Macintosh Plus power supply troubleshooting and repair
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- Опубликовано: 12 май 2023
- #apple #macintosh
On today's video, I'm going outside my comfort zone and working on analog stuff. It's a very broken Macintosh 128/512/Plus power supply/analog board. It was sent in by Tommy of the RUclips channel Arctic Retro as a broken part. He didn't know what was wrong with it other than the machine wouldn't power on. So let's find the faults with this board and get it working again!
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Oh, I can tell you why it tripped the breaker. Your step up transformer is wired in split phase. European power is single phase 240. So in Europe neutral is at ground potential. Your step up transformer is neutral at 120v with respect to ground. This isn't safe and you should not run that step up with that configuration as you can short 120v directly to ground if you have a neutral bonded to ground in a chassis for some reason. Don't use that step up transformer for anything. You need a proper one that keeps neutral at ground potential and the hot needs to be 240v. This is why your circuit breaker tripped. (edit: since the tranformer probably has split phase output you just need to rewire it correctly so the center tap isn't connected to your ground, but remains capped off and disconnected.)
Even at Apple we called that the devil board. So many CRT's & motherboards were killed by it. I was with Apple way after, these machines, but they came in for repairs. Even some of the New Old Stock boards were wonky & I was a rare soldering tech at Apple. So I saw about 50 or so
Can confirm it can be devil to troubleshoot. I had to hand these boards off to the master analog repair shop manager sometimes to shorten the troubleshooting time. I eventually got into the groove with several brands of monitors.
The damn things are proof of "spooky interaction at a distance" when you get incoming RF and magnetic fields affecting the picture.
Thanks for featuring my board. I knew you could figure it out. Great work!
In case anyone wonders how these dual supply voltage switch mode power supplies work:
They're basically 220-240V power supplies, only the big charge capacitor on the primary side is replaced with two capacitors in series. On 230V everything works as normal, the mains AC is rectified and goes to the capacitors.
If you switch the power supply to 120V (either with an external switch or, like in this case, with a wire link), the center point of the two capacitors gets connected directly to the AC supply, before the rectifier. That way you turn the full bridge rectifier into two half-wave rectifiers, basically creating a voltage doubler circuit: the positive half wave of the input AC charges one capacitor with a positive voltage, the negative half wave charges the other one with a negative voltage (referenced to the center point). That way you get two capacitors charged to roughly 170VDC, which in series is equal to the roughly 340VDC you would get when rectifying 230VAC.
It's simple, elegant and quite ingenious!
voltage multiplier circuit for 100V and 230V as regular rectifier switched with one jumper link
I always wondered how they did that! I just figured it involved a center tap on the primary of the transformer or something.
@@seancurtin6103 I would have thought so, too. The PF on that circuit can't be too great, since you would have to go with a passive PFC coil, and optimize it for either voltage setting, or the midpoint between them. (I think.)
@@nickwallette6201 Any ATX PSU that I've seen with a voltage toggle is passive PFC.
@@steeviebops Active PFC circuits usually are step-up converters that step the momentary rectified mains voltage up to 350V to 400V, doing so with a good power factor. The regulation range of the active PFC circuit is wide enough that it can cope with 90V-240V input without any circuit modifications. That's why you don't find that toggle switch on active PFC supplies.
A step-up transformer is a transformer that takes on extra responsibility for the good of the team. ;)
And 240v is more common in the world than 110v
@@steves009 Saves copper. The US standard of 100 goes back 100+ years ago. Since we started early our standards meet older needs.
笑っちゃう
It's not my real transformer. It's my step-transformer.
@@AndrewTubbiolo
The US ended up with roughy 110V because light bulbs in the 1880s could only handle that. Europe’s systems started being built out a decade later by which time bulbs that could deal with ~220 V had been developed.
(@51:05) “None of those (capacitors) needed to be changed.” Except the infamous RIFA cap! 😂
Pretty advanced and even a bit scary - you're a great technician Adrian.
I just developed a great big smile when I saw you using the mirror to look at the screen while tinkering with the set electronics! That brought back so many memories from 53 years ago, as I was a bench tech at Bernie's Radio & TV.
