Apple Multiple Scan 15AV Display.Help identifying a resistor

[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.

[R_J]

The built in degaussing circuit will not fix that issue, you can try an external degaussing coil, or it is possible the tubes shadow mask has been damaged, this can happen if the monitor was ever dropped. It is also possible there were some strong magnets stored against the crt The fact they are in circles is odd.
I would look into why the 70v line is low, it is likely the source for it is damaged

[tony359]

Interesting. As you might imagine I don't have a degaussing coil. other options?

I've had a look at the service manual and... I don't know, it feels something might be missing.

It says to start with Brightness and Contrast at maximum. Then to adjust the Sub-Brightness to max with a black picture.

The manual never mentions again to touch the Contrast or the sub-brightness controls. The front panel Contrast I might understand, it might mean "adjust at will" but is the sub-brigthness on the neckboard supposed to be set at max all the time?

[petehall347]

not something i would recommend but years ago i used a magnet to clear a crt . started at one corner then moved to other side then down a little and back across and carried on to the last corner and beyond .

[momaka]

Originally Posted by tony359

Good news!

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

Good job!

Originally Posted by tony359

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.

Yes, your picture looks very blue-ish. It looks as if the red and green guns are barely firing.

I see two potentiometers on the picture of the neck board you provided earlier: "R-Cut-Off" and "G-Cut-Off". These are your red and green gun cut-off adjustments. Might have to play around with them... but before you do, take a sharpie / permanent marker and mark their original position (this is always a good idea when adjusting potentiometers in general.) Then adjust and see if that changes anything on the picture. With higher red and green colors, the brightness should also come up, since those colors will now be firing on the screen as well and helping with the overall light output. If these don't seem to change anything on the picture, check that these potentiometers have not gone open-circuit from old age (and especially from moisture.)

As for the "Sub-B" pot... not sure what that does. Perhaps something with the Blue gun/gain adjustment?

All of that said, before you do anything, it might be even a better idea to consult with the service manual for any of these pots / adjustments.

Originally Posted by tony359

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

Can you invoke the degauss function from the monitor's menu / OSD? If so, try that first and see if they disappear (even if only momentarily while the degauss is working.)

If there is no menu option to degauss the monitor and you don't hear the monitor try to degauss when starting it from cold (let it sit turned Off and disconnected for 20-30 minutes if doing that), then check the series PTC and degauss relay. If needed, you can always externally force the relay contacts on. The relay is probably RL901, and associated PTC likely TH901.

If the monitor's degauss doesn't fix or improve the blotches, you might have to degauss with an external coil, like R_J suggested. I've also seen people (on Youtube) do it by using very strong Neodymium magnets attached to a shaft and spun on a drill. Not sure how effective or good that would be... and obviously I find such "hacks" to be a little risky if any of the magnets fly off and get too close to the CRT... but it is an option, I suppose.

And if neither an external degauss nor rotating magnets "hacks" can touch the blotches, then indeed the monitor's shadow mask might have suffered fall damage or something. Though the way those circular patterns appear really does seem like something highly-magnetic and very sharp came in contact either with the tube or some of the metal work on the back of the tube.

Originally Posted by tony359

- 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!

30V does seem quite low for a "70V" line, so I don't expect that to be normal. But in any case, do check with the SM first.

Originally Posted by tony359

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

Congrats! Really great to see that CRT light up, even if it still has issues.
FWIW, I am one of the forum's CRT "junkies", so it always brightens my day when I hear of success CRT repair stories.

Originally Posted by tony359

I can confirm R959 and R960 are both resistors, 1K and 33K 5%. I've placed an order. Shall I be concerned by the blue coloured body of R960? Might that mean something?

On newer electronics, blue resistors are usually of the 1% variety... but not necessarily, of course. This being an old CRT, I doubt the manufacturer used 1% resistors for anything. CRT circuits tend to be fairly forgiving with tolerances on most things, being analog and all.

Originally Posted by tony359

Great idea about sharing the whole board for future reference, it's attached.

Thank you for posting it!

Originally Posted by tony359

Q953 had died as I said but Q954 is still ok. Thankfully I bought a bunch of those 1013 transistors - it would have been sad to smoke my only £5 transistor!

Yes, whenever replacing failed transistors, always buy at least one extra and never assume the original one burned "for no reason". I've seen/had exceptions, of course... but when it comes to name-brand manufacturers for semiconductors, generally these don't fail by themselves only, unless there was an error in the design, making said semiconductor run too hot or overloaded.

