[sdiy] Murata AFL811WF / Ursa Major 126

[Jim Credland]

Getting there, I think.  

It might be a dodgy comparator in the ADC.  (A CMP05: http://www.analog.com/static/imported-files/data_sheets_obsolete/OBSOLETE%20WATERMARK/CMP-05.pdf) 

I'm presuming that when the +ve input is at a higher voltage than the -ve input I'm expecting to see a HIGH on the output. 

However instead, when the +ve input goes substantially more negative than -ve input I get a HIGH output. 

Also, having removed the feedback from the SAR to the comparator, and feeding in a sine wave, the thing only outputs a 1 for 25% of the cycle.  I'm guessing this should be 50%!

I can't think of why this should be unless it's no longer a proper comparator?   

I'll try and get a shot of the scope output...

J. 

PS. It's also running a bit hot.  Easily the hottest chip on the board at about 50 degrees C. 


PPS. If you're interested and fancy double checking my logic, the circuit is here:

http://www.sevenwoodsaudio.com/MSP126_Preliminary_Service_Manual.pdf

Top of sheet 5, I've removed U25 from it's socket to make things a little easier to follow. 



On 30 Nov 2013, at 21:15, Jim Credland <jim at cernproductions.com> wrote:

> Cheers everyone, really helpful.  We were guessing that was their purpose. There's three of them, a mono input and a stereo output pair. 
> 
> However we suspect the problem is on the digital side.  There's some pretty crazy distortion going on.  I'll post a shot from the scope later - it's like a cryptic crossword clue!!
> 
> On 30 Nov 2013, at 20:59, Veronica Merryfield <veronica at merryfield.ca> wrote:
> 
>> It’s a 22Khz filter, active I think. Search for AFL811 yields a few more results. I can’t find a modern replacement that will work on a +/-15v supply. You can probably create a replacement although given the post filter op-amps have some gain, it looks like there is a fair amount of loss through the filter.
>> 
>> Schematic here http://www.sevenwoodsaudio.com/MSP126_Preliminary_Service_Manual.pdf
>> 
>> 
>> On Nov 30, 2013, at 9:18 AM, Jim Credland <jim at cernproductions.com> wrote:
>> 
>>> Hello knowledgable people - I've got an Ursa Major MSP-126 in pieces on my lap.  I've been meaning to fix it for years.  There's a large silver part i don't recognise and on which google doesn't help. 
>>> 
>>> Does anyone know what a Murata AFL811WF would be?  I'm guessing it's an obscure filter... rather hoping it's not the faulty part :)
>>> 
>>> cheers! J.
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>> 
> 
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