Repairing MS8-29FSG

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grantspain
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Re: Repairing MS8-29SU

Postby grantspain » March 8th, 2017, 9:46 am

the fuse is blowing because of a short
your next step is to isolate the b+ pin of the flyback and install a dummy load on the b+ line-normally that fuse blowing is a short on the horizontal output transistor
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Re: Repairing MS8-29SU

Postby hindered » March 9th, 2017, 1:27 am

grantspain wrote:the fuse is blowing because of a short
your next step is to isolate the b+ pin of the flyback and install a dummy load on the b+ line-normally that fuse blowing is a short on the horizontal output transistor


I'm not sure which pin of the flyback is b+. From looking at this picture, my best guess is that it's pin 5. If that's the case, what do I need to do to isolate it with a dummy load?

MS8-29 Flyback HOT parts side.JPG
MS8-29 Flyback HOT parts side


I'm assuming the C4157 transistor at Q901 is the HOT? If so, instead of isolating the b+, can't I just desolder Q901 from the board and test it for a short with a multimeter? Q901 is the voltage regulator.

MS8-29 Q901 HOT.JPG
MS8-29 Q901 HOT


EDIT: Info about how to test flyback with a dummy load found here: viewtopic.php?t=33054 Sounds like grantspain is recommending to isolate the b+ pin, you need to desolder the whole flyback? That seems incorrect but it's how I'm interpreting the instructions.
Last edited by hindered on March 9th, 2017, 11:57 pm, edited 1 time in total.
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Re: Repairing MS8-29SU

Postby hindered » March 9th, 2017, 2:59 am

I removed the transistor at Q901 and tested it. B->C, C->B, B->E, E->B, C->E, C->B, all tested open shorted using continuity/diode test. Looks like we for sure have a shorted HOT voltage regulator! Is this a symptom or the cause? I'm assuming this is a symptom of the shorted HOT.

EDIT: In addition to strikethroughs above, removed bit asking for help finding a replacement. Will worry about replacement parts once bad parts are identified.
Attachments
T C4157 31.JPG
C4157 Q901 Horizontal Output Transistor
Last edited by hindered on March 9th, 2017, 11:59 pm, edited 1 time in total.
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Re: Repairing MS8-29FSG

Postby grantspain » March 9th, 2017, 7:26 am

that is the voltage regulator
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Re: Repairing MS8-29FSG

Postby hindered » March 9th, 2017, 1:44 pm

grantspain wrote:that is the voltage regulator


Well, okay. So I have a shorted voltage regulator at q901, thank you for clarifying. Where is the HOT? I'm assuming, from your previous posts, that the hot is shorted as well, which took out things further down the chain?

Your post previously said to check Q901 when F902 blows, so I made the assumption Q901 was the hot.
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Re: Repairing MS8-29FSG

Postby PrincessPrinPrin » March 9th, 2017, 2:51 pm

There is a universal rule for understanding where the HOT is on a chassis: the collector is directly connected to the flyback and the other side of that winding is the B+. In your case it's pin 1 and 2 of the flyback.

Also, when you say that "B->C, C->B, B->E, E->B, C->E, C->B, all tested open" and your conclusion is that "we for sure have a shorted HOT" it is a bit contradictory as open is the opposite of shorted. Anyway, an NPN transistor is good when B-C and B-E are the only junctions that give a reading and all the others read open, like in the pic below:

Image
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Re: Repairing MS8-29FSG

Postby hindered » March 9th, 2017, 3:30 pm

PrincessPrinPrin wrote:There is a universal rule for understanding where the HOT is on a chassis: the collector is directly connected to the flyback and the other side of that winding is the B+. In your case it's pin 1 and 2 of the flyback.

Also, when you say that "B->C, C->B, B->E, E->B, C->E, C->B, all tested open" and your conclusion is that "we for sure have a shorted HOT" it is a bit contradictory as open is the opposite of shorted. Anyway, an NPN transistor is good when B-C and B-E are the only junctions that give a reading and all the others read open, like in the pic below:

Image


Thanks for taking the time to educate me. What you say makes sense and I will read more about it after work tonight, and update my previous posts with accurate information for anyone in the future reading this. I'm learning as I go, and couldn't do it without the help of everyone here.

I'll also re-test the voltage regulator using the resistance method as a learning exercise.
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Re: Repairing MS8-29FSG

Postby hindered » March 10th, 2017, 4:50 am

OK, from what I understand, PrincessPrinPrin was saying that pins 1 and 2 are the primary winding of the flyback. Also, the collector of the HOT directly connects to the flyback. Tracing pin 1 backwards, I found it directly connected to the collector of a Toshiba 2SC4288A at Q533.

MS8-29 Flyback HOT parts side (2).JPG
Diagram showing HOT and B+ attempt


Is this image accurate?

So, understanding what I do now, what would I need to do to "b+ pin of the flyback and install a dummy load on the b+ line". My educated guess is that I could lift one leg of the diode at D904 (other end of the red line in my picture) to isolate the flyback, and then jumper a resistor or lightbulb between the lifted diode leg and ground. What would a good ground location be?