Originally Posted by tony359

Correct me if I am mistaken please (one reason I do these repairs is to learn new things): if Collector and Emitter short out, then I directly connect the PSU to whatever is shorted on the neckboard. Probably a larger component which could withstand the extra current without leaving visible clues while the PSU shutdown.

Yes, you are correct.

Originally Posted by tony359

When Base and Collector shorted - and Q954 opened - the current from the PSU flowed through towards the short on the neckboard. That is a 1/8W resistor so it couldn't bear the extra current.

More than likely, that is what happened, indeed.

Originally Posted by tony359

This is pretty fun - Can't wait to try again.

I always get that vibe with CRTs as well.

Originally Posted by tony359

Is the MX-4 conductive??

No. But it is slightly capacitative. So with high-frequency pulses at several hundred volts on the HOT's Collector, even a few uA to mA of current will quickly generate an alternate path for current to flow and short-out. In my case, that small current going through the MX-4 quickly heated it up and the thermal pad, causing both to burn a little. Once burned / charred, it was even easier for current to flow through. This agrees with what I saw/heard: CRT HV started coming up and I could hear some static from the tube... but also some arcing that progressively increased in loudness until the PSU shut down.

Originally Posted by tony359

Definitely a shorted CRT driver I'm afraid. I can see the physical short inside.

It would only draw 100mA with 5V but when I put 7V on the line it started snowballing and my bench PSU started limiting.

Oh yeah, that's definitely done for.

Originally Posted by tony359

Those ICs are 9.99 on Ebay - I think I'll swap the neckboard for now, I have plenty of spares apparently as some of those monitors have smashed CRTs unfortunately.

That's really nice that you have a source for these parts.
I myself have saved boards from CRTs (monitors and TVs) when I saw scrappers bust them on the side of the road to take the yoke coils for the copper. Some of these CRT boards still fuel my workbench with all kinds of useful parts, from small SMD resistors to large resistors and transistors.

[tony359]

Originally Posted by momaka

Good job!
All of that said, before you do anything, it might be even a better idea to consult with the service manual for any of these pots / adjustments.

Indeed the manual has the full procedure, I'll try tomorrow.

As for the "Sub-B" pot... not sure what that does. Perhaps something with the Blue gun/gain adjustment?

That would make sense. No, it's "Sub-Bright" so definitely related to brightness. To be fair I gave it a go with the current settings and it caused the picture to smear - I'll try the full procedure tomorrow but I am not optimistic.

One other thing is that I am using a Mac to VGA "adaptor" (made with dupont cables). Maybe I mixed something up. But since I have Red, Green, Blue and the picture is there...

Can you invoke the degauss function from the monitor's menu / OSD? If so, try that first and see if they disappear (even if only momentarily while the degauss is working.)

No unfortunately. There is no OSD. only geometric adjustments. The RGB signals go straight to the neckboard from the cable!

If there is no menu option to degauss the monitor and you don't hear the monitor try to degauss when starting it from cold (let it sit turned Off and disconnected for 20-30 minutes if doing that), then check the series PTC and degauss relay. If needed, you can always externally force the relay contacts on. The relay is probably RL901, and associated PTC likely TH901.

It's pretty complicated to access the bottom of the board as it's enclosed in a metal protection. But yes, that is the relay indeed. I'm not sure I heard it clicking - but it was clicking when the PSU was struggling. Weird.

If the monitor's degauss doesn't fix or improve the blotches, you might have to degauss with an external coil, like R_J suggested. I've also seen people (on Youtube) do it by using very strong Neodymium magnets attached to a shaft and spun on a drill. Not sure how effective or good that would be... and obviously I find such "hacks" to be a little risky if any of the magnets fly off and get too close to the CRT... but it is an option, I suppose.

I tried with a small magnet I had. I think I saw some changes on the blotches but not on that circular spot. Uhm... Let me focus on the adjustments first.

30V does seem quite low for a "70V" line, so I don't expect that to be normal. But in any case, do check with the SM first.

The IC datasheet says 80V... 31V feels wrong. I shall check on the IC itself - I was checking on the 70V line on the main PCB.

Congrats! Really great to see that CRT light up, even if it still has issues.
FWIW, I am one of the forum's CRT "junkies", so it always brightens my day when I hear of success CRT repair stories.

Thanks! I'm happy too - in the end I can fix what I can - I cannot fix a bad CRT!
And thanks a lot for your help!

