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Mitsubishi CS27201 repair

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Auburn Boy
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Mar 16, 2018, 11:23 PM

Post #26 of 37 (1257 views)
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Re: [Auburn Boy] Mitsubishi CS27201 repair [In reply to]	Can't Post

Hello again..,

I've checked another part of the Video Buffer circuit around Q250.

The blocking diode D500 diode cathode measures 2.1v and the anode 1.92v.

I checked the voltage at pin 5 of T551 and find 5.78v there.

Also I noticed that the main P.S. return load resistor, R957 is super hot to the touch! Not at all comfortable hot.., But holding up to the temp.., Maybe that should operate hot????

( I'm thinking since everything on CRT seems overdriven, there is way too much current being drawn through the main P.S. but not enough to blow the fuse or the load resistor..,)

(This post was edited by Auburn Boy on Mar 16, 2018, 11:44 PM)

Auburn Boy
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Mar 16, 2018, 11:37 PM

Post #27 of 37 (1255 views)
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Re: [jts1957] Mitsubishi CS27201 repair [In reply to]	Can't Post

Please check my follow on post which was just after your post here.

I'm thinking about pulling out that Q250 unless you have any other ideas relative to D500 and R957.

What do you mean my "major discrepancy" where you have mentioned it. Pin 21 is the -y out and det pin, Discrepancy being it is far to low???

The shutdown voltage seems to be lifted to Vcc on pin 24.

NOTE: pin 21 is actually 2.166 VOLTS!!!

I was measuring pin 22 earlier!!!!

Thanks for all your help.

jts1957
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Mar 17, 2018, 1:24 AM

Post #28 of 37 (1246 views)
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Re: [Auburn Boy] Mitsubishi CS27201 repair [In reply to]	Can't Post

Quote

The blocking diode D500

Where? I can see both (D500/D501) in parts list, but can only find D501 on schematic.

Quote

voltage at pin 5 of T551 and find 5.78v there.

A/CV - DCV - RMS? and how will you know if is good or bad?

Quote

( I'm thinking since everything on CRT seems overdriven, there is way too much current being drawn through the main P.S. but not enough to blow the fuse or the load resistor..,)

Unplug CRT Board being sure nothing can short. Measure the three RGB Color Output Collectors and see if voltages become higher and resistor runs cooler.

-------------------------
Location: Far, Far Away

jts1957
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Mar 17, 2018, 1:35 AM

Post #29 of 37 (1244 views)
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Re: [Auburn Boy] Mitsubishi CS27201 repair [In reply to]	Can't Post

Quote comes first, then my response to something in the quote.

Quote

I'm thinking about pulling out that Q250 unless you have any other ideas relative to D500 and R957.

See post #25

Quote

What do you mean my "major discrepancy" where you have mentioned it.

Both refer to Pin #24. Silly me, I assumed that you were measuring to pin #24 of IC200 in both statements. One is 35 volts higher than the other.

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Location: Far, Far Away

Auburn Boy
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Mar 17, 2018, 2:32 AM

Post #30 of 37 (1235 views)
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Re: [jts1957] Mitsubishi CS27201 repair [In reply to]	Can't Post

Hello again..,

D500 is connected to pin 21 of IC200, to R211 and to Base of Q250.

I measured 5.78 VDC between pin 7 and pin 5 but I see that at those pins AC would be found. But that voltage gets rectified in two places D501 at pin 22 of IC200 and at D500 up near Q250.

Putting my scope on those T551 voltages might "show" me what the output is up to better than just a voltmeter..,

Friday night now. So I might not get much more updated until later in the next couple of days.

Thanks for all your help.

Auburn Boy
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Mar 18, 2018, 1:24 AM

Post #31 of 37 (1222 views)
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Re: [jts1957] Mitsubishi CS27201 repair [In reply to]	Can't Post

In Reply To

Quote

The blocking diode D500

Where? I can see both (D500/D501) in parts list, but can only find D501 on schematic.

Quote

voltage at pin 5 of T551 and find 5.78v there.

A/CV - DCV - RMS? and how will you know if is good or bad?

Quote

( I'm thinking since everything on CRT seems overdriven, there is way too much current being drawn through the main P.S. but not enough to blow the fuse or the load resistor..,)

Unplug CRT Board being sure nothing can short. Measure the three RGB Color Output Collectors and see if voltages become higher and resistor runs cooler.

Hello again,

I unplugged the CRT board from the tube neck and propped it up to safely measure the voltages. I am measuring everything in DC V for now.

The voltages on Q250, Q251, Q651, Q653, Q652, Q654, Q656, and Q655 are all the same as when the CRT Board is plugged in.

R957 still seems overheated.

Key voltages at the collectors of the color output are still:

R = 71.4, G = 63.4, and B = 64.6..,

Q250 checks out in perfect shape tested on my ESR tester when removed from the PCB.

Are there any resistance checks that can be done for the CRT tube itself?

Thanks,

jts1957
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Mar 18, 2018, 2:54 AM

Post #32 of 37 (1219 views)
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Re: [Auburn Boy] Mitsubishi CS27201 repair [In reply to]	Can't Post

You say voltages and overheating is same whether the CRT board is plugged in or not. CRT should NOT be cause.

