I've recently picked up a Nuova Simonelli MAC Digit, single group. Manufactured Dec 1993.

I used it for about 2 weeks. Everything worked fine. I would leave it on for about 2 or 3 hours at a time, but at the end of that, I would turn off, and then unplug from the wall. Everything was fine, the machine was doing nothing out of the ordinary, so I decided to turn off, and leave it plugged in. When I tried to turn it on the next morning, there was no response. The T100mAL250v fuse had blown, so I ordered some replacements. After replacing the fuse, the machine went right back to work for a few days, but popped again. The next replacement fuse only lasted 30 seconds or so.

The control circuit board looks good. No blown caps, broken traces, etc, and it was working fine.

I am handy with a voltage meter, and with parts replacement, but not an Electrical Engineer. Any suggestions on what/how I should check?

I could make guesses, but I've no idea what the control board is, could you maybe show us a picture of what the fuse is connected to and the control board?

You could always try unplugging all the connectors from the board and then plugging them back in one at a time to see if it's the board or something connected to the board.

Ira

On the top row, there are two grey fuse holders. The one without the cover, is the fuse that is blowing

Not much in those pictures to take a guess about. One of the brown wires one the right in the last picture looks funny, almost like it's shorted to the other brown wire.

Ira

Double checked that brown wire, there is not short. The discoloration is a remnant of the deteriorated plastic that used to cover the terminals.

I have the parts manual, but I do not have any idea of what values should be measured for any of the components. Any idea where that info might be obtained?

I'm going to go back to my original suggestion, unplug everything but the power to the board in the lower picture and if the fuse stops blowing find out which plug causes the problem. What size is the fuse?

Ira

The fuse in 100mA. Almost certainly for controls.

Unfortunately, of the three things I can unplug, two prevent the machine from turning on, and the third plug makes the pump run non stop.

After reconnecting, the fuse popped in a matter of seconds again. This would seem to imply a direct short, but I do not see one.

So unplugging the connector with the red wires stops the fuse blowing?

So likely there is a problem on the board with LEDs. I have the tools to find the problem, but I'd start by replacing what I guess is the 3 pin voltage regulator in the corner, probably a 7805, 7812 or some variation of those numbers. I'd also on principal replace the 3 cylindrical blue capacitors and they tend to get old and die. There is also an 8 pin chip which looks like it's socketed, what are the numbers on that. I'd also like to know what the four small 3 pin parts are on the edge of the board.

Ira

Unplugging the red wires does stop it from blowing, but it also stops the unit from powering on. Will the fuse blow, if no power goes past the relay?

It is a 7805. Just hooked it up to a power supply, and get 4.97v coming out.

The 8pin is 93c46a81

The 4 NPNs, are BC 337-25

It's blowing what you said was a 100ma fuse which is so small I would guess it should only be protecting the electronics on the board with the LEDs. If you can make an extension cable and leave one wire at a time disconnected you might find the problem. Try to figure out which is ground and don't disconnect that ever. Better pictures of the board with the transformer might help, there is something hidden below what I think is the transformer. Also a good picture of the back might help.