I have a Cambridge P40 (mark 2, 1980 type) integrated amp that has just gone bad. When you power it on the speakers' bass drivers jump forward and just sit there humming.
It happens with or without any source or source cables connected, and no music plays. Switching input selector switch has no effect on it, nor does adjusting the volume or any of the other controls.
The plug and socket, speakers and speaker cables have been switched out. Internally I can't see anything wrong.
I can't find a schematic for this amp but photo of internal attached. I have a multimeter which I am competent to use and recently came by an oscilloscope, which I am not.
Could anyone take a guess at what's up or suggest a basic troubleshooting sequence please?
It happens with or without any source or source cables connected, and no music plays. Switching input selector switch has no effect on it, nor does adjusting the volume or any of the other controls.
The plug and socket, speakers and speaker cables have been switched out. Internally I can't see anything wrong.
I can't find a schematic for this amp but photo of internal attached. I have a multimeter which I am competent to use and recently came by an oscilloscope, which I am not.
Could anyone take a guess at what's up or suggest a basic troubleshooting sequence please?
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Intermittent and common to both channels. Sounds like a supply issue, possibly a dry joint.
Without speakers connected, you can put a voltmeter on the output and tap around the circuit board with an insulated probe to see if you can make the fault come and go.
If this works, where the tapping is most sensitive will be near where the dry joint is. A lightbulb current limiter would be a good idea, or a variac and run it on say 25% voltage.
Without speakers connected, you can put a voltmeter on the output and tap around the circuit board with an insulated probe to see if you can make the fault come and go.
If this works, where the tapping is most sensitive will be near where the dry joint is. A lightbulb current limiter would be a good idea, or a variac and run it on say 25% voltage.
I came across a similar case on the web.
a Cambridge P40 ( the board looked the same ) had similar problem, DC on the output.
The owner re-solder all the joints on the board and switches, change the 4 rectifiers, two big capacitors 6800u...
and the P40 went back to normal....
here's the link
Cambridge P40 - UK Vintage Radio Repair and Restoration Discussion Forum
FYI
a Cambridge P40 ( the board looked the same ) had similar problem, DC on the output.
The owner re-solder all the joints on the board and switches, change the 4 rectifiers, two big capacitors 6800u...
and the P40 went back to normal....
here's the link
Cambridge P40 - UK Vintage Radio Repair and Restoration Discussion Forum
FYI
Those round red parts with ROE written on them are 220uF 6v capacitors, which are known to go bad with age. Replace them with some good quality 220uF 16V or better parts
edit: In fact, those are probably the DC blocking capacitors in the negative feedback path of the power amplifier... which if bad would cause exactly the symptoms described
edit: In fact, those are probably the DC blocking capacitors in the negative feedback path of the power amplifier... which if bad would cause exactly the symptoms described
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Thanks jaycee, as luck would have it I have a pair of 220uF 50v Nichicon capacitors, ordered in error a year or two back, which I'm hoping have just found their reason for still being in my house. Is the 50v rating going to be too much? I'm reading about ESR and higher rated caps giving lower than stated capacitance in a low voltage circuit.
It looks like the VC was replaced in the past. PCB tracks lift relatively easily without proper desoldering tools and practice.
The resolution of your photo isn't quite clear enough to be sure, but I suggest you desolder and re-solder all the connections for all the power supply components inside the two red circles in the attached photo.
The resolution of your photo isn't quite clear enough to be sure, but I suggest you desolder and re-solder all the connections for all the power supply components inside the two red circles in the attached photo.
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