Ha Adrian. You can test that SCR with your meter in continuity/diode-check mode. Put the black wire to the cathode of the SCR, and the red lead to the anode of the SCR with the gate floating. You should get an open circuit. Now, briefly connect the gate to the anode. You should get continuity (or a few volts drop) and it should stay that way until you disconnect it, wherein it’ll go back to being an open.
For low-power applications, you can replace an SCR with a 2N3904 & 2N3906 (i.e. one NPN & one PNP). The emitter of the NPN is the cathode of the SCR. Wire the base of the NPN to the collector of the PNP, and bring this out as the gate. Wire the collector of the NPN to the base of the PNP, and the emitter of the PNP now becomes the anode of the SCR.
Theory of operation: initially, both transistors are off (SCR not conducting.) When you pull the gate positive w/respect to the cathode, the NPN transistor conducts, bringing it’s collector close to ground. This turns on the PNP transistor, which now conducts, bringing it’s collector to the voltage at its emitter. This keeps the NPN transistor on so the gate signal can be removed and it will still conduct until the circuit is broken (no more current.)
Finally, this mock-up of an SCR is just for testing; use a real SCR in the circuit!
This was a great troubleshooting video. I really enjoy these.
You may find this helpful. Going back into my memory from my tech days, square pads on the circuit boards indicate the positive pin for electrolytic capacitors and the direction for other directional components such as diodes. For ICs, the square pad is for pin no. 1.
This makes troubleshooting a lot easier.
It may be a good idea to wire that step-up transformer to have a CEE7/7 (Schuko, or type E/F universal) socket, so you don't accidentally plug in a 115V device into the 230V part of the transformer. Schuko to IEC power cables can be found practically everywhere.
Another masterclass in diagnosing compound faults. Awesome repair!
27:16 I think you need to look at how your step-up transformer is wired. It appears the switch does kill the power (line - neutral), but it's leaving 120V from line (or neutral) to ground. That could give you a nasty surprise in the future.
cheap ones are usually an autotransformer, for safety use an isolated step up transformer.
@@fumthings It's not the isolation I'm concerned about. It appears the power switch on the unit opens the neutral and not the hot, leaving line power on the output. Although, maybe the problem is the power plug it's plugged into isn't wired right.
@@russellhltn1396 ok, understood.
It is hard to top your previous troubleshooting videos, but you still manage to create an even better one. Thank you for these videos.
for my .02 any diag vid by this guy is on my top 3 must watch now list.....
I remember getting those red battery holder with black silkscreen analog boards as service parts in about 1991. My guess is Apple decided to standardize on a single repair part that late in the original Mac life cycle. It's funny what gets stored in your brain.... I remember a note in the package about making sure the W12 jumper was set up correctly for the voltage it going to be used with. Being a service tech in those days was a different experience.
Nowadays a service techs only job is to tell people how stupid a repair is and sell them a replacement
You inspired me to recap my sad MacPlus board after your video last week and it is working now. Thanks for cool videos!
Congratulations! 👍
I like your dedication to fight against mindless recapping :)
Great video as always Adrian. I used to do a fair bit of board repair with 2 years straight just working on CRT's. Watching your video's brings it all back. Superb. Keep em coming. EDIT: The NEC Multisync monitors I think were the most difficult patients of them all for me. That one used to come in with some very random faults. And the harder they are the more fun they are.
I remember reading that those monitors are also quite complicated with a bunch of boards inside -- making troubleshooting that much harder? I've never had to work on any luckily!
@@adriansdigitalbasement Indeed. Shango66 had one on his channel.
Short circuits are easy. Use your bench power supply and inject 5V. Limit current so not to burn tracks and the offending component will warm up. If you have a thermal camera, it’s even quicker. Great work with finding the faults. Regards Ron
Absolutely love your diagnostic repair videos. Have learned so much about the methodology and reasons behind why it's so important to learn and know how different components behave and interact.
It's a great day when I see a video from you Adrian! You have a very good way of visually explaining things, your videos are always top-notch. Love from just north of you in BC :D
Damn good job, lots of danger and new experiments/repairs with this one 🤘
And finally, something that actually had a failure (failures!) instead of it just fixing itself
Nice! Love watching you troubleshoot. I always learn so much. Thanks.