[R_J]

Go back and find out why your 70 volts is low, It looks like it comes from the power supply, and if it is only half, nothing is going to work correctly, Check you don't have a bad diode or partly open resistor in the power supply for this 70v line
Usually a purity problem would not show such defined circles, it would be more of a blotch covering part of the screen

It may only be the picture but is this trace from fbt pin 6 ok?

Also check that D954 (70v source diode) is ok, and that the ground connection on that transformer is ok,

[tony359]

I've now checked that 70V line and probed the INPUT of the regulation. I read 71.5V so it looks like the PSU is ok with that.
I noticed the output voltage changes with brightness - not by much, from 27V to 35V approximately. I wonder whether this is just a board design, the picture tube being used does not require the 50Vp/p the CRT driver can output and 35V VCC input are enough. Still weird they called the line "70V" though. However, the CRT driver is rated 80V so maybe that's totally fine.

I guess I will need to attempt a calibration to find out whether I can get anything out of this tube. I'll decide next step based on my findings.

@R_J
Interesting, I'll check that trace at the first occasion, unfortunately the board is not easily accessible. Thanks for spotting that.

[tony359]

Weird update.

I unintentionally shorted the 70V line again (I soldered a pin to the VCC of the CRT IC to check voltage at the IC, then I put the metal cover back without removing it...).

Same transistor and same resistor went up in smoke. I replaced them.

I went to power up again, monitoring the 70V while doing it.

Now it reads 67V! And the picture is definitely much better, clearly it just needs white balance and cutoff and it will be ok - besides the blotches.

Very weird. I guess this happens when you buy stuff from Ebay?

@R_J
The trace you pointed out is fine. There's a little flux residue from factory which made that weird effect on camera! Well spotted though!

[momaka]

Nice work again!

Sorry to hear about your mishap with shorting the transistor again... and at the same time, I guess we can say it's a good thing you did. Otherwise, you might just have went on a wild good chase with playing with adjustments to no end, when the trouble was the 70V line being low the whole time.

Yeah, Ebay transistors can be quite crap. No idea what you're going to get until you use it. A lot of times, the "cheap" transistors will have smaller physical dies inside, and thus not be able to handle the stated current. I say "cheap", because sometimes they are not even cheaper than buying from a reputable place like Mouser, Digikey, RS, and etc. If this transistor goes up in smoke again, perhaps see if anyone is offering TTA004B. As per my post earlier in the thread, this transistor should also work in the circuit, and it has a larger TO-126 case. So if the Chinese cheated with the die on that one too, perhaps it will be just as good as a genuine 2SA1013.

Speaking of which, my package of 10x 13009 transistors arrived not too long ago. They are cheap no-name ones from eBay, so I have to see if they are even remotely capable of what a true genuine 13009 can do. My guess is, they will be about as good as a 13007, at best. Probably going to do worse, though. Time for a test!

[tony359]

You're right, I would have probably abandoned the project - or resumed it once I fixed another one of those boards and discovered you actually have 70V on the line called 70V.

I did test those transistors in a tester (those arduino-based ones) before using them and they checked out ok. I know it's not the same thing but... that line is very very low current I think, the cable that goes to the CRT driver is tiny. The IC datasheet says 40mA max so you'd expect that even a crappy transistor could make it.

Well, as you say, I'm actually lucky I burnt it - and I spotted the orange light from the PCB before I had sparks this time so I did not cause any further damage to the traces.

I used to buy my stuff from Farnell UK - less stocked than RS but they would allow you to buy 1 or 2 of anything - RS is usually minimum 10 or much more.

Shipping was free over £5.

Today shipping is free over £35 and there is a "small order handling charge" for small orders so ordering 10 transistors from them is not feasible.
Even Ebay can be expensive. There's a large Ebay seller that has "everything" but everything is a rip-off. Those 1013 for example: £4.99 EACH! Still cheaper than RS unfortunately.

Back OT, how do I check that the degaussing coil is working? I guess I should read a short on the coil, what about the connector? Can I disconnect the coil and expect to read a voltage when I power up? What kind of voltage should I expect?

The thing is that I hear the relay clicking but I don't hear the usual "WOANG" (I'm making this up!) noise when I power up.

