Repairing MS8-29FSG

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hindered
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Re: Repairing MS8-29FSG

Postby hindered » March 12th, 2017, 7:37 pm

PrincessPrinPrin wrote:There are two resistors in the PTC with a common side (the middle pin). One resistor is much higher than the other so you need to test resistance (not continuity) and adjust the meter manually if it's not autoranging.


OK, the thing that I didn't understand is that the PTC and NTC were thermistors (a word I didn't know). Googling PTC eventually led me to PTC and NTC thermistors, which I read about and understand at least a little bit now, thanks. :)

I was unable to find a datasheet for my specific NTC thermistor, but I looked up how to test thermistors in general. Pins 1 to 2 showed a steady resistance, until I applied heat to pin 2, at which point the resistance dropped steadily. Same thing for pins 2 to 3 -- steady resistance, but applying heat caused the resistance to drop. Like you said, pins 2-3 had a much lower resistance than 1-2, low enough to trigger the continuity beep on my diode tester, but I don't think it's actually shorted. I think this part is good, and I put it back into the chassis.

Going back to testing whether there are any other shorted components on the board -- I have a small concern about the area around the voltage regulator. When I do a continuity test (diode mode) on the traces where the voltage regulator was, I get the following:

B-E - no beep, resistance remains steady
B-C - no beep, resistance slowly rises until it reads open
C-E - Initial short beep, but resistance rises until it reads open. To me,

My concern is that initial short beep on C-E, indicating low resistance between the collector and the emitter. But then, something happens (a capacitor filling up?) that causes the resistance between C-E to increase. Does this seem normal?
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Re: Repairing MS8-29FSG

Postby hindered » March 16th, 2017, 2:33 am

OK. I got some replacement HOT and VRs in today.

I replaced F902 and the VR, but not the HOT. I powered it on, the fuse doesn't blow. I metered between C and E on the traces of the HOT and got 93.9v. So far so good.

MS8-29 Removed HOT parts side.JPG
MS8-29 chassis with HOT removed, solder side.


I powered down, soldered the new HOT in, and powered back up. FSSSSSSS sound, F902 blown, VR and HOT shorted.

I replaced F902 with a 1A circuit breaker for temporary testing, and replaced the VR.. Removed the HOT so the traces were empty again. Powered it on.. the circuit breaker holds, and I have 93.9V across C and E of the HOT again. Not sure where to go from there.

Is this connector connected to the correct header, marked low? Should it be on the header marked high?

High or Low.JPG
Is this connector right?


My brain keeps going back to that non-polarized cap at C446. Could it be the cause of the issue? Should I replace it with a polarized cap? If so, which side is negative?

Nonpolarized cap removed.JPG
Nonpolarized cap at C466 removed
Last edited by hindered on March 20th, 2017, 11:18 pm, edited 1 time in total.
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Re: Repairing MS8-29FSG

Postby grantspain » March 16th, 2017, 8:21 am

the np cap won't cause it
the header marked high and low are for frequency,15khz input then jumper goes to low and 25khz jumper goes to high
did you put an insulation sheet behind the hot-unless its an isolated package of course and then it don't need it

things that will short the hot are hv caps and flyback and shorted deflection yoke
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Re: Repairing MS8-29FSG

Postby hindered » March 16th, 2017, 11:27 am

grantspain wrote:the np cap won't cause it
the header marked high and low are for frequency,15khz input then jumper goes to low and 25khz jumper goes to high
did you put an insulation sheet behind the hot-unless its an isolated package of course and then it don't need it

things that will short the hot are hv caps and flyback and shorted deflection yoke


I used the existing insulation sheet for the HOT even though the package itself is also insulated.

I metered the yoke resistance and got ~0.7 ohms on red/blue (assume horizontal winding) and ~6.7 ohms for yellow/brown (assume vertical winding). These values seem low to me, and they fluxuated wildly with the amount of applied pressure of probe-to-yoke connector. However, red/blue to yellow/green tested open. Does that mean the yoke is not shorted?

I replaced the HV caps when I did the cap kit earlier in the thread. Does that mean the flyback is the most likely problem then?

I spent a couple of hours reading https://www.repairfaq.org/samnew/tvfaq.htm. Good resource for anyone looking for basic information.

EDIT: though, doesn't the circuit flow flyback -> HOT -> yoke? If so, doesn't getting proper b+ at the HOT traces with the HOT removed indicate the flyback is OK? Am I misunderstanding/oversimplifying the situation?

