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Old 26th August 2012, 04:34 AM   #1
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Default Linn Numerik Muting after playing a while

I have a problem with my Linn Numerik. After it has been playing a while it will mute (you can still hear a very faint output.) I am using a squeezebox v3 as the input (was working well for years).

Opening the unit up and debugging show that the TP22 and TP23 flip from -14V to 14V muting the output.

The LED's indicate it is still locked, at 44.1Khz and still on the correct input. I can even still hear the output (all be it very quiet).

I found the schematic online but I am having trouble with the muting section.

Most other voltage test points seem ok except TP12. This says it should be between +/-5V but it is registereing -14V even when the unit is playing normally.

To get it to unmute, I can power it off and back on and then it will play normally for a couple more minutes before muting again.

Anyone any ideas on what could be going on?

This is where I found the schematic.

Linn Numerik Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics

I would really appreciate any pointers.

Thanks

Pen
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Old 26th August 2012, 04:17 PM   #2
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Not because I know much, but because it took quite a long time to find TP22/23:

One is mute, the other de-emphasis. Is it odd that they should be related? Functionally they shouldn't be, I guess, since there's no point in de-emphasising a mute signal.

If the mute is triggered by the "digital mute" signal, then that same signal should also ground the data to U8 (DF1700), so I wonder how analogue output is still present, however quiet.

So, what happens at TP6, and pin 5 of U1 (SAA7274)?

As for T12, that might be a red herring. It appears to be adjustable using VR1, but Q2 can be turned off if TP9 is negative, in which case T12 will be close to the -15V supply, AFAICS. Also if the +15V supply to VR1 is not present.

I would check the voltages at all three pins of VR1, and TP9.

These checks may get you closer to finding where the problems are coming from. Maybe more data will generate some interest from the experts. People who know what varicaps do, perhaps.
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Old 26th August 2012, 09:30 PM   #3
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Arrh, Now I see. The problem is actually with the power supply. After it has been running a while the +8V drops from about 10v to 6V and the 24V goes from 25V to 16V. This drop is enough to trigger the mute circuit. Digi-mute, Deem etc are all still normal.

Now I have moved on to the Power supply. This is the updated Brilliant SMPS. In there there is what looks like a small round elecrolytic cap that has split up the side. The board however is maked L2 here so I suspect this is actually an inductor of some sorts. The only marking on it is

^
221K
. 9

I have found a circuit diagram to a later version of this PSU here.

http://www.diyaudio.com/forums/attac...n-smps_ckt.pdf

It seems to indicate L2 is a 220uH inductor.

I guess I am off to frys to see if I can find a substitute.

I will keep you all updated of my progress.
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Old 7th September 2012, 09:01 PM   #4
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Just a followup for those that may read this in the future.

Changing the inductor on its own did not fix the issue. So I then moved on to changing some of the electrolytic caps. I am not sure exactly what fixed the issue as I did things in batches but I suspect it was the caps connected to the bad inductor. I changed these along with a small cap between two transistors on the control board.

Upshot my real old Numerik is back up and working.

Thanks All.
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Old 8th September 2012, 01:28 AM   #5
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Quote:
Originally Posted by pendoric View Post

...Upshot my real old Numerik is back up and working.
Hooray.

An SMPS is a scary place for a cap to find itself.
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