Thread
Mac 128k blown fuse on analog board
Have tested SCRs and they all tested okay. If I get time I may start testing each component individually out of circuit but I think repairing this has become beyond my expertise! I've spent far too many hours with this.. My brother-in-law is quite clued up in electronics so next time I see him I will ask him to take a look. I'm sure I've overlooked something simple and possibly related to original fault of shorted diodes. I hope that one day I can have this 128k running again! Thanks again for everyone's help with this 
Sometimes there is no substitute for testing out of circuit, but you can check a lot of parts in place. If a diode shows a reasonable forward drop in one direction and nothing to speak of in the other then it's probably good. Transistors can be tested like a pair of end to end diodes from the B-E and B-C junction. It takes experience to know how to interpret the readings you get from some parts, but with each repair it gets a little easier.
I still remember when I first started working on CRT monitors. It was a struggle at first, but after a while they all started to look the same on the inside, and I started seeing the same 2-3 faults over and over. It got easier and easier to guess at the bad part with just a quick look at the symptoms.
I still remember when I first started working on CRT monitors. It was a struggle at first, but after a while they all started to look the same on the inside, and I started seeing the same 2-3 faults over and over. It got easier and easier to guess at the bad part with just a quick look at the symptoms.
I always hated the multisync monitors, especially sony because of thier extreme complexity.
For the most part they weren't really that bad to work on and had the same sort of faults as any other monitor. Bad capacitors, the occasional open resistor or shorted diode, blown HOT. Sometimes it was just a bad solder joint. Once I had a good multimeter and an ESR meter I had a pretty solid success rate.
The most interesting was the series of Viewsonic flat CRTs that all seemed to develop the same fault. They would come to me with a blown vertical output chip, replacing that and the fusible resistors feeding it would usually get it going again, only to fail a while later. I finally ended up tacking wires to one of the power supply outputs and using the monitor for a couple weeks with a bench multimeter connected until I finally caught the voltage creeping up. Some freeze spray found a bad opto-coupler in the power supply. It would work when cold, but eventually start to fade, causing the output to rise up until it got high enough to blow the vertical output. Once I figured that out, I must have fixed a dozen 17-22" Viewsonics with the same problem and never had one come back.
The problem I had with monitors, I would get them running again, just for them to scramble up and short the HOT after maybe an hour or more of operation. You knew when it was going to do it, as the screen would slowly get wider, then the picture would loose sync and the infamous squeek, tick-tick-tick-tick of the SMPS when the HOT goes short.
Replace the HOT, and it was good to go for anywhere up to another hour, to days... Never figured htat one out. It was a viewsonic if I remember correctly. I had an apple powermac 5300 do that too. would randomly short the HOT.
it was always the ones that shorted the HOT or had other random horizontal issues that drove me nuts. All the other stuff was easy.
I had an apple multiscan i think... 15" display I worked on long ago, that would pulse a bright raster with an image for the upper half the screen, then the lower half would be a bright squiggly center line almost question-mark like in striation, and almost to the burn-in point, and it would shut down. this would happen every 1 second. fire up, pulse raster, bright line, shutdown. Turned out to be a bad transistor in the pincussion circuit. Of all things, and it was on a seperate PCB near the flyback, seperate from the mainboard. (going from memory about 12 years ago, I was 15 at the time)
Replace the HOT, and it was good to go for anywhere up to another hour, to days... Never figured htat one out. It was a viewsonic if I remember correctly. I had an apple powermac 5300 do that too. would randomly short the HOT.
it was always the ones that shorted the HOT or had other random horizontal issues that drove me nuts. All the other stuff was easy.
I had an apple multiscan i think... 15" display I worked on long ago, that would pulse a bright raster with an image for the upper half the screen, then the lower half would be a bright squiggly center line almost question-mark like in striation, and almost to the burn-in point, and it would shut down. this would happen every 1 second. fire up, pulse raster, bright line, shutdown. Turned out to be a bad transistor in the pincussion circuit. Of all things, and it was on a seperate PCB near the flyback, seperate from the mainboard. (going from memory about 12 years ago, I was 15 at the time)
Macdrone has very kindly sent me a 110V analog board from the US. Was wondering if there is any easy way to covert this to accept 240V or would it be best to use with a step down transformer?
I've heard bad things about step-down transformers and old Macs
Didn't know about this. Do you know what bad things you heard? Would it be better to covert power supply to 240V?
From what I've heard, the old analogue boards don't tolerate the fluctuations that occur with cheaper/lower quality transformers very well and develop severe problems or burn out all together in short order. So, if you are going to use a step-down transformer, it has to be excellent quality with very tight tolerances, which is not cheap.
That could very well have been the same opto-coupler problem in the power supply that I saw on numerous PF series monitors. In those, the vertical output IC was the first to succumb to the over-voltage but it's entirely possible that a different monitor would blow the HOT instead.
Most of the time when I ran into chronic blown HOTs, it was bad capacitors causing weak horizontal drive. Either the drive level was too low or the waveform was ugly, either way the HOT would spend too much time in its linear region where it is acting like a resistor rather than switched fully on or off and the result is a tremendous amount of heat and a fried HOT.
The horizontal deflection circuit, traditionally combined with the HV circuit but frequently separated in multisync monitors, is a resonant tank with a great deal of current circulating around. Everything has to be just right or things get ugly really fast. A 150W bulb in series with B+ would often prevent the HOT from blowing up so you could run it long enough to see what was going on.
They make travel power plugs for American travelers. I am sure a 240 to 115 plug can be got for pretty inexpensive and then you will be all set. Just a suggestion. Will save a lot of work.
Paralel mentioned that he had heard of issues with step-down transformers so looking for a quality one at a decent price. I would prefer to convert the analog board to 240V but have no idea what parts would need replacing. I originally thought it would be a matter of just replacing the transformer on power supply circuit but perhaps this is not the case.
Yeah, when I was in the UK, I had a friend that collected vintage macs and he managed to blow out two 120v systems with cheap transformers. When we looked at the voltage/amperage, as well as the waveform, there were wide fluctuations in the transformers output. In some instances there were deviations of more than 20% from ideal, far worse than you would see in a US household with no major wiring issues. I would definitely use a quality transformer which lists its tolerances relative to ideal output.
You have to be careful. A lot of these travel converters are not transformers at all, but triac based circuits that work like a light dimmer with a fixed output. They are intended for purely resistive loads only and will usually fry electronics connected to them. Transformer based travel converters are normally only rated for about 50W, and they are undersized, providing very poor regulation. Larger autotransformers can be had and work well, but they are bulky and tend to be expensive.
You would have to look at the schematic or compare the 120V and 240V analog boards to see what the difference is. Many dual voltage switching power supplies use a 330VDC bus and the voltage is selected by jumpers that change the way the input rectifier/filter is wired. They use a full wave rectifier and a pair of filter capacitors in series to get 330VDC from 240VAC. For 120V, the rectifier is configured as a voltage doubler using only two of the four diodes and charges the series capacitors to 330VDC in alternating fashion. Look up Cockroft-Walton voltage multipliers for an example of how this works.
If the 120V version uses a single filter capacitor and 170VDC bus, the transformer is likely different, as is the main filter capacitor, but the differences are likely only on the primary side. You shouldn't need to replace the whole board though, the problem is likely something simple. For the record, in ~20 years of repairing this sort of stuff I have *never* seen a bad transformer in a switchmode power supply. They just don't fail, the electronics blow long before the transformer takes enough abuse to overheat.
What kind of fuse holder did you buy that fits?