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While on a witch hunt from some road tech's note on a Marshall JCM 900-5881 100W head, stating 'bad cap' on the cabinet, and not finding anything wrong yet, I began monitoring the 1st filter stage on the HT supply, that cap being both sections of a 50uF/50uF/500V clamp-mounted electrolytic. With scope + X100 probe on the buss cap to watch the charging current, and a Fluke 8060A DMM monitoring both DCV and ACV on it, the rippple looked nominal. About 7V P-P @ idle, increasing to around 10V P-P at the moderate signal I was running...using burst pink noise. 2 sec burst, 3 sec off-time.
Not knowing if this 'bad cap' statement only occurs during a set on stage, I let the amp run for a good hour. Then, I took to powering back down to Standby, and back to operate numerous times, noting the discharge time being quite slow. While in S/B mode, I discharged the 3 supply caps, then watched the 1st stage cap charge back up to around 120VDC. Discharged it again, same thing...slow charging to that 120V level. Turned the amp off, and then it dropped down to what one would typically see from the dielctric absorption...a few volts residual. Discharged it again, and saw it came back up to that normal DA residual. Powered back on, still in S/B mode, and again, saw the cap charge back up to around 120VDC.
Looked at the schematic, finding the S/B switch opens one side of the HT winding to the diode bridge, so there's no complete circuit to charge those caps up in Standby...unless there's leakage current thru the S/B switch.
Why would there be residual charging of the buss caps in Standby.?
jcm900_dualrev_100w_4100.-1.pdf
Not knowing if this 'bad cap' statement only occurs during a set on stage, I let the amp run for a good hour. Then, I took to powering back down to Standby, and back to operate numerous times, noting the discharge time being quite slow. While in S/B mode, I discharged the 3 supply caps, then watched the 1st stage cap charge back up to around 120VDC. Discharged it again, same thing...slow charging to that 120V level. Turned the amp off, and then it dropped down to what one would typically see from the dielctric absorption...a few volts residual. Discharged it again, and saw it came back up to that normal DA residual. Powered back on, still in S/B mode, and again, saw the cap charge back up to around 120VDC.
Looked at the schematic, finding the S/B switch opens one side of the HT winding to the diode bridge, so there's no complete circuit to charge those caps up in Standby...unless there's leakage current thru the S/B switch.
Why would there be residual charging of the buss caps in Standby.?
jcm900_dualrev_100w_4100.-1.pdf
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