Burnt resistors and interesting caps

[phildil]

Hello all

I am repairing a rectifier board for a film viewer (a Steenbeck table). See pic of the board attached. Evidently at some time there has been a puff of the magic smoke from what was formerly a resistor. The circuit diagram is also attached FYI.

Two questions come to mind:
i) should the burnt resistor (1000 ohm / 2 watt) be replaced with a higher-powered, perhaps a 5 watt, version;
ii) looking at the capacitors on the board, there is only one polarized electrolytic: the 15uF just above the four diodes. What are the types of the other caps? Bipolar / unpolarized electrolytics perhaps? Would it be "usual" to replace these or just leave them alone?

cheers
Phil

[Station X]

Is the 1000R in series with 0.25uF wired across the mains by any chance? If so the cap needs to be Class X.

You won't find electrolytic capacitors in values less than 1uF. Polypropylene caps would make good replacements for what appear to be metal cased paper caps.

[phildil]

Yes mains voltage.

[Station X]

Class X then. The existing cap probably went short circuit causing the resistor to burn out.

[joebog1]

And that being so, replacing the resistor with a five watter may cause a fire!!.
Mains components should remain the same for safety

Joe

[Diabolical Artificer]

What are those lines on the resistor symbols on the schematic? There's two lines at one end at an angle, other R's have Roman numerals, so wattage/power rating? Also worth trying to find out why the resistor burnt out.

Andy.

[Nuvistor]

The lines may be the type of resistor, WW, metal film, carbon, safety etc, there will probably be a key somewhere on the circuit diagram.

[phildil]

Just to clarify, I should replace the existing 0.25 uF / 1000V cap with a 0.25uF / 275V Class X cap, yes?

For the same reason should I also replace the other 0.25 uF / 1000V cap connected in series with the (currently intact) 5.6 ohm resistor between pins 13 and 27?

As for the markings inside the resistor symbols then according to this page:

https://electronics.stackexchange.co...identification

they are indeed the power rating of the components. However, if the above link is accurate, the markings on the diagram don't tally with what is on the board. Meanwhile the numbers by some of the resistors, eg 111, 112 etc, relate to a descriptive text in the service manual.

thanks
Phil

[Station X]

It's difficult to answer you questions as we only have a detached part of the circuit to look at. It looks to me though that the components between pins 13 and 17 are in the DC part of the circuit.

Are the bridge rectifiers feed directly from the mains or are there mains transformers?

[phildil]

Ah I see. Yes I suspect you may be right about DC.

I've attached the diagram for the whole of the motor control circuit in the film viewer. Card (Teil) A is the rectifier board in question. The other boards are: B (pulse generator); C2 (25 fps frequency comparator) and D (relays). C1 is an option and is not fitted.

Also attached is the first part of the descriptive text, which runs to ten pages total.

Does this help?

[Station X]

Hard to make any sense of the circuit, it's too small. Can you post just the top right hand corner please. It's hard to tell whether the symbol at top right represents mains input or the secondary of a transformer.

[Lucien Nunes]

It seems the resistor that went phut is the R part of the RC suppressor across the motor supply transformer secondary. Agree that the probably cause is the capacitor going S/C and that must be replaced. Note that the 1000V rating is DC, whereas the voltage rating of a Class X capacitor is normally the AC voltage, which is lower for the same capacitor hence the disparity. I do not think a class X capacitor is required here as it is on the secondary of the transformer, although it should do no harm.

The other 0.25uF cap in series with 5.6R that I think you are referring to is the snubber for the SCR. This experiences high dV/dT and is probably best a normal polypropylene capacitor, not a Class X. Failure would cause the motor to run at full speed and probably another resistor burnout.

What age is the table? It's a long time since I had to repair one of these; the motor controller in my ST701 is a later generation.

[phildil]

A better scan of the cct diagram, and a detail of the top right corner, are attached. Note the upload process seems to squash them down.

Lucien, this is a ST400W four-plate 35mm table from the early 70s. Example date codes on caps are typically 1972. AFAICS it is all original although it's a FOC donation to my workplace and we don't know the history of it. We also have another Steenbeck of similar vintage in situ which has some other faults which I am hoping to tackle next. Would you mind if I ran them past you? Finding anyone with Steenbeck tech knowledge these days is rare.

thanks
Phil

[HamishBoxer]

That suspect cap appears to be across the transformer secondary if I can see it right?

[Station X]

Agreed. In that case a class X capacitor is not required.

[barrymagrec]

In my experience of this type of motor drive worn out brushes cause excess sparking which can damage the drive circuitry so I`d check those as well as everything else.

It`s a long time since I`ve played with a Steenbeck (35+ years) and then I think it was just the sound amp.

[phildil]

Originally Posted by barrymagrec

In my experience of this type of motor drive worn out brushes cause excess sparking which can damage the drive circuitry so I`d check those as well as everything else.

It`s a long time since I`ve played with a Steenbeck (35+ years) and then I think it was just the sound amp.

Thanks all for the useful info on this one.

Barry I have sent you a PM about sparking motors, hope that's OK.

[Lucien Nunes]

Agree, motor faults tend to rapidly lead to dead electronics in one way or another. But, does this drive actually work nonetheless? The shorted suppressor might have blown the fuse but it's not essential to the working of the motor and can be cut out of circuit for test. What testing have you done on it?

Re. other faults, ask away although my knowledge of that generation of flatbed is rusty and was never great. But I do look after a lot of old gear (including DC motor drives) and we have in our warehouse an ST400 that belonged to a very well known film-maker. I've never had cause to look inside it much, as it worked perfectly when it was last used about 15 years ago. Which is about the same time as I last ran sepmag 'for real' in preview work, although I do still handle film as part of the day job.

[phildil]

Yes - the table drive continues to work with the busted resistor in situ. Now that I know that it's part of a RC suppressor, that makes sense.

Thanks Lucien I will PM you re Steenbecks.

cheers
phil