Concerning SCR testing with a multimeter, normally Gate-cathode junction act like a diode. But anode-cathode should be open both direction.
I am so impressed by all those diagnosis videos. Still stuck with my own analog board repair for months 😂 Great video. Thanks
A very enjoyable video. I enjoy watching your analytical skills in action.
My favorite part of Saturday mornings :) Nostalgia and edification
Oh, I thought it was nostalgia and petrifaction. 🤔
@@williamsquires3070 It is of course subjective. I can only speak for myself. 😎
Your troubleshooting videos have taught me so much. This is another great one. Thank you.
Adrian is a heat seeking missile towards the weirdest problems.
Great video as usual. I really enjoy watching you troubleshooting these issues. Can't help flinching when your hands go near the HV! (I know you know what your doing!)
23:42 That's a needlessly complex turn signal for your car! 🤣
I've seen some actual cars with fancy animated turn signals, presumably controlled by a computer. Wouldn't be surprised if that computer is faster than this Mac.
Loved every minute of this! All the best from the UK
You have good analytic skills, and the fault finding was exciting to watch! Also nice to see a board from Tommy :D
45:19 Discord notifications, thought it was mine
Great analog troubleshooting, I learned something today.
Man, vids like this are sooo satisfying... Thanx Adrian!
@Adrian, the SCR is a "crowbar" circuit that's used to protect the power supply rail from excessive voltage. Sometimes if they do overvolt, the SCR can fail shorted. SCRs operate like diodes, but they have "trigger" input (gate) that once is powered the diode part begins conducting and latches on as long as there is a certain threshold of current flowing. This makes it a perfect choice for the latching function of the crowbar. It literally shorts out the rail if it's trigger circuit detects an overvolt on the rail (usually a zener circuit).
Nothing can make me nostalgic for old Macs like a great diagnostic/fixit video.
Great Fix Adrian Good Approach. The Repair
41:43 clearly that dead RIFA cap is still good for something. 😂
The only thing it's good for :P
Adrian get a ESR meter. As much as you work on older electronics you really need a good one. EDS-88A CapAnalyzer Series II Electronic Design Specialists Cap Analyzer I can wholeheartly recommend. I own and use that one and a Capacitor Wizard Analog ESR Tester with charged capacitor protection board added. The digital unit (EDS-88A) will perform a short circuit test before the esr test as a short circuit can simulate low ESR. The cap wizard does not have this capability. But both are excellent testers and the results are accurate. Also when I have a power supply that has not been on in a long time, I will put the dim bulb tester in series and bring up the power supply slowly (unloaded) with a variac. I feel it has definitely saved me from exploding electrolytic capacitors. It will do nothing for the rifa cap issue so snip those out before starting!!
I had a GME Professional In-Circuit ESR Capacitance Meter Capacitor Tester for a while and sent it back. It had quality issues and would lock up if testing caps in rapid succession. Do not recommend. Price was nice but no comparison to the other units mentioned above!!
Another benefit is the ESR tester uses high frequency so inductors look open instead of short. This helps you determine if a short is "real" or not.
Excellent detective work!
Amazed! You have a gift my man.
The cherry on the top of a Saturday!
This was quite the adventure. Thanks.
It's so much fun getting a CRT to fire up and show a picture after going through all that work to find one small and annoying part that died... :P
That SCR was part of a power supply crowbar circuit, just in case the 5 volt supply went too high. So not absolutely needed but maybe so if someone went and tweaked the voltage too high.
Ah yes! I would have to think with that crowbar design that it should have been able to handle the over voltage without failing short?
@@adriansdigitalbasement I could have lost an eye once when a crowbar circuit tripped and a 1 ohm 1 Watt resistor in series with the SCR exploded.
The only problem I've seen with the Mac analog board is the vertical yoke capacitor. It handles several amps of current and tends to go bad or heats up its terminals and unsolders itself. You can see premonitions of this if the vertical height of the screen jumps around a little bit.