[R_J]

The degaussing circuit consists of the coil itself, TH901 (PTC) thermistor and in this case there is a relay across the a/c line. Once the thermistor is warm from running a degaussing cycle it won't operate again until it has cooled.
It is there to clear up MINOR purity problems only.
That ring is likely there permanently, it does not take that much of a magnetic field to change the purity, even the earths magnetic field effects it. If you screwdriver tip is magnetized, move it along the screen and you will see that doesn't take much. I fear that spot is permanent

[momaka]

Originally Posted by tony359

I did test those transistors in a tester (those arduino-based ones) before using them and they checked out ok. I know it's not the same thing but... that line is very very low current I think, the cable that goes to the CRT driver is tiny. The IC datasheet says 40mA max so you'd expect that even a crappy transistor could make it.

Well, here's how it works:

40 mA might not seem like a lot... but if the input voltage is, say, 90V for example and the output voltage is 70V... then the differential is 20V across C-E terminals. Remember, Power = Voltage x Current. So with 40 mA going through the transistor and 20V across C-E, that's 0.8 Watts of heat dissipation, which is more or less right around the limit for a TO-92 case transistor. If any of those knock-off transistors have a smaller die that can't handle this much heat dissipation, it would be overheating, causing hFE to drop significantly... and hence the likely reason why the voltage was sagging.

Originally Posted by tony359

Back OT, how do I check that the degaussing coil is working? I guess I should read a short on the coil, what about the connector?

Yes, the degauss coil shoud appear like a short-circuit or very low resistance - I'm guessing around a few Ohms tops or 10 at most. If it does, then it's OK.
I think it's unlikely you will find the degauss coil burned. It will trip a breaker way before it can burn out.

Originally Posted by tony359

Can I disconnect the coil and expect to read a voltage when I power up? What kind of voltage should I expect?

With 230/240V AC mains... 230-240V AC. Simple.
This is why you hear that buzzing "WOANG" / "BONG" / "WAAAAAA" sound - it's the 50/60 Hz buzzing from the AC mains, vibrating the wires in the degauss coils from the strong magnetic fields.

Originally Posted by tony359

The thing is that I hear the relay clicking but I don't hear the usual "WOANG" (I'm making this up!) noise when I power up.

Check the PTC/thermistor TH901. It's a positive temperature coefficient (hence PTC) resistor. Basically, its resistance increases as it heats up. With its resistance increasing, it gradually reduces the current going through the degauss coil to a very low value. So the magnetizing current from the degauss coil starts off very high when the PTC is cold and slowly diminishes. This is important, because otherwise if there is no PTC and the relay coil cuts off when the AC mains is at non-zero value, then there may be a strong magnetic current as the relay cuts off. This will leave things magnetized behind the CRT, causing the picture's colors to be really messed up and hue-ey in circular patterns. (And I can tell you from personal experience too - I had a CRT whose relay decided to cut off right away after starting, with the PTC still cold and all. The result was a very messed up picture.)

Anyways, if the PTC is not open-circuit or really high resistance, then check that the contacts on the relay work. Connect an incandescent light bulb in place of the degauss coil on the main board. When you power up the monitor, you should see the light bulb light up. If it doesn't and you've already checked the PTC, then check if the relay is getting a signal to turn On.

Originally Posted by tony359

I used to buy my stuff from Farnell UK - less stocked than RS but they would allow you to buy 1 or 2 of anything - RS is usually minimum 10 or much more.

Shipping was free over £5.

Today shipping is free over £35 and there is a "small order handling charge" for small orders so ordering 10 transistors from them is not feasible.
Even Ebay can be expensive. There's a large Ebay seller that has "everything" but everything is a rip-off. Those 1013 for example: £4.99 EACH! Still cheaper than RS unfortunately.

Yeah, times have changed for sure.
Here, neither Digikey nor Mouser have any minimum order. For small packets, I think the cheapest option is USPS priority or something like that - IIRC, around $7 for shipping for up to about a few ounces. Don't quote me on that, though, I haven't checked in a long time (well, only 8-9 months, but I forgot.)

[tony359]

Thanks @R_J I appreciate the integrated degaussing is not going to fix those concentric things, it's just that I don't hear the circuit working...

Originally Posted by momaka

Well, here's how it works:

40 mA might not seem like a lot... but if the input voltage is, say, 90V for example and the output voltage is 70V... then the differential is 20V across C-E terminals. Remember, Power = Voltage x Current. So with 40 mA going through the transistor and 20V across C-E, that's 0.8 Watts of heat dissipation

I thought that that calculation applied to linear voltage regulators (like a 7805) not to transistors. Interesting.

That said, the input voltage is 71V and output is 68V so more like 0.12W dissipation.

