K7000 Blowing Fuses :(

[MK3FAN]

Hey everyone. I have a K7000 that I was working on out of the frame. I bumped it and the red wire contact coming from the big square ceramic resistor (180 ohms) touched the monitor frame. The chassis shut off and the fuse blew along with a flash from R103. Now the fuse blows every time power is applied. I checked all the normal things. ALL the power diodes check good. The HOT and IC4 check good. All the power circuit resistors check good for the correct values out of the circuit. What's strange is that the fuse only seems to blow when the chassis is hooked up. When I power it up completely removed from the monitor (I know, not a good idea, but it's only for a sec), the fuse doesn't blow. It only blows when everything is hooked up. There is a glow from R103 and it along with the fuse get RED HOT almost immediately. Any ideas? Thanks - Mike.

[lilshawn]

define what "everything" is, it seems that something in your "everything" is what's causing your short.

resistor + hot = not supposed to happen - r103 should be 2.7 ohms. if you measure it right now it may be in the 10's of kilo ohms from being overheated.

all the monitors voltage flows through this resistor...if it's out of spec it will load down the fuse till it blows....it is supposed to do this...

if you find you need to replace it...and i know what your thinking...i should replace it with a big 10 watt resistor so it doesn't blow again.

REPLACE IT WITH A 2.7 OHM 7WATT RESISTOR, as the schematic says. it is a SAFETY CRITICAL COMPONENT!!!!

[Kevin Mullins]

And how exactly did you check IC4 ?

[MK3FAN]

And how exactly did you check IC4 ?

Across pics 1 and 4. Does not read shorted.

[MK3FAN]

define what "everything" is, it seems that something in your "everything" is what's causing your short.

resistor + hot = not supposed to happen - r103 should be 2.7 ohms. if you measure it right now it may be in the 10's of kilo ohms from being overheated.

all the monitors voltage flows through this resistor...if it's out of spec it will load down the fuse till it blows....it is supposed to do this...

if you find you need to replace it...and i know what your thinking...i should replace it with a big 10 watt resistor so it doesn't blow again.

REPLACE IT WITH A 2.7 OHM 7WATT RESISTOR, as the schematic says. it is a SAFETY CRITICAL COMPONENT!!!!

R103 reads 3.4 ohms when removed from the circuit.

[lilshawn]

although r103 only being a little high, because of it's size it's likely that once it gets loaded down and starts heating up to "normal" operating temperature, that value may get to be triple that.

the drift is only supposed to be 2% or 5% AT normal operating temperature, your there at room temp.

3.4 ohms is almost 10% out....at room temp.....once it starts heating up...resistance goes up....then  more heat is produced....which means more resistance... until it catches on fire or blows the fuse cause the load on the fuse is too high.

IMHO change that resistor, and go from there,
also: take an old fuse and solder some wires to the end caps and attach them to a 75 or 100 watt lightbulb...youll save of fuses and if it comes on super bright and stays that way, you still have a problem someplace, but the lightbulb will take the load instead of the resistor, if it comes on bright but settles down, you may be alright.

[Kevin Mullins]

And how exactly did you check IC4 ?

Across pics 1 and 4. Does not read shorted.

What about pin 3 and any other pin ?

Pin 3 is ground.

[MK3FAN]

Well, like I said in the first post, the fuse only blows if the chassis is hooked up. I started ohming out all the yoke connection posts and the BLUE post was shorted to ground. Turns out it was caused by a shorted C38. It must have shorted when the red post touched the monitor frame. I had an extra 19" K7000 chassis layout around that I use for parts and it still had C38. It was the same part number as the one for the 25". I replaced it, hooked everything back up, turned on power, and...nothing. The fuse doesn't blow anymore, but the monitor doesn't power up. You can hear a SLIGHT chargeup of the tube, but it doesn't actually power up. So, I add 2 and 2 together and ohm'd the yoke coils. BOTH of them are shorted. DAMN! Looks like I need a new yoke to get me going again. Thanks for the suggestions guys. I appreciate it.

[MonMotha]

Dead shorted?  As in darn near 0 ohms on the lowest setting your multimeter's got?  Deflection yokes don't have overly high resistance to start with.

[Kevin Mullins]

Well, like I said in the first post, the fuse only blows if the chassis is hooked up.

And what do you mean by "only when it's hooked up" ?
You should never run a chassis that's not "hooked up" aside from the video input.

If you ran it without the yoke connected then there's a chance you damaged many things.

And if you checked the yoke with a continuity test, then yeah, it's gonna show as shorted.
You should Ohm it for resistance.

[SirPeale]

Well, like I said in the first post, the fuse only blows if the chassis is hooked up.

