Some help with a Cowells speed controller needed.

[HollowpointParticipant @hollowpoint]

Hi guys. I bought a Cowells CW90 a few months ago which I have been slowly restoring. Got it all back together yesterday and noticed that the speed controller has some issues. It works, but when the potentiometer is in certain positions the motor rpm fluctuates and it is unstable.

I assume it’s likely that the potentiometer is faulty and I am going to replace it. However upon opening the controller up I have noticed a few other components that have clearly gone kaput. (Two capacitors and a resistor).

I need some help with identifying and sourcing the parts needed.

Coincidentally a search lead to some useful information from in an old thread. Pictures below.

 

Image found online of a better condition controller.

[HollowpointParticipant @hollowpoint]

My controller

Clearly blown components

[Grizzly bearParticipant @grizzlybear]

Hi,

It looks repairable.

Capacitors, probably mains voltage.

Resistor looks like 150 ohms quarter watt.

The T03 2n…… transistor could be short circuit.

All the diodes will need checking.

Good luck……….

[Robert Atkinson 2Participant @robertatkinson2]

The capacitors are RIFA X type paper capacitors often used in EMC filters. Thay have suffered a common internal failure mode. There is probably a nasty smell about them. Replace with 47nF / 0.047uF “X” or X2″ plastic film capacitors. Thes will have a rating of about 275 V AC.
The “transistor” isn’t. It’a a 2N3525 Sillicon Controlled Rectifier or SCR.  One of the diodes has been changed to what is almost certainly one with two low a current rating. It the part number is not visible on the original 3 I’d suggest replacing all 4 with 1N5408 diodes.
If you are getting some speed control it’s likely that the SCR is OK.
The burnt resistor is 150 Ohm 1/2 W.

Robert.

[HollowpointParticipant @hollowpoint]

Thanks for that, I’ve ordered a potentiometer and the capacitors, just need to find some resistors now.

I tested the diodes, all seem fine.

[HollowpointParticipant @hollowpoint]

Another quick question. Do the capacitors have a correct way around? Negative/Positive?

[Robert Atkinson 2Participant @robertatkinson2]

No. The 47nF / 0.047uF capacitors are not polarised. They can go either way.
On your board only the black plastic covered capacitors (electrolytics) are polarised.

Robert.

[HollowpointParticipant @hollowpoint]

Thank you

[Kiwi BlokeParticipant @kiwibloke62605]

The Rifa paper dielectric capacitors are known here as ‘smokers’ (and other, coarser names). I’ve replaced several, over the years, which have failed explosively, or smokily, and once with flames… I have replaced like with like, but wondered whether I was being silly.

It seems that they are still available, and are relatively expensive. Given their limited in-service life, can anyone explain what, if any, redeeming features they possess? Is it reasonable to replace them with suitably-rated metallised-film caps, in all cases, or do they possess characteristics that make substitution risky?

[SillyOldDufferModerator @sillyoldduffer]

They’re designed to fail safely by not exploding, catching fire or going short-circuit.   Also rated to take a couple of kilovolts peak, necessary because of high-voltage transients due to lightning, solar activity, supply-side switching, and equipment failures.

Just a guess, but as their function includes filtering out high frequency electrical noise, they might also be low inductance and low ESR (i.e. they charge and discharge faster than most capacitors.)

They fail, I think, because they’re the first line of defence against dirty mains and because they might be worked hard by the  system normally.  Sparking commutator, on/off transformer spikes, or a switch-mode power supply: they’re all stressy!

Dave

[Robert Atkinson 2Participant @robertatkinson2]

Going off topic a bit.
The old RIFA paper capacitors fail due to moisture ingress and runaway breakdown / self-healing / breakdown. It appears that the original encapsulation resin was not impervious to moisure. I have new old stock ones that have swollen and cracked the encapsulation.
The modern ones, this century, seem to be OK.
We see more failures in the UK due to the higher main voltage.

Robert.

[Julie AnnParticipant @julieann]

The function of X/Y capacitors is to filter high frequency noise on the mains and prevent it from reaching sensitive equipment. They also filter general mains noise and, since spikes are essentially wide band noise, they help with mains spikes.

The capacitors come in two basic types. An X capacitor is designed for line to line (live to neutral) and deals with differential noise. A Y capacitor is designed for line to ground (live, or neutral, to ground) and deals with common mode noise.

The capacitors are normally metalised plastic film and are designed to have specific failure characteristics. An X capacitor is designed to fail short circuit thus causing a protection device, such a fuse, to fail. A Y capacitor is designed to fail open circuit.

Depending on application the capacitors come in several grades, X1/X2/X3 and Y1/Y2/Y3/Y4.

Basic EMC testing of mains power supplies involve single and repetitive pulses, of both polarities, at peak voltages of 500V, 1kV and 2kV. In my experience a well designed power supply should have no problem in passing these tests.

Julie

[Kiwi BlokeParticipant @kiwibloke62605]

Apologies for continuing the topic drift…

It has been suggested to me that the Rifa caps’ problem is also possibly due to the paper dielectric containing moisture at the time of encapsulation. Whatever, the wretched things seem to fail, after ten years or so, by swelling causing the encapsulant to crack (or crack more), before they explode or burn. In my experience of several, the fuse did not rupture. So, they fail, even unused, in dry storage, and do not reliably fail short-circuit. And they’re expensive. Is there any reason not to replace them with, say metallised polypropylene X caps? What is the justification for their continued existence?

[Robert Atkinson 2Participant @robertatkinson2]

The Rifa X and Y caps were / are cost effective products and were very widely used from the late 70’s at least. I don’t think the failure mode is trapped moisture from manufacture. That would not cause swelling. It’s absorbsion of atmospheric moisture. Many years ago I bought several boxes of RIFAs that had been stored in a stack for years. The capacitors were in a cardboard grid inside the boxes giving a defined layout. The capacitors in the top box were all badly swollen. In a box from the middle of the stack the capacitors in the outer rows were badly swollen but the ones in the middle were fine. I posit that this was due to them being more protected from the atmosphere.

Robert.

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