EDIT2: looks like it's more complicated than that. Reading more here: http://marsonotv.blogspot.com/2010/09/t ... ction.html
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Re: Repairing MS8-29FSG

Postby hindered » March 17th, 2017, 1:42 am

OK, from watching this video: https://www.youtube.com/watch?v=CXxkOQOK_uA and reading this site http://www.electronicrepairguide.com/te ... ormer.html I learned quite a bit about the primary and secondary windings of a flyback. How I understand it:

The primary winding of the flyback (among maybe other things) accepts b+ from the voltage circuitry and feeds it into the collector of the HOT. On my flyback, pin 1 of the flyback connects to the HOT.
The secondary winding of the flyback is responsible for producing the high voltage. The anode cap is the positive end of the secondary winding. One of the pins on flyback should be the ground of the secondary winding.

I'm assuming that's the primary/secondary delineation by the big white line on the chassis. In other words, pins 1-5 are the primary "side", pins 6-10 are the secondary "side".

MS8-29 Flyback parts side.JPG
MS8-29 FSG Flyback pins solder side of chassis


To help me better understand how flybacks work, I removed my flyback from my chassis to do some of the tests shown in that youtube video. First, I tried to identify the primary winding (even though I understand it should be pins 1-2). When I tested adjacent pins as outlined in the video, I never got readings in the kOhms range, like in the video. I'm using an autoranging MM and made sure of the units -- all values below are in ohms:

1-2: .6
2-3: .9
3-4: 0.L
4-5: .7
5-6: 0.L
6-7: .5
7-8: 0.L
8-9: 0.L
9-10: 1.0

Conclusion: I have no idea if this is good or not, but it's wildly different from the results in the video.

Next I tried to identify the secondary winding, putting two 9v batteries in series, as outlined in the video. When I tested pins 1-5, I was getting basically 0 volts (the meter was reading about 0.002v the whole time, whether I was making contact with a pin or not). When I tested pin 6, 7, 9, 10, I got about .392v. When I tested pin 8, I got about .447v.

Conclusion: The fact that A: I got voltage on all of the secondary pins, and b: I did not get the expected voltage on _any_ of the pins, makes me think that the secondary winding of my flyback is messed up somehow. Can anyone confirm this?

One thing that is different in my test than in the video -- my flyback was still connected to my neckboard (which was not connected to the tube). Would that affect the results of the tests above?

Am I even on the right track here? I'm trying to figure things out here but I'm kind of feeling in the dark...
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Re: Repairing MS8-29FSG

Postby hindered » March 20th, 2017, 1:55 am

I've been going over this in my mind the last few days, and I'm pretty convinced that I don't have a bad flyback or yoke (but if anyone thinks I do, please chime in). Here's why:

Going back and rereading the thread, here's where I started: Great looking picture, that at the end of a ~30 minute play session, had a wigged out picture. Adjusting the pots could get the picture to snap back in temporarily, but eventually the picture died to this: https://www.youtube.com/watch?v=DrPcA_mXUqk Conclusion: At the time of that video, the flyback, yoke, HOT and VR were not shorted, and fuse F902 was not blown.

So, I got a cap kit and replaced the caps. Buttoned everything back up -- ZAP. Shorted Hot, Shorted VR, blown fuse at F902. Conclusion: Something I did during the cap kit caused a short. Whatever I did, it has not yet been fixed.

I replace the fuse and the VR, test -- I get b+ to the C/E pins of the HOT (with no HOT in place). Conclusion: The primary winding of the flyback is properly sending b+ to the HOTs traces. The winding is not open.

I replaced the HOT, and tested. Fuse F902 blows, VR, and HOT become shorted. Conclusion: Whatever was shorted previously is still shorted.

I really feel that I should not be focusing on the flyback / yoke and instead should be looking for a problem caused by me while doing the cap kit. It's possible that the problem I introduced during the cap kit in turn shorted the flyback / yoke, but I do not think they were damaged prior to the cap kit.

How can I find this short? My mind keeps going to the two non-standard things I did during the cap kit: The non-polarized cap at C446 (grantspain says this is not the problem, and I believe him), and bridging the capacitor at C560 to the resistor at R560 due to a damaged trace (see picture).

Bridged trace.JPG
Bridged trace C560 to R560


Is there anything wrong with my logic? How can I track this problem down?

Edited to fix error, original post called out c433 instead of c446
Last edited by hindered on March 20th, 2017, 11:27 pm, edited 2 times in total.
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Re: Repairing MS8-29FSG

Postby grantspain » March 20th, 2017, 12:05 pm

i have 2 x ms8-29 in for repair,neither have a cap installed at c433
do you actually remember taking one out?
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Re: Repairing MS8-29FSG

Postby hindered » March 20th, 2017, 1:23 pm

grantspain wrote:i have 2 x ms8-29 in for repair,neither have a cap installed at c433
do you actually remember taking one out?


It was over a year ago, so I am not 100% sure, but I believe so, yes. I replace caps one at a time, meaning I remove a cap and then solder in the replacement before moving on to the next location. If the location was empty I would have stopped to ask here. The cap kit listed here viewtopic.php?t=18737 has a cap value for c433, making me think it is typically populated.

