I am restoring an ancient Sherwood solid state integrated amp and am ordering new power supply and output capacitors. I see this one odd cap that has one leg soldered to the Left (only) channel output fuse and the other to chassis.
What type of cap is it and what is it used for? Thanks in advance
What type of cap is it and what is it used for? Thanks in advance
That could be a "death cap". If it shorts the entire chassis becomes hot with mains voltage. Remove it before work or returning to service. It is not even rated at the 1300 V that motor turn off surges can reach.
That is such an old practice I'm surprised this unit isn't vacuum tubes.
To reduce big pops from lightning down the AC line or electric motor shutoff, you may install UL/VDE/CE rated MOS supressors between hot (after fuse) and neutral and between hot (after the fuse) and safety ground. Not to the chassis. The voltage shown on these is the rated AC voltage they are supposed to protect; not the peak voltage that occurs on such a line. MOS supressors are also rated in joules of energy they will absorb before failure. The 7 mm ones are toys in throwaway consumer equipment like PCAT switcher supplies. The 20 mm diameter ones are used in industrial motor drives that see motors shut off dozens of times a day. Domestic ones are blue with an "S" with a slash through it, for Schurter, the original vendor. The import ones I've found in ATX supplies are green. These prevent surge voltage from over stressing electrolytic caps after the rectifiers.
To reduce pops from AC shutoff, Y rated capacitors are sometimes installed after the power switch between hot & neutral of a transformer input or after the rectifier of a switcher supply mains cap.
That is such an old practice I'm surprised this unit isn't vacuum tubes.
To reduce big pops from lightning down the AC line or electric motor shutoff, you may install UL/VDE/CE rated MOS supressors between hot (after fuse) and neutral and between hot (after the fuse) and safety ground. Not to the chassis. The voltage shown on these is the rated AC voltage they are supposed to protect; not the peak voltage that occurs on such a line. MOS supressors are also rated in joules of energy they will absorb before failure. The 7 mm ones are toys in throwaway consumer equipment like PCAT switcher supplies. The 20 mm diameter ones are used in industrial motor drives that see motors shut off dozens of times a day. Domestic ones are blue with an "S" with a slash through it, for Schurter, the original vendor. The import ones I've found in ATX supplies are green. These prevent surge voltage from over stressing electrolytic caps after the rectifiers.
To reduce pops from AC shutoff, Y rated capacitors are sometimes installed after the power switch between hot & neutral of a transformer input or after the rectifier of a switcher supply mains cap.
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Okay, sorry. A proper zobel is something like a .22 uf series 1000 ohms from speaker hot to speaker return terminals, usually. To chassis is a bit cheap. Zobel helps prevent radio interference to the feedback from the speaker wires. Also helps prevent oscillations caused by certain loads. My 1966 Dynakit amp was shipped without a zobel, then one was added in a service bulletin about 1970.
BTW experience has proved an output transistor will always blow to protect a speaker fuse. Speaker fuse is entirely obsolete.
BTW experience has proved an output transistor will always blow to protect a speaker fuse. Speaker fuse is entirely obsolete.
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It's a Mylar film capacitor, on the power supply positive rail. Seems like it was an afterthought.
It just looks like a bog standard supply RF bypass cap to my ignorant eyes. I would suggest it was found to be necessary on one rail, but not the other, due to the actual layout of the amplifier, and as the other rail didn't need it there is none. It is standard practise in mass produced items to minimise the parts count. The reason for the 250V rating is likely because that's what the company had in its parts bin in a component with suitable RF performance.
Hopefully someone can put me straight!
Edit: I agree with rayma.
Hopefully someone can put me straight!
Edit: I agree with rayma.
It wouldn't be there like it is, if there hadn't been some kind of problem, like AC line noise
coupling into the circuit, etc. That's definitely a hack done under duress.
And look at the solder by-products scattered around, Sherwood was better than that.
Maybe a previous owner did it, I hope so.
coupling into the circuit, etc. That's definitely a hack done under duress.
And look at the solder by-products scattered around, Sherwood was better than that.
Maybe a previous owner did it, I hope so.
It wouldn't be shown in the factory schematic if it was done by a subsequent owner. The power supply capacitors are obvious replacements becasue they don't fit the retainer clips on the chassis; the originals were skinnier and longer, and probably not as large a value I'd guess.
I have two of these amps, the one below is all original which I am using as a reference, the one in the photo in my first post was worked on and will be restored, not sure if that green cap in my first post is original
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Yes, those solder bits were from the previous owner who replaced the two output caps with some used caps pulled from a more modern amp, the glue is still stuck to the base of them, he may have replaced the cap in question as well
Yes, the originals were either 1500 uF or 2700 uF (the parts list shows both). The replacement caps are 5600 uF and still have the adhesive from the amp they were pulled from, they test at 6800 uF
Thanks for all of the replies. I'm sure I'll have more questions once I get the new caps in and start testing.
Neither a death cap nor a Zobel nor a hack (it´s shown in the Factory schematic) but a plain rail decoupling cap.
Why all the fuss?
Why all the fuss?
Thanks for the reply. I'm new to electronics so I just want to proceed with caution