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Old 14th January 2012, 11:46 AM   #1
hemanth is offline hemanth  India
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Default Problem with MyRef Rev C based on LM3886

Hello everybody,

This is my first DIY amplifier project. I bought the Mauro Penasa MyRef Rev C kit Ver 1.4 from linuxguru. I populated all the components on the PCB (two monoblocks). Iím using a transformer Primary:0-230 Volt Secondary: 24-0-24 V/300 VA. I checked the transformer by connecting to the AC main and it is reading 23.5 and 23.6 respectively on the secondary.

After the full assembly Iím facing the following issues:
Channel 1: When I switch it on, first it makes the relay triggering sound and then after a couple of seconds it makes screeching sound intermittently, I checked for any loose connections, and did re-soldering, but the problem still persists. When I pause the source there is no screeching, but when I start again, it starts screeching intermittently.

Channel 2: When I switch it on, there is no relay triggering sound, but when I switch it off, exactly after 6 seconds I hear the relay triggered on and triggered off( two beeps) and I observed the LM3886 getting warm.

Please guide me on how to debug. what could be the problem?

I have attached the snaps.

Thanks in advance.
Attached Images
File Type: jpg IMAG0173.jpg (361.2 KB, 579 views)
File Type: jpg Working with Screeching.jpg (406.4 KB, 548 views)
File Type: jpg Non_Working.jpg (440.6 KB, 534 views)
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Old 14th January 2012, 02:53 PM   #2
AndrewT is online now AndrewT  Scotland
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Send a PM to Linuxguru.
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Old 14th January 2012, 03:45 PM   #3
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Is your mains ground tied to the bolt that goes thru the center of your toroid?

Hhhmmmm........
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Old 14th January 2012, 04:34 PM   #4
hemanth is offline hemanth  India
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Yes, my main grounds are connected to the bolt which goes through the center of the toroidal. Linuxguru suggested that.
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Old 14th January 2012, 04:40 PM   #5
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How much you brought it for.
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Old 14th January 2012, 05:15 PM   #6
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i had the same problem that you're having with channel 2. my issue turned out to be op-amp. try swapping the lm318.
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Old 15th January 2012, 08:07 AM   #7
hemanth is offline hemanth  India
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Hi kuldeepsingh,
The PCB with components cost me 3K + 1K for upgradation of some components. The transformer cost me Rs.1700.

Hi PreSapian,

I did swap the LM318, but no luck. the issue remains as it is.

Thanks.
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Old 15th January 2012, 12:19 PM   #8
bcmbob is offline bcmbob  United States
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hemanth - How confident are you that the correct resistor values are in specific locations? I thought I had checked everything with a VOM before mounting but twice found a K-value resistor where a value resistor should be. In one case that was preventing the relay from functioning. Otherwise, any other problems I had were born of cold solder joints.
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Old 15th January 2012, 01:10 PM   #9
hemanth is offline hemanth  India
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Hi Bob,

I did verified them multiple times and even Siva(linuxguru) verified it and it seems to be fine.

Today I took help from Syed (who is one of the forum member) and he asked me to check the DC offset and the DC offset on the channel with no sound(delayed relay triggering) is 31.6V and on the channel with screeching sound is having a DC offset of 1.2V. According to him the Dc offset if too high(it should be <100mV). So I will talk to Siva on this to check if I can replace some of the components.

Thanks.
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Old 15th January 2012, 01:14 PM   #10
AndrewT is online now AndrewT  Scotland
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Quote:
Originally Posted by hemanth View Post
I did verified them multiple times and even Siva(linuxguru) verified it and it seems to be fine.
having seen the pics you showed in post1, I cannot believe that Linux gave your assembly the OK.

The assembly is strewn with bad practice and errors.
That's why I said contact the seller. I think you have broken the amplifier sent to you.
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