@@adriansdigitalbasement These crowbar circuits are designed to actively short the rail that overvolts. Because they short that rail, the voltage across the SCR is "not that high", so the amount of power dissipated by the SCR is limited. Yet, an triggered SCR still has some forward voltage (IIRC, around 1.5V), and a broken supply putting several amps into it will dissipate some more several watts in the SCR. Crowbar circuits are not meant to operate continuously, but it is expected that the supply breaks down (blows a fuse, activates short-circuit protection) within some time. If that doesn't happen, the user is expected to power off / unplug the device. As you see no heatsink on the SCR, it may overheat if left in shorting mode for too long. The typical failure mode of overheating power semiconductors is going short. That's likely what happened on your supply, as the +5V setting was tuned too high, so the crowbar did it's job, sacrificing itself.
The crowbar circuit *still* *seems* *broken*. You shouldn't be able to get 5.8V out of that supply without the crowbar triggering. At the moment when you got 6.3V, the regulation and the crowbar was not connected, so it didn't have a chance to kick in. For continued protection, please check the crowbar operation, you might have a broken component in the SCR drive circuit.
Another great Video, thank you so much. You just scared my quite a bit when you touched the hi-voltage part of the switching power supply with your hands.
Maybe show us how you discharge that part of the power supply and make sure it is fully discharged, too? Greetings from Germany!
45:35 , when you put the Prob on the video Signal pin from the motherboard is when the Picture appeared on the screen, the fault is on that pin, not the connector going to the neck connector on the crt.
Exactly. Review the video Adrian. :D
good detective work - I don’t think I would have know where to start
nicely done!
Most 5v chips from that era can handle 7v absolute max before the magic smoke is permanently allowed to escape, I've certainly done exactly the same before with no ill effects. Thank goodness the error margins for overvolting are quite large. Nicely repaired Adrian, also Deoxit to the rescue again ;)
Thanks for the video Adrian, good as always!
If I may suggest, you should invest in a face shield designed for electrical works, safety goggles sadly aren't enough for two reasons: first, goggles will protect your eyes against incandescent flying debris, but not the rest of your face, burning pieces of metal, plastic and electrolytes will bury very deep into the skin, to the bone in fact and sometimes deeper, causing irreparable damages to nerves, muscles, etc... that can be lethal, if you are unlucky.
Second, the flash can burn your flesh and retinas, even wearing goggles, the UV light is so intense, comparable to arc welding and maybe more, that simple goggles wont protect you. electrical face shields have UV filters capable to withstand a flash, at most you may have a temporary bright dot in your vision and maybe a light sunburn on the cheeks, but that's rarely the case.
you can add to that a pair of electrical gloves if you need to approach live circuits and a closed long sleeved vest, preferably without metallic buttons (for obvious reasons), made in cotton (synthetic fibers will melt and stick to the skin, cotton wont) and a high collar (to protect you throat).
I don't know what the electrical protection ratings are in the US, I'm in France, you might need to do a bit of research, still, electricity is the same either side of the pond, it's dangerous and it can kill but can do wonderful things if properly handled ;).
51:00 we will just forget about that RIFA, even though it was probably bad and mere moments from releasing the wonderful fish-scented perfume they're known for anyway, despite not being the source of the breaker tripping. I know you were referring to the electrolytic you removed and replaced, but you did also remove the RIFA, which was necessary anyhow.
