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VT100 keeps blowing main fuse

hush

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hey all, another issue in my lab :) i just received a VT100 from someone with better diagnosis and repair skills than myself, and while it left the shop fully functioning, it arrived to me non-working. it seems that the main fuse blew at some point between then and now, and while i replaced it (exact part here), the new fuse blew instantly when i turned the terminal on. unfortunately the person who repaired it will be on vacation for the next couple of weeks, so i'd like to try and fix it myself if i could. i'm not really sure where to begin looking, so any suggestions are greatly appreciated. it only has the basic RS232/current loop I/O card, nothing else.

for what it's worth the terminal is plugged into a surge protector with a decently well-built power cable that i've used on my servers in the past, so i don't think there's an issue with the power source. maybe something shook loose during shipping? i can't see or hear anything when i move it, so i'm really not sure.

thank you in advance, as always!
 
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pbirkel@gmail.com

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A hard-blow after being shipped strongly suggests to me that a conductive part came loose and lodged in an unfortunate location. Likely a piece of mounting hardware, perhaps a nut or lock washer. Perhaps even a "foreign body" left behind during a pervious repair (bit of snipped wire, solder ball, ...). Suggest carefully shaking the monitor while held in various positions while listening for a rattle or sound of loose debris shifting. Then removing the cover and carefully inspecting throughout.
 

MattisLind

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I have had the “instantly blowing fuse” in a few VT100 terminals. As far as I remember this has always been the main switch transistor that was short-circuited.

Now it was supposed to work before tranportation so it could be something that has came lose in the PSU.

If you cannot find any lose parts in the PSU I would recommend checking the switch-transistor and the high-voltage fast recovery 1A diodes that is surrounding the switch transistor. If these go bad the switch transistor may blow.
 

daver2

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The schematics are here https://archive.org/details/bitsavers_decterminachematicFeb82_6949589/page/n26/mode/1up. See page 27 of 64 for the power supply.

It would be worth double-checking that the voltage selector is correct for the voltage supply you are running it from. Also, check that the fuse is the correct rating (and type) for the voltage. The fuse will be approximately twice as large for 110V as 220/230V. Also make sure that it is the correct 'type' (anti-surge or not) when compared to the parts list.

The mains switch appears to be ahead of the fuse. So that cannot be the culprit.

+1 for looking for something that has come adrift and shorting something out.

After that, it is working through the power supply schematic looking for faulty components I am afraid.

Dave
 

hush

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thanks all for the responses. had a chance to look at this over lunch today, and it doesn't seem like there's anything floating around in the chassis- pulled both the terminal controller board and the power supply board and took a look, shook the chassis out a bit while they were both out as well to no avail. seems like it's just bad timing and bad luck, unfortunately. my inexperience is absolutely showing here but i can't actually seem to identify the type of fuse i need from the parts list, i see a few listed but they all appear to be surface mount components. if an incorrect fuse type could be the culprit then that's my best guess, it's hard for me to imagine a component dying after a few days in transit right after a repair, but that may just be me coping. :)
 

hush

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I have just checked the VT100 User Manual - and it states 3A normal blow (irrespective of the line voltage). So do not use a Slow Blow (anti-surge) fuse...


Dave
yep, that's where i looked initially when i did my home depot run yesterday, i was just under the impression the parts list would have more detailed specs. i am using a fast-blow fuse that's rated for 3A 250V so it shouldn't be the fuse... very unfortunate :( i'll try and do some diagnosis this evening, see if i can't come up with something. thank you all again!
 

thunter0512

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yep, that's where i looked initially when i did my home depot run yesterday, i was just under the impression the parts list would have more detailed specs. i am using a fast-blow fuse that's rated for 3A 250V so it shouldn't be the fuse... very unfortunate :( i'll try and do some diagnosis this evening, see if i can't come up with something. thank you all again!
I would change the fast blow fuse to a standard blow fuse. Fast blow won't survive the inrush current when you turn it on.
 

daver2

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The manual recommends a standard fuse. If there is a genuine fault, and you incorporate a slow blow fuse, you stand a chance of doing more damage.

It is not good practice to replace the correct type of safety device for another one.

Dave
 

MattisLind

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It is quite interesting to note how the fuse blows. When there is a short in the switch transistor or rectifier bridge the fuse blows in a quite spectacular fashion with metal fumes covering the glass. At least if the fuse is a standard or slow blow.
 

Roland Huisman

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A blown mains fuse in a switching power supply is almost always a (partly) shorted mains rectifier and/or a shorted switching power transistor. When the transistor is also shorted there is a chance that the driver electronics is also bad... Recently I fixed a VT320 power supply and you can see quite a chain reaction of bad components... Don't know about the VT100, but switching power supplies often have similar problems... The most important thing is to find the root cause first, otherwise you will blow out the new components again.
 

hush

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it seems to be blowing the fuse before i even turn it on (when the power supply is plugged in), and not in spectacular fashion. i did notice a small flash of light at one point from the rear of the tube at one point, but nothing really beyond that. there is a bare wire touching the side of the CRT, but it looks as though it's supposed to be there so i won't do anything to it. picture attached:

IMG_0951.jpg

i've been prodding at the power supply on and off but haven't been able to identify a failure point yet. i'll keep working at it.
 
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daver2

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Can I just confirm what you mean by "before i even turn it on". Turn what switch on?

The fuse is supposed to be after the ON/OFF switch, so is the fuse blowing with the VT100 switched OFF on the unit, but ON at the wall socket?

I just want to clarify this point.

Dave
 

Roland Huisman

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That wire is to connect the conductive layer on the CRT to gnd. Does your Vt100 have a mains filter in it? Maybe some versions do have a filter. These can go bad too... Can you post some pictures from where your fuse is located?
 

daver2

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I was thinking a similar thing Roland... That was going to be my next suggestion if the response to my last email was YES.

Dave
 

jackrubin

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Assuming from his location info that hush is resident in the US and is plugging his VT100 into a wall socket, there is no ON switch for the individual socket. Our mains outlets have the "feature" of being always on, unless switched off at the central breaker panel. Sounds like at least a shorted AC input filter.
 

hush

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reading the schematics and technical manual, it seems like the metal assembly that houses the power plug also houses the AC input filter? if so, i'm getting continuity between the ground prong of the power plug and the exterior of the assembly. would this be a simple repair or should i consider replacing the power supply with the ATX conversion i've been reading about?

edit: also yes, currently in the US
 
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hush

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ok, so it seems like i was mistaken... just disconnected the whole switch and fuse assembly from J1 on the board and tried inserting a fuse then adding power, it didn't blow. only when the power is switched on. so the issue is further down the line. i think it must've been a fluke the first time, because i wouldn't have left power on without a fuse inserted. back to working through components...
 
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Roland Huisman

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I would not do a ATX rebuild or something like that.. But you said: "the person who repaired it will be on vacation for the next couple of weeks, so i'd like to try and fix it myself if i could."

So then I would think: sit back and relax.... Don't do more harm then good... Wait until he is back, check it together and post what you have learned here on the forum...
 
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