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RS Function Generator RS 435-226: RS 434-289: RS 591-124

 
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Re: RS Function Generator RS 435-226: RS 434-289: RS 591-124

Post by crustytv » Tue Jun 02, 2015 8:53 pm

Had an idea, I found an empty DVD player and wondered if this would be a good candidate.

It would certainly give plenty of room to fit all the boards and allow additional features to be added to the device as discussed such as frequency counter and amplifier.

All I will need to do is fashion a fascia and trim the knob shafts, its a bit larger than I wanted though and I'm not really sure I like it.

case1.jpg

case2.jpg

case3.jpg

 
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Re: RS Function Generator RS 435-226: RS 434-289: RS 591-124

Post by Refugee » Wed Jun 03, 2015 3:30 am

Look out for any old tin sky boxes. I was given one a week or so ago and it looks like it has a UHF modulator in it.
They are smaller than old VCRs.

 
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Re: RS Function Generator RS 435-226: RS 434-289: RS 591-124

Post by TVJON74 » Wed Jun 03, 2015 9:05 am

Hi Chris,
I had a look through my transformers last night, I'm sorry to say I have nothing suitable!
If Alistair still has the one he offered you I would take that :aad

 
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Re: RS Function Generator RS 435-226: RS 434-289: RS 591-124

Post by Mark Hennessy » Wed Jun 03, 2015 12:06 pm

I've often "up-cycled" enclosures from consumer gear, but in practice, it's not as easy as it first appears. Replacing the front panel can be difficult because the bottom/top/sides usually slot into it for rigidity, and there isn't always a clear way to attach the new panel. Sometimes, retaining the old front panel but "re-skinning" it with a new sheet can work well, but that relies on the original being largely flat. For something like a hi-fi amplifier - which has almost no controls (thanks to audiophile "minimalism") - you can can re-use the positions the old controls occupied.

Another problem is that the steel can be incredibly thin and hence hard to work with - as counter-intuitive as that sounds...

Old test gear is usually more promising, but of course old test gear does tend to be more fixable and hence more valuable. IT equipment - especially the more professional kind - can also be very good. Think about hubs/switches, modems, terminal servers, UPSs, etc. Nice rectangular steel boxes that are well-shielded and solid.

But yes, enclosures are expensive - especially from the big suppliers like RS, Farnell, etc. Whenever I rummage through the skip, I always carefully check anything that might yield a useful box, whatever it is.

Of course, the DIY route might be feasible - e.g. a simple MDF surround and aluminium front/rear panels. If you need to screen a wooden box, aluminium tape is often available at Lidl really cheaply (very handy stuff!). Aluminium extrusions are very handy, and I always rescue them when I can (I use Aluminium Warehouse if I need to buy something). For example, angle that is 2" by 1/2" makes for good front/rear panels, which can be attached to a sheet of aluminium that forms the base, then followed with 2 bits of wood for the sides and top. If you like, you could use a sheet of perspex on the front panel, held to the aluminium angle by the controls and connectors - then, a sheet of paper with the control legends could be sandwiched between the perspex and aluminium. If you're feeling especially swish, you can even back-light the legends...

Last time I did any of this sort of stuff, I only had access to basic hand tool, plus a jigsaw (which surprisingly cuts sheet aluminium better than timber), and a woodworking router, which I found worked especially well with aluminium provided you go slowly. It's time-consuming, but at the time (pre-kids!), that was part of the fun.

Anyway, hopefully some good general ideas in here. As far as I can remember, I used one of these and mounted the controls on the lid. It was a bit of a squeeze, but it all fitted. For the PSU, I used an RS PCB, but is was smaller than one of the ones you have, and it generated both rails from one chip. The transformer was a 6VA frame type. But no space for a power amp or anything else - I built the amp in another box of the same dimensions.

 
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Re: RS Function Generator RS 435-226: RS 434-289: RS 591-124

Post by crustytv » Wed Jun 03, 2015 8:56 pm

I decided to wrap this project up and go with Marks PSU.

With some thought into placement of the board and trimming one corner of it, PSU, controls and output BNC, I've managed to shoehorn it all into the small project box. I need to shorten the leads to the various controls, secure the mains cable with a grommet clip and use a letraset to add some control legends. Agreed it leaves no room for expansion however at the end of the day the project brief was to have a function generator, that's what I've now got.

I will use the PSU boards on another project as and when I find one that inspires me.

Thanks to all who have contributed both with advice and parts for this thread, it has been fun and as I mentioned elsewhere a much needed diversion.

case4.jpg

case5.jpg

case6.jpg

case7.jpg

 
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Re: RS Function Generator RS 435-226: RS 434-289: RS 591-124

Post by Mark Hennessy » Wed Jun 03, 2015 9:02 pm

Looks good :thumb

Small is always useful in a busy workshop. Having the power amp as a separate box is a good idea because it can be used for other things - certainly, that's what I found.

