25K5501 short.......

[modessitt]

Picked up a game that somehow smoked (and partially melted) its isolation transformer.  This was not it's original iso (had obviously been installed later).  I installed a big, older one I had lying around and unplugged everything until I could check the voltages to the various points.  Everything looked good, so I hooked everything up and hit the switch. 

Good news?  The boards power up and I can hear gameplay!

Bad news?  Monitor (which seems to power up on a delay of about 5 seconds) blew the fuse immediately!  Don't know why it hadn't blown when the iso went (especially as it had another fuse wired in series between the iso and the monitor power plug sometime in the past - and that fuse wasn't blown either).  Anyway, I pulled the chassis and checked the HOT (2SD870) and found it blown.  Checked IC501 (STR381) and it seemed okay.  Replaced the HOT and the fuse, hooked it all back up and powered it up.  Blew the fuse again.  Pulled the chassis and found the HOT blown again.  This time IC501 was also showing a short (both checked out of circuit).  Used the flowchart for the K49xx (seems to be the same) and the diodes D501-D504 are okay.  With the HOT out of circuit, I'm measuring a short across the two holes for the base and emitter, but that seems to be because they are connected through T351.  I have not done the cap kit yet (was hoping for a quick fix) but that will be my next step.  I don't recall bad caps causing the HOT and IC501 to blow, though I could be wrong.  Am I?  Also, I know the flowchart states that the flyback rarely fails on these monitors, but I wonder if I should change it anyway?  I have another K49xx series chassis with the same flyback that DIDN'T blow the fuse, although it had other problems which ended up turning it into a parts chassis.

I know, I know.... DO THE CAP KIT FIRST!  This is usually my advice, also.  Since my caps are in the shop (and I'll have to go get them later), I thought I'd ask for other ideas, in case I need to grab other parts while I'm there.

Thanks in advance...

[modessitt]

Forgot to mention that the big white resistor on the frame which is soldered to the chassis between position 51 and position 52 (as labeled on the back) gets extremely hot to the touch in the 2-3 seconds between power on and fuse blows.  It is held in place by a metal bracket that covers the value, so I can't read it right now, although it measure about 42k-ohms across those points.  just thought I'dmention it....

[grantspain]

maybe worth checking any high voltage polypropylene caps on the collector of the hdt,if you have a faulty ploy cap it will carry on blowing the hdt,is this a wg chassis?

[modessitt]

Hey Grant!

Yeah, it's a WG.

As far as I can tell, the poly caps seem okay, as they do the standard cap measure (start at 0 and start climbing) on the meter.  Since I've got a flyback lying around that's supposed to be good, and I need to do a cap kit anyway, I'm going to go ahead and do it just for fun.  I'll probably pull  T351 out of circuit and start looking for a short somewhere, even pulling those caps out of circuit to test.  Unfortunately, the wife came home and now I probably won't be able to mess with it more until tomorrow.  I'd really like to play this game, though....

[grantspain]

i bet ken would know exactly what the problem is with this one,maybe worth send him a pm.
i have just looked at the schem and for the fuse to blow after a few seconds as opposed to straight away it would point to a problem in the circuit of ic501,most likely the 130 v-but why thats killing the hdt i am not sure,the 2sd870 has a diode built into it,unless of course the flyback is completely shot

[modessitt]

Well, the only flowchart I have to work with is for the K4900, which seems to be very similar in circuit design, if a little different in a few parts.  I've also had to use a K5515 manual as that's the only one I can find in the K55xx series.

Here's the update so far.

I replaced all the caps.  During my capping, I found two things of interest.  C301 (a ceramic cap) was in it's spot but both legs had no solder.  The part was just rattling around in the holes.  Went ahead and soldered it into place.  Also, on the protection adjust mini-board, Q101 had one leg hole lifted and separated from it's trace.  I reattached it to the trace.

After plugging into power, the fuse didn't blow.... but the monitor didn't power up.  After a bit more flowcharting, i determined that R601 was bad.  Replaced it.  Powered up the monitor.   There seems to be a delay of a few seconds before the monitor tries to power up.  After the delay it powered up, then blew the fuse about 2 seconds later.  Checked and found Q352 blown again.  Also, R601 was extremely hot, but did not go open.  IC501 was still good.  I'm guessing the next step is to swap the flyback out and see what happens.

Anyone have any bright ideas of other things to check?  I've checked for other shorts and not found any....

[Ken Layton]

I would definitely check those Polypropylene capacitors (colored either blue, brown, or orange) in the horizontal output/width circuit for a shorted one. Test them out of circuit to be sure as the very low resistance of the flyback will give you false readings in circuit on these caps.

Also look for shorted diodes in the horizontal output section. Another thing to do is leave the degaussing coil wrapped around the picture tube unplugged in case that is shorted to the frame.

Oh yes something else important..... I believe the horizontal output transistor on this model needs to be insulated from the metal frame. Be sure there's a clear mica insulator with silicone heat sink grease applied to it. Some monitors have a gray flexible "sil-pad" insulator between the transistor and metal frame. The "sil-pad" does not need heat sink grease.

[modessitt]

Hey, Ken!

Yes, it has the insulator and grease on the HOT.  I've also checked the diodes for shorts, but not found any.  I've even tried the "unplug the degauss connector" but it didn't work.

I guess I'll try to test those poly caps out of circuit next.   Probably easier than replacing the flyback, but that will be next if the caps don't show a short...

[grantspain]

the only things i have found to kill a hdt are bad poly caps on the collector of the hdt,bad flyback,yoke(once) or a diode on the h circuit

[modessitt]

Ok, small update....

I pulled every Poly-cap out of circuit in the Horiz and PS sections and tested them, but found no shorts.  This included C363 (which is a pain the butt to get in and out with the flyback in place) and the two caps soldered between the collector and emitter of the HOT assembly itself.  I also pulled one leg of all the diodes to check them, and they are also not shorted.  When reading shorts in-circuit, I have traced back, pulling legs until the short goes away, always finding it to be caused by a false reading through the horiz. width coil, T501, a low value resistor, or the flyback itself.

I guess I'm just going to replace the flyback next, since I can't find an obvious answer.  I'm starting to wonder if the problem could lie in the "protection adjust" mini-board?  I have never dealt with one of these, and the K5515 manual isn't specific enough about it's function....

[grantspain]

the strange thing is the fuse blowing,this should not happen if the faulty component is in the secondary p/s circuit.normally a blown fuse would only occur due to some kind of component failure in the primary p/s circuit,and another thing is the constant destruction of the hd transistor-all very strange

[Kevin Mullins]

Typically speaking... seems that when I find a blown fuse I find the HOT is shorted. (almost hand in hand)
If the flyback is the cause of the blowing HOT... then it could then also be taking out the fuse.

I'd remove the flyback and then apply power power after replacing the HOT.
(hopefully all will remain good and stable)
Measure the B+ on the collector of the HOT and see if it's abnormally high.
(without the specs, I'd assume it to be around the 120V range)

I'd be leaning towards looking at the flyback.
Does it appear "crispy" or "bubbled" around the top where it is usually sealed?

Just throwing some ideas out there.........

[grantspain]

i have never had a fuse blown with a short hdt,only a trip in psu circuit .the only fuse blown i have seen are with degauss component failure or primary psu diode fail or dead short cap,again in the primary psu circuit