MAS SPA-100 left channel weaker sound - diyAudio
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Old 17th September 2013, 04:20 AM   #1
FPN is offline FPN
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Default MAS SPA-100 left channel weaker sound

Hi Guys,

I just obtain a Music And Sound (MAS) pre-power combo.
The pre is DCC-1 and power is SPA-100.
I just realize that the right channel sound weaker than the left channel.
It will balance if I adjust the balance to right or I adjust the pot on the power (there are pot in from of the power amp for both left and right)
When I adjust the pot from power amp (to achieve balance sound), the difference is pot is significant (around 1/4 turn).
I try with 3 different speaker and the result is similar.
Anyone able to help what is the "potential" problem with my power amp?
I try to open the case and there are 4 fuse and one fuse is different (maybe the old owner change it).
Is the fuse different between left and right channel cause this problem?
Or there are other "potential" problem like pot problem, transistor problem or cap problem?
I'm not electronic expert but I'm quite handy with solder and able to perform light repair.
Thanks in advance.

Last edited by FPN; 17th September 2013 at 04:29 AM.
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Old 18th September 2013, 03:46 PM   #2
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Anyone able to help me?
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Old 18th September 2013, 03:55 PM   #3
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Static on Right Channel Realistic SPA-100
It seems this guy also had your problem but didn't proceed. The solution will depend on your skills at problem finding, I'm afraid.
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Old 19th September 2013, 07:29 AM   #4
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Quote:
Originally Posted by Ian Finch View Post
Static on Right Channel Realistic SPA-100
It seems this guy also had your problem but didn't proceed. The solution will depend on your skills at problem finding, I'm afraid.
I see, let me take a look on the thread you provide.
Thanks mate..
Cheers.
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Old 19th September 2013, 07:34 AM   #5
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Hi Ian,

Looks like the problem and the product is different.
That guy has Realistic integrated amp, mine is MAS pwr amp.
His problem is static sound and mine is weak sound (no hissing or hum).
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Old 19th September 2013, 10:07 AM   #6
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I found no web refererence to MAS other than as "Music and Sound" but apologies for wasting your time. Perhaps it's generic brand. The thread starter did refer also to weak sound in one channel.

You will still have to methodically examine and test the amplifier to trace a gain loss, assuming that it is just a stereo pair of power amplifiers. Start by removing all connections but the power cord. Power up, relays turn on the speakers now so measure the DC offset; the small residual DC measured across each pair of output terminals. If that's below say, 50mV, then fine - go check inside on the PCBs.

Cast an eye over suspicious replacements. Guys bust stuff, do cheap repairs with odd parts and sell the real problem - as you probably realise, so you will get stung some time if you buy without knowing the true condition. Look for messy residue on the board, heatsink grease everywhere etc.

Power up again and when testing amps, don't feed the amplifier signals when speakers are attached - don't fit them when testing anything or youll kiss it all goodbye. You'll need a steady signal tone source and audio millivoltmeter or DMM of suitable AC bandwidth to measure and compare the output test tone accurately, though an Oscilloscope is the preferred tool.

Using ground as a reference to attach the black probe (use a clip-lead or such) and compare as many voltages at the same points on each channel on the PCB as you can and note those points with differences >5%, say on the PCB, particularly around the output transistors.
Take great care not to slip when taking measurements which is easy to do if you are juggling pencil, paper, schematic, probes, meter face etc. (yes you'll need to locate and download a schematic if you want to know what you are testing and why, or even communicate about the amp.) Link to or post it so others know what you are talking about too. Perhaps someone can help out there

Lets see how you go with this so far.
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Old 20th September 2013, 12:43 AM   #7
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Hi Ian,

No worries mate.
Thanks for your advice.
I'll try your sugesstion step by step.
Cheers.
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Old 20th September 2013, 03:27 AM   #8
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OK. good.
Just clarifying, as I seem to have mixed 2 different tests together in #6.
What I describe there first, is a comparison of DC voltages at like points in each channel. This is not the same as comparing the relative sensitivity (output level for a given input) which directly concerns you. That's an AC (audio) test but we'd like to actually measure the difference and compare with the spec. of the amp, if available, as a sanity check.
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Old 20th September 2013, 03:32 AM   #9
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Good advices from Ian Finch.

Also, are you sure the problem is not the pre? you need to rule out things. To check this out reverse the interconnecting signal wires between the pre and the power units. If the problem shifts sides then the trouble is in the pre.
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Old 20th September 2013, 03:03 PM   #10
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Quote:
Originally Posted by sextaafondo View Post
Good advices from Ian Finch.

Also, are you sure the problem is not the pre? you need to rule out things. To check this out reverse the interconnecting signal wires between the pre and the power units. If the problem shifts sides then the trouble is in the pre.
Hi Pablo,

I did check with connecting pwr amp straight from cd player without pre (I adjust the pwr gain through pot and my cdp had volume gain) and got similar result.
I'll try Ian diagnostic and if not succeed, I'll bring my pwr amp to repair.
Anyway thank for your response.
-Frans
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