net/ic-datasheet/Y10GA-1922183.html Good company to deal with. It may take a while to get your parts. Don't be dismayed by the RFQ. Prices are reasonable.
Thanks, I asked for a quote and I'm waiting for their answer.

here is a company I have used before.
http://www.voltagemultipliers.com/start.htm
I can't find the diode I'm looking for there, and as I said above, unless I find a part with the exact specs, I wouldn't be able to pick a replacement.
 

Thread Starter

adam555

Joined Aug 17, 2013
858
I posted a high voltage probe In the earlier post you may have missed it. I just edited it
It mite get you by to see the high voltages
Thanks, just saw it. I looked for HV probes on ebay before, but I couldn't find any at a good price. I don't usually mess with HV so buying an expensive probe to fix an old oscilloscope that cost me $100 looks like a waste of money.

D1038 voltage sounds ok it a 22v Zener diode I think.
And did i get this right the voltage at C1021 only went from 71.3 to 72.4 thats not much.
that could be the reason for only a small change in briteness.
Yes, I tried it with the CRT Bias at different positions, and the variation on C1021 is only 1.1V from maximum to minimum brightness (by brightness I mean the brightness knob in the front panel).
 

sailorjoe

Joined Jun 4, 2013
363
I don't feel comfortable enough to attach a variac or the 120V transformer; I'm just a beginner. But I measured all the voltages, including the 120V, and couldn't find anything wrong. The only thing I could find was that diode, which someone recommended me to check.

I also don't think it's the crt cathode, as at the beginning the dimming went on and off suddenly. It was like that for a few months, and then it just stayed dim until now.

I'm attaching the whole diagram for the HV and Z-amp, and the HV section with the Y10GA marked.

View attachment 96856 View attachment 96857



But then, do you think that the 5V drop on the HV diode instead of the 22V could be the cause of the problem?
Thank you for the schematics, that helps a lot. Others can correct me if this is wrong, but I think the diode is a half wave rectifier, followed by a filter that provides the -1650 volts to the cathode of the crt. A partially operating diode could reduce the final voltage, thus dimming the scope beam spot. If it was mine, I'd find a replacement, with all the most important parameters intact.
Reverse Voltage at least 6.0KV.
Reverse current at 6.0kv less than or equal to 4 u-amps.
Forward surge current at least 500ma.
Forward Voltage equal to or lower than 22v at 5.0ma forward current.
Forward current steady state at least 5.0ma.
Digikey has a search capability that might help. http://www.digikey.com/product-search/en/discrete-semiconductor-products/diodes-rectifiers-single/1376383
This could be a bit of a slog.

P.S. Don't recommend a varied on the mains kin this instance. Crank it up too high and all the high voltages will go over some component's maximum, and then things could really spark!
 

sailorjoe

Joined Jun 4, 2013
363
Check out the UX-FBR8 here. http://www.deantechnology.com/catalog/hvca/high-voltage-diodes/axial-lead-power-diodes/ux
After looking at the schematic a bit more, it occurs to me there may be a different problem. The -1650 V output from the regulator to the cathode, also supplies the voltage for the first screen stage, which should accelerate the electrons heading for the phosphors. If that part of the circuit is off, that could also dim the beam. You checked all those parts, yes? Oops, behind on the posts, I see you're on the right trail.
 
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