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Digital electronics repair

Postby BJG145 » Wed Sep 19, 2018 7:50 pm

...so, for sale: some obsolete audio tech based on digital electronics with an external PSU, that won't switch on one day. PSU still lights up. Is that 'worth a punt' or 'run away'...? I don't know whether there are characteristic failures or simple fixes for this kind of thing. If the default answer is "new motherboard", that's not going to happen.
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Re: Digital electronics repair

Postby James Perrett » Wed Sep 19, 2018 9:35 pm

Not really enough info - some digital devices are designed to be serviced while others are going to be a nightmare. I'm guessing that you don't want to alert people to a potential bargain but we'd really need to know what it is you are talking about to be able to give you any idea of what is involved.
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Re: Digital electronics repair

Postby Folderol » Wed Sep 19, 2018 11:44 pm

Sorry, but I'm confused by the whole post.

What do you mean by 'won't switch on one day'? does on other days?

'obsolete audio tech' ~ 'motherboard'? Is this hardware modules in a computer or a rack system or completely independent units?

'PSU lights up' means nothing really, might be the wrong voltage/polarity, might have a broken lead to the kit.
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Re: Digital electronics repair

Postby BJG145 » Mon Oct 01, 2018 7:21 pm

The mystery object was a defunct Soundbeam 5 with a couple of sensors, listed under Violins on eBay, and which I nabbed for £100. You don't see many for sale, and they cost over £3K, so it would be a nice bargain if I can get it working.

SOS reviewed V2 back in 2001. It's basically a long-range sonar-based movement-to-MIDI system, originally inspired by the theremin and designed for things like dance performance, though its main userbase nowadays is in special needs. I think it has loads of untapped performance potential though.

The power supply lights up, but the display on the base unit is blank. I have a multimeter, and a basic understanding of how to use it. I guess I should start by testing the output of the three-pin power cable, and maybe reseating the internal connectors, but I thought I'd consult the experts before touching anything. (It all seems quite clean; I can't see any obviously swollen or blackened components. The seller simply said it stopped working one day.)

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Re: Digital electronics repair

Postby BJG145 » Mon Oct 01, 2018 8:29 pm

...just wondering, the holes like the ones shown in the above pic labelled "GND" and "+5V"...are they put there deliberately for diagnostics...? So, safe to try probing those...?
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Re: Digital electronics repair

Postby Folderol » Mon Oct 01, 2018 9:00 pm

BJG145 wrote:...just wondering, the holes like the ones shown in the above pic labelled "GND" and "+5V"...are they put there deliberately for diagnostics...? So, safe to try probing those...?
Yes.
Also, it looks like you've got rectifiers and nice big smoothing caps, so probably + & - supplies. In which case the 'power supply' is just a transformer sending low voltage centre tapped AC.

That area would be my first bit to check.
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Re: Digital electronics repair

Postby Sam Spoons » Mon Oct 01, 2018 9:25 pm

Not an expert (except at buggering them up) I say that while there are a fair few SMDs on there that was designed to be serviceable with sturdy boards and plenty of room around the components. Good luck....
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Re: Digital electronics repair

Postby BJG145 » Tue Oct 02, 2018 7:29 pm

...damn, I'm so rubbish at this. I'm messing around with a UT30B.

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I tested with a battery to make sure I understood how to read small voltages, then tested various combinations of 0V/GND and +3, +5, +12 in holes around the board, but got nothing.

So I wondered if the PSU was producing any output. I plugged the probes between the "10A MAX" and "COM" sockets on the multimeter, experimentally turned the dial to 200 in the squiggly "V~" section, and stuck the probes in a couple of the sockets on the disconnected 3-pin power cable, which resulted in a hum and a spark.

So I guess it was live then. How do I know which pin is which, and check that voltage correctly...? Any other suggestions...?

(The PSU is labelled: "Output voltage 25-0-25 AC rating 160VA, primary current 1.0A secondary current 6.4A")

*edit* d'oh, I get it, two red wires and one black one for the internal connections on the PSU. And I guess I should RTFM for the multimeter.
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Re: Digital electronics repair

Postby BJG145 » Tue Oct 02, 2018 8:22 pm

OK, so yeah, I get a reading of 29 between the centre pin of the PSU and either of the side pins. I think I'm going to abandon this idea of diagnosing it myself though.
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Re: Digital electronics repair

Postby Folderol » Tue Oct 02, 2018 10:00 pm

Just take a deep breath and switch brain into high gear.

OK so transformer is fine, and power is getting to the unit.

Unless I'm much mistaken, that should result in a + & - 40V main DC supply, so you will need to proceed with some care.

Can you get to the underside of those two big caps? If so, each of them should have 40V across them, so switch to the 200V range - and you definitely want to be in the V socket, not 10A one :)

I'm off-line from tomorrow till after SynthFest, so that's plenty of time to have a bit of a look around.

TTFN
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Re: Digital electronics repair

Postby ef37a » Wed Oct 03, 2018 6:51 am

There is what I am certain is a power audio chip mounted on the big heatsink left of the caps and front of that what could be a "slab" bridge rectifier? You might be able to get at the + an - pins of that to check the supply DC volts. NB! Do not let the test probe slip and short pins. Take a file and sharpen the probe tips to lethal points then they will dig into the copper. Sharp probes can also puncture cables and make quick checks easier if there is no handy bare connector.

If you have no volts there is a fuse. I am sure the small round object, marked 330 above the caps is a "Wickman" fuse, bloody daft idea, soldering in a fuse! If it has blown I would replace it with a 20mm fuse carrier or an inline fuse holder. You could be replacing fuses a few times here!

That power chip could be short and blowing the fuse, is there a number on it?

Sad to say but old displays are wont to just fade out, remember Ford dash clocks?

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Re: Digital electronics repair

Postby BJG145 » Wed Oct 03, 2018 9:45 am

Thanks for the ideas folks...! Have to say I'm reluctant to explore this further with these high voltages flying about in case I do more damage, so I think I'll find some local repair place to take a look at it...will let you know if it comes alive.
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