 
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Re: RS Function Generator RS 435-226: RS 434-289: RS 591-124

Post by Alistair D » Wed Jun 03, 2015 9:21 pm

One small suggestion. Fit another BNC socket. Wire it across the amplitude pot's track. You can use that to drive a frequency counter. That way you will have enough signal to drive the counter no matter what the amplitude is set to.

Al

 
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Re: RS Function Generator RS 435-226: RS 434-289: RS 591-124

Post by Niall » Wed Jun 03, 2015 10:05 pm

I have bought a number of cheap things at ham rallies and the like "for the box" but have had to set stricter criteria for what is inside - has to be nothing, incomprehensible, or clearly beyond repair, otherwise I just end up getting it going or setting it aside in case a circuit turns up and still needing a box for whatever I originally needed it for.

 
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Re: RS Function Generator RS 435-226: RS 434-289: RS 591-124

Post by crustytv » Thu Jun 04, 2015 12:02 am

Tidied up the wiring, installed some PCB mounting posts and did a final test, all OK.

func1.jpg
Its all a snug fit in there

func2.jpg

Alistair D wrote:One small suggestion. Fit another BNC socket. Wire it across the amplitude pot's track. You can use that to drive a frequency counter. That way you will have enough signal to drive the counter no matter what the amplitude is set to.

Al

Thanks, I will do as you suggest :thumb

 
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Re: RS Function Generator RS 435-226: RS 434-289: RS 591-124

Post by Lloyd » Thu Jun 04, 2015 12:19 am

Looks good Chris :thumb

I've been after a function generator for some time, been eyeing up the cheap Chinese bare board ones with the LCD displays.. This looks much simpler! I might have to have a go at building such a thing.

I reckon you still have room in that box for a built-in frequency display!

Regards,
Lloyd.

 
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Re: RS Function Generator RS 435-226: RS 434-289: RS 591-124

Post by Alistair D » Thu Jun 04, 2015 12:23 am

Looking good. Very professional.

Al

 
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Re: RS Function Generator RS 435-226: RS 434-289: RS 591-124

Post by crustytv » Thu Jun 04, 2015 12:42 pm

This really is the last two pics and sign-off.

As Al suggested I've added the extra output tapped from the amplitude pot for frequency counter connection. Applied the legends to the various parts, not perfect but will do.

The End.

extrabnc.jpg

letra.jpg

 
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Re: RS Function Generator RS 435-226: RS 434-289: RS 591-124

Post by crustytv » Thu Jun 04, 2015 4:24 pm

Having a little play this afternoon I discovered an oddity and was wondering if anyone could shed some light on it.

When I adjust the amplitude on Sine of Square from min to max, all is well. However when on triangle wave when almost at max amplitude, the last volt between 27.2V - 28.2V, the waveform is clipped. Why? and is there anything I can do?

Edit: The Amp pot was supposed to be a 4K7 but all I had was a 5K would this be the reason?

noclip.jpg
Amplitude of wave form is uniform right up 27.2V

clipped.jpg
At 28.2V the max setting for the amplitude pot, it clips the waveform

 
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Re: RS Function Generator RS 435-226: RS 434-289: RS 591-124

Post by Mark Hennessy » Thu Jun 04, 2015 5:07 pm

The output amplifier has slightly too much gain. You could try altering the resistors around it - perhaps reducing the 22k to 20k or 18k might do the trick? Or, if it is just the triangle waveform that clips, then you could increase the 3k9 resistor (to 4k3 or 4k7) ahead of the waveform switch (from pin 14 of the op-amp). Those resistors before the waveform switch are chosen to that the 3 waveforms have nominally the same amplitude as you switch between them, but there will be some variation in practice.

While the pot value will have an influence on the amplitude - because it forms a potential divider with those resistors feeding the waveform switch - potentiometers tend to have rather "loose" tolerances, and certainly 4k7 is comfortably within 10% of 5k. A good designer would avoid relying on precise values of potentiometers.

Or, you could just be aware of the issue, and decide not worry about it. It's arguably unlikely that you'd ever need such a big waveform anyway. My complicated Wavetek only gives 20V peak to peak at full output, and that is plenty. It's a personal choice thing :bba

This has triggered a distant memory. I *think* I added preset resistors in that part of the circuit so I could tweak the outputs to be exactly the same amplitude, and also to avoid clipping (which is inevitable as supplied). Having had that recollection, perhaps the details might bubble to the surface later on...

 
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Re: RS Function Generator RS 435-226: RS 434-289: RS 591-124

Post by crustytv » Fri Jun 05, 2015 8:28 am

Hi Mark,

Once again many thanks for your ever useful input and explanations, I think I will just live with it for now. I'll start another thread soon as I have an idea for the PSU PCB's that I want to run by folk.

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