Scope repair: meguro mo-1251

[fezder]

i'll take measurements tomorrow, gotta go to sleep....

 

[fezder]

small trouble, almost all measured, but i can't find datasheet/pinout for q17/18 (2sc1628(y) and q19/20 (2sa818(y)
both are to-126 case, at least they look much like them (so pretty certain) of course, i could take them out circuit for polarity testing, but what do you think? could polarity be determined from schematics (probadly can....) but, here'e what i got so far ignoring those:

Q9, Q10: 0,75, 0,75

Q11, Q12: 0.644, 0.679

Q13, Q14: 0.673, 0,9

Q15, Q16: 0.7, 0.7

Q21, Q22:0.709, 0.705

Q23, Q24: 0.303, 0.694

 

[KeepItSimpleStupid]

OK, check all of the small resistors connected between Q19-Q24 IN CIRCUIT. There is a bunch of 22 ohm resistors. I f the values seem way off, just report them for now.

Q23 is getting replaced, 2 resistors (metal oxide), on regulator, and add a 1n4001 diode, so far.

 

[KeepItSimpleStupid]

The next thing, I'll likely ask you to do is remove Q23 and Q24 and lift R74 and R75 at the collector sides

Then we want to look at the 12 V supply again. if that's good. I want the voltage BETWEEN
Base Q10 - Base Q9
Emitter Q12 to Emitter Q11
Emitter Q14 to Emitter Q13
Base Q16 to Base Q16

 

[fezder]

Q23 is getting replaced, 2 resistors (metal oxide), on regulator, and add a 1n4001 diode, so far.

just for learning purposes, why Q23? i know 0.33volts is too low, but is there other reason? i don't question your knowledge! and question about that 1n4001 (don't have those, 4007 should suffice?) is for regulator reverse-voltage?

 

[KeepItSimpleStupid]

Note D1 in this http://www.ti.com/lit/ds/symlink/lm317.pdf datasheet.

Now you only tested the B-E junction. In all likeliness it's bad, and it would also cause the symptom. I have one of these: **broken link removed** I actually won it. The list price was about $500 maybe in the 80's. That transistor is likely leaky and it might show conduction with an ohmmeter. R79 is popped and r76 is popped. You could also test it out of circuit too.

And you mentioned that the complementary transistors are hot/

 

[fezder]

The next thing, I'll likely ask you to do is remove Q23 and Q24 and lift R74 and R75 at the collector sides

Then we want to look at the 12 V supply again.

oh dear, started doing just that, smoke started appear. ''luckily'' it was only resistor, r89/1k, between Q19/20. seems something is terribly wrong with this 'scope...

 

[KeepItSimpleStupid]

I was, sort of, afraid of that. Check everything between the emitter of Q15 and C36? C38? With Q23 and Q24 removed, I really don't see anything that could cause that yet.

Otherwise, we can try replacing all of the parts determined to be bad and see what happens.

Possibly, try to bring up the scope with a variac. I have to think about what to monitor.

 

[fezder]

I was, sort of, afraid of that. Check everything between the emitter of Q15 and C36? C38? With Q23 and Q24 removed, I really don't see anything that could cause that yet.

haha, me too, somehow at first, i thought fizzing-noise was from transformer or similar, then i saw smoke...
i suppose those measurements are as without power applied? no point to burn stuff now anymore further
and by everything, do you mean all components in that section? quite pile to check....

i did bring it alive with variac, dunno would bulb saved anything, didn't used it (yeah, it is recommend to use bulb but it has issues, like startup-current limiting....)

 

[KeepItSimpleStupid]

But, check in-circuit. Check for shorted caps, diodes and DRASTIC differences in resistor values. There is about 10 components.

The main voltage to monitor is the +12 when we get there.

 

[fezder]

well, i couldn't find anything alaming, hopefully i measured them right, and all of those in area. however, i did find something:
I measured all b-e junctions with diode test/DMM in circuit, and Q14 result was changeg from 0.9 -> 0.673, so now it's same with its counterpart Q13.
also, two legs from Q19 (''unknown'' 2sa818(y)) are shorted
hopefully this bit helps at all....

 

[fezder]

any thoughts about matter?

 

[KeepItSimpleStupid]

I need to get up to speed again. The problem is a tough one. Right now, I'm swamped with stuff to do and my brain has been migrainy. Usually, it's all or nothing in terms of replacement. Sometimes, you scan disconnect in certain areas to get parts of the circuit to work. that's what I sort of, failed at. I had a washer, dryer and wet vac failed at the same time. The wet vac might be unrepairable. Replaced switch and it ran for a few seconds and smoked. Brushes are HARD to get to. I like the wet vac because it's small.

I was considering that you replace some of the transistors and the obvious stuff that popped

The gas dryer, is, get this 48 years old. not much maintenance at all, but it does get regular cleanings (yearly ducts) and this last one was never done before, because I never had the top off. The drum belt was ORIGINAL until replaced. The blower belt was replaced too. I remember taking the blower assembly out once to thoroughly clean and lubricate. I found two thrust bearings bad on the idler so I replaced them. The trust bearings I used are better than the originals. PTFE on a hard surface. No wear at all on the oilite bearings. Selective painting (mostly interior) occurs periodically.

So I have a few odds and ends that need fixing, like the filter hinge and filter door bumpers and some painting in the summer. The hinges arrived today.

I plane to re-build the lint filter and characterize a new ignitor (a spare) which is now unobtainable. That part, unless I make it the dryer becomes junk. I'd have to build buy a CD welder and figure out the material of the ignitor. Only one symptom was a very tough nut to crack. The burner would run for about 10 min and shut off. That was metal fatigue in the burner assembly.

The washer is on it's way to being completely fixed, but I have to re-engineer something and it's usable.. The bumpers are too small and deform too much so the fragile lid switch gets damaged. Supposedly, the front, sides and top lift off as a single assembly to replace the lid switch.