And what do you mean by "only when it's hooked up" ?
You should never run a chassis that's not "hooked up" aside from the video input.

If you ran it without the yoke connected then there's a chance you damaged many things.

And if you checked the yoke with a continuity test, then yeah, it's gonna show as shorted.
You should Ohm it for resistance.

Modesitt @ KLOV routinely runs G07s not hooked up.  He says you have to do it "very carefully".

[MK3FAN]

Well, like I said in the first post, the fuse only blows if the chassis is hooked up.

And what do you mean by "only when it's hooked up" ?
You should never run a chassis that's not "hooked up" aside from the video input.

If you ran it without the yoke connected then there's a chance you damaged many things.

And if you checked the yoke with a continuity test, then yeah, it's gonna show as shorted.
You should Ohm it for resistance.

The readings on both vertical and horizontal are 00.1 ohms. I am not checking them for continuity. I am reading ohms. Something else that I am not sure of is IC4. Looking at the bottom of the chassis, with the flyback close to me, pin 3 (middle pin) to the pin just in front of it reads 00.1 ohms. From pin 3 to the other pins do not read shorted. What does this mean? Am I reading it correctly? Is it 1,2,3,4 from flyback back? If so, then pins 2 and 3 are shorted. If it's the other way, then pins 3 and 4 are shorted (to each other).

[MK3FAN]

OK. New developments. Checked the resistance of the coils on another yoke I had. They read the same as the one on the tube I am testing the chassis on. So the Yoke is most likely NOT the culprit. After changing out C38, the fuses stopped blowing, but I still couldn't get the monitor to power up. I removed it from the tube again, checked, re-checked, and triple checked D19, D20, D21, D22, D23, D24, HOT, IC4, C38, R103, and R301. NOTHING is shorted. I am an aviation electrician by trade, so I'd like to think I know what I am doing. After re-checking everything, I went to try it again and now the fuses are blowing again! I am stumped, a bit frustrated, and ready to stomp this thing into oblivion! It's not a connection problem or anything like that. Everything that is normally the problem for fuse blowing checks good. Any further assistance would be appreciated greatly. Thanks - Mike.

[lilshawn]

are you sure there isn't something that is sticking out of the monitor mounting area that would cause the chassis to short out?


a screw or a metal peg?

extra long lead sticking out and grounding on the frame?

[MK3FAN]

are you sure there isn't something that is sticking out of the monitor mounting area that would cause the chassis to short out?


a screw or a metal peg?

extra long lead sticking out and grounding on the frame?

No, because the fuse blows even when the chassis isnt sitting in the frame.

[lilshawn]

you are running this monitor with an isolation transformer aren't you?

Running this monitor with NO isolation transformer will do the following:

-Power supply diodes

-voltage regulator

-burn trace foil

-or all the above

The chassis MUST be tested with a tube to produce the proper load. It will do a high voltage shutdown if no tube is attached

Try unplugging the degaussing coil. Sometimes it can get shorted against the frame.

check c36 and d18.

c36 should read OPEN it is a 4 legged cap. The ends are a common connection. Put a wire jumper in its place and use a common 2 legged capacitor if you need to replace it.

[MK3FAN]

you are running this monitor with an isolation transformer aren't you?

Running this monitor with NO isolation transformer will do the following:

-Power supply diodes

-voltage regulator

-burn trace foil

-or all the above

The chassis MUST be tested with a tube to produce the proper load. It will do a high voltage shutdown if no tube is attached

Try unplugging the degaussing coil. Sometimes it can get shorted against the frame.

check c36 and d18.

c36 should read OPEN it is a 4 legged cap. The ends are a common connection. Put a wire jumper in its place and use a common 2 legged capacitor if you need to replace it.

D18 checks good. How am I supposed to read C36? After poking around and taking some more readings, I found C37 was shorted. I replaced it and now the fuse has once again STOPPED blowing, but the monitor still doesn't power on. R301 (big ceramic resistor on the side) get REALLY HOT almost immediately upon power application. There is definitely still a short somewhere, but you got me where it is. I'll keep poking around.

[MK3FAN]

Further t-shooting has discovered that I now have no High Voltage. Nothing at Pin 4 of IC4 and nothing on pin 3 of P202 at the neckboard. The K7000 manual trouble tree is of little help. There is 68VAC at the fuse, but nothing anywhere else. I replaced IC2 and IC4 with known good ones. Nothing. Replaced the B+ pot, but without proper operation, I can not adjust it. This is where I am now stuck. What's the first step. Thanks - Mike.

[Ken Layton]

Remember that IC4 and the horizontal output transistor both need mica insulators and silicone heat sink grease.

[MK3FAN]

Remember that IC4 and the horizontal output transistor both need mica insulators and silicone heat sink grease.

