19" K7000 Blows HOTs

ManiN

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My K7000 keeps blowing the HOT and not any fuses. I have done a repair kit from Bob R. that is the caps, flyback, fuse, HOT, and I think that's all. Anyway, this thing has done this twice now. My thoughts were that it was the flyback, so I put the 'kit' on and it blew the new HOT within the first few power cycles! At $5 a pop, this is getting irritating. The same thing happens every time in that one side of the hot is shorted, and the large 220ohm resistor takes the load like a damn toaster.

I have checked through and through, and now am stumped. My last hunch is that it may be the voltage regulator. Any hints on how to check that? Specific values I should look for with a meter?

Thanks!
 
You can test a voltage regulator similar to any other transistor, but it may perform differently under load.

I'd say replace the regulator anyway, since you're going to get another HOT - buy a regulator while you're at it.

Good advice I got from Chris25810 is to double check ALL of the solder on the foil side of the board. Check for loose pins, lifted traces, solder splashes, cracked joints.

Have you tried a 2nd flyback? There's a few threads here about bad flybacks right out of the box.
 
Hmm, yeah I think I'll put a new one on anyway. That would suck if the flyback isn't even bad and I got another one, but I understand it being bad out of the box.

What would cause a transistor to short? Higher than rated voltage / watts fuzing the plates together?
 
C36, C38, D18 should be tested.

Also, you are installing a fresh mica insulator between the new transistor and metal frame, aren't you? And smearing said insulator on both sides with silicone heat sink grease?
 
yeah, the HOT was bad when I first got the monitor. Guy I got it from said he swapped the yoke wires to 'flip' it, and heard a pop. After that the monitor was dead. I got it from him and traced the short to the HOT. I slapped another one in it and it worked for like 6 months. It randomly died on start-up on day, then I put a 'kit' on from Bob. R. thinking the flyback was going bad and causing the HOT to short. With the fresh kit, the damn thing only lasted like 30 min. and about 3-4 power cycles. So before I start throwing HOTs at it like fuses, I thought I'd ask for help.

So if the flyback is possibly bad out of the box, do we just eat it for cost from Bob R.? I noticed the adjustment knobs acted kinda funny giving wiggly signals on the screen if you jiggled them while adjusting the flyback screen etc.
 
C36, C38, D18 should be tested.

Also, you are installing a fresh mica insulator between the new transistor and metal frame, aren't you? And smearing said insulator on both sides with silicone heat sink grease?

I didn't replace the mica insulator, but I cleaned it and doped both sides and the HOT itself before installing. It seems to die right on startup, never during operations, so I'm not convinced heat is the culprit.
 
Is the dag wire being reconnected? Was the fuse blown when you got it?
 
by dag do you mean the ground wire from the neck board? yeah, I reconnected that. The fuse has never been blown since I had it.
 
so I'm not convinced heat is the culprit.

The concern is really whether or not it is shorting to the heatsink. That will take a H.O.T. in a heartbeat. (although it usually blows a fuse too)
The "insulator" is there to keep them separate and not physically touching or have a potential arc between the H.O.T. and the heatsink.
 
Gotcha, yeah I don't think there is any contact going on. the mica was in pretty decent shape. I'm going to pull the board and do some checks on the caps n' resistors mentioned. Be nice if it ends up that simple.
 
Jake, what is it with k7000's blowing the hots in the pac group today? One of mine blew a fuse and hot a month ago so I got a new fuse and hot and it worked fine for a while but today when I was testing another monitor in my dual monitor cyberball, after the power was on for probably about 10 seconds I hear the crackling of a monitor that has just turned off and sure enough, it has happened again. Stupidly I did not put new heat sink grease on even though I have some on hand. So now I'm left wondering if that is the problem or if it's something else that is randomly blowing the hot. Where do you even get new mica insulators? Bob doesn't include it with the hot.

The worst part about this? I placed an order with Bob earlier today. Ugh.

Guess I'll do the same. Pull out the soldering iron and check other components. Don't think it would be the fly. I changed that out 1.5-2 years ago and all that time the monitor worked fine.
 
yeah no kidding! Hey, let me know what you find, I'll do the same. If I have time tonight, I'll pull it and look.

I'd say as long as you used a mica piece period, you're probably ok there. Though next time you should get off the lazy train and put the heat transfer paste on haha.
 
I see a pattern here.

I also have a 25k7000 chassis blowing hots. I would replace flybacks on all my rebuilds and didn't have the proper test equipment to test the new flybacks on arrival.

