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105/1 LED question

 
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jifcom
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Joined: 24 May 2015
Posts: 7

PostPosted: Sun Jun 14, 2015 9:13 pm    Post subject: 105/1 LED question Reply with quote

Hi all
This is only my second post since joining; I am in the middle of restoring a pair of 105 series 1.

They are coming along nicely, Xovers have been re-capped with the Falcon acoustics kit, cabinet has been rubbed down, repaired a few bits of damaged veneer, re-stained and in the process of oiling.

Head units are next.

But to come to the point, the LED is not working one head unit. Does anyone know how to test the LED or the LED circuit board, if itís the LED anyone know what sort to replace with, i.e. Voltage/Amperage ect.

Any help appreciated

Jon
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speakerguru
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Joined: 18 Nov 2005
Posts: 954
Location: Green Hut, Tovil

PostPosted: Sun Jun 14, 2015 10:31 pm    Post subject: Reply with quote

I had a brief spell troubleshooting/repairing the production failures of those LED circuit boards when I was at KEF. It's very unlikely that LEDs themselves fail. If it has, it's very easy to check and any red LED will work as a replacement.
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jifcom
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Joined: 24 May 2015
Posts: 7

PostPosted: Mon Jun 15, 2015 5:23 am    Post subject: Reply with quote

Hi Speakerguru

Thanks for reply.
How would I go about diagnosing the board if the LED checks out ok?

By searching the internet, I now know how to check a LED is working Smile

Jon
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Garrard 401, custom plinth
Yamaha CA-810 as a Pre.
Yamaha CA-810 as a Power
KEF 105/1 Speakers
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audiolabtower
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Joined: 06 Jan 2009
Posts: 544

PostPosted: Mon Jun 15, 2015 12:46 pm    Post subject: Reply with quote

I traced the circuit a while ago



I would start with perhaps dry joints and the switch contacts, the least likely is perhaps the semiconductors? the most likely maybe the electrolytics have dried out after all these years, maybe the series input reversible cap if the switch contacts are fine?

As guru has said the led is a bog standard red you can test with a 9 volt battery and say 620 to 820 ohm series resistor to give around 10mA current?
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jifcom
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Joined: 24 May 2015
Posts: 7

PostPosted: Mon Jun 15, 2015 4:06 pm    Post subject: Reply with quote

Thanks for that audiolabtower.

I have just tested the LED and that's fine
Much appreciated the schematics, I will start testing this evening

Jon
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Garrard 401, custom plinth
Yamaha CA-810 as a Pre.
Yamaha CA-810 as a Power
KEF 105/1 Speakers
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speakerguru
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Joined: 18 Nov 2005
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Location: Green Hut, Tovil

PostPosted: Mon Jun 15, 2015 5:12 pm    Post subject: Reply with quote

The 10 Ohm input resistor is the first thing to check. Early ones could go open circuit (like a fuse) due to over-current, despite not exceeding the power rating.

As ever electrolytic capacitors can go high resistance and/or low capacitance with age.

audiolabtower, I don't recall those transistor types. Are you sure?

I seem to remember workiing very hard to source low cost, high voltage, low power, npn and pnp types for safe use with very high power amplifiers; e.g. Motorola BC449A and BC450A. In fact I still have some in a box somwhere.

They became unavailable and later there was a 2N5401 and it's opposite polarity complement.
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audiolabtower
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PostPosted: Mon Jun 15, 2015 7:25 pm    Post subject: Reply with quote

Hi Guru, yes, luckily I took some pictures just before I assembled everything again, you should just be able to read the types on the transistors:



I seem to remember looking them up at the time and they were 20 or 25V transistors, and I assumed that once the speaker voltage was attenuated through the 1M and 1:10 divider they never saw a huge voltage? Good tip about the 10 ohm, worth remembering!
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SaSi
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Joined: 24 Aug 2008
Posts: 223

PostPosted: Mon Jun 15, 2015 9:56 pm    Post subject: Reply with quote

I'd say keep them. They are getting rare and as the cones are heavy, drivers just sitting vertically doing nothing eventually will start rubbing. Better have spares.
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speakerguru
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Joined: 18 Nov 2005
Posts: 954
Location: Green Hut, Tovil

PostPosted: Tue Jun 16, 2015 8:51 am    Post subject: Reply with quote

There's no arguing with a picture. Now that I see it, I do recall those lockfit transistors. In fact I now remember recommending them because they stayed in the pcb without bending leads over and did not require cutting after soldering; all lower cost production options at that time.

The other types I mentioned would have been in the later Reference Series models with relay protection circuits.
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