Repairing MS8-29FSG

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grantspain
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Re: Repairing MS8-29FSG

Postby grantspain » March 26th, 2017, 3:19 pm

your previous photo did not show that but yes judging from this photo you have connected the bulb to the primary side
so your regulation circuit is not working
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hindered
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Re: Repairing MS8-29FSG

Postby hindered » March 26th, 2017, 3:27 pm

grantspain wrote:your previous photo did not show that but yes judging from this photo you have connected the bulb to the primary side
so your regulation circuit is not working


Ok, sorry for the confusion with the previous picture. I'm glad I redrew it for more clarity.

I'll focus on understanding the regulation circuit.
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hindered
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Re: Repairing MS8-29FSG

Postby hindered » March 28th, 2017, 12:36 am

OK, I started from the MS8-26SU schematic and compared it to my PCB:

MS8-29 FSG Partial Diagram.JPG
Partial MS8-29 FSG diagram


Noted differences from the original:
1: C905 and C904 switch order
2: R915 and R911 are parallel for me but in series in the original
3: Many cap/resistor values are different, which I'm capturing elsewhere.
4: The capacitors leading up to BD901 (c916/915/917 etc) are different. In fact I think my PCB doesn't have one of them at all, and I think a couple may be series instead of parallel (I will have to look again).

How I understand it, the AC comes in and goes through BD901 (which is a bridge rectifier? I still can't find a datasheet for this part, it's marked BF40E3E). It then goes through the filtercap and the voltage regulation circuitry and ends up at F902.

Assumptions (please correct me if I'm wrong).
A: The circuit before BD901 is not shorted, otherwise we would be blowing F901 (not shown, but both sides of the fuse feed into BD901 eventually).
B: I should focus my efforts between the negative leg of the filter cap, and the primary side of F902, because that's where we hooked up the light bulb for the test.

Questions:
1: My first gut feel is that BD901 is the first thing to check, but like I said, I can't find a datasheet for this. How do I test it? Can it be tested in circuit?
2: Are there known "expected voltages" at certain points in this voltage regulation circuit? That would help me narrow down the part of the circuit that is failing. For example, what do I expect as voltage out of BD901? What do I expect as voltage out of Q901 (voltage regulator)? Honestly, if I knew what voltage / current to expect out of BD901, I could probably use the schematic to figure out basically the rest?
3: What does the symbol on R903/R904 mean? It's a resistor but then next to it is what looks like a fuse. Is it a resistor that we expect to act like a fuse by opening if too much power goes through it?
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grantspain
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Re: Repairing MS8-29FSG

Postby grantspain » March 28th, 2017, 1:30 am

ms8-26 and ms8-29 are completely different chassis
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hindered
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Re: Repairing MS8-29FSG

Postby hindered » March 28th, 2017, 2:04 am

grantspain wrote:ms8-26 and ms8-29 are completely different chassis


I understand this. But I used the ms8-26 schematic as a starting point as a way to visualize the circuit, and compared it to my pcb. The pen drawing on top of the printout matches reality, at least in terms of how the components are connected. Like I said, I found differences beyond resistance and capacitance values, and I documented those in the drawing.

In other words, the pen drawing is an accurate wiring diagram for the voltage regulation circuit of my monitor (barring any mistakes I made).
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Re: Repairing MS8-29FSG

Postby hindered » April 1st, 2017, 11:04 pm

I had some time to look at this today. I suspected my VR was shorted again. Removed it from the chassis, confirmed it is shorted via diode test.

I powered it up in this state: No F902, No Q901 (VR), no HOT, Pin 2 of flyback isolated.

I get 120v AC into the AC pins of the bridge diodes at BD901. I measure 170v dc across the - and + pins. Without a datasheet, I have no idea if this is the expected output.

Checking voltage between the negative of the big filter cap and the emitter pad of where the voltage regulator would be, I get 93.7v dc (expected b+ voltage). I think this is a good sign?

My plan is to order replacement VRs, and when they arrive, do the light bulb test again (F902 removed, connect F902 primary to filter cap negative) and measuring b+ voltage. I suspect that test will work fine.