I really can't explain what happened before. Sure, I might have made a mistake somewhere but I did triple-check everything...

With 230/240V AC mains... 230-240V AC. Simple.

Ah, it's basically a 240V momentary short?
Then I know what's happening. Currently I have the monitor connected to my light bulb current limiter. The degausser is being stopped by the light bulbs for sure

I'll connect directly to mains when I stop burning things

This is important, because otherwise if there is no PTC and the relay coil cuts off when the AC mains is at non-zero value, then there may be a strong magnetic current as the relay cuts off. This will leave things magnetized behind the CRT, causing the picture's colors to be really messed up and hue-ey in circular patterns. (And I can tell you from personal experience too - I had a CRT whose relay decided to cut off right away after starting, with the PTC still cold and all. The result was a very messed up picture.)

This is quite interesting thanks!

Yeah, times have changed for sure.
Here, neither Digikey nor Mouser have any minimum order. For small packets, I think the cheapest option is USPS priority or something like that - IIRC, around $7 for shipping for up to about a few ounces. Don't quote me on that, though, I haven't checked in a long time (well, only 8-9 months, but I forgot.)

The issue here in the UK is that there aren't many options left. RS have everything - but usually not in small quantities. Then it's the Farnell Group. Then there are a few small ones which have just the basics.

Then there is Ebay.

[tony359]

Good news!

I was able to do white balance and cutoff and the picture looks great (besides the blotches).

Just for the records, I think there is a mistake in the service manual.

On page 32 it says to turn all front controls to maximum and also "sub-brightness" on the neckboard to maximum but it says that is achieved in a counterclockwise manner.

That should give a blueish 100% black picture. That was not the case as if I turn the control counterclockwise I am REDUCING the brightness, not increasing it.

I turned it clockwise and I had the blueish black required.

Eventually I got a pretty good result - I even used my colour meter to try to get a good balance. I didn't strive for perfection, particularly because the tube seems to be damaged.

But hey, I did it!

I plugged the monitor straight to mains - no current limiter - and I still don't hear the "WOANG" of the degaussing coil. I checked with a multimeter set to record maximum in manual range and I only got 68V. I'm not sure whether it's the multimeter not fast enough to record that or something's wrong with that circuit.

I was thinking of unplugging the coil and plug the multimeter there: because there is no current being drawn, the NTC won't get warm and I should read 240V until the relay cuts the power, would that work?

The coil reads 18 Ohm if memory serves.

[R_J]

The monitor would likely need to sit for 1/2 hour for the PTC to cool down, then you might hear the degaussing coil but might not, in any event it is not going to clear up the purity problem, That looks like someone placed a couple of permanent magnets close to the screen for a long time to magnetize the shadow mask like that.

[momaka]

Originally Posted by tony359

I thought that that calculation applied to linear voltage regulators (like a 7805) not to transistors. Interesting.

It all depends on how you're using the transistor.
When used as a linear regulator, then it's the same exact calculation (V_drop... i.e. voltage difference between input and output... multiplied by the current draw is your total power dissipation). When used as a switch... well, it's the same again, except when the transistor is used as a switch, the voltage drop between input and output will be very small (typically 0.7-0.8V for a BJT.) Multiplying that by the current gives the device dissipation power. This applies to MOSFETs too, except MOSFETs will have almost no difference between Source and Drain when fully driven On.

Originally Posted by tony359

That said, the input voltage is 71V and output is 68V so more like 0.12W dissipation.

I really can't explain what happened before. Sure, I might have made a mistake somewhere but I did triple-check everything...

Could have been a dud transistor, then.
I still haven't tested my 13009's. Was going to do that today, but didn't get a chance. I'm really really curious.
At around $0.50 a piece, I think I can afford to burn a few to see what they really are. (Though I will try doing it without burning anything.)

Originally Posted by tony359

Ah, it's basically a 240V momentary short?
Then I know what's happening. Currently I have the monitor connected to my light bulb current limiter. The degausser is being stopped by the light bulbs for sure

Yup. that would do it.
Actually, I'm quite surprised the monitor even worked at all with the series incandescent bulb. Usually, you cannot use it to test a power supply with much of a load on it. You do have 240V AC mains, though, so that does help a bit, since this monitor's PSU uses wide voltage input discontinuous topology, and those can work OK with anything down to 100V. So the series bulb dropping the voltage still allowed the PSU to work. But the degauss coil - yeah, not so much.

Originally Posted by tony359

Good news!

