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Old 3rd January 2013, 08:58 PM   #1
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Default Oscilloscope repair - diode ID..?

Hello all,

I have an old Hung Chang 3502 scope, bought it at an audio fair for 40. It has done me well up until tonight when something iside it started to arc, or fail, and sparked in quick succession.

I took the cover off and the culprit(s) is in a metal case with a seperate transformer. They look like germanuim diodes, or some kind of LED. From the picture you can see there are 3 in a row, each one flashes simultaneously.

I do not know much about scope circuits, and could not find a schematic so any advice here would be helpful.

It would be good to know what the circuitry is in this seperate enclosure - possible they are used with LDRs in the same case?

Whether I can simply replace the diodes, and with what type?

Or, do I just cut my losses and buy a decent scope as it only cost me a few quid?

Thanks for any help

Charlie
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File Type: jpg Hung Chang.jpg (620.3 KB, 113 views)
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Old 3rd January 2013, 09:21 PM   #2
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Those are neon lamps. They fire at 85 volts and clamp to 60 volts. Only one side is lighting up meaning they are getting DC. They most likely are being used as a 180 Volt zener and look good.

Last edited by simon7000; 3rd January 2013 at 09:23 PM.
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Old 3rd January 2013, 09:21 PM   #3
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Quote:
Originally Posted by CSlee View Post
Hello all,

I have an old Hung Chang 3502 scope, bought it at an audio fair for 40. It has done me well up until tonight when something iside it started to arc, or fail, and sparked in quick succession.

I took the cover off and the culprit(s) is in a metal case with a seperate transformer. They look like germanuim diodes, or some kind of LED. From the picture you can see there are 3 in a row, each one flashes simultaneously.

I do not know much about scope circuits, and could not find a schematic so any advice here would be helpful.

It would be good to know what the circuitry is in this seperate enclosure - possible they are used with LDRs in the same case?

Whether I can simply replace the diodes, and with what type?

Or, do I just cut my losses and buy a decent scope as it only cost me a few quid?

Thanks for any help

Charlie
The service manual is here: http://www.electro-tech-online.com/c...Man_P3502C.pdf

Those three flashing bulbs are neon lights. I'm not sure they are your problem, they are in a separate enclosure because they are a part of the HV CRT circuit. Are there no burnt marks from the arcing? Alos try reading the manual to see if it can provide some more clues.

Good luck,
Jaz
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Old 3rd January 2013, 09:30 PM   #4
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Thanks for the help,

The photo was taken whilst they were arcing, there are no burn marks around them as they seem to still be firing up.

There are however scratch marks underneath them, inbetween the pins. So maybe this is a common problem and the manufactuerer or customer had already tried to prevent this.

Cheers

Charlie
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Old 3rd January 2013, 09:39 PM   #5
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Manual says diodes are ISS83, so I could find the equivalent and replace them.

But as SImon mentions they look good, or at least they are firing up in some way. SO possibly there is something else that is faulty that is causing this..
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Old 3rd January 2013, 10:01 PM   #6
Enzo is offline Enzo  United States
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But they are not diodes, they are neon lamps. Right below them in the photo is a diode. D407 is the glass part just above the red wire, for example. THAT is a diode, and if your part list says ISS83 for D407, it is to that small part they refer.

If by "arcing" you mean the orange glow inside the glass bulbs, that is normal operation, not really what we call an arc. I don't know if it is normal for them to be lit, but being lit is not a defect, it is just a sumptom.

Look next to each bulb on the board, does it not say N2, N3, and N4 there? N for neon bulb.

On the schematic Jazbo posted, thank you to him, they are located next to the CRT. They are wired between the cathode and first grid of the CRT.
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Old 3rd January 2013, 10:22 PM   #7
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I beg your pardon, wasn't paying any attention to what I was doing then. Good job people here do!

I said arcing becuase I did not know how else to describe it. I am assuming the neons are always meant to be on, but tonight there was a very fast repeated spark from the under side of the circuit. When I opened up the smaller enclosure the neons were blinking at the same rate.

I thought maybe there was an arc across the pins of the neon because of the spark, and also because of the scratched circuit board surface where it seems someone has already attempted to prevent this.

So if the neons are still lighting up, is it likely that these are not the issue and something else is causing this?

Sorry for my confusion, and thanks for all your help.
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Last edited by Brouilly; 3rd January 2013 at 10:26 PM.
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Old 3rd January 2013, 10:42 PM   #8
Ketje is offline Ketje  Belgium
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It looks like the neons shoud'd lite up at all.The are there to protect the wehnelt of the CRT from a to high tension.If that 's the case here,the HT goes up to high.The HT is regulated via a feetback loop trough the focus potmeter.Check the plugs and pot around the focusregulator for bad contacts.
Mona
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Old 3rd January 2013, 11:00 PM   #9
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"very fast repeated spark from the under side of the circuit."
You mean directly below the neon lights? In any case, if you can see where the sparks are generated, you are already half way to fixing the problem... Must be a bad bad trace on the board or component failure in the particular area of the board...

Enzo, good to see you here as well, do you know what's happening at music-electronics-forum? I can't access it for the last few days, may be just my bad connection?...

Jaz
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Old 4th January 2013, 12:41 AM   #10
Enzo is offline Enzo  United States
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No, I can't get on there either. Have not found it up a couple days at least.


OK, if the circuit is arcing elsewhere, it certainly could cause the neons to blink, however I still do not think the neons themselves are at fault. If they can light up and go out, that is pretty much all a neon bulb is designed to do. If an arc is causing an excessive voltage difference between grid and cathode on the CRT, they will blink trying to limit it.

If your arc spark is dancing between the actual wires of the neons, that too is not likely the fault of the neon bulbs, rather they just have legs close enough together an arc can strike.

Ketje, I see you are Belgish, but what is wehnelt in English? I get the idea, but wonder the translation.
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