Dishwasher Brain surgery

[Brian Gaff]

I think you just answered you own questions. They did not want them to fall out, could not be bothered with a zig Zag bender and they did not even consider repair so got the ones they could get at the time and put a case to fit on the whole lot. Is this a Bosch?

Anyway, why not fit anything that will work, and see if it fixes the problem then worry an making the case fit later on, say by making a hole in it and coating over the top with sealant. I've seen this sort of thing done on new equipment, but it was some time ago now, but I'd not expect it to be dangerous in any case as long as the case is earthed. Brian

[Brian Gaff]

Yes used to do this a lot on old portable radios to fit stuff in the case. Of course these one assumes are working on a higher voltage. Brian

[Tim+]

No I didn’t. Alan suggested that. I just agreed that it was a possibility.

Seriously?? How likely do you think it is that I’m gonna unsolder a brand new capacitor from £169 of control board to satisfy your curiosity? ;-). I’m not *that* curious myself.

Well I understand your plan but I think the idea of sticking a couple of different shaped capacitors with the same quoted rating elsewhere in the control boards box is a better one.

Tim

[Tim+]

FWIW the old caps are reading about 830uF. Would this be enough out of spec to stop it working?

Tim

[Joe]

I wouldn't have thought so. Aluminium electrolytics generally have wide tolerances, typically 20%, so they are never used where precision is required. Many types have values which are multiples of 1, 2.2 and 4.7.

Some electrolytics leak under some circumstances, usually heat, and the electrolyte not only has a low resistance but may dissolve copper, so there may be consequences of them failing.

[Theo]

That sounds within tolerance. What can happen with bad caps is the Equivalent Series Resistance goes too high. If the cap is being used to smooth a voltage rail, this makes a problem when something tries to take a big gulp of current. ESR is analogous with battery internal resistance. A low ESR cap would be able to fulfill the current demand, but a high ESR cap can't and so the voltage sags.

You can't measure that on most multimeters, you need a specific ESR meter (they can be had for £10 from China).

It is possible to buy specific low ESR caps for this application but, in the absence of a spec for the old ones, we don't know what the ESR is supposed to be. If you have a choice of replacement, worth picking the one with the lowest ESR.

Theo

[alan_m]

Select one with the lowest ESR from the list of zero found ?

[Tim+]

Measured ESR is 0.29 ohms.

Tim

[Vir Campestris]

100v seems an odd choice for a PSU. Nowhere near mains, but far too high for anything intelligent.

Or is it just I don't know enough about PSUs?

Andy

[Theo]

It appears to be a motor control board. Not unusual for motors to take 60V DC or more.

Theo

[The Natural Philosopher]

Yes. at high power it often makes sense to push up towards 100V, which is the sort of cliff edge for cheap transistors, and it keeps currents down and avoids massive wires.

Above that your transistors tend to become more pricey. A 100v supply rail is just enough to get 125W into 8 ohms, give or take...we used to use that pretty much all the time in medium power audio.

Not much point on going much higher since the loudspeakers to take it didnt exist.

[tony sayer]

Haven't got the cheapie from China but this one is excellent as is their back up and service!

[Tim+]

Gah! I’ve been duped. I feel such a fool.

I should have prodded the tops of the capacitors a bit more firmly. Turns out the “bulging” was just the plastic cover on the end of the cap that bulged, not the capacitor can itself.

Tim

[Animal]

Maybe I'm misunderstanding, but it looks like there's loads of room to add other caps, lying them onto the low profile parts. You can connect Cs on long leads, but when doing so do have at least some capacitance in the original position. Fix your Cs mechanically in anything that shakes or moves.

[Animal]

830uF is ok, but a) if they're bulging they're failing or failed b) you need to also measure ESR & leakage (Vloss) to see if they're bad.

[Animal]

Generic cheap chinese component testers will tell you if a cap's faulty, ESR meters sometimes do, sometimes don't.

[Theo]

How do you get on with the dishdrawers by the way? They seem like an interesting solution to having to unload the dishwasher before putting dirty things in it.

How does detergent work? It mostly comes in tablet form nowadays. Do you split tablets or just use a whole tablet per drawer?

I'm assuming the drawers are the same size? Does that mean less space for pans, tall plates and similar?

Is everything duplicated - two pumps, two salt reservoirs, etc, or are some parts shared?

Theo

[Tim+]

With hindsight, I’d say one of the best appliance purchases we’ve made.

Designed to work with powder but we generally just chop Aldi tablets in half and toss it in the machine and ignore the soap dispensers.

Yep, identical.

A bit I guess. No problem with 99% of things we put in.

All duplicated. Might sound like overkill but in practice it’s proved very reliable.

Tim