Once that is done, what do I need to do to test? I'm assuming I will need to replace F902, and test for 94 VDC at F902 and a capacitor negative? TP1 looks like it connects to F902, so that is probably the designated b+ test point. Does anyone know what TP2 or TP3 are?

WIll this test be affected by the fact that I have removed the shorted voltage regulator at Q901? I'm assuming missing component = no b+ voltage at all.
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Re: Repairing MS8-29FSG

Postby PrincessPrinPrin » March 10th, 2017, 12:50 pm

You have pulled the voltage regulator. With that off circuit you cannot have b+. If it's actually bad replace it. If it's good put it back in and go ahead with bulb test etc.
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Re: Repairing MS8-29FSG

Postby hindered » March 10th, 2017, 1:40 pm

PrincessPrinPrin wrote:You have pulled the voltage regulator. With that off circuit you cannot have b+. If it's actually bad replace it. If it's good put it back in and go ahead with bulb test etc.


Thanks. Yes, the voltage regulator is completely shorted. There is no resistance in any direction on any of the pins. I will try to find a suitable replacement for the VR but I am concerned it is a symptom and not the core problem.

In my previous post, did I understand how to do the light bulb test correctly?

Edit: I removed the 2sc4288a at q533. It is shorted. No resistance Between any pins.
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Re: Repairing MS8-29FSG

Postby PrincessPrinPrin » March 10th, 2017, 6:30 pm

Replace the regulator and the HOT but make sure that no other components connected to the HOT collector are shorted to ground, i.e. check that there is no short across the C and E pads now that the HOT is off. The light blb can be installed in various points. What grantspain suggested is as close as possible to the flyback, i.e. desolder pin 2 so it doesn't touch the pad and solder the bulb to the pad (or a point in common with it). Your point is a step earlier. It's also possible to put it after the flyback, on the HOt collector line after removing or disabling the HOt.
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Re: Repairing MS8-29FSG

Postby hindered » March 10th, 2017, 9:28 pm

PrincessPrinPrin wrote:Replace the regulator and the HOT but make sure that no other components connected to the HOT collector are shorted to ground, i.e. check that there is no short across the C and E pads now that the HOT is off.


OK, I tested this. Measuring the traces on the PCB, C-B and C-E show no continuity, but B-E shows continuity. I'm assuming this is bad, but from doing some reading on the subject, it could be normal if the transistor is biased.

Also, Pins 1-2 of the flyback are showing continuity/shorted as well. Is this expected?

PrincessPrinPrin wrote:The light blb can be installed in various points. What grantspain suggested is as close as possible to the flyback, i.e. desolder pin 2 so it doesn't touch the pad and solder the bulb to the pad (or a point in common with it). Your point is a step earlier. It's also possible to put it after the flyback, on the HOt collector line after removing or disabling the HOt.


Thanks, this all makes sense and makes the process more clear to me. I have ordered a replacement HOT but am still having trouble finding a replacement voltage regulator. Google is indicating that NTE2311 is a suitable replacement, but I have a friend with a bunch of spare parts who might have something on hand that I can use. Like you said, wIthout the voltage regulator, this test is not possible.
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Re: Repairing MS8-29FSG

Postby PrincessPrinPrin » March 11th, 2017, 4:07 am

hindered wrote:
PrincessPrinPrin wrote:Replace the regulator and the HOT but make sure that no other components connected to the HOT collector are shorted to ground, i.e. check that there is no short across the C and E pads now that the HOT is off.


OK, I tested this. Measuring the traces on the PCB, C-B and C-E show no continuity, but B-E shows continuity. I'm assuming this is bad, but from doing some reading on the subject, it could be normal if the transistor is biased.


It's because one side of the secondary winding of the driver transformer is connected to the base of the HOT and the other side is ground. It's normal that the tester shows continuity although it's not exactly what it is.

hindered wrote:Also, Pins 1-2 of the flyback are showing continuity/shorted as well. Is this expected?


yes, same reason as above.

hindered wrote:Google is indicating that NTE2311 is a suitable replacement, but I have a friend with a bunch of spare parts who might have something on hand that I can use. Like you said, wIthout the voltage regulator, this test is not possible.


I wouldn't use anything NTE and that stuff is not even available outside the US afaik. I would use any transistor that has HOT specs provided it's not one with internal diode.
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Re: Repairing MS8-29FSG

Postby grantspain » March 11th, 2017, 11:28 am

the nte2311 does check out ok against the 2sc4157 for the voltage regulator but will need an insulation sheet between transistor and heatsink
nte2365 checks out as the replacement for 2sc4288a the horizontal output transistor-package looks similar but i would expect the use of insulation sheet

i have never used any nte components,like prinprin said they are not available outside of north america

i would be very wary about getting anything from ebay and certainly not from china-virtually every high spec transistor on ebay is a fake,same goes with anything from china.The amount of 2sc5143 and 2sc5144 fakes on ebay is shocking.
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Re: Repairing MS8-29FSG

Postby hindered » March 11th, 2017, 2:56 pm

PrincessPrinPrin wrote:I wouldn't use anything NTE and that stuff is not even available outside the US afaik. I would use any transistor that has HOT specs provided it's not one with internal diode.


grantspain wrote:i would be very wary about getting anything from ebay and certainly not from china-virtually every high spec transistor on ebay is a fake,same goes with anything from china.The amount of 2sc5143 and 2sc5144 fakes on ebay is shocking.


