Apple Multiple Scan 15AV CRT monitor repair

[tony359]

Hello all,

This is my first post here!

To start: I (hope) I know what I am doing inside a CRT, that I need to discharge it and ground it before touching anything.

I've got a number of these Apple monitors. They're somehow brand new but likely DOA or used by service centres as there are RMA numbers on the boxes. I was trying to make one or two working out of a pile I found
I brought home a couple, one unfortunately had a broken neck.

The other one would make a fast clicking noise when powered up. The Power LED also flashes. I know this usually points to the flyback but it looks like the PSU section to me. I do not see arching and the clicking noise comes from the PSU (I've confirmed using a plastic hose!). I've monitored one of the test points mentioned by the service manual for voltage and it goes on and off with the clicking.

So I swapped the boards between the two monitors and... same clicking. Now, I cannot be sure these monitors ever worked. Chances are those boards have the same fault on them. Again, both the boards and the tubes are spotless.

I took a look around, I could not find anything shorted around the PSU area and my thermal camera does not show anything warming up.

Unfortunately I do not have schematics - only a service manual which is pretty generic.

The question I have for you is: can a faulty CRT cause this kind of issue? When I swapped the board, I swapped the neckboard with it so there is nothing left in the monitor!

I have a feeling this is a "simple" SMPS issue but it's not easy with no schematics. The board is also pretty much inaccessible - I guess I would need to take it out and power it up on a side to test something, but still pretty difficult without schematics.
The board is manufactured by LG and it's PN 6870T024A

Any pointers would be welcome! Thank you!

Service manual can be found here: https://1drv.ms/b/s!AukePwK7XyEPhOZL...X5v0Q?e=64MaJY

Pics

[zeno]

To discharge short out the anode to the bare ground wire. It probably
is bleeding off anyhows. Wont kill you, I got bit once a week for 40 yrs !

Ticking usually from SMPS. Lets say there is a short on the chassis, most
likely the Hoz output transistor. The PS comes up then detects overcurrent.
It turns the PS off then repeats until the short is cleared.

Set looks like a Gold Star A.K.A. LG. At least the CRT is. To find manual go
to FCC.gov, enter the FCC ID number OR type acceptance number.
This will USUALLY cross to the OEM & OEM model ##. Look for a real
manual from the OEM.

BTW since the chassis has almost everything you may try unplugging the yoke.
Do it only as long as you need to see if it clears the short. Also there
may be a B+ jumper on the plug !

Good luck
73 Zeno
LFOD !

[tony359]

ahah good to know it's not going to kill me - but I'll be careful anyways! I'll check the horizontal transistor, thanks (if I can find it!).

One of the tubes on another identical set is a Goldstar indeed - too bad it did not survive the shipping.

I'll try unplugging the Yoke - won't allow the CRT to burn, no worries! Can I check what you mean with "B+ jumper on the plug"

Thank you!

[zeno]

Jumper just kills the B+ to the hoz / hv so if you forget to plug it
in you wont fry the CRT. May have it, maybe not.

Zeno

[tony359]

ah I see. So a safeguard in case I don't plug it back to stop the CRT from being damaged by the lack of deflection.

Would the set be able to power up with that safeguard in place?

[zeno]

If the yoke is shorted & jumped it should. Maybe not full HV but
it will stop ticking. Yokes rarely go BTW

[tony359]

Update.

Horizontal transitor (C5129) showed a short between base and emitter (short in diode mode and 0.4Ohm in resistance mode). I took it out but the transistor is fine.

I inspected the PCB and I see the Base is going back to ground via a small transformer (labelled T701). This small transformer reads indeed 0.4 ohms on my meter.

BBut I believe this is normal?

Removing the Yoke plug did not improve things.

I took out the main switching transformer to understand how many voltages are generated. I counted 5. I can follow them on the back of the PCB, they all go through diodes and capacitors (obviously!) and then disappear around the board. I tested all I could think of but I did not find a short.

I've removed a connector which seems to take filament voltage and 12V to the neckboard but nothing.

The outputs sometimes go through transistors and voltage regulator but, again, I don't see a short there.

Any other ideas?

I might want to inject some voltage on the various rails to see if by any chance my thermal camera sees something?

I have a twin board which - surprisingly - is showing the same fault, I could use that one.

Thank you!

[tony359]

Well the thermal camera worked. I found a shorted small transistor A1275-Y. Datasheet says it's used in TV for vertical deflection or sound - I'd say it's for sound.