I believe doing the second light bulb test as recommended by PrincessPrinPrin (F902 in, connect filter cap negative to diode that feeds pad 2 of the flyback) is what shorted my VR again.
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Re: Repairing MS8-29FSG

Postby grantspain » April 2nd, 2017, 1:10 am

why don't you admit defeat and install a decent ms9-29 chassis-save yourself a lot of hassle
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Re: Repairing MS8-29FSG

Postby hindered » April 2nd, 2017, 6:27 pm

grantspain wrote:why don't you admit defeat and install a decent ms9-29 chassis-save yourself a lot of hassle


Well, my friend, I agree with the point of your message - I already know that I am in over my head on this repair.

But, I'm not going to give up on it, for several reasons. A: I don't want this chassis to end up in the garbage. B: I'm enjoying learning as I go. In addition to the posts here, I am doing a lot of reading of textbooks, repair manuals, and schematics. I'm still beginner level, but I've learned so much in the past few weeks, and I'm still learning. It's enjoyable. C: I'm stubborn, and once I decide to do something, I usually see it through.

If this is your way of saying you don't want to provide any further assistance, I accept that, and thank you for all the information you've given me so far. Some of it didn't make sense at the beginning, but as I learn more, it makes more and more sense (although, to be fair, some concepts are still beyond me at this point :)). But, I'm not going to give up at this point. I'll seek help elsewhere though, if I've worn out my welcome here.
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Re: Repairing MS8-29FSG

Postby xga » April 3rd, 2017, 5:09 am

Hey Hindered,

Although I kind of agree with grantspain's last comment, I really want to see you bring this MS8 chassis back to life. I check this thread every couple of days to see if you've managed to fix it and I imagine a lot of other people are too with over 3,000 views.

I'm sure what you've learnt over the past few weeks has been invaluable and I've also been learning bits and pieces from it along the way too (I especially appreciate the photos you've posted which make things a little easier to visualise), so please continue to keep us updated on your progress and don't give up!
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Re: Repairing MS8-29FSG

Postby hindered » May 7th, 2017, 3:15 am

xga, thanks for the kind words. I hope to be able to fix this, and even more than that, I hope to continue to learn. Though it has been frustrating, working on this chasis has been overall a positive experience and I'm glad to have tackled it. Grantspain and PPP have been unbelievably patient with their help and for that I'm grateful.

I'm picking this back up after a time away and more study (Reading through my earlier posts is embarassing at how ignorant I was. I'm still going to say ignorant things, but hopefully I can discuss at least a little more intelligently now). I want to give it at least one more try before I find someone else to repair it for me. I understand this power supply section much better now after doing a lot of reading about how they work (though I'm obviously still not an expert by any means).

PrincessPrinPrin wrote:I think you should do a proper light bulb test.

Start with this:
MS8-29bulb1.jpg


and if the bulb lights up and B+ is the correct voltage [snip]


So, I ordered a new replacement VR and popped it in. I did the test above with pin 2 of flyback isolated and HOT removed. I get 148-150 vdc across the lightbulb, which is far from the 94 vdc I'd expect. This means the short is in the power supply circuit, correct? Based on other repair guides I have read, the next step may be to desolder 1 leg of the components in the circuit to try to find anything out of tolerance. Is that folly?

I wanted to scope the DC output of the bridge rectifier to see if it looked OK but my brand new digital scope quit booting up after only 3 weeks. ;( Sending it back for a replacement.
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Asayuki
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Re: Repairing MS8-29FSG

Postby Asayuki » May 7th, 2017, 3:21 pm

hindered wrote:my brand new digital scope quit booting up after only 3 weeks. ;( Sending it back for a replacement.


Wow, that really sucks. :( What brand/model is that, if I may ask? ...just curious
My 15kHz cabinet Peplos will never power up, with any item, and I am quite proud of that.
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hindered
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Re: Repairing MS8-29FSG

Postby hindered » May 7th, 2017, 3:41 pm

Asayuki wrote:
hindered wrote:my brand new digital scope quit booting up after only 3 weeks. ;( Sending it back for a replacement.


Wow, that really sucks. :( What brand/model is that, if I may ask? ...just curious


Siglent sds 1102cml+. Supposedly hitting the math button repeatedly during startup will usually fix this problem, but it didn't work for me after ~20 attempts. At least it's withi the 30 day Amazon return window so it's going back.

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