Understood, and after doing some reading about NTE I can understand why you may want to avoid those as well. A US-based monitor repair guy recommended a FJL6920 as a suitable replacement for the HOT. I have reached out to Mouser to see if they have suitable replacements for both the HOT and the VR.

Grantspain, you have recommended elsewhere to check these components (below).

Q901 - Voltage Regulator - Transistor C4157 - Shorted
BD901 - 4 Pins, marking BF40E3E -- ???
NTH901 - Green ntc thermistor, 2 pins - Markings 05 050 30
PTH901 - Black cube, ptc thermistor, 3 pins, marking 8ROM 30.

The Q901 is bad, shorted. Should I check the other 3 as well? If so, how?
Last edited by hindered on March 12th, 2017, 12:04 pm, edited 2 times in total.
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Re: Repairing MS8-29FSG

Postby grantspain » March 11th, 2017, 4:35 pm

bd901 is the bridge rectifier so read with diode test-if faulty it will read dead short across one of the rectifiers
nth901 is negative thermal coefficient-if faulty it will blown apart or cracked
pth901 is postive thermal coefficient-if faulty it will read short-this is the degauss thermal switch,you can remove it for the time being
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Re: Repairing MS8-29FSG

Postby PrincessPrinPrin » March 11th, 2017, 5:24 pm

hindered wrote:A US-based monitor repair guy recommended a FJL6920 as a suitable replacement for the HOT. I have reached out to Mouser to see if they have suitable replacements for both the HOT and the VR.


It's more beefed up than the 2SC4288A, it will work fine. Toshiba recommends 2SC5589:

https://www.westfloridacomponents.com/m ... c4288a.pdf

Get whatever is available/cheaper to you.
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Re: Repairing MS8-29FSG

Postby hindered » March 11th, 2017, 8:13 pm

grantspain wrote:bd901 is the bridge rectifier so read with diode test-if faulty it will read dead short across one of the rectifiers
nth901 is negative thermal coefficient-if faulty it will blown apart or cracked
pth901 is postive thermal coefficient-if faulty it will read short-this is the degauss thermal switch,you can remove it for the time being


BD901 - I was having trouble desoldering this component even at 380c and I didn't want to crank my desoldering station up more. I tested for continuity from each of the 4 pins to the others and found no shorts.
NTH901 - visually looks fine. I left it in circuit and didn't bother testing it.
PTH901 - I removed it from circuit and tested continuity. Pins 2-3 have continuity, 1-2 and 1-3 do not. I searched and could not find a datasheet for this part to better understand how to test it..

PTH901 Parts.JPG
PTH901 Parts.JPG (16.45 KiB) Viewed 480 times

PTH901 pins.JPG
PTH901 pins.JPG (15.93 KiB) Viewed 480 times


PrincessPrinPrin wrote:
hindered wrote:A US-based monitor repair guy recommended a FJL6920 as a suitable replacement for the HOT. I have reached out to Mouser to see if they have suitable replacements for both the HOT and the VR.


It's more beefed up than the 2SC4288A, it will work fine. Toshiba recommends 2SC5589:

https://www.westfloridacomponents.com/m ... c4288a.pdf

Get whatever is available/cheaper to you.


Thanks for this information. How did you determine Toshiba recommends 2SC5589? I can't find that cross reference on the sites I have available to me. Regardless, I was able to pick up a couple of FJL6920 for $3 each, while the cheapest 2SC5589s I can find are $10+.

Re: A replacement for the C4157.. I did find a couple of 2SC4157 on ebay but they both seem like fakes, so I'll avoid them. Using Mouser.com's smart filtering system, I took the values from the 2SC4157 datasheet and found all transistors that met/exceeded those ratings. I ended up filtered down to one part, the MJW18020G, http://www.mouser.com/ProductDetail/ON- ... RvyQ%3d%3d Is this a suitable replacement?
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Re: Repairing MS8-29FSG

Postby PrincessPrinPrin » March 12th, 2017, 9:09 am

double post.
Last edited by PrincessPrinPrin on March 12th, 2017, 9:31 am, edited 2 times in total.
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Re: Repairing MS8-29FSG

Postby PrincessPrinPrin » March 12th, 2017, 9:30 am

hindered wrote:Thanks for this information. How did you determine Toshiba recommends 2SC5589?


Page 20 of that PDF.

hindered wrote:MJW18020G, http://www.mouser.com/ProductDetail/ON- ... RvyQ%3d%3d Is this a suitable replacement?


Yes.

hindered wrote:PTH901 - I removed it from circuit and tested continuity. Pins 2-3 have continuity, 1-2 and 1-3 do not. I searched and could not find a datasheet for this part to better understand how to test it..


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.
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