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.
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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.
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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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.
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.
The transformer has 2 wires on the primary. Should I put the varistor directly across the two and since it's 110 volts in what value should I use.
So it's best then to get a 1 kv ceramic disc cap rather than the gumball looking mylar radial film one's found on my guitar tone pots to replace the 104m's? Also, I thought the M was a tolerance code for 20%.
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There is a military code that uses a trailing letter for voltage on caps. No telling what your cap manufacturer meant, but obviously the voltage performance was inadequate whatever the rating was. Buy 1000 V ceramics they are cheap compared to the freight bill.
For the MOS supressor, I find 300 V ones in PCAT power supplies designed for 120/240 VAC, and 500 V ones in motor drives for 460 VAC 3 phase. Most transformer wiring is suitable for 600 VDC, so I reuse 500 V supressors in my 120 V hifi equipment, but If I was buying new I would buy 300 V ones. Yeah, the varistor goes after the power switch and across the input of the power transformer (and anything else). I mount a Cinch terminal strip (solder type) up near the power switch to make a safe place for it.
Film caps are fine for tone controls for the 20-15000 hz human ear, but power line spikes can contain frequencies over 1 mhz (fourrier analysis) so the inductance of a wound cap is not suitable for suppressing those. 1000 V disc caps for across the transformer output, or X2 rated caps on the input if you want to get modern and UL/CSA/VD compliant . Disassemble a dead PCAT power supply and look how they do it these days. Not blowing up due to refrigerator motor turnoff spikes is useful even in a $29.95 commodity product. Your MOS suppressor should come with a UL/CSA/VDE logo, too.
As michael bean suggested, most high end equipment also has a .01 uf 1000 V ceramic cap across the power switch contact to eliminate turnoff pop. I installed those years ago in my dynakit equipment, following TV practice I saw on the Howard Sams prints, and when lightning strikes with power off it usually takes out the 1000 V turn off pop capacitor and also arcs over the slide power switch without damaging anything else. Just a nice extra you could order now at the same time as the other things.
For the MOS supressor, I find 300 V ones in PCAT power supplies designed for 120/240 VAC, and 500 V ones in motor drives for 460 VAC 3 phase. Most transformer wiring is suitable for 600 VDC, so I reuse 500 V supressors in my 120 V hifi equipment, but If I was buying new I would buy 300 V ones. Yeah, the varistor goes after the power switch and across the input of the power transformer (and anything else). I mount a Cinch terminal strip (solder type) up near the power switch to make a safe place for it.
Film caps are fine for tone controls for the 20-15000 hz human ear, but power line spikes can contain frequencies over 1 mhz (fourrier analysis) so the inductance of a wound cap is not suitable for suppressing those. 1000 V disc caps for across the transformer output, or X2 rated caps on the input if you want to get modern and UL/CSA/VD compliant . Disassemble a dead PCAT power supply and look how they do it these days. Not blowing up due to refrigerator motor turnoff spikes is useful even in a $29.95 commodity product. Your MOS suppressor should come with a UL/CSA/VDE logo, too.
As michael bean suggested, most high end equipment also has a .01 uf 1000 V ceramic cap across the power switch contact to eliminate turnoff pop. I installed those years ago in my dynakit equipment, following TV practice I saw on the Howard Sams prints, and when lightning strikes with power off it usually takes out the 1000 V turn off pop capacitor and also arcs over the slide power switch without damaging anything else. Just a nice extra you could order now at the same time as the other things.
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michael was right. I looked at it and itis in fact a cap. It says 103 on it. I just assumed it was a varistor because it looked like one at first glance. I'll redraw the drawing with the switch and cap and where I think the varistor should be incase I'm mistaken where it should go. But these could actually plug into a wall socket then without damage? Thats basically what I'm doing then by putting it across the line in of the primary? I was thinking I could solder it across the terminals where plug comes in. It uses a computer type 3 pin IEC 320 C14 panel socket with a built in fuse. I could solder it on the backside of it. I'll also order a couple of ceramic disc 1kv caps at the same time as the varistor. Thanks btw for all the info in your reply . As always, this is the place to go for these technical questions. I've received great help as well in the tube/valve section.
It is a bit more traditional to put your surge suppressor MOS devices after the switch and after the fuse or circuit breaker, then before the transformer or rectifier. Yes they have 120 VAC + spikes on them, that is what they are for. That is why UL/CSA/VDE rate them for the proper uses. I suggest drilling a hole for a cinch type solder terminal strip, to mount the mos suppressor on. That is how I usually do it. Tubesandmore.com, triodeelectronics.com, electronicsurplus.com, apexelec.com sell them, not mouser or newark. MOS supressor Varistors are blue and have an elongated S with 2 slashes through it on them. MOS suprossors will have a 3 digit number, which is the Voltage they are inactive to. Above that voltage they short out the spike, up to a certain rated energy.
Caps don't always have uf or pf on them, but usually now have a 3 digit number unless very precise 4 digit units. Caps often have a trailing letter for the voltage class, unless the voltage is shown explicitly in numbers with a V. To see all this stuff at look, cutting up a PCAT power supply is very instructive, it has all this stuff, which I recommend copying but stopping before the rectifiers into the DC storage caps for the switcher power supply. I even salvaged the toroid noise suppressor from a PCAT supply to put inseries with the AC hot and neutral line to keep RF from CB'ers out of my disco mixer I use instead of a preamp. The E-core transformer does that job on an old amp that still has one of those.
Caps don't always have uf or pf on them, but usually now have a 3 digit number unless very precise 4 digit units. Caps often have a trailing letter for the voltage class, unless the voltage is shown explicitly in numbers with a V. To see all this stuff at look, cutting up a PCAT power supply is very instructive, it has all this stuff, which I recommend copying but stopping before the rectifiers into the DC storage caps for the switcher power supply. I even salvaged the toroid noise suppressor from a PCAT supply to put inseries with the AC hot and neutral line to keep RF from CB'ers out of my disco mixer I use instead of a preamp. The E-core transformer does that job on an old amp that still has one of those.
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I'm not sure what a PCAT supply is. so I googled it but couldn't find anything.
I also tried to find a .1uf 1kv cap. You would think they would be a dime a dozen, as well as the 600v MOS. I usually buy direct from China on Ebay so I'll have to check out your links. One thing I will be doing, at least while at my house, is running it through a Furman PL-8C.
It seems like all the ceramic disc's are 50V. Very few are 100v. I think I'll try a couple of 50v that I have and try the MOS if I can figure out how/where to put it. Either that or I'll buy a power strip with surge protector for when I use it away from my Furman. I don't really care to start splicing up the wiring of the amp and I'm still not exactly sure where to put it. And a surge strip won't cost much more than a varistor by the time I end up finding a 600v ( I think I found a 1000v one on ebay but I dont know if it's ok to use ) one. It sure seems like it would be the simple way out. And like you said earlier, the caps are just for noise. It is nice to know, thanks to you guys, that it was probably just a surge that took out the caps in the first place and they were just probably 50v ones anyway.
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