Haha yeah I just left it off entirely --- they just do filtering so really it's only needed for EMI anyway :-)
Funny: You took the Rifa Cap of the Board @ 26:48 and @ 41:42, put the Board on the Rifa Cap! LOL 😀
A few comments --- 1) Your house has 240 V for a range ot something else, if you just dig it out. Your 120 V is duplicate in voltage, but just one half of the 240 V each, because it is a center tapped arrangement, with the tap grounded. 2) Your step-up transformer is not isolated between primary and "secondary" - it is another center tapped gizmo, just like a Variac, only not adjustable. That explains the ground fault trip until you added the isolation transformer in front. 3) I like to use a Diode Test mode on the DMM when checking semiconductors instead of plain ohms. The diode mode typically drives the circuit with a 1 mA current and displays the developed voltage "drop" in direct reading. That tells the 0.6 or so voltage of small silicon junctions. More on Darlingtons and less on Schottky. 4) Switching power supplies I like to start analyzing with a pair of field probes ( magnetic field or electric field ), attached to my oscilloscope. 5) Inductive, as well as capacitive components I usually can measure with my DER 5000 meter, using its 100, 120, 1k, 10 and 100kHz frequencies and a little reasoning. The IMPEDANCE of both inductances and capacitances varies by the frequency, but possibly parallel connected resistors don't vary. 6) When dealing with unknown way grounded systems, I have some tricks in addition to an isolation transformer. I have battery operated differential probes and even a Tektronix dual channel probe isolator device. But often I can use a Tektronix current probe. I have two, one small, 10 or 20 A maximum at 50 MHz and a big one up to 500 A and up to 2 MHz. Finally, 7) I have an old HP probe set that consist of a pulser, a logic probe and a pulse current tracing probe. Works great, as long as I can avoid the apparent short circuits that any capacitors show on the fast pulses.
.
I hope this gives another view to means and instruments that can be used for troubleshooting unknown devices.
Excellent repair mate, just one thing. But!!!But!!! you need to replace all the caps😅😅😅
It's always best to work on CRTs standing up and with a clear path - that way if you do manage to get a shock you'll have an easier time removing yourself from the situation!
One hand rule is one I like to follow when dealing with a sketchy situation like that. It'll hurt like hell still but won't stop your heart!
I feel like if I ever started servicing picture tubes or other high-voltage equipment, I'd just get some really heavy rubber gloves. I've never understood why that's not a normal thing to do.
@@stevethepocket good luck using a soldering iron through those extra thick Marigolds ;)
@@hgbugalou Yep, one bit me once, and I lived to talk about it. I was one handed and standing. What I didn't expect was my elbow being pushed by a curious tech next to me when he leaned over.
From that point on, I told everyone when I was servicing monitors to stay away unless invited.
The shop jokes about my hair standing up and smelling cooked hot dogs went on for a few days.(groan)
Cool, brings back to life
You found RIFA caps' true purpose: a spacer for propping up other equipment 😂
Those pots are super common.
I want a tshirt that has "It's freakin' Workin'!" on it!
Hi Adrian, I have one of those component testers, and I found even not being used it would go flat fairly quick. I added a switch in the positive line between the battery and the PCB to isolate it when not being used. Otherwise its a great tool! Love your videos mate, from Sydney, Australia.
👍 👍 👍 Well done! I also like repairs of old electronic equipment...
Arctic retro!! What a great channel!
Interresting that you did all the troubleshooting on 240V. If I troubleshoot power supplies, I always use my isolation transformer to generate 115V for safety reasons. The transformer itself adds a lot of satety, the reduction to 115V makes it even more safe. Typically a power supply like this runs on 115V, there is usually no need to switch it into 115V mode. The amount of power it is able to generate is greatly reduced and there is good chance it won't be able to drive the computer unless you reconfigure it to 115V. However, to check wether it is generating the right voltages, the low voltage is often sufficient.
When all faults are removed, I plug it into the 230V output of my isolation transformer and then test it at full capacity. Usually I connect a few halogen lamps and let it run for an hour or so. Only when it passes such a test, I am willing to let it power a computer.
You do have 240V in your home if you live in the US, its just not present at the outlet, all of your high energy appliances such as Central AC, Stove, and Clothes Washer/Dryer run on 240V, we have Split Phase power in North america.
30:00 Mad scientist vibes ⚡️
thyristors in computers are usually only there as a overvoltage crowbar ,so imagine my horror when you were about to power up the logic board @19:35 without looking into why that scr went short or what it does! thanks for not supporting the badcaps myth!
SCRs used to be quite common in CRT drive circuitry back in the day, I think, probably because there were limited options for high-power semiconductor switches.
