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Old 24th February 2009, 09:51 AM   #1
roro1 is offline roro1  Canada
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Default Help identifying a capacitor

I'm recaping a sherwood s 7700 circa 1962.
I have recaped the powersupply caps and the pre and amp sections too.
I now have a loud hum when before i had a small hum and a midrange buzz.It's why i recaped it.
I have one cap im not sure of the value.I'ts at the end of the feedback loop.
It's orange.It looks like this 22k
47
I have measured it but can't get a good reading.
Any idea of the value.
My schmatic says 47
I tried .47 and got motorboating.
Any help greatly apreciated!
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Old 24th February 2009, 10:20 AM   #2
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Hi,

A pic of the orange cap would would help.

Midrange buzz is usually noise from CFL lights in the house or dimmer switches induced through the line or input jacks or rectifier switching noise. Unlikely the latter unless the rectifier was replaced with the wrong types.

Loud hum... I would double check your soldering joints on the filter caps or look for broken leads. If the caps are NOS, they may be dried out.

Loud hum could also be caused by the power supply being loaded down. Is anything heating up more than normal?

Cheers!
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Old 24th February 2009, 03:12 PM   #3
roro1 is offline roro1  Canada
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I reheated the joints.I'm sure i put the right caps in the right place.
I think the hum is because i havent replaced the electrolitics.
Most of them were from 1962.One small electrolytic had a crack half way down it.
The cap is just a disc type cap of that era.I will try to get a pic up later.
I read on another site it could be a 047.
I have tried a 47 it moterboated.I tried a .0022 and got hum with the high frequencys cut off.
Again the cap is orange on it it says 22k
47
My schematic shows it as 47.
I think the schematic might be wrong.
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Old 24th February 2009, 03:32 PM   #4
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Quote:
Originally posted by roro1

Again the cap is orange on it it says 22k
47
My schematic shows it as 47.
I think the schematic might be wrong.

I think it must be 47 pF. Is the capacitor in parallel with the feedback resistor?
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Old 24th February 2009, 07:12 PM   #5
roro1 is offline roro1  Canada
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Quote:
Originally posted by Tube_Dude



I think it must be 47 pF. Is the capacitor in parallel with the feedback resistor?
Yes its parrellel.To lead to further confusion the schematic says the resistor is 22k just like the writeing on the capacitor.
When i recaped the rest of the amp there was a .0047 cap.
It's size was much smaller.The 22k
47 cap i have is normal size like the .02 and.05 caps i replaced around it.
My cap tester read it as .19 n
It's a auto rangeing multimeter.
The problem is i'm haveing a hard time getting a reading.
It will read 2.19 n or other readings..19 n is the one i get the most.
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Old 24th February 2009, 07:12 PM   #6
roro1 is offline roro1  Canada
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Ok i found a link on the web.I think it's .047u.
The 22k is the multiplier.47 is the value.But bu going by what i read on that site it would be.047 u.
Anyone think i might be dead wrong?
I have a couple of .047's on the way.
It would make senses as the cap is the same size as the .05 near it.
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Old 24th February 2009, 07:12 PM   #7
roro1 is offline roro1  Canada
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Or is .o22 wth 47 as its operateing voltage.Like i said it's at the end of the feedback loop.Would a 47 voltage rateing be enough?
I don't think so.So i'm thinking it must be .047 and the schematic must be wrong.
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Old 24th February 2009, 10:03 PM   #8
roro1 is offline roro1  Canada
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It's .0022 my meter keeps comeing up with that number.
I have been all over the net checking capacitor codes.
So confusing.
2.2n turns into .0022u.
I also checked the resistors around it and none of them are no where near 22k.
So i figure it's a typo on the schematic.
Sherwood were famous for changeing things on the assembly line without telling anyone.So who knows.
The only thing i;m afraid of is that someone was in the amp and changed out a couple of caps.There were a few different newer caps in my unit.
My problem now is i already had a .002 in that position.Still had a hum.
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Old 24th February 2009, 10:13 PM   #9
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What is the value of the feedback resistor, in parallel with the capacitor?
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Old 25th February 2009, 02:35 AM   #10
roro1 is offline roro1  Canada
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Well i asked on another fourm.I got this answer which made alot of sense.It's a combo cap and resistor.That's why i could not get a reading.Their was a dashed box around it on the schematic.I never understood what it was for.
Its a 22k resistor and a 47 pf cap wired in parrellel.It looks just like a regular ceramic disk.
Aparently in old machines like mine they used combo's like that in the feedback loop.
So i have a 22k kwamie and im going to get a 47 pf cap wire them together.
Hopefully this will get rid of most of my hum.
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