U5000 no high voltage

JukeboxJon

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Have a u5000 im working on had some power supply issues fixed those it passed the light bulb test fine, b+ was around 125v dc which should be fine, oddly hooked up to the tube it reads about the same even thought the high voltage never kicks on. I have tested all the usual culprits HOT even thought it makes no ticking i hear absolutely nothing, r760, r765, r728 even out of circuit, q703, q708 etc etc pretty much the entire horizontal side compared with a working u5000 and have found not a single thing, all i have noticed is q703 is getting really hot and it has the heat sink mod on it, i know it gets hot already but i think way hotter than normal. I went to test it out of circuit and the original one the legs literally fell off so I replaced it and the new one still gets really hot and no high voltage so i dont let it run for long. Swapped the LA chip since it already had a socket installed and it was easy still nothing. Tested continuity everywhere found nothing out of the order. It also has a new cap kit installed so that should not be in question and double checked my work, i did the cap kit to rule it out actually.

Any thoughts?


I think my only next move is to either change the HOT even though out of circuit it test fine and is an 86 not an 88 or to replace the flyback, im a bit suspicious of it anyway because its the bleeder box type. But im not sure if either of those could cause q703 to burn up.

Or possible a weird issue like a ceramic cap or poly cap even though i inspected all those and tested for shorts and found nothing.
 
Is @MKL @zenomorp @mecha all on vacation or something? Someone convinced me its rare that the flyback shits on these and that it could still be a power supply problem even though im getting b+, since it cost like 2$ if that for 25 of every resistor that encompasses the start up/high voltage circuit of a 7400 series ima replace them all lmao.
 
You should first measure voltage on pin 10 of U701 (LA7850). Then, if +12V isn't there you go check R760, R765, Q708 etc. There's no point in pulling those first and not checking for the voltage that makes the horizontal pulse come out from pin 12 of U701 and drive the HOT/flyback.

And are you sure all the other secondary DC voltages from the flyback are absent? You have +15V (K D301), -65V (A D303), +30V (K D302), +24V (K D711) and +160V (K D310).

Also, check the voltage on the side of R728 that goes to the primary side of T701, same as the positive of C716. Compare to what you have on the good chassis.
 
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You should first measure voltage on pin 10 of U701 (LA7850). Then, if +12V isn't there you go check R760, R765, Q708 etc. There's no point in pulling those first and not checking for the voltage that makes the horizontal pulse come out from pin 12 of U701 and drive the HOT/flyback.

And are you sure all the other secondary DC voltages from the flyback are absent? You have +15V (K D301), -65V (A D303), +30V (K D302), +24V (K D711) and +160V (K D310).

Also, check the voltage on the side of R728 that goes to the primary side of T701, same as the positive of C716. Compare to what you have on the good chassis.
Thank you MKL that why i was trying to tag you lol, I will check those tomorrow

I will check those diodes as well thats great information.

To rule out the transformer? Have you actually ever seen one of those go bad or to make sure im getting the right voltage from the power supply section?
 
The diodes are the rectifiers for those voltages. I mentioned them as test points not because I think they could be bad.

You're talking about the power transformer T101 in the power supply? That can't be bad if your bulb test is OK even if the voltage is slightly higher (can't you adjust it down to 117V?).

As for flybacks, after ruling out all possible causes, if I'm still in doubt I test it with the STVDST-01. There's a few on ebay right now. If you're into fixing monitors that's a must have tool.
 
Is @MKL @zenomorp @mecha all on vacation or something? Someone convinced me its rare that the flyback shits on these and that it could still be a power supply problem even though im getting b+, since it cost like 2$ if that for 25 of every resistor that encompasses the start up/high voltage circuit of a 7400 series ima replace them all lmao.
I think I'm allowed to be away for 2 days to attend one of the annual conventions I actually want to go to

also M K L is way more qualified on this subject than me
 
The diodes are the rectifiers for those voltages. I mentioned them as test points not because I think they could be bad.

You're talking about the power transformer T101 in the power supply? That can't be bad if your bulb test is OK even if the voltage is slightly higher (can't you adjust it down to 117V?).

As for flybacks, after ruling out all possible causes, if I'm still in doubt I test it with the STVDST-01. There's a few on ebay right now. If you're into fixing monitors that's a must have tool.
So i was getting 12v to u701, so I went and checked r760 etc again and found the problem, when i got the chassis it was already missing r760 and r765 for what ever reason.

R760 i had put in 100k ohm resistor and here is why the u5000 specifications sheet is wrong, it says 100k ohms on the spec sheet from wgs website not 100 ohms. If i remember right now that i think about it I believe some revisions had a 470ohm resistor in r760. Thats the second error/typo ive found on this spec sheet, throwing this sheet in the trash the full manual appears to be right. So evidently because the 100k resistor was so out of spec being the wrong one it was stopping the horizontal circuit from starting. First time firing it up i had some weird vertical double image problems, someone already installed a socket so i changed u701 thinking i might of damaged it and it fixed the problem, put the u701 I had back in it with vertical problems and it works fine so not sure what that is about will have to double check some traces/soldering, but bleeder box flyback is perfectly fine and it looks great 😌
 

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This sheet here, garbage. Second time it caused me to cause myself problems lol. R760 being wrong as mentioned about, and q9-q11 are 2n3906 not 3904 complete diff transistor type.
 

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R760 was 470 ohm originally. the NFL Blitz HOT service kit changed it though.


that manual scan is a newer one. I'm curious what my schematic sheet at work says for the part.

also I remember them changing it to 100 ohm, I have no idea where 100k came from as that's significantly too much
 
R760 was 470 ohm originally. the NFL Blitz HOT service kit changed it though.


that manual scan is a newer one. I'm curious what my schematic sheet at work says for the part.

also I remember them changing it to 100 ohm, I have no idea where 100k came from as that's significantly too much
Yeah lol why I did not catch it the first time I was like 100 that seems right wait 100k?? Then crosschecked with the manual. I guess I also didnt think that sheet would be wrong and its been a while since ive had time to work on my chassis I use to have those 2 memorized r760 and r765, as far as Ive checked so far the parts list in the full manual everything is correct even the 100 ohm r760 revision. I just was using that sheet because I didnt need theory of operation etc etc lol.

I printed out that copy too of the blitz HOT service kit so I have it never know when links to these documents will go away, they have before on WGs website.
 
Yeah lol why I did not catch it the first time I was like 100 that seems right wait 100k?? Then crosschecked with the manual. I guess I also didnt think that sheet would be wrong and its been a while since ive had time to work on my chassis I use to have those 2 memorized r760 and r765, as far as Ive checked so far the parts list in the full manual everything is correct even the 100 ohm r760 revision. I just was using that sheet because I didnt need theory of operation etc etc lol.

I printed out that copy too of the blitz HOT service kit so I have it never know when links to these documents will go away, they have before on WGs website.
yeah much of the parts that nuke the HOT are listed there. in my experience if you find an open R760, Q708 is probably shorted. 😮
 
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