@@makomkyes that was mostly sony trinitron in the line output stage and they were an absolute bugger to diagnose ,often failing Again after 2 weeks of soak test. Dont miss that one ha
Look up dim bulb tester for troubleshooting electronics. You insert this in series with the power supply with an appropriate wattage incandescent light bulb. If there is a catastrophic short or if the power supply is drawing too much current the light bulb will limit it to a safe value and you will tend not to blow things up. Super easy to construct. All you need is an AC socket and AC plug a lamp socket and a light bulb. If you want to get real fancy you can install a switch to bypass the light bulb. People who troubleshoot switching power supplies for a living always use something like this.
I think he has one.
Good work Adrian :)
I remember when I was kid in 80's people smacked tv a lot when they loose pic. I know now that that is usually bad soldering joint due to heating and cooling all the time but people had a myth that tv is fixed if you smack it.
30:13 12,000 volts scares the crap outta me. My palms sweat when you touch the yoke or any part of the CRT cause I dont understand what part kills you or when. You're brave I'm not touching that stuff.
There was an inductor in series on the missing supply, with the diode looking short removing one leg of the inductor narrows down the possibilities, my first suspects are Always secondary diodes never caps.
Still, at 15:10 the capacitor he pulled should have been checked with a meter for high resistance going down to know that it wasn't shorted. He discounts it as probably good and decides to put it back in. Might as well quick test it before putting it back in. Also LCR meters are cheap sub $50 devices now to know the value of the cap hasn't changed drastically. If it works, who cares about the ESR value as this is a 60 hz power supply thing.
That would also help in chasing multiple part failures causing a section of circuits looking like it's all shorted. :}
Ah, bad connectors. A friend had a monitor that kept shutting off at random times. It would turn on again after a while, or smacking it typically brought it back. I looked at it. The signal cable connected internally to a bunch of plug connectors. By sheer luck I noticed that one of them did not make great contact. So I soldered the plug to the pin and it was rock solid ever since.
On another note, it's possible that the plug in that Mac needs new pins though. It's not always bad contacts. Sometimes the springs are too weak.
I was screaming at my screen.
Yeah new Vid!!
Nice video, thanks :)
I don't know why that was tripping your GFCI, because it seems like it shouldn't have been anyway, but your step-up transformer is also arguably not wired right for this sort of appliance. It is common for 240 volt circuits in the US to have two hot legs, both at 120V relative to ground; however, for most other countries (Europe, etc) which use 220/240V, I'm pretty sure that the neutral wire should have a potential of (more or less) 0 volts relative to ground (and the hot should be 220/240V relative to ground).
It looks like your step-up transformer is wired with the ground connected to a center tap of the output, which is not ideal. Instead, I would recommend connecting the ground straight through to the input ground, but _not_ to any part of the output of the transformer. This effectively turns your step-up transformer into a combination step-up and isolation transformer, and assures that (no matter what you plug into it) you will never have current travelling back along the ground line or elsewhere which could trip a GFCI.
(Note that if you do this, then the output of the step-up transformer should show 0V between both ground-to-neutral and ground-to-hot (but 240V neutral-to-hot). This is normal, and is because both neutral and hot are now completely isolated from ground.)
I agree. In Europe, the "ground" is an earth and a protective earth only, which is isolated from the rest of the wires flowing current. On the circuit breaker panel, usually earth and neutral are connected together. There should be 230V between output hot and neutral, and the input windings should be hot and neutral only as well. Ground should pass straight through and also connect to the casing of the transformer.
The step-up transformer has three leads internally, there's no way it can be used for isolation. However I agree he should make it be 0-240 output instead of 120-120. He probably just needs to swap the incoming live and neutral wires.
You should look into a Peak DCA55 component tester for those ICs. They do a more advanced DCA75 version, too. I've had a DCA55 for a while now, I love it!!
A hard-fought successful repair!
18:43 - I believe that Igt is reason why this component wasnt read properly. TC1 states it can measure Thyristor / Triac IGT < 6 mA, and data sheet for 2N6395 says typical Igt is 5mA, so its on limit. I believe that TC1 is weak and cannot trigger Thyristor gate.
Edit:
Known errors and unsolved problems:
5. The output current of the tester is 6MA/voltage
the only thing that i was screaming for was you just touching stuff just after disconnecting the power without discharging the caps
I say there were no mistakes made. It is just the process of the repair. Find something broken - fix it. Find out something else it broken, fix that too. Find another thing broken, fix it some more, keep finding and fixing until it is working again. It is not like many Classic Macs during the 1990s where only 1 or 2 things were broken and were easy to fix. Classic Macs today will have a whole host of things wrong with them, this is not the 1990s, but 30 years after the fact.
