Please see attached part of the schematic. This is a delayed turn on and protection circuit for an SS power amp. There are two electrolytic capacitors, 220uF and 470uF. Before repair, the 220uF cap was completely dead, showed 0uF and many parts such as surface mount resistors were all burned out around it. I replaced the bad parts, the amp now turns on and off properly and performs as expected.
But these two electrolytic capacitors get very hot. Too hot to the touch. Beyond 60 degrees Celsius. I’m assuming this heat is what caused the original cap (cheap 85 degree Lelon) to fail and take out the circuit. The new caps I put in are 105 degree Panasonic FR.
Any idea of why these caps are running so hot by looking at the section of this schematic? Any help or advice is appreciated.
But these two electrolytic capacitors get very hot. Too hot to the touch. Beyond 60 degrees Celsius. I’m assuming this heat is what caused the original cap (cheap 85 degree Lelon) to fail and take out the circuit. The new caps I put in are 105 degree Panasonic FR.
Any idea of why these caps are running so hot by looking at the section of this schematic? Any help or advice is appreciated.
Attachments
Your circuit as shown has the VCC supply at +70VDC and the caps in question are only rated at 16VDC as per the schematic.
What is the actual voltage on the 2 caps - what ever it is, add 20% safety factor to get the properly rated DC voltage for the caps.
For example if it was 50VDC, then use 63V rated electro caps - definitely not less than the actual voltage as measured with a DMM.
No wonder they are running hot and have failed to an OC.
What is the actual voltage on the 2 caps - what ever it is, add 20% safety factor to get the properly rated DC voltage for the caps.
For example if it was 50VDC, then use 63V rated electro caps - definitely not less than the actual voltage as measured with a DMM.
No wonder they are running hot and have failed to an OC.
Ripple current or too high a DCV. Try and confirm the DC supply voltage on a scope has an ok DCV and ripple is symmetric (120Hz, not 60Hz).
There’s 8VDC on these caps. Originals were rated at 16VDC, the new FRs that I installed are 35VDC.
If you don't have a scope, then can your meter measure ACV across the cap?
What is the power supply circuit feeding the cap? Edit: what is the circuitry supplying the approx 8V DC bias to the opamp input?
Have you confirmed and double checked the cap polarity, as some caps have ambiguous markings?
What is the power supply circuit feeding the cap? Edit: what is the circuitry supplying the approx 8V DC bias to the opamp input?
Have you confirmed and double checked the cap polarity, as some caps have ambiguous markings?
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Caps have 4.5mV AC on them. Polarity is golden. Main power supply is dirt simple toroid, NXP Schottky diode discrete bridge, and 3300uF cap. There are also two LM317T regulators which feed the chips.
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Hi,
When you say
When you say
means the band in the capacitor. Normally the band in the capacitor means it is the ground.Can you explain what do you mean "the golden is the polarity".
What is the capacitor brand and type number - hopefully not some garbage Chinese types or indeed fakes.
The schematic shows two power rails. The +70V rail is not connected across the cap - it is just shown as connected to separate test points/terminals, so a few people are mis-reading the schematic.
Would be useful to see more of the schematic to see what is going on.
Are the connectors TP24 to TP27 and TP28 to TP31 connected somewhere else in the amp?
Must be something driving power into the capacitor if the capacitor itself is heating up and not being heated by parts close to it. It is just connected to an op-amp input according to the small part of the schematic you have shown (i.e. serves no purpose?).
Unless there is another fault like the divider (R15,R16,R17 etc) on the 70V input to the opamp has failed, the opamp won't survive 70V on the inputs, who knows what failure that would cause.
Are the connectors TP24 to TP27 and TP28 to TP31 connected somewhere else in the amp?
Must be something driving power into the capacitor if the capacitor itself is heating up and not being heated by parts close to it. It is just connected to an op-amp input according to the small part of the schematic you have shown (i.e. serves no purpose?).
Unless there is another fault like the divider (R15,R16,R17 etc) on the 70V input to the opamp has failed, the opamp won't survive 70V on the inputs, who knows what failure that would cause.
Are you sure that polarity marked on the silk screen on the PCB is correct?
Perhaps it's wrong, and the original caps were installed all wrong, and that's why they failed.
Check the actual traces on the pcb.
Perhaps it's wrong, and the original caps were installed all wrong, and that's why they failed.
Check the actual traces on the pcb.