It's been almost 2 years since I did any work on Oscar, and the urge got to be too strong to resist. I'd cut two of the three PFs that came with him, but I never used the 3rd - so I fitted a pressure gauge to it this afternoon. We all know that a stock Oscar pushes well over 9 bar into the puck, which I confirmed by pegging the needle on a 200 psi gauge, and this was enough to push me into finding a fix.

I resisted a dimmer because the knobs and levers don't look right. But I found a wonderful little Lutron called the Credenza that's just the right size and on a zip cord for remote hard-wired use. It'll go wherever it looks right and fits well - I Velcro'ed it under the flip panel over the space for the water tank (my Oscar's plumbed).



The hot lead to the pump comes off the main power switch and is continuous with the hot lead to the heater coil. As I didn't want to alter the stock parts and configuration in case this idea didn't work, I devised a simple way to separate the heater feed from the pump without cutting or altering anything. I disconnected the hot wire from the main switch and the heater, leaving the distal end of the wire connected to the pump. I made a new hot lead harness with a 3-wire splice tap, connecting one wire to the main switch where the pump/heater lead was connected, one lead to the heater coil directly, and one to the dimmer. I'm a big fan of strain relief, which is why you see a loop in the lead:



This is what it looks like from the top -



and this is what it looks like from the right side -



I disconnected the female spade lug fitting from the lower heater terminal, replaced it with a hot wire from the splice tap, and secured the now unused connector to the new hot wire to avoid risk of a short -



I ran the wires to the dimmer through one of the square openings in the back corners of the top cover piece -



As others have reported, the dimmer has a narrow range of effect - the pump starts making an odd clicking noise at about 2/3 between off and full on, and it only takes a tiny shift to change the pressure considerably. It starts pumping as I move the slider above about 3/4 and sounds normal once I crank it over 6 bar or so. With the dimmer fully on, the dimmer's out of the circuit and the gauge needle is pegged. Interestingly enough, the pressure ramps up over several seconds when I adjust it for a maximum of 140 psi into a blind basket. So I'm going to stick with this for preinfusion as it's too hard to adjust the slider perfectly to 9 bar every time. My pressure gauge now confirms that it's consistently where I want it, and Oscar's completely stock from the outside.



Yes, my Redbird has even better mouth feel now and is absolutely delicious - richer than ever, with a bit more chocolate, a little more nut, and some more caramel in the finish. Assuming it doesn't kill the pump, this seems like a great mod.

Hmmm - there's a downside to this mod. It appears that the heater light is always on now - I haven't seen it go out since I did the deed. The element does seem to cycle, since it makes the sound of an immersion coil in a cup of water in the dorm room when it's on. Has anyone with a dimmer in the vibe pump wiring noticed this?

As the boiler seems to function normally, I'm going to leave everything as is for a few more days and see what transpires. So far, my espresso is the best it's ever been. I'll look at the wiring diagram in more detail to see if I can figure out why the light stays on when the element is off.

Addendum: the light glows very dimly when the heating element is off and is fully lit when on. There's obviously a small potential across the bulb that was shorted by the now-disconnected blue wire that joined the hot terminals of the heater and pump with the switched hot terminal on the power switch. This has not triggered the ground fault outlet, so I think it's OK.

I dont have an Oscar o intent to do the mod in my machime but I want to give you my congratulations as I love mod and user customizations !!
Well explained and illustraded, great post !

Demian.

I may try this, I have an old Bezzera I am having trouble finding fittings for an opv, maybe they are serto, thats my last possibility. Or just not around , this option beats machining.

forbeskm wrote:I may try this, I have an old Bezzera I am having trouble finding fittings for an opv, maybe they are serto, thats my last possibility. Or just not around , this option beats machining.

Well, so far so good! The pressure into a blind basket is still stable @ 140 psi and there's very little splatter in the cup above the gorgeous and delicious espresso gently cascading into it. I'm happy now and happier every day that the pump stays fine. I assume I'll reach full comfort after 6 months.....