Check.

[lilshawn]

How am I supposed to read C36?

measure between the farthest left and right legs should read open...

the shorter distance is hooked together... like this...

http://arcadecontrols.com/BBBB/csc.html

[MK3FAN]

How am I supposed to read C36?

measure between the farthest left and right legs should read open...

the shorter distance is hooked together... like this...

http://arcadecontrols.com/BBBB/csc.html

Thanks. I'll check it out. But I have TWO caps installed there. Two blue ones, not one green one. Also, I am going to finally admit my ignorance because I have searched without success to find out what "VR" means. Anyone wanna fill me in? Thanks.

[SirPeale]

voltage regulator?

[MK3FAN]

voltage regulator?

I found that out a few minutes ago. I have replaced it and double checked it (across pins 1 and 4) and it checks good.

[lilshawn]

But I have TWO caps installed there. Two blue ones, not one green one.

i would pull that and mod according to the directions I/arcadecontrols have provided above.

i don't know why the did that, but with 2 caps instead of one.

[MK3FAN]

But I have TWO caps installed there. Two blue ones, not one green one.

i would pull that and mod according to the directions I/arcadecontrols have provided above.

i don't know why the did that, but with 2 caps instead of one.

Done. Made no difference. I think the insulator between the HOT or the VR may be bad. I am going to check them out when I get home from work.

[lilshawn]

i make sure there is no burrs or anything on the edges of transistors and the like that have to be insulated, even the smallest of burrs is enough to poke right through.

especially since it looks like the stamp them out using a dull punch

[MonMotha]

Also make sure that the insulator on the screw that holds the tab to the heatsink isn't cracked.  These often crack due to heat stress and being under constant compression.  The result can be a somewhat intermittent failure that won't show up until something knocks a little of the insulation away and allows the screw to short the tab to the heatsink.

[MK3FAN]

Also make sure that the insulator on the screw that holds the tab to the heatsink isn't cracked.  These often crack due to heat stress and being under constant compression.  The result can be a somewhat intermittent failure that won't show up until something knocks a little of the insulation away and allows the screw to short the tab to the heatsink.

Well, I moved the HOT and IC4 away from their respective walls and left them free floating to see if they are shorting to the case, and it made no difference. Still no High Voltage. I did find out that legs 3 and 4 of T2 are shorted to each other. Is that how it's supposed to be?

[MK3FAN]

Well, I removed R103 from the circuit and it read 01.1 ohm. That's a short in my book. It's supposed to read 2.7 ohms. It reads 3.1 in the circuit, but 01.1 out of the circuit. I'll replace it and see what happens.

[MK3FAN]

Well, I removed R103 from the circuit and it read 01.1 ohm. That's a short in my book. It's supposed to read 2.7 ohms. It reads 3.1 in the circuit, but 01.1 out of the circuit. I'll replace it and see what happens.

Nope. No difference.

[lilshawn]

I did find out that legs 3 and 4 of T2 are shorted to each other. Is that how it's supposed to be?

well yes and no...

it's a transformer, it's not really supposed to have resistance in it, otherwise you would be wasting energy, effectivly to test a transformer you need to test between the coils...in this case seeing if there is a short between them... in this case 1/2 is one coil and 3/4 is another.... so test between 1/3 or if you like 2/4 or 1/4 or 2/3 (you only need to do one.)

Well, I removed R103 from the circuit and it read 01.1 ohm. That's a short in my book. It's supposed to read 2.7 ohms. It reads 3.1 in the circuit, but 01.1 out of the circuit. I'll replace it and see what happens.

resistors, when stressed don't typically lessen in resistance, they go up in resistance...replace it though.


my thoughts for further diagnosis:

since you stated first that you shorted r301 to the chassis ground...i would concentrate on everything UPSTREAM of the resistor... this includes...

r103
ic4
r84, r85, r86
c55, c56, c57, c59, c60, c61 (59, 60, 61 should read open)
d19, d20, d21, d22 (i know you checked them, but be sure..on DIODE check setting of your multimeter...they should all read nearly identical reverse voltage)


also....

triple check C36 and C38...  the capacitors should both read "open." You'll have to pull them out of circuit to be sure. The value of C36 is 6100 picofarads, 1500 volts. This value may be a bit different in your monitor due to changes in circuit design to accommodate different CRTs and horizontal scan rates. Also, some monitors use a capacitor with four legs instead of the normally found pair of component leads. If you are using a two legged replacement component, you'll need to add a couple of small jumper wires. The value of C38 is .39 microfarads, 200 volts.

if your fuse is still blowing, you have a short someplace in the B+ line or power supply section... you just need to track it down.