The PID I use is a Sestos D1S, aka "A Set Dual Digital PID Temperature Controller OMRON relay & 25A SSR & K Sensor" on eBay
It comes with thermocouple and SSR, is self-tuning and has a full set of features (vastly more than required for our purposes)
You'll need to carefully cut the thermocouple fitting off the sheath, which will leave the thermocouple bead bare (if you damage it you can always re-twist/solder)
The thermostat you'll be replacing (situated down the side of the boiler, not the steam thermostat on top) is M4 threaded, so get some "10x Brass Standoff Spacer M4 Male x M4 Female - 10mm"
Drill a small (2-3mm diameter, I think?) hole down the centre of the stand-off, with the aim being to come out the other end.
Do yourself a favour and take the four allen-bolts out of the grouphead to give yourself enough room to move the boiler around when fitting. Better yet, take the whole boiler out completely if you can be bothered.
I filled the threaded hole where the TStat was mounted with some spare heat transfer compound, but it's probably not necessary, then inserted the bead through the brass standoff and bent it slightly to the side, so that when screwed in, the standoff would press the TC bead against the inside of the threaded hole. Use your judgement about how hard is secure without breaking the bead. you can secure the sheath in the other end of the standoff with some high-temp silicone if you want but once again, not strictly necessary.
As far as the wiring goes, you'll want to piggyback the switched active, which is (looking at the switch-set from the connector side) on the far right, second from the top. Go to Jaycar and get some 6.8mm piggyback spade crimps and a cheap ($5-$10) crimper, unless you have something suitable at home. Run this to one of the AC power terminals of the PID(terminals 9 & 10).
The neutral, you'll want to take from the topmost terminal of the IEC socket at the back of the machine and run to the other power terminal of the PID.
When you disconnected the thermostat, you'd have had two connectors to detach. those will be wired to the two switched terminals of the SSR (marked 24-240V AC). Once again, use the 6.8mm spades, since you don't want to be cutting up your machine for the sake of five bucks and might want to sell it stock one day). Probably goes without saying, but wire one connector to one terminal and the second connector to the other.
Wire the thermocouple leads to terminals 3 & 4 (3=blue, 4=red).
Last bit is to wire the SSR power terminals (marked 3-32VDC) to the SSR terminals on the PID (6 & 8). These are polarity conscious, so match them according to the markings on the SSR/PID. + goes to +, - to -.
Once all that's done, double-check your wiring and fire it up. If you want a dry run, disconnect the connectors supplying the elements, making note of which goes where, then turn it on and check the operation of your PID and SSR.
Pressing the up or down buttons will let you change your setpoint, holding "set" will open up the menu (which you want to leave alone for now), pressing "set" quickly will toggle between displaying the setpoint and displaying the output (0-100% heating)
Here's the cool/tricky bit. Where most people run PIDs to hold an exact boiler temp, what we're going to do with the Classic's tiny boiler and low process lag is purge the boiler a little to cool the too-hot boiler and group, then grind/dose/tamp and then pour our shot while the boiler is still in recovery. The shot will start at the correct brew temperature (when the setpoint is dialled in correctly for your thermocouple and prep-time) and the PID will hold the heating signal relatively steady while cool water is entering the boiler, keeping the discharge water at a constant temperature.
I would suggest setting your temperature at 111.5 to start with, then operating the machine as following:
Turn it on, fill the boiler and leave it until it's had time to heat up. A good guide is that the left-hand side of the top section of the machine will get warm and the PID output will settle at a range of 3-9% (as the metal components aren't sucking all the heat from the boiler housing).
Hit the brew switch and watch the PF. With a naked PF you'll see the water bubbling out in a steaming cone, then the flow will stop, then start again with more uniform dispersion. With a regular PF you won't see it, but should notice that the flow stops and starts. Let it flow for ~2 seconds after the flow starts again (or until you've pulled 40-60ml of hot water, total) then turn it off. Great way to preheat your glass/cup, by the way. Unseat the PF just enough to let the remaining water drain from the basket then do your thing (weigh/grind/dry-PF/dose/tamp). It takes me one and a half minutes, which is what my variables are optimised for. If it takes you substantially more/less time, pull more/less water through in the purge to compensate (don't mess with it too much though unless you're able to measure your brew temperature at the puck).
Lock your PF in and brew, then remove your shot and portafilter and give the brew-switch three quick bursts to clean the shower-screen. If you want more shots before steaming just repeat the process, grinding/pulling back to back, with no purge. The grouphead/water temp shouldn't vary more than a degree as long as you leave 1.5-3 minutes between shots.
If you're steaming milk, hit the steam switch, bleed the water out and wait until the boiler temp hits ~150 before doing your thing. This will vary between machines/users; the idea is to leave it as late as possible without the steam thermostat tripping and cutting power to the elements (at which point the temp will drop massively before the TStat resets). Because the alarm contact on this model of PID is dry (no voltage) it's not possible to bypass the steam thermostat without using a discrete power supply to power an extra relay/SSR (do NOT try and switch the elements directly using the PID's internal relays).
After steaming and cleaning your wand, hit the brew switch, wait for the gush of steam to give way to the regular trickle of water (once again, the flow should stop, then start again at a trickle) and you're ready to go again (or walk away, knowing your machine's boiler is full).
Once you have your shot-weight/time sorted, then you can adjust your temperature setpoint to your taste/TC/climate, I'd suggest one degree at a time. Once you've worked that out, you can program in an offset (SC in the menu) and if you like a hi-temp alarm to let you know when to start steaming). My offset is -18.5°C, which results in my setpoint matching my initial brew temp (which in my case rises half a degree, then drops one and a half degrees over the course of a double shot).
If you don't have access to a calibrated thermocouple/meter, just get it to a point where it's tasty then make your offset -x
°C, where x
is the difference between your "tasty" temp and 94°C. Set your setpoint to 94°C, which will be close enough.