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Cracked MoV's in 'scope (leader LBO-514)

fezder

Well-Known Member
Reason why I dug this from storage was because I need to order parts for that kikusui, and this leader has working XY-mode. I recalled couple things when I placed this on storage;
Slowish boot time (for beam to come in screen) and some issues with controls. Opening case to clean contacts, which should cure control issue, I noted couple MoV's cracked
IMG_2568.JPGIMG_2569.JPG
Mov's that have visible cracks are D117 and D118 (Schematics attached).

Obviously those cracks got me worried. But is it only cosmetic fault?
 

Attachments

sagor1

Active Member
If possible, I would replace them regardless. While they may be still good, they are showing some age or stress related cracks. They seem to be a 1000V and a 750V MOV. Original may be 15 to 17mm in size, try to get some at least the same size (joule rating)
Usually when a MOV goes, it burns up noticeably, but for a HV supply with limited current, there may not be other visible signs other than cracks.... The MOVs are probably designed to fire before you get an arc in the CRT.

Perhaps some helpful information:
 
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fezder

Well-Known Member
Fair enough, this 'scope is old. Now that I used it, there are couple more issues (surprise), minor but affects usage (now that I used it more after cleaning);
-most notable is horizontal jitter on XY-mode. Jitter is there whether or not signal is applied to inputs, but jitter is not seen on normal operation. This jitter is worse when unit is cold, so I personally suspect bad solder joint, or bad cap.
-Some minor offset issues with variable/div, which I can live with, but with channel 2 voltage magnification, beam jumps around one division upwards.
...the more you know. Sometimes I "hate" these old equipment, but at least keeps life interesting.
Could measure voltage lines (low voltage for now mind you) in case of bad ripple during startup. Thankfully there aren't that many electrolytics to be swapped.

Also, for MOV matter; I couldn't find datasheet for those current ones, but is voltage and joule rating enough to determine good replacements?

edit; thanks for link.
 

sagor1

Active Member
I would say the same voltage rating and a 20mm size would be more than enough, provided the 20mm size fits ok. Size basically determines the Joule rating, other things being equal. Other sizes now are 14mm, hard to find any 15mm or 17mm.
Watch carefully the voltage ratings. Like you say, can't find specs for the old ones. Littlefuse has common 1000V MOV types, but not 750V. Other vendors may have 750V - you need to shop around at places like Mouser of Digikey..
 

fezder

Well-Known Member
I thought it would be simple to buy 1kV and 750V varistors, but now I noticed that there is VDC, VAC, VRMS ratings. According to schematic, these varistors should be 750VDC and 1kVDC.

But like this example, 750V varistor, but 460VAC, 615VDC (didn't compare physical size)
 

sagor1

Active Member
If you want to search by VDC ratings, there are MOVs that meet your needs. Search Mouser.com for such. From the Canadian site, I get:
which shows Bourns makes 20mm and 14mm sizes rated for 750VDC and 1000VDC
But, (there's always a but...) what are the true ratings of the old ones? That is a key. Most MOV don't really start conducting until they have like 20% overvoltage (VDC). So, a 1000VDC MOV would allow 1000VDC to just sit there, and conduct very little....
Look at a typical 20mm 102K MOV Rated 625VAC and 825VDC continuous. At around 1000VDC, it starts to conduct 1mA, which is relatively low current, but it is starting to trigger at that point. Thus, that MOV protects for anything over 1000VDC, but expects less than that in normal conditions (825VDC). MOVs are not clear cut on/off, they start conducting along the way..
Thus, a 102K MOV is called a "1kV MOV" because that is where the conduction starts in earnest, it will generate a bit of heat in doing so, but does not full conduction until it hits around 1600VDC
 
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sagor1

Active Member
I would add, if the original markings are 751 and 102, I would order the same rating in the part numbers...
 

fezder

Well-Known Member
Oh yes, properties of original ones are still mystery, good point.

edit: well this datasheet shows something relevant of TNR varistor, V/I curve is sharper (page 5)
 
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sagor1

Active Member
That datasheet shows they have 751K and 102K types. I would just use those, and use a bigger size if possible. A bigger size (20mm) may dissipate a bit more heat at the 1mA conduction point, letting the device run a bit cooler.
I can't see the cracked ones with 751K and 102K being anything different than the current designations of 751K and 102K
 

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