Need help repairing couple 'scopes (different faults)

[fezder]

First things first, look at the +-12, 5 and +140 supplies before and after the fault.

hmm, good point, haven't measured those yet ( or located that matter) but should be easy enought task.

Is the Y-axis (vertical deflection) circuitry working correctly?

dunno yet, haven't looket at that portion yet.

 

[fezder]

kikusui's 'been running for about 2hrs, still no sign of fault! it shows signal just right, nothing faulty at all....what do you guys think?

 

[alec_t]

I think if it runs faultlessly for many more hours the fault has cured itself. Perhaps an electrolytic cap has reformed after a long period without use? Perhaps operation of switches has removed an oxide layer?

 

[fezder]

I think if it runs faultlessly for many more hours the fault has cured itself. Perhaps an electrolytic cap has reformed after a long period without use? Perhaps operation of switches has removed an oxide layer?

hmm, newer thought about those, i did wiggle with switches some time ago. But, about 'lytics, i suppose ESR test shouldn't be waste of time? i mean, if that would expose the ''faulty'' one

 

[cowboybob]

I think if it runs faultlessly for many more hours the fault has cured itself. Perhaps an electrolytic cap has reformed after a long period without use? Perhaps operation of switches has removed an oxide layer?

Back in the day, we routinely "exercised" (rotated many times, back and forth, stop to stop) ALL switches of equipment that came into our calibration lab from the submarines. Corrosion/fouling (oil, dust, cig smoke - you name it), however slight on the contact surfaces, played heck with all the input and trace control circuit responses, despite the switch contacts being gold plated.

 

[KeepItSimpleStupid]

Guess you didn't know about Stabilant22 at that time. https://www.stabilant.com/ Note, it's even recommended for submarine systems.

 

[cowboybob]

Guess you didn't know about Stabilant22 at that time. https://www.stabilant.com/ Note, it's even recommended for submarine systems.

Nope, sure didn't. Their stuff came after my time ('67 - '74). Although we did occasionally get into switches with alcohol and/or acetone and cotton swabs, but the hassles outweighed the effects, as compared to our mechanical method(s). Essentially, the subs' gear did not gets used enough... Our cal standards and test gear never suffered from fouling, despite being in identical (for the most part) environmental conditions.

 

[fezder]

aha, fult occured this time, now as it should be, around couple minutes. gonna measure voltages. things of note:
#1 backlight of screen is ALSO blinking at same speed as signal. backlights are neon bulbs, at least they look like them.
#2 when i turn ILLUMINATION knob for those neon bulbs, which affects only bulb brigtness, now signal brightness, counter-clockwise (towards off) signal disappears also.
#3 when fault occurs, backlights are MUCH bighter than when in normal state, even at full brightness
#4 when INTENSITY knob is full (i know, not wise for crt, but it was just for testing purposes) signal ''buzzes'' at middle of the screen, heavily distorted

that's all, i'd suspect fault is somewhere in HV (140v) but of course, it could be also drive circuit which drives that HV section.
should i still measure 12/5/140v sections as you suggested earlier keepitsimplestupid?

 

[KeepItSimpleStupid]

The #1 thing to is Inspect and nose test which you did. The #2 thing is usually measure the supplies and ripple.

A faulty 140 V supply at a fault, won't necessarily mean that the 140 V supply is bad.

You could have a bad solder joint or component that gets upset when heated.

 

[alec_t]

With that interaction between signal and illumination I'd be looking for a power supply fault.

 

[fezder]

i just hope it's not bad solder joint, annoying fault to locate...
any opinions about coolant spray? i do have -50c available

 

[KeepItSimpleStupid]

Don't go spraying willy-nilly. Localize the fault first. It might take a while, but you will have some before/after fault voltages.

The s[ray also enhances the cold solder joints. You can see the difference as it cools around the joint interfaces.

 

[fezder]

oh no, i wasn''t going to just spray around, only if i can locate suspicious place.....

The s[ray also enhances the cold solder joints

what do you mean ''s[ray''? or was it just typo? thanks for help so far by the way!

 

[tvtech]

Hi Guys

As embarrassing as this sounds....(tvech should get a scope up and working in no time)...I prefer to send the thing away for repairs/calibration.

I have enough stuff to fix for Customers...never mind my own stuff.

Being a Hobbyist is one thing. A hobbyist can tinker and lovingly restore things. (I miss that part where Electronics was my hobby...and not my main source of income Years ago). Those were the truly fun days

Now days, I tend to toss things that are a PITA. No time for electronic passion or restoration. Big pity.

Either it works or I toss it. And get a new one.

Tough stuff.

Regards,
tvtech

 

[fezder]

there are couple reasons i try to repair these stuff:
if it isn't use for me, it serves other hobbyist well enought, and the new owner doesnt need to repair it.
also, need to make room for possibly new stuff (hehe) like digital oscilloscope, well okay it doesn't take much space but you get the idea. as for why, there are some situations when i'd like to see a very small time signal: inrush current for an example. of course, good multimeter would do that job too, but i'll buy both in ''near'' future (either rish or finest model, price is around 300-400€ and scope another 300-500€, should get serious lab stuff for those costs.

and then there's always hobby-side which you tvtech pointed out, i just love this electronic hobby.

 

[tvtech]

there are couple reasons i try to repair these stuff:
if it isn't use for me, it serves other hobbyist well enought, and the new owner doesnt need to repair it.
also, need to make room for possibly new stuff (hehe) like digital oscilloscope, well okay it doesn't take much space but you get the idea. as for why, there are some situations when i'd like to see a very small time signal: inrush current for an example. of course, good multimeter would do that job too, but i'll buy both in ''near'' future (either rish or finest model, price is around 300-400€ and scope another 300-500€, should get serious lab stuff for those costs.

and then there's always hobby-side which you tvtech pointed out, i just love this electronic hobby.

Heya Fez

Don't let me put you off. I always like your teardown's of stuff you have bought.

Me, a little bit cynical right now.

That's life I guess when (as I did) you turn a passionate hobby into a career....as I did. Everything becomes a mission and passion flies out the window.

Enough from me

Keep going Buddy,
tvtech

 

[fezder]

trouble is, nowadays it's hard to make career fixin' things when all is made in china with a dollar.....dunno, if this is even near the truth though!

 

[fezder]

measuring those voltages you asked, will post results soon

 

[fezder]

and here are the results:
(measured from sockets, indicated at datasheet, p27 and after resistor r639 (-12v)
voltage before fault during fault
+5v +5.18v +4.8v
+12v +12v +8v (fluctuates between +8v-12)
-12v -12v -12v
-16v -19v -28v
+140v +145v +141v
my dmm is pretty slow and otherwise too cheap for better measurements! (victor 88b...)

 

[fezder]

Is the Y-axis (vertical deflection) circuitry working correctly?

after long work, i managed to locate that from the circuit....but, how do i test it to verify it works as intended? it's on 2:nd page on pdf i posted.