Hi everybody, long time no see
After almost 4 years of absence I'm trying to slowly do something electronic related and need some help with a very strange issue that is driving me up the wall.
Cambridge Audio A5
Original fault was a failed power transformer that was replaced with the wrong one, someone fitted a 25-0-25 instead of the 30-0-30 original one but it worked.
I bought it before my health problem and was in the wardrobe until last week.
Current issue:
Sound volume to low, need to turn up to 12 o'clock just to have a moderate sound level, I checked everything, every single resistor even put opamps from pre and buffer on bases to make it easier to replace, replaced them (NE5532) to no avail and just don't know where to look any more, the amp works fine just the sound level is to low, I repaired so many of this amps (A4/A5/A300/A500) and never seen anything like this.
After almost 4 years of absence I'm trying to slowly do something electronic related and need some help with a very strange issue that is driving me up the wall.
Cambridge Audio A5
Original fault was a failed power transformer that was replaced with the wrong one, someone fitted a 25-0-25 instead of the 30-0-30 original one but it worked.
I bought it before my health problem and was in the wardrobe until last week.
Current issue:
Sound volume to low, need to turn up to 12 o'clock just to have a moderate sound level, I checked everything, every single resistor even put opamps from pre and buffer on bases to make it easier to replace, replaced them (NE5532) to no avail and just don't know where to look any more, the amp works fine just the sound level is to low, I repaired so many of this amps (A4/A5/A300/A500) and never seen anything like this.
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If you have checked everything including resistors have you checked the capacitors in the signal path?
A low voltage will produce clipping if turned right up which in turn can blow loudspeakers maybe its affected a capacitor ?
A low voltage will produce clipping if turned right up which in turn can blow loudspeakers maybe its affected a capacitor ?
Hi there
Yes checked everything with the scope, no oscillation, no clipping.
The strange thing is that it's common on both channels so it must be on the preamp and tone control board the LM337 gets really hot.
I replaced the transformer with the right voltage one and now all voltages on the supply and amp are correct (from service manual)
Going to try and figure out the the current that the LM317/337 are supplying to make them so hot
Yes checked everything with the scope, no oscillation, no clipping.
The strange thing is that it's common on both channels so it must be on the preamp and tone control board the LM337 gets really hot.
I replaced the transformer with the right voltage one and now all voltages on the supply and amp are correct (from service manual)
Going to try and figure out the the current that the LM317/337 are supplying to make them so hot
If I remember correctly those voltage regulators can output up to =1.5A
that's way above a normal pre-amp unless it has a power-amp type output .
What I was getting at was maybe a signal capacitor was slightly faulty.
If you input the standard RMS signal required for a given output on an aux input how much down is it with or without a load on it?
that's way above a normal pre-amp unless it has a power-amp type output .
What I was getting at was maybe a signal capacitor was slightly faulty.
If you input the standard RMS signal required for a given output on an aux input how much down is it with or without a load on it?
You got me there...
I was injecting 1Vpp In the aux and was getting exactly the same at the output of the preamp/tone stage (wire that goes to power amp) on power stage it just went OL (was on wrong divider).
What I just found out is that the signal is fine with square/sinus signal but when I use my sound source it's just low (source is fine with other amp).
I was injecting 1Vpp In the aux and was getting exactly the same at the output of the preamp/tone stage (wire that goes to power amp) on power stage it just went OL (was on wrong divider).
What I just found out is that the signal is fine with square/sinus signal but when I use my sound source it's just low (source is fine with other amp).
Can someone suggest a quick or simple test I can do to trace this fault ?
I tested everything on the amp board, preamp/tone control board and can't find nothing wrong with it but the sound level is still to low.
My brain tell me it's the preamp/tone board because the fault is common to both channels but right now I'm starting to doubt my capabilities...
I tested everything on the amp board, preamp/tone control board and can't find nothing wrong with it but the sound level is still to low.
My brain tell me it's the preamp/tone board because the fault is common to both channels but right now I'm starting to doubt my capabilities...
Your going to have to split up the testing ,in other words test the preamp and power amp separately .
It might seem non conformist but I would start with the power amp .
Input a 1Khz sine wave at the specified voltage to obtain full output ,don't bother with a dummy load unless it tests okay then test again .
Only then input the same frequency test signal at the preamp input but at the sensitivity of that input for full output and check at the preamp output .
That then narrows down where the fault lies .
If its the power amp then test using an oscilloscope probe at the input drivers to the output BJT,s watch for reduced signal as you do it,if okay then put probe on the output.
If its the preamp remember to use the tone defeat before you start testing.
If you do not own a scope then use an RMS reading -volt/millivolt meter make sure its got a good bandwidth and sensitivity .
It might seem non conformist but I would start with the power amp .
Input a 1Khz sine wave at the specified voltage to obtain full output ,don't bother with a dummy load unless it tests okay then test again .
Only then input the same frequency test signal at the preamp input but at the sensitivity of that input for full output and check at the preamp output .
That then narrows down where the fault lies .
If its the power amp then test using an oscilloscope probe at the input drivers to the output BJT,s watch for reduced signal as you do it,if okay then put probe on the output.
If its the preamp remember to use the tone defeat before you start testing.
If you do not own a scope then use an RMS reading -volt/millivolt meter make sure its got a good bandwidth and sensitivity .
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But that is what I'm doing, have done and always did I always do the tests in separate stages (where possible).
There is nothing wrong with the Power amp board, I checked every single component on it and no faults found same on preamp no faults (there are only 4 resistor 2 capacitor and 1 opamp) tone control same thing no faults.
The only think that is odd it the power supply for the preamp/tone control, the LM317/337 get really hot because they are dropping 15V without an heatsink so no wonder they cook.
Going to put this one back in the shelf until I get another one to compare to.
There is nothing wrong with the Power amp board, I checked every single component on it and no faults found same on preamp no faults (there are only 4 resistor 2 capacitor and 1 opamp) tone control same thing no faults.
The only think that is odd it the power supply for the preamp/tone control, the LM317/337 get really hot because they are dropping 15V without an heatsink so no wonder they cook.
Going to put this one back in the shelf until I get another one to compare to.
Why don't you disconnect both voltage regulators and test for a short circuit from their output rails ?
So what voltage gain do you see with each stage? How does that compare with the specs?
To me having the volume at 12 o'clock for a moderate sound level seems quite normal with a log-taper volume pot - I saying you need measurement evidence to be sure its not up to scratch.
To me having the volume at 12 o'clock for a moderate sound level seems quite normal with a log-taper volume pot - I saying you need measurement evidence to be sure its not up to scratch.
I have another power amplifier board that I was cannibalizing for parts that I'm going to rebuild, just ordered the parts and once they arrive i going to try it in this amp to see if it makes a difference.
For now going to give it a rest have other stuff that need my attention.
Thanks guys
For now going to give it a rest have other stuff that need my attention.
Thanks guys
I don't like to give up on stuff I'm working or building but sometimes the the lack of parts or the time to source then takes the better of me...
In this case I was right the fault was in the preamp tone control board but not on the opamps or any other component passive or active it was the dam "Direct" or tone defeat switch swapped it with the "Tape/MD Monitor" one and problem solved amp working as it should now.
In this case I was right the fault was in the preamp tone control board but not on the opamps or any other component passive or active it was the dam "Direct" or tone defeat switch swapped it with the "Tape/MD Monitor" one and problem solved amp working as it should now.