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Old 6th June 2012, 08:24 AM   #1
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Default Power Supply ceramic caps burnt

Hello, I have a solid state 100 watt bass guitar amp that I just bought and I opened it up to change the input jack and noticed two burnt open ceramic capacitors next to the secondary transformer fuses. I searched around the internet and found someone else with this same problem on the same amp. The caps are 104M. I couldn't find any schematics on this amp so I traced it out and drew one up for the power section. I was wondering why these burnt up and if I could or should use beefier mylar caps rather than ceramic. I haven't had a chance to see what the transformer voltage is yet because the input jack was broken when I bought the amp. The amp powered up ok and I could hear that the amp still works by the treble control hiss and other controls when turned up all the way. I read on the Toshiba data sheet for the two power transistors that they will become erratic when pushed to their limits so a fail safe circuit should be engineered in to prevent this from happening. Maybe this is why the caps burnt up? Here's the schematic I drew up. The arrows point to the burnt caps. Thanks for any help.
Click the image to open in full size.
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Old 6th June 2012, 09:58 AM   #2
DF96 is online now DF96  England
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Some capacitor types can't cope too well with continuous AC voltage stress, even when well within their DC voltage rating. It could be that the manufacturer saved a bit of money and used the wrong component. The value of the capacitor is probably not too critical, but for reliability choose one designed for AC use. If you can't find this, use one with a DC rating 3 or 4 times bigger than the peak AC voltage.
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Old 6th June 2012, 11:47 AM   #3
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You might put a 300V or 500V MOS surge supressor on the input of the power transfomer. I like the 14 mm ones, they have longer life. PCAT power supplies usually have the 7 mm versions on the input, they are blue. I salvage 14 mm supressors from dead motor drives but distributors sell them. Instruments in parallel with a refrigerator or air conditioner get power line transients of 4-10x the nominal power line voltage when the motor turns off at the peak of the voltage waveform. A film cap will not surpress sharp edged transients from either motors or lightning surge due to the inductance- better stay with ceramic. Unless you buy an expensive stacked (non-inductive) film cap. Ceramic caps can be quite durable, but some manufacturers don't do enough incoming inspection tests on the bargain parts the purchasing agent finds. 104M is a 0.1 uf high voltage cap, I think above 600 but I don't have the chart. M is a voltage code. You don't show the rail voltage, but 600 v spikes are quite common on the input AC line. 1000 V ceramic caps don't cost much more, certainly not compared to the freight of the box from the distributor.
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Last edited by indianajo; 6th June 2012 at 11:56 AM.
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Old 6th June 2012, 06:07 PM   #4
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Indeed, a surge is about the most likely cause for the damage. It's a 100 watter, so I guess rails would be +/- 55..60 VDC or thereabouts? That would mean a 350 V part minimum, 630 V to be on the safe side.
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Old 6th June 2012, 10:03 PM   #5
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what's the function or reason these 2 caps are there. Is it for surge or high frequency filtering?- The amp has a tweeter.in it.
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Old 7th June 2012, 01:52 AM   #6
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Quote:
Originally Posted by sgrossklass View Post
Indeed, a surge is about the most likely cause for the damage. It's a 100 watter, so I guess rails would be +/- 55..60 VDC or thereabouts? That would mean a 350 V part minimum, 630 V to be on the safe side.
would that for be a mos like indianajo suggested?
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Old 7th June 2012, 01:59 AM   #7
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Quote:
Originally Posted by indianajo View Post
You might put a 300V or 500V MOS surge supressor on the input of the power transfomer. I like the 14 mm ones, they have longer life. PCAT power supplies usually have the 7 mm versions on the input, they are blue. I salvage 14 mm supressors from dead motor drives but distributors sell them. Instruments in parallel with a refrigerator or air conditioner get power line transients of 4-10x the nominal power line voltage when the motor turns off at the peak of the voltage waveform. A film cap will not surpress sharp edged transients from either motors or lightning surge due to the inductance- better stay with ceramic. Unless you buy an expensive stacked (non-inductive) film cap. Ceramic caps can be quite durable, but some manufacturers don't do enough incoming inspection tests on the bargain parts the purchasing agent finds. 104M is a 0.1 uf high voltage cap, I think above 600 but I don't have the chart. M is a voltage code. You don't show the rail voltage, but 600 v spikes are quite common on the input AC line. 1000 V ceramic caps don't cost much more, certainly not compared to the freight of the box from the distributor.
Thanks indianajo, now that you mention it, I think there's one on the power switch. I forgot to put that in the drawing. It's the blue round part on the power switch. Should that've stopped the burnt caps. Click the image to open in full size.
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Old 7th June 2012, 02:08 AM   #8
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The caps are just for supply noise reduction. They shouldn't really effect the overall operation of the amp.
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Old 7th June 2012, 02:17 AM   #9
yajnaS is offline yajnaS  India
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Quote:
Originally Posted by Clemford View Post
I think there's one on the power switch. It's the blue round part on the power switch.
A MOV across the poles of a switch makes no sense ?
Unless its something new/subtle that I'm unaware of.
It should be across Live and Neutral.

In that location, a capacitor makes more sense.
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Old 7th June 2012, 02:32 AM   #10
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Actually suppressing the switch contacts does make some sense, but I would still put across Live and neutral after the switch. It's probably failed open anyway so fit a new MOV of adequate rating.
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