I purchased a flyback tester for around $25.00 to test this flyback in particular. FLYBACK IS FAULTY.

I sent Bob a message regarding my findings. I am not looking for a replacement from Bob for the faulty flyback because I purchased it about 6mos. ago. But I did ask him to confirm it is faulty. I am sending it back shortly. Good luck with your findings everyone, but I would recomment testing before installing.
 
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I'm in that same boat as I purchased mine 6 months or so ago... it'd be cool if Bob could just throw it on a tester before he packed it if this is such a huge deal. Shit, I'd even pay him a lil extra to do so.
 
Well apparently I will not be sending it back to Bob Roberts for testing. Here is Bob's response to my inquiry. It appears that statistics show I don't have a bad flyback.


Hello bob,

I have a question relating to a past order, and it may have been some
time ago.

I purchased a K7000 flyback for a monitor repair and could never get the
chassis repaired. I recently purchased a flyback tester to test flybacks
before replacement. Usually G07 flybacks get replaced when I do a
chassis rebuild because of the faulty originals. Before purchasing the
flyback tester I was taking the same approach with k7000 chassis because
the flybacks are cheap and I want the chassis to be reliable.

Back to the chassis I couldn't repair. I pulled the flyback and tested
it, and it's also bad. Due to my lack of test equipment at the time I
wasted countless hours and many HOTS trying to solve this problem.

I would appreciate it if I could send this flyback back for you to
confirm it's shorted/faulty. I am not looking for a credit, or
replacement. At this point I would just like to know I found the
problem. Unfortunately I don't have a replacement and I will order a new
one but will wait to hear back from you on this particular unit.

In the future I will test them on arrival and contact you in a timely
manor.

Bob's Response.

As voted on 16 years ago... all sales are final. As for flybacks, they are
one of the few things that I can't test before shipping, but in 55 years of
dealing with flybacks I have never seen a bad replacement nor ever had any
of my techs find a bad one. I can only remember of one that was ever
physically damaged & I cannot say for sure it was shipped that way. My tech
at the time brought it to me with the top snapped off evidently from being
dropped & swore it was in the box that way when he opened it.

I've been around longer than flyback ringers & have tested just about every
new one that hit the market that D'mambro Electronics distributed & have yet
to find an accurate one. They will drive you insane... good shows bad... bad
shows good... they are simply worse than the old time vacuum tube testers
that had about a 15% accuracy! Back in the late 90's I had let about 20 of
the guys send back flybacks at their expense for me to test... the right
way, substitution... because they were soooo adamant that theirs was bad.
Not a single one ever was, so no one spoiled my record. I swore I'd never
waste my time like that again, but about 6 months ago one of the regulars
said he had a bad flyback for sure... absolutely positive... and he said I
just had 55 years of good luck. Then he sent it to one of the well known
online shops to be checked out. He repairs them every day & helps the hobby
greatly, so I won't disclose the shop, but they confirmed that the flyback
was defective... which I had never heard of...ever!!! So I told them to send
it back. I popped into a $15 clunker without doing anything else & it fired
right up! I took my test K7000 rig & removed the flyback & installed it onto
it.... fired right up without incident. Took picks & sent it back to him. I
was told on this & another occasion that a Antec tester had given the false
readings. In this case it turned out to be C36 according to the shop..."C36
had changed values a little and was just enough to keep the B+ off" .

While I cannot test your flyback for you, I feel very confident in telling
you that yours is a 99.9% good flyback. You'd be breaking a long standing
record if it were not. Granted while in the business I only went through
15,000 or so, replacement flybacks without incident, but since retiring
we've more than tripled that number bringing it to about 60,000 lifetime
flybacks with only 21 claims of defective product all disproved. It's pretty
hard to mess up a coil of wire in production... I'd almost have to say it
would have to be something deliberate to have a mfr'g defect.
 
Or not... haha. Hey thanks for relaying your correspondence man! Well, should we figure our fly-backs are fine?
 
I will keep this flyback for a spare for now and order another one to put into this chassis. Looks like I have my work cut out for me. I will post my findings once this chassis is back in action.
 
So I pulled the board finally and found R301 had gotten cooked again like it took the shorted load. Must have had it on longer cause the wires were a bit cooked on either side of it. I'm surprised that circuit isn't fused... Anyway, poking around the only thing I found that didn't seem right was that pins 2 & 4 on the VR (IC4) seem to be shorted. That's not normal right? All the local components seem to be fine. Would that explain the unit working for a while then dying on startup later? Or did the real culprit take out the VR with the HOT?
 
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