Reason I could not spot it is that it's connected to ground via 40Ohm resistor. I did notice one rail was showing this weird low-ish resistance but I thought it was ok.

Anyways, I'm looking for a replacement. Ebay sometimes is nasty as there are sellers selling ONE transistor at stupid price.

I've been looking for a replacement.

1275 datasheet: https://datasheetspdf.com/pdf-file/637525/KEC/A1275/1

I found this: http://www.unisonic.com.tw/datasheet/2SA1013.pdf

The original is Y so I need to make sure the replacement is P (does it matter if it's not as I see different manufacturers have different naming).

Would that work?

Thanks!

[zeno]

I sniffed around a little & didnt find much. Odds are the ones on E-bay are
Chi-Com bootlegs. Almost every STK amp IC is. They fake EVERYTHING. Did you try the one from the other chassis ??

Zeno

[tony359]

Hey thanks for that.
I did find some from Ebay UK which were delivered today so I should be able to test soon.

How do I know if the ones I have are fake or not? I used this seller before and don't remember having issues - that doesn't mean I got some fake stuff and they just won't perform as per specs!

[tony359]

Update.

I swapped the transistor and checked things around.

The monitor powered up and stayed in standby happily (Yay!). When I applied VGA signal, the monitor fully powered up, I had HV, a weird image on screen and then The Magic Smoke!

The same transistor shorted (Base with Collector) and a resistor connected to it went on fire.

I've looked around and here are my notes:

- The transistor seems to be generating +70V for the neckboard. On the Neckboard there is a CVA2422TL CRT driver. I've noticed that I've got 90Ohm between GND and the 70V line on the neckboard - my other neckboard reads some Kiloohms and climbing.

CVA2422TL datasheet: https://pdf1.alldatasheet.com/datash...CVA2422TL.html

Shall I move my attention to the neckboard? Could it be the reason why the 70V rail fails every time?

Could a bad CRT cause this issue on the neckboard? Please note I read 90 Ohm with the neckboard disconnected from everything (but the Flyback)

I don't want to give up know. The area which went up in smoke seems pretty simple - I'm sure some of you can recognise the topology, I suspect it's some voltage regulation?

When the screen came up it was pretty weird, white with blotches. Maybe someone can recognise the symptom of something? See video below.

Also, I cannot assume that the trimmers around the PCBs haven't been touched. Still, I wouldn't expect components to go up in flame if something is misadjusted?

Here is the video of the accident (I have a small youtube channel where I publish my repairs):

https://www.youtube.com/watch?v=YqpgZc0HX2M

Attached some pictures - the main PCB is single layer so the circuit can be easily followed there!

Thanks so far!

[tony359]

Latest diagnostic: I've injected 7V into the 70V line and the CRT driver IC lit up!

You can actually see the short inside the IC!

[zeno]

First most new hoz outs have a damper & base resistor built in so
readings may be off. In 40yrs I had only one Hoz out open, All others
dead shorted. Hundreds of them.
Here are typical CRT voltages.

Cathodes R,G, &B 130 v
G-2 400v
So the lower the cathode V is the brighter the pix.
Go too low & you get over current then HVSD.
If G-2 gets too high same thing.
MOST the time the outputs run on a 200 V source from a flyback winding.
On TV's the filter cap for that is a super common thing on all brands.
Loose the 200 V & the K's go low then HVSD.
BTW I have no idea what that 70 V does !

Zeno

[tony359]

it powers the CVA2422TL which I believe powers the RGB signals? To be fair the CVA2422TL says 80V, but the PCB says 70V...

[tony359]

Good news!

I've replaced the resistors and the transistor. Swapped the neckboard. The monitor powered up... and stayed up!

The picture is all messed up but I believe this is normal when you swap a neckboard between tubes - all the adjustments are in there. Colours are off, brightness is too high and I can't turn up contrast without causing major smearing on the picture.

Only two potential issues
- Those blotches are still there. I am not sure the de-gaussing circuit works? I remember hearing this loud "boing" (sorry!) when turning on a TV and you'd see the picture wobbling for a few seconds while the coil was discharged. I don't hear/see anything here
- The 70V line. I was monitoring it and my meter only reads 31V. I'm wondering whether the picture is dim because the CRT driver is getting half of the expected voltage? Unless the voltage rises when adjusting the drive on the neckboard? Do you think that second transistor could get some feedback from the neckboard and drive the rail harder if necessary?

Anyways, the service manual has the complete procedure to re-adjust it. I'll put a mirror in front of it and try tomorrow!

For now: Yay! I found the fault! My first CRT!

Thanks so much for your help so far.