How does it know?

[Chris Hogg]

I have a Philips PT860 rotary electric razor that takes HQ80 cutting heads. Every couple of years or so, a little three-point light comes on, telling me that the blades should be replaced. So I take them off, totally dismantle the entire head, clean it in an ultrasonic bath and re-fit, but it still tells me the blades need replacing. How does it know that I've not put new blades in rather than just putting back the old blades? It even tells me it needs new blades when I run it without the entire blade assembly.

So, how does it know, and more to the point, can I fool it into thinking I've changed the blades even if I haven't?

[Fredxx]

I'm wondering if there if the ride height can be detected, where the blades have worn and the drive sprockets have moved up in sympathy. The fact the light comes on when its run without the assembly gives further credence to the theory.

[Chris Hogg]

Hmm...I had much the same thought. But examining the drive sprockets, they're just simple spring-mounted dogs sitting on the end of three rigid drive shafts. If there are sensors somewhere there, they're very clever, and very sensitive to slight wear.

Another recent thought is that perhaps something monitors the motor current, and when the blades are worn there is less friction against the three foils, the current falls, and 'it knows'.

[Richard]

This any use?

[John Rumm]

Its probably just a timer...

[Fredxx]

That sounds more likely. Depending on control electronics it could be measuring motor speed, which may also rise if there is less friction against the foils.

If brushless, there will be some means of determining motor position and hence speed.

[Jimmy Stewart ...]

was it not meant to be reset at the philips service centre whan new blades were fitted

[Jimmy Stewart ...]

just a simple timer? ...

[Rambo]

He needs to hold the power button down for 30 seconds or so......

[Jimmy Stewart ...]

totly

[Fredxx]

It does seem that holding the on/off button for more than 10s will reset the reminder:

[Chris Hogg]

Except that what triggers the reset? Simply taking off the blades and replacing them, either with clean or new ones, doesn't seem to.

[Chris Hogg]

Bingo! Yes, that's fixed it. Thanks.

[Jimmy Stewart ...]

ha ha hours of pontification and bingo rambo has the answer as usual....what a man.....but then again most of the replies on here are like that ...you have to be able to separate the wheat from the chaff ...tee hee

[Brian Gaff (Sofa]

Does it stop working then if you don't?

Brian

[Brian Gaff (Sofa]

That would need a very accurate sensor, something optical I'dhave thought. That sort of shaver would be lost on me as you can shine as many lights as you like but i still won't see it.

Brian

[Brian Gaff (Sofa]

That would be prone to other friction issues and the wear in the motor though, Brian

[John Rumm]

You need to do that manually:

"Replacing shaving heads To maintain your Philips Shaver, it is important to regularly replace its shaving heads. Depending on your shaver model, you may see a shaving head symbol flashing on your shaver or hear a beep as a reminder to replace the shaving heads.

For most shaver models, the shaving heads should be replaced every two years. For instructions on how to replace the shaving heads of your shaver, refer to your user manual.

***Note: After replacing the shaving heads, you need to reset the shaver by pressing the on/off button for a few seconds.***"

[Chris Hogg]

Yep. I got there eventually! But more than a few; more like twenty.

[Dave Plowman (News]

They would say that, wouldn't they, given the high cost of genuine replacements? (About 50 quid) Wonder if they make printers too? ;-)

Before replacing, try sharpening them. A bit of flat glass and some Solvol Autosol car chrome polish. Do this every 6 months or so and they'll likely outlast the shaver.

If they are very blunt one of those fine diamond dust plates might save some work to start with.