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Saturday, 19 January 2019

Linn Majik - humming fault

"Ho, ho, ho , It's magic you know , Never believe it's not so"

Except for this one. Magic it isn't...

It's another of Colin's Job lot of Linn stuff.

A Majik integrated amplifier.


Connecting up and switching on it works, but is humming badly from the right hand channel. 

Probably more caps I'm thinking ... let's get it in bits and have a look.

Same MO as the other Linn products in this range, four screws holding the top cover on, are found underneath. Slide off the top cover and put it to one side...
We'll need to remove the remote / balance input board from the rear panel. Two screws, and a couple of plastic support, and it lifts out. Talking of balanced audio... why oh why oh why an RJ11 socket?? XLR's not good enough? 

OK, having got that out of the way, we're going to need to remove the main PCB. Flip the unit over, and remove the 5 screws holding the heatsink to the base plate. 


Remove the screws holding the phono sockets to the back panel.

There are four screws to remove to remove the PCB from the base plate. Two by the capacitor bank, and two on the other side, marked "tool hole" and "sneaky"!

The front panel needs to come off so we can get the headphone jack to clear.

Two screws at the top, left and right...

and two underneath (the silver ones).

Disconnect the ribbon and the two pin connector to the power LED, and remove the front panel.

One the links for the power / amp amp are removed from the back panel, the main PCB easily lifts out. A cursory visual check shows nothing obvious. The main electrolytics are still in fine shape, as they're still holding charge.
Right, we're going to need to get the thing back together to begin testing... so the transformer is plugged back in, and the front panel. It's all a bit dicey. Be careful not to allow the board to short out on any of the case! There's no need to plug the balanced board back in, it'll run happily without.

Without the heatsink bolted to the base plate, it does get quite warm quite quickly, so keep running times whilst testing to a few minutes, then let it cool down.
Powering on shows all the LT rails (+/- 26V, +/-15V and +5V) are all present. The power amp is now isolated from the pre-amp, and the hum remains. So our right hand power amp is at fault.

Comparing differences, both static resistance measurements, and using the scope between left and right amplifiers under identical conditions, I can't see any difference between the amplifiers except for the output of the right amp.

I short the input of the right hand amp to ground, to see if the hum is getting in through the input somehow. It makes no difference.

It must be a fault within the output IC's (TDA1514A) of the right hand amplifier (there are two IC's per channel, in parallel). The problem is the TDA1514A is obsolete. I won't risk the eBay IC's... they're bound to be fakes.

Damn.

One thing I have noticed is there's a bit of 100Hz ripple on the -26V rail. But both amps share the same +/-26V supply (It's actually +26/-25)... so surely it can't be that, can it? I change C602 and C603 which are the local decoupling capacitors to the power amp IC's. There's slight "change" to the hum, but it's very much still present. Encouraged by this change I look at the supply.

Now the -26V supply is regulated by a 2SC3519 pass transistor which is controlled by an LM337. The adjust pin of the regulator is decoupled with a 100uF capacitor. The input is decoupled by another 100uF capacitor (C308 & C310). These two caps are right up against the tab of the regulator. They may have well been cooked by the heat. Decoupling the input to the pass transistor is yet another 100uF capacitor, C320. I change the lot. 

Shush! Can you hear it? Me neither, the hum is gone! 

So why on earth was the left hand amp not affected?? One of life's little mysteries..

Now the +26V rail has a similar set up, but uses a 2SA1386 pass transistor and an LM317 regulator. The components are mounted in a similar manner. I change that lot too, so we should have a reliable amp.

There , all playing nicely again. Another saved from landfill...




*STOP PRESS*

Steve's been in touch. He writes "The board is a Knekt receiver board and the RJ45's are for balanced audio & 2 way control from main room sources, the other socket is for the RCU (Room Control Unit) a wall pad to control the amp and local or remote sources."

Now the RJ45 makes sense!

Thanks Steve.





2 comments:

  1. Hi can you please let me know what is the value of R311 got one pcb in damaged mode

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    Replies
    1. Sorry, I neither have the amp, nor a schematic.

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