If I can make a suggestion:

I didn't see a test of each leg of the heater to ground - with water in the boiler, and the machine unplugged. There should be no continuity. If the heater failed internally, then you would expect a ground path between heater/water/boiler/ground.

I would also not plug the heater directly to mains without a sufficient ground with this out-of-box arrangement, and preferably with a suitable fuse in place on the hot leg(s). If you just plug the two terminals to power without a ground path, it's possible the heater will function to some degree and not trip your house breaker, but the entire frame of the machine will be live.

Hope that helps,
Mickey

Is the solid-state relay accessable for a continuity test? Could it have failed ON, resulting in an overheating boiler, triggering the thermal? And then, when reconnected without the heating element, no evidence of the short?

Joe

Let me just say what I have done now. I connected the heating element through a fused power strip directly into the mains, with the machine housing connected to ground. Before starting the test I ran the machine to be sure the boiler had all the water it thought it should have. Then I cycled the mains power to the heating element several times, letting it cool down between power ups. Each time the heating element seemed to work fine, with the boiler pressure coming up. So, I guess this test, along with the reasonable resistance across the heating element I measured earlier, shows that the heating element is ok?

Other than the previous two posts, which I will think about tomorrow, what next?

Mickey, I just checked and there is "infinite" resistance between each leg of the heater element and ground with water in the boiler and no power to the machine.

Assuming you've snooped around the wiring with a flash-light and there are no obvious defects :

Last test would be to connect just blue element lead and power on the machine. Machine should not heat.
If that goes OK, remove blue lead and try with black only. Machine should not heat.

Again if all goes OK, connect both.

Sounds like the element is ok.

Have you checked for short from PStat terminals to PStat casing/ground?

Mickey

However, this morning when I turned it on there was immediately a very loud pop and the circuit breaker for the kitchen was tripped, indicating that a short had occurred somewhere in the machine.

The above is from William's first post on the problem. What's throwing me off a little is the loud pop and no visible signs of any resulting damage. Hence my original suggestion that the heating element had suffered a localized "explosion" and tripped your RCD.

You have looked at the other side of the control board circuitry?

Are your kitchen outlets of the ground fault circuit interrupter type (RCD in Europe)? This thread by Paul Pratt - Gaggia Orione restoration (see middle of page) describes how a heating element can test out just fine using an ordinary multimeter but fails miserably when tested using a megometer (commonly referred to as meggers). This is not a common household tool.

Eric, each local circuit in our house is controlled by a Garo MCB, which I believe is just a circuit breaker. But the main control panel also has a Garo RCCB (residual-current circuit breaker for ground fault). When we first moved into the house I tripped the RCCB while installing a fixed stair lamp (with the local circuit breaker switched off), and, as I recall, the RCCB cut the power for the entire house. With my current problem, when I switched on the Isomac and got the instantaneous very loud pop in the machine, only the local circuit breaker was tripped--I did not have to reset the RCCB.

Anyway, if the heating element had failed, would I not get a repeat circuit breaker trip or no heating when I had the heating element connected directly to the mains?

I checked continuity at the PStat and found no shorts.

Since everything (??) seems to check out ok, I am wondering if the problem could be in the power switch? I find it hard to imagine a simple switch shorting and then somehow healing itself, but if the short were inside the switch maybe it would not be evident looking at the outside. What do you think?

Maybe I have found the problem. In doing a bit more checking of the wiring I found a little knot in the black heater core wire coming from the thermal fuse, see the first photo below. It is near the bottom of the loop in the wire, and if I press the thermal fuse back against the boiler and push the loop toward the middle of the machine, the knot gets positioned close to a brass nut in the water line, as you can see in the second photo. You can see that there is some discoloration of the insulation at the knot. I probed around the spot with the needle tip of my DVM and I can get continuity with the core of the wire. Also, I can imagine that I see some discoloration of the brass nut when I look at it with the naked eye. So, I am thinking that there must have been some breakdown in the insulation at the point of the knot, so that there was an arc to the brass nut when I turned on the machine. Sound reasonable? Then I should be back in business if I can just replace the wire (any suggestions for where I can buy this kind of wire?).

Anyway, if the heating element had failed, would I not get a repeat circuit breaker trip or no heating when I had the heating element connected directly to the mains?

You would think so, (and I would think so also) BUT, if you read some of the paragraphs in the link I provided to Paul Pratt's Gaggia Orione restoration, you would see that that was not the case.

In any event, it looks as though you have found the problem and that's good news.