STM control input filtering

[DrV]

I've just been working on some STM units and noticed that on some (about half) of the modules the capacitor, C4, that filters the control input has been changed form 2.2uF to 1uF. The original caps would give an input time constant of about 20mS so the new ones make this 10mS. My question is why would anyone change this? With a time constant of 20mS you get full output from a 0-100% step in 100mS and with the reduced value you get it in 50mS. Would this be visible? It's obviously been done fairly recently on site as the snipped off 2.2uF caps are scattered in the fluff all over the bottom of the case. Insights anyone?

 

Dave

[oilyspark]

I don't think you'd see much on conventional TH lamps although you might get snappier results with something else like chasing CCFLs. My guess would be that the caps had started deteriorating and were replaced with whatever was to hand, possibly whe it was discovered that a channel was flickering or failing to come on fully, depending what was wrong with the cap. I have changed these in the past, when the originals had gone high resistance.

 

Lucien

[DrV]

Thanks Lucien, I'd agree with that. It seems to have been done a bit carelessly too as all the replacements have been fitted the wrong way round! I'll be back inside the units in a fortnight so I'll sweep up some of the old caps and test the theory.

 

Dave

[oilyspark]

the replacements have been fitted the wrong way round!

Little alarm bell starts ringing. It's not part of a ham-fisted / misguided attempt to convert the dimmers to positive control is it? Of course you don't need to turn the cap round to do that, you need to turn the whole card round by interchanging ground and control input leads. It works in the modular dimmers because each has its own floating power supply and can have its 'ground' terminal at any arbitrary voltage. A bit naughty though, because the common of the circuit is no longer earthy but riding up and down on the control voltage. Anyhow I have seen this done quite a few times and wonder whether your dimmers have been 'got at' for that reason.

Not often I get to talk about these old things any more though!

Lucien

[DrV]

the replacements have been fitted the wrong way round!

Little alarm bell starts ringing. It's not part of a ham-fisted / misguided attempt to convert the dimmers to positive control is it? Of course you don't need to turn the cap round to do that, you need to turn the whole card round by interchanging ground and control input leads.

 

I wondered about that too... but only after I had left the venue and wasn't able to check the polarity of the input voltage. I don't recall what type of demuxes are in use but they are being driven by a D54 line so they are almost certain to be Strand and, therefore, -ve. In any case only about half of them had been done and they were in random positions over several racks. I'll be back there the week after next and plan to check the control voltage then. By chance the two modules I brought back to the workshop for repair were one of each type - I made a point of checking the capacitance and leakage current of the caps and they were fine so if they were changed because they were going dry or leaky it clearly didn't affect all of them.

 

Not often I get to talk about these old things any more though!

 

Bit like working on a Morris Minor - sound, straightforward engineering and loads of elbow room (notwithstanding the live heatsinks!)

 

Dave