"This is where the fun starts." As you flip the board over and back to chase the circuit without a schematic. At some point in the past I would get out a few sheets of paper and start doodling up a schematic if there isn't one printed for the board.
Later on, if I saw the same board again, I would have my notes-n-jumpers settings with a hit list of common things to check and replace.
I knew a guy who could do this in his head when reworking a large pile of similar parts. It borders of OCD/rainman level of attention at first. But then after he solved the typical failures, it became automatic. I would jot notes for myself and shorthand for repairs that I knew would be coming in again from the field.
23:42 Power on, yellow light starts flashing, "OK, so it's turning" 😅
Usual rule. Turn the house lights down and look to see if the CRT gun heaters are glowing. Always a dead giveaway if not that the supplies are down.
Crowbar thyristor. You should test zener which drives crowbar thyristor.
*Safety First!* It's true that there *is* danger from voltage potentials from the inside of a CRT Mac, requiring one to take the necessary precautions, but at least there is no threat of lethal levels of ionizing X-radiation, such as that experienced by technicians repairing early color television sets.
Nice video! Trying to find the cause of a short using a multimeter is difficult. You tend to measure low resistance values everywhere without finding the actual short. Using a simple current source and your rmultimeter as described by the youtube channel Learning Electronics Repair's video "Convert Your MULTIMETER into an accurate SHORT CIRCUIT TRACER Finder" makes this much easier. It might be worth your while to look into this.
That SCR is the crowbar. If +12 goes too high it will trigger the SCR and short the power supply out. The SCR will only turn off once the power supply shuts down, which produces the chirp-chirp-chirp sound if the over voltage condition remains. That SCR probably failed because the regulation went out of spec and kept cycling... it just kept dumping current until it finally blew up and shorted permanently.
correction, you house voltage is 240. Here in the states we split that at the breaker box unless you need a 240 draw like a dryer or range. So yeah, you house gets 240 then it's split at the box.
36:26 The squiggly line is the filament, and the box around it is the cathode.
Just need to review some vacuum tube theory. A vacuum tube’s cathode emits electrons when heated. The anode is charged to high voltage potential and draws the electrons to the anode (or “plate.”). The control grid will be negative with respect to the cathode. The more negative it is, the more it repels electrons back to the cathode. The closer to zero it is, the more electrons are allowed to flow to the anode. A picture tube is a fancy vacuum tube, but it’s just a vacuum tube.
"one of these jobbies" he says as he uses his EEVblog multimeter :)
The footprint for that potentiometer looks pretty standard (0.2" pin pitch, likely could plug into a breadboard), but what's likely the "difficult" part is the actual knob on it -- whatever the connection to that large plastic shroud placed over it is the really important part. If that's actually just a 1MΩ pot, those are super common and easy, but a photo of the knob would probably help.
So close. Those old parts used to have an absolute maximum spec of 7 volts.
i've got a tough mac plus thats been one of my on again off again projects, recapped the power board and fixed some cracked solder the usual culprits, had some trouble with the ram slots and there were a few traces that didnt appear to be connected anymore, bad ram sticks, one of the rom chips was bad and its still not quite booting. Im pretty sure ive got a few more traces to check on the logic board but the schematics for that are hand drawn and missing some pieces between where they were scanned in many years ago, i started to try and piece them into one sheet with an image editing program but it only got me so far
what a drama Adrian, now I need a shot... 🥃
for the SCR maybe look at the Peak atlas SCR100 Atlas SCR Thyristor and Triac Analyser
I got an number of there small tester and they work great
Watching the start when you hook up the CRT tube…Don’t assume that a ground to the implosion band mount makes a connection to the aqua-Dag ! That’s not a given !
There’s a reason you see the braided grounding strap arrangement across the back of the tube.
This is not just safety issue, it actually forms part of the anode high voltage circuit by using the “capacitance” of tube as part of the HV smoothing.