You mentioned before that you think I have a ms8-29 SU, not FSG. Can you please explain why you think that? Maybe that is part of the difference?
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Re: Repairing MS8-29FSG

Postby grantspain » March 20th, 2017, 2:03 pm

take a photo of the solder side around c433
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Re: Repairing MS8-29FSG

Postby hindered » March 20th, 2017, 10:37 pm

grantspain wrote:take a photo of the solder side around c433


With and without C446 installed (I initially desoldered a jumper instead of the cap :oops:)

C446 - 1.JPG
MS8-29 FSG C446 solder side, component installed

C446 - 2.JPG
MS8-29 FSG C446 solder side, component removed
Last edited by hindered on March 20th, 2017, 11:26 pm, edited 3 times in total.
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Re: Repairing MS8-29FSG

Postby grantspain » March 20th, 2017, 11:05 pm

i can't see c433 anywhere there,c433 position is right in front of the vertical deflection ic
that cap you have pointed on the photo is c446 and in the horizontal size circuit
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Re: Repairing MS8-29FSG

Postby hindered » March 20th, 2017, 11:24 pm

grantspain wrote:i can't see c433 anywhere there,c433 position is right in front of the vertical deflection ic
that cap you have pointed on the photo is c446 and in the horizontal size circuit


My apologies, I specified the wrong cap in my previous post. I've gone back and fixed the error. The non polarized cap is at c446, as pictured above. My c433 is also empty, though the pads are filled with solder.
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Re: Repairing MS8-29FSG

Postby grantspain » March 21st, 2017, 12:27 am

ok well the 2 chassis i had here are repaired and on there way back to holland so i will have to see if i have an old chassis in storage to check the caps
although i doubt that cap would cause an issue as you can actually remove that horizontal control ic pcb and the chassis will still work

i am interested in something you mentioned in an earlier post,you said you changed all the hv poly caps-is that correct,because there are quite a few
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Re: Repairing MS8-29FSG

Postby hindered » March 21st, 2017, 3:27 am

grantspain wrote:ok well the 2 chassis i had here are repaired and on there way back to holland so i will have to see if i have an old chassis in storage to check the caps
although i doubt that cap would cause an issue as you can actually remove that horizontal control ic pcb and the chassis will still work


Interesting. But removing the pcb is different from having the wrong type of cap, right? Is it possible that the cap is causing a short? Logic is telling me no, but I'm having trouble letting go of it.

grantspain wrote:i am interested in something you mentioned in an earlier post,you said you changed all the hv poly caps-is that correct,because there are quite a few


viewtopic.php?t=18737 per the checklist here, I changed all the caps except c905, the big filter cap. I checked to see if it was shorter, and I used my esr meter on it, both checked out fine. I also changed c332 on the neckboard.

Differences on my board vs the checklist: c910 is 25v instead of 50v. C911 c562 c446 c574 are 50v instead of 25v. C521 c449 are 25v instead of 16v. C446 is np instead of polarized. C910 being a lower voltage is concerning, but if the voltage was an issue I'd expect the cap to burn up, not cause a short.
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Re: Repairing MS8-29FSG

Postby grantspain » March 21st, 2017, 3:49 am

so you did not change any hv polypropylene capacitors then(box type)?
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Re: Repairing MS8-29FSG

Postby hindered » March 21st, 2017, 3:56 am

No, only the cylindrical electrolytic caps.
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Re: Repairing MS8-29FSG

Postby grantspain » March 21st, 2017, 12:22 pm

c910 100uf 50v
c446 is 33uf 16v bipolar
links either side of c446
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Re: Repairing MS8-29FSG

Postby hindered » March 21st, 2017, 2:19 pm

grantspain wrote:c910 100uf 50v
c446 is 33uf 16v bipolar
links either side of c446


Thank you for verifying that. Ok, so that means the bipolar cap is standard and we should probably edit the wiki to reflect that.

I'll try to find a 50v cap to replace 910 but that wouldn't cause the short...

That means the short is elsewhere, right? If so I'm probably at a loss for how to repair this. Did you have a chance to look at the cap I bridged to a resistor above? I think that's fine but am not 100% sure.

If I isolated the b+ on the flyback and put in a dummy load (lightbulb, resistor) in place of the flyback so reduced current b+ makes it to the HOT, what does that let me do? I'm assuming the reduced current means the short could be found by measuring voltage on the board while not being strong enough to short out the replacement HOT?
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Re: Repairing MS8-29FSG

Postby PrincessPrinPrin » March 21st, 2017, 2:31 pm

I would remove the b+ fuse and connect a light bulb to the fuse holder to see if the PSU is OK or not.
intrinsically safe
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Re: Repairing MS8-29FSG

Postby grantspain » March 21st, 2017, 2:35 pm

the cap solder link is ok

is d530 ok?

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