Mac 128k blown fuse on analog board

[James1095]

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.

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.

 

[Macdrone]

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.

 

[td75]

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.

 

[Paralel]

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.

 

[James1095]

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.

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.

 

[LOOM]

I have now replaced the fuse holders and fuse (1.6A 250V slow blow)

What kind of fuse holder did you buy that fits?