By the way, Did/Does Shutdown still occur with the CRT unplugged?

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Location: Far, Far Away

Auburn Boy
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Mar 18, 2018, 7:19 AM

Post #33 of 37 (1214 views)
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Re: [jts1957] Mitsubishi CS27201 repair [In reply to]	Can't Post

Good evening,

Yep. All the voltages I measure are still the same ranges.

Remember, I have the R521 removed to PREVENT the shutdown whilst I test.

I'm leaning toward the possibility that IC200 is somewhere near the root cause of this.

I intend to start looking at signals around it with my scope and see.

Is it possible that the 40.1 volt waveform is what is triggering the
X-Ray PROT input to shutdown the horizontal oscillator. I'll have to SCOPE that line too.

This is quite a component level easter-egg hunt isn't it?

Thanks and goodnight!!

jts1957
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Mar 18, 2018, 7:44 AM

Post #34 of 37 (1213 views)
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Re: [Auburn Boy] Mitsubishi CS27201 repair [In reply to]	Can't Post

NO 40.1Volt waveform!
53VPP waveform. Rectified by D582 to produce (a nominal) 40.1Volts which is applied to D510. When D510 turns on (by the 40.1Volts increasing), the voltage at pin 24 of IC200 raises which shuts down the Horizontal Oscillator.

Well reconnect R521 and see if it still shuts down with CRT unplugged.

-------------------------
Location: Far, Far Away

(This post was edited by jts1957 on Mar 18, 2018, 7:50 AM)

Auburn Boy
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Mar 19, 2018, 2:19 AM

Post #35 of 37 (1190 views)
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Re: [jts1957] Mitsubishi CS27201 repair [In reply to]	Can't Post

Interesting circumstances today.

Yesterday you asked me to test the RGB transistor outputs with the CRT board disconnected. I gave you all the weird voltages listed above.

Okay. So I decided to poke around with my scope and check a few voltage waveforms. Lots of the T555 voltages look exactly as pictured in the schematic. Great.

So I decide to follow your other tips, I reconnected R521 and it still shuts down with the CRT board disconnected.

So I disconnect it again to see if I can continue following the signal waveforms on the video processor output to see what was might be working with them. (With a cable signal provided.) So I can measure low amplitude modulated on all the transistors I've previously checked.

So far seeming good.

Now I connect the CRT board back on the tube and power on. Now is where it got confusing.

Recall that earlier with all the seeming unusual voltages on the output transistors I had an extremely bright white screen?? This time nothing on the video, but the startup crackle of the high voltage was present.

BLACK screen. So check the sound (connect the speaker plug) AUDIO plays!! But BLACK SCREEN with audio.

So what might be different? I decided to check the high voltage outputs on the RGB transistors. Guess what? They now sit at about 229, all three of them!!

Head scratcher. (X-RAY Prot still disconnected.)

So, what's different? SCREEN. Hmmm. I recalled I had turned the Screen control DOWN to keep the APK from turning off the set..,

Hmmmm..,

So I nudge up the screen a little bit at a time. Viola! The screen lights. VIDEO!!! Now the three video out transistors sit at about 189 with an intensity based modulation on them.

So, I'm wondering why all this fuss came up??????? Comments??

I will have to let the set cool overnight and see if the LONG, LONG warmup symptom is gone.

There is some 60 Hz rolling flicker on the screen that I might need to address. (This problem wasn't there before I started all this.)

The X-Ray PROT might have to be reconnected and see if FINE-TUNING the SCREEN setting can settle down that shutdown problem.

What do you think of this outcome? Any hints on what I still might need to do other than what I've mentioned? Could the value of R957, the power supply neutral load return have anything to do with this? I have several values of the same wattage resistor I could try.

Sort of a frustrating effort, but at least the set is now mostly "healed."

(This post was edited by Auburn Boy on Mar 19, 2018, 2:20 AM)

jts1957
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Mar 19, 2018, 2:52 AM

Post #36 of 37 (1188 views)
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Re: [Auburn Boy] Mitsubishi CS27201 repair [In reply to]	Can't Post

Rod Serling:
You're traveling through another dimension, a dimension not only of sight and sound but of mind. A journey into a wondrous land whose boundaries are that of imagination. That's the signpost up ahead - your next stop, the Twilight Zone!

or perhaps: “last season was all a dream/fantasy” as in "The Last Newhart."

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Location: Far, Far Away

Auburn Boy
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Mar 20, 2018, 3:47 AM

Post #37 of 37 (1180 views)
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Re: [jts1957] Mitsubishi CS27201 repair [In reply to]	Can't Post

Thanks for all your help!

I do understand the "twilight zone" nature of electronics. My last real job was assembling and provisioning mobile computing platforms for engineering qualification. Pulling together 20 test boards, and as many pieces of android OS and driver updates. Talk about it only works 10% of the time..,

Now that I've dug into this thing pretty deeply, do you think there could be reason to do a full re-CAP operation on it. Maybe to clear up lingering "out-of-spec" signal conditions?

Thank you..,

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