I was able to do white balance and cutoff and the picture looks great (besides the blotches).

Sweet!

Originally Posted by tony359

I plugged the monitor straight to mains - no current limiter - and I still don't hear the "WOANG" of the degaussing coil. I checked with a multimeter set to record maximum in manual range and I only got 68V. I'm not sure whether it's the multimeter not fast enough to record that or something's wrong with that circuit.

Like R_J said, if the PTC was hot from previously running the monitor, then you need to turn off the monitor and keep it off for a few minutes. Otherwise, with the PTC hot, the degauss won't fully run. Though the fact that you only hear the relay click and not even a faint "BUMMM", I'm inclined to think the PTC may be open or high resistance or the relay contacts are gone.

Originally Posted by tony359

I was thinking of unplugging the coil and plug the multimeter there: because there is no current being drawn, the NTC won't get warm and I should read 240V until the relay cuts the power, would that work?

Connect a 25-40W incandescent light bulb to where the degauss coil normally plugs in. Then connect a multimeter to measure the voltage across the bulb. Don't use "Max" value. Indeed some multimeters can be a little too slow. If it's an auto-ranging one, just set the voltage range directly to the highest (i.e. 400-600V for most meters) and see what the voltage measures when you power on the monitor. Should get 240V AC for a good few seconds, since the incandescent bulb won't pull as much power as the degauss coil, and thus would make the PTC heat slower and drop the voltage slower.

Originally Posted by tony359

The coil reads 18 Ohm if memory serves.

OK, that still looks alright.

[tony359]

Originally Posted by momaka

It all depends on how you're using the transistor.
When used as a linear regulator, then it's the same exact calculation (V_drop... i.e. voltage difference between input and output... multiplied by the current draw is your total power dissipation). When used as a switch... well, it's the same again, except when the transistor is used as a switch, the voltage drop between input and output will be very small (typically 0.7-0.8V for a BJT.) Multiplying that by the current gives the device dissipation power. This applies to MOSFETs too, except MOSFETs will have almost no difference between Source and Drain when fully driven On.

Great, I learnt something new today thanks!

Actually, I'm quite surprised the monitor even worked at all with the series incandescent bulb. Usually, you cannot use it to test a power supply with much of a load on it. You do have 240V AC mains, though, so that does help a bit, since this monitor's PSU uses wide voltage input discontinuous topology, and those can work OK with anything down to 100V. So the series bulb dropping the voltage still allowed the PSU to work. But the degauss coil - yeah, not so much.

I have multiple bulbs connected - it's a box I made, I showed it here: https://youtu.be/38jQtb0LAhA
For the monitor I selected all the bulbs so I did not have a voltage drop when the monitor was powered up - but indeed a big flash when turning on from cold, indication that the degaussing is somehow working? (But it could also be capacitors around the board even though I see an NTC before the power supply)

Like R_J said, if the PTC was hot from previously running the monitor, then you need to turn off the monitor and keep it off for a few minutes. Otherwise, with the PTC hot, the degauss won't fully run. Though the fact that you only hear the relay click and not even a faint "BUMMM", I'm inclined to think the PTC may be open or high resistance or the relay contacts are gone.

I did test after 30m rest (thanks R_J) and only got 68V and no "BUMMM" at all - but I can hear the relay.

Connect a 25-40W incandescent light bulb to where the degauss coil normally plugs in. Then connect a multimeter to measure the voltage across the bulb. Don't use "Max" value. Indeed some multimeters can be a little too slow. If it's an auto-ranging one, just set the voltage range directly to the highest (i.e. 400-600V for most meters) and see what the voltage measures when you power on the monitor. Should get 240V AC for a good few seconds, since the incandescent bulb won't pull as much power as the degauss coil, and thus would make the PTC heat slower and drop the voltage slower.

Ah, that's a good idea. I was also planning to scope that connector - I have differential probes so it's safe to do. A small lightbulb would help giving a visual feedback and also a little load to trigger the PTC I guess.

I did disable auto-range when I checked - and it's a reasonably good meter, Fluke 117 - but might not be fast enough anyways.

[tony359]

This is what I get out of the degaussing circuit - with the degaussing coil attached.

416V p/p are 147V RMS. Do you think this is what I should expect from a degaussing circuit?

This is with the monitor cold, previous attempt was an hour earlier.

With a 15W light bulb I get full power (see pic) so two scenarios here:

1. Everything is working totally fine
2. The Thermistor is reducing the power too early.