Scope repair: meguro mo-1251

[fezder]

Hi!
previously, i made an error by repairing many scopes in same thread, but now, as fellow member pointed out, different scopes=different thread. Sorry for previous confusion folks!. but now for this issue:

The faulty part, at least one of them, is located at y-amp section (vertical), and it's on ''positive'' portion of mirrored section. Burned component, is r95 in schematic (page 27, on REAL pdf page numbering, 25 on scanned manual). resistor is rated 100 ohms
now, if i just replace resistor, it probadly just burn out again, as something is the reason why it burned out.


obserwations from front panel settings: x-y setting doesn't work, do dot beam, beam slides from left to center, about 2 div high. volts/div knob doesn't do a thing at signal.
outside of x-y setting, volts/div still won't affect, but time/div knob does work.
position knobs won't work either well, at either state. beam ''squeezes'' at the edges if i try to push it out the horizontal edges.
and, trace disappear at about 2 divs before right edge (even at x-y when i should see only dot)

what would be best way to start further troubleshooting?

edit: forgot to add manual:
https://onedrive.live.com/view.aspx?resid=477358247C3BC88D!229&app=WordPdf

edit2:wrong resistor number....sorry

 

[KeepItSimpleStupid]

What's first?:
1) Symptoms (OK, for the most part)
2) Inspect (uncovered something)
3) Check the supply voltages and ripple on P3. (I don't expect anything wrong)
.............me: I'll print out the page

(I needed better directions) e.g. connected to Emitter of Q23, near top left corner of page.

 

[fezder]

1) Symptoms (OK, for the most part)

cinda hard to describe any better, i'll post video in some time

2) Inspect (uncovered something)

only thing that caught my eye was that burnt resistror

3) Check the supply voltages and ripple on P3. (I don't expect anything wrong)

that should be good place to start actually, i'we been just pretty busy at work lately (yesterday, evening shift 14-22, next day ''morning'' evening shift 8-16, so i didn't have any spare time lately. But now, thank god it's weekend!

(I needed better directions) e.g. connected to Emitter of Q23, near top left corner of page.

sure, here's some: top-right corner, between Q19 and Q23. resistor also has 100p cap parallel with it.

 

[fezder]

measured voltages, two voltages were quite off: 12v measured 20.7v and 120v measured 64v
ripple on those: 12v : ~1vpp 120v ~4.5vpp

 

[KeepItSimpleStupid]

That's a big Oops and changes the tactic quite a bit. It re-enforces the importance "power supply" check.

That 100 ohm resistor is effectively a fuse. The metal oxide types are good for this application.

 

[fezder]

That's a big Oops and changes the tactic quite a bit. It re-enforces the importance "power supply" check.

yeah, it's pretty alarming, and i douple checked it

That 100 ohm resistor is effectively a fuse. The meta oxide types are good for this application.

yet again new info to me!

 

[KeepItSimpleStupid]

Typo: Fixed meta oxide = metal oxide

 

[KeepItSimpleStupid]

Hopefully, your power button isn't too bright, otherwise, I would suspect IC2 (7812 regulator). So look at the voltage dropped across IC2, input and output voltages. Look at a datasheet. Measure, in circuit, the low value resistors nearby after the power supply has discharged. If the regulator is defective, I' pay attention to how it's heat sinked and probably would replace the cap following the regulator. The regulator should have gone into thermal limiting, if the heatsink was big enough.

 

[fezder]

oh right, those regulators do have thermal limiting. Oh no, power light isn't too bright, pretty normal brightness. I'll be in touch once i know something!

 

[fezder]

pretty strange results came up, measured 7812's voltages: input is 19.3v and output 21v. Shouldn't input voltage be HIGHER than output voltage? and output voltage should be closer to 12v?.
I measured it like this: ground was chassis, input pin 1, output pin3
As for heatsink, it's huge, surrounds whole crt circuit.
measured also small resistor (R8/4.7ohm, before 7812) 5 ohms

 

[KeepItSimpleStupid]

Yep. Odd.

Trace centered. X-Y mode. Intensity down or off.

Measure the voltage on the CRT plates to ground. Connector P2; Voltage could be high (e.g. 120 V).
And the B, E and C on Q13 and Q15.

 

[fezder]

trouble is, i can't control trace at all, but i'll try. post results in while

 

[KeepItSimpleStupid]

Fine: x-y mode then. Position controls midrange. Not - position centered. PS: I'm supposed to be sleeping.

 

[fezder]

don't lose your rest on my account!

 

[fezder]

ok, here's what i got

y: 47vdc
_y: 47vdc

q13:
b=16.5v
c=gnd
e=17v

q15:
b=18.8v
c=37v
e=21.3v

oh, and i noticed q17/19 are either one getting hot, fast, but their counterparts are running cool

 

[KeepItSimpleStupid]

OK, with the 12 V supply messed up, I'm suspecting backfeeding from the 20 V supply. I don't suspect the 12 V regulator at this point.
I will keep in mind of replacing it eventually with reverse voltage protection.

That said, do the same b-e tests with your DMM with power off for all the transistors in that amp. List as counter parts
e.g.

Q9, Q10: 0.6, 0.66
,,,
Q23, Q24:

 

[fezder]

ok, takes a while....i suppose you mean from vertical amp section, right? and 3:rd ''section'' or page

 

[KeepItSimpleStupid]

Basically Q9 through Q24. Top of one page. Because of a broken R95, I think, we ave to assume at least one active element bad.
Because of the 12 V supply being backfed voltages are going to be tough. Just like an audio amp, you have to fix EVERYTHING or it pops again. SO, hoping we can locate at least one bad active device and then start looking at low value resistors. These usually change value.

 

[fezder]

Just like an audio amp, you have to fix EVERYTHING or it pops again

reminds of that one ''1kw'' (what a joke now afterwards) which i tried to fix: found couple bad components, but they blew up right on again....
still trying to locate those things, no point measuring wrong place. seems i have to take pcb off, at least that time i'll be able mark measure points somehow (pretty narrow space to check out component markings...) but, i found another burnt resistor, it's on the same side as previous one, new one is R76/22ohm

 

[KeepItSimpleStupid]

You may have heard the story, but I happened to make a mirror image of the Leach Amp circuit board and really had a hard time determining what was wrong. The most interesting part of the story was that I was able to USE the board. I just reversed the locations of the NPN and PNP transistors, cut two traces and re-routed them,