G07 fuse blowing- this one has me stumped

Tubular Gearhead

Active member
Joined
Dec 15, 2011
Messages
983
Reaction score
88
Location
Orangevale, California
Hi,
I have a G07 that was blowing fuse 902 every time it was powered up BEFORE any work was done to it. The chassis looked like nobody had worked on it before. I recently decided to just rebuild the whole thing, thinking that whatever problem it was would probably get fixed along the way. It still blows F902 EVERYTIME it is powered up. I carefully installed a Bob Roberts cap kit (noting the polarity of all electrolytics, including the one that is frequently mis-labeled on one side of the pcb), new HOT, new flyback, new power diodes (D901-4), new C1106 power regulator, and new width coil. I have checked the degauss circuit and it is fine. I even checked the tube and frame by installing a known working G07 chassis and it works fine, so I know the problem is not the tube or degauss circuit. I have compared readings all over the pcb on both chassis and they seem to be the same for the most part. I have checked most all the transitors on the chassis and they look good. Ohm resistance going into the ac plug is the same on both chassis. Large R01 reads 220ohms on both the bad and working chassis.

This one really has me stumped... I feel like I have looked for all the usual suspects and still don't understand why it still ALWAYS blows the fuse. Maybe the focus block???

I know this one is a tough one, but if any of the gurus have any advice I would sure appreciate it!

Thanks in advance. :)
 
I didn't see this in your listing of stuff that was done.

Did you re-flow all the connector pins? That is what caused my fuse to blow.

Another idea - since you have multiple chasis, you could put them side by side, and measure resistance from point to point starting at the fuse, to see if you could find the low resistance short or high resistance connector.

Also (long shot) but you could have a bad batch of fuses. Before you do anything, try your "blowing" fuse on the other chassis, and see if it goes. I have seen bad fuses in the past on occasion.
 
you put the insulators back in between the H.O.T and the VR correct.

Peace
Buffett

Yes I did. I have since verified several times that the two big transitors are not shorted and working right.

As far as the header pins- could a bad solder joint on them cause the main power fuse to blow? I thought that would just give me bad sync, or colors, or some other problem.

Thanks guys for the suggestions so far.
 
I didn't see this in your listing of stuff that was done.

Did you re-flow all the connector pins? That is what caused my fuse to blow.

Actually, come to think of it the yoke conector has header pins... I did not check that for bad solder joints. Would a open circuit/high resistance there cause the fuse to blow?
 
look for bridged solder joints and verify that you have the rectifier diodes in the correct way.

Peace
Buffett
 
Fixed!

Well, I wanted to update this thread in hopes that this helps others out. I finally got the chassis fixed and this one really was strange (in my opinion). To cut to the chase, it turns out that one of the capacitors was in backwards, causing the fuse to blow. However, the reason it was in backwards seems very, very unconventional to me. After checking my work over and over again I finally noticed the markings on this one cap and could not believe it. I did not take a picture of it last night, but cap 511 in the kit I bought from Bob Roberts (which contained various manufacturers) has the stripe on the cap GOING DOWN THE POSITIVE LEG! To the manufacturers credit (I guess) the stripe is labeled >+ >+ >+. Yep, in decades of guitar and HI-FI tube amplifier contruction and repair experience I have never seen an electrolytic with a clearly labeled stripe that was not negative. Oh well, learn (to check)something new every day...

Anyway, the monitor works great now- crisp, bright and clear. I just wanted to give this "heads up" to others who have recently ordered this kit from Bob to CHECK the markings on cap 511!

That closes this mystery.
 
Back
Top Bottom