K7000 Blowing Fuses :(

[zenomorp]

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.

 

[KenLayton]

The red wire is the input voltage to the resistor R301. Shorting that to ground could have damaged capacitors C55, C56, and C57.

Check capacitors C64, C59, C60, and C61 for shorted condition. Possibly your degaussing circuit diodes D23 or D24 are shorted.

 

[cadillacman]

may or may not be related but still also worth testing c36 just to make sure its not shorted out...

 

[zenomorp]

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.

 

[zenomorp]

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.

 

[DogP]

Have you checked D18? And you say it doesn't blow when not hooked up... have you tried narrowing down what blows it when you hook it up? I assume you've tried running it w/o the degaussing coil connected?

DogP

 

[modessitt]

Are you reconnecting the ground wire from the tube to the neckboard during testing? if you don't the fuse will blow, and you'll most likely burn up some parts like the VR, HOT, or the big ceramic resistor on the opposite side of the chassis from the VR...

 

[zenomorp]

Are you reconnecting the ground wire from the tube to the neckboard during testing? if you don't the fuse will blow, and you'll most likely burn up some parts like the VR, HOT, or the big ceramic resistor on the opposite side of the chassis from the VR...

Yes, I hook it up 100% every time I check it.

 

[zenomorp]

Have you checked D18? And you say it doesn't blow when not hooked up... have you tried narrowing down what blows it when you hook it up? I assume you've tried running it w/o the degaussing coil connected?

DogP

D18 checks good. (.532 with the diode checker setting)

 

[R3LL1K]

I'm sure you checked C36 and C37 as well?

 

[zenomorp]

I'm sure you checked C36 and C37 as well?

Yes, they both read the same and definitely not shorted. I am at work right now, so the exact value excapes me, but they are not bad. Something else though, pins 1 and 3 of IC4 read shorted. When I remove IC4, the actual pins on the regulator do NOT read shorted, but the pads on the chassis DO. I can follow the trace to R103. It read 2.8 Ohms, but if you follow the traces right to pins 1 and 3 of IC4, they read shorted. Don't know if that makes sense to you all or if it's even an issue, but IC4 is not shorted.

 

[zenomorp]

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.

 

[modessitt]

Check D13 and D14. Check C38. Check your VR...

 

[R3LL1K]

This sounds just like the issue I was having. Find one issue only to find out that another one popped up. I hope you figure it out.

 

[zenomorp]

Check D13 and D14. Check C38. Check your VR...

D13 and D14 both read .534 on the diode checker setting. Not shorted. C38 WAS shorted and has been replaced. That stopped the fuses blowing, but now I am experiencing what is described two posts up.

 

[zenomorp]

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.

 

[racer2e]

I've got one of these with a very similar problem and this thread has been very helpful. Mine has d18 shorted. Does anyone know of an NTE part number to replace this?
Jayme

 

[modessitt]

i've got one of these with a very similar problem and this thread has been very helpful. Mine has d18 shorted. Does anyone know of an nte part number to replace this?
Jayme

nte558..............

 

[zenomorp]

Ok, so I am going to admit my ignorance because I haven't been able to figure it out, but what is "VR?" Thanks.

 

[modessitt]

Ok, so I am going to admit my ignorance because I haven't been able to figure it out, but what is "VR?" Thanks.

Voltage Regulator. Otherwise known as IC4....