Checking capacitors

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Max N
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Checking capacitors

Post by Max N » Fri Feb 14, 2020 3:07 pm

Looking for a bit of advice please....
I'm working on an old Technics amp, and trying to decide if I should replace the main PSU caps
They are nominally 8200uF at 50V. They both measure about 7200UF. I would just replace them to be sure, but they appear to be 'glued' to the PCB, so I think it will be a bit of a job, so don't want to do it unless they really need to be replaced.
This is a picture of the 'glue'
IMG_0047.jpg
Unless that brown stuff around the base is evidence of a blown cap?

They don't hold voltage for very long, less than a minute, but obviously I am testing them in situ, so maybe there is a leakage path.....

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IslandPink
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Re: Checking capacitors

Post by IslandPink » Fri Feb 14, 2020 3:49 pm

I suppose the first question is how old are they ?
If they're 15 years or more, then you should get new ones in there - the dropping value may be a sign their electrolyte is drying out.
Ps. what do you use to measure such high C values ? My L/C meter tops out around 200uF.
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ed
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Re: Checking capacitors

Post by ed » Fri Feb 14, 2020 3:51 pm

cheesewire or top E guitar string should defeat the glue.
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Max N
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Re: Checking capacitors

Post by Max N » Fri Feb 14, 2020 8:08 pm

IslandPink wrote:
Fri Feb 14, 2020 3:49 pm
I suppose the first question is how old are they ?
If they're 15 years or more, then you should get new ones in there - the dropping value may be a sign their electrolyte is drying out.
Ps. what do you use to measure such high C values ? My L/C meter tops out around 200uF.
I use this - possibly recommended by someone on here many moons ago?
IMG_0304.jpg
IMG_0307.jpg
The amp is about 31 years old, but has not been used much for the last 20 or so....I remember it sounding really good though

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Dave the bass
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Re: Checking capacitors

Post by Dave the bass » Sat Feb 15, 2020 11:15 am

IME.... Measuring caps in situ can sometimes give false results I've found Max, if you can unsolder one leg to isolate each cap you'll be sure to be measuring only that one cap and not anything else in the cct.

Once you're sure that it does need changing you can tackle the glue problem. A lot of 80s and 90's stuff I worked on had big electrolytics bonded to the PCB with that Gorrila snot as we dubbed it at the time.

We got good results by gently heating that area with either a hairdryer or a hot air gun on a low setting then slicing through the warmed glue with a scalpel that was also heated by the heat source.

Try and remove as much of the original solder 1st then apply the hair dryer and scalpel technique, apply soldering iron heat to the 2 or 4 pins that hold the walloping big caps in the pcb and they should come out cleanly.

I'd remove the old glue snot before fitting a new cap.

That's how I used to do it, hope that helps.
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rowuk
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Re: Checking capacitors

Post by rowuk » Sat Feb 15, 2020 2:43 pm

especially with amplifiers - we listen to the power supply. After 31 years, I would do a recap on the electrolytics. It is not crazy expensive and you will be surprised!
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Max N
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Re: Checking capacitors

Post by Max N » Sat Feb 15, 2020 3:19 pm

Dave the bass wrote:
Sat Feb 15, 2020 11:15 am
IME.... Measuring caps in situ can sometimes give false results I've found Max, if you can unsolder one leg to isolate each cap you'll be sure to be measuring only that one cap and not anything else in the cct.

Once you're sure that it does need changing you can tackle the glue problem. A lot of 80s and 90's stuff I worked on had big electrolytics bonded to the PCB with that Gorrila snot as we dubbed it at the time.

We got good results by gently heating that area with either a hairdryer or a hot air gun on a low setting then slicing through the warmed glue with a scalpel that was also heated by the heat source.

Try and remove as much of the original solder 1st then apply the hair dryer and scalpel technique, apply soldering iron heat to the 2 or 4 pins that hold the walloping big caps in the pcb and they should come out cleanly.

I'd remove the old glue snot before fitting a new cap.

That's how I used to do it, hope that helps.
Thanks Dave, that’s really useful advice from a pro. I think I need to order some new solder braid, then I’ll give it a go. Cheers

Max N
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Re: Checking capacitors

Post by Max N » Sat Feb 15, 2020 3:20 pm

ed wrote:
Fri Feb 14, 2020 3:51 pm
cheesewire or top E guitar string should defeat the glue.
Thanks Ed. I think I would need to remove a lot of other components off the board first to get access....to get the wire all the way through...

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Re: Checking capacitors

Post by Max N » Sat Feb 15, 2020 3:28 pm

rowuk wrote:
Sat Feb 15, 2020 2:43 pm
especially with amplifiers - we listen to the power supply. After 31 years, I would do a recap on the electrolytics. It is not crazy expensive and you will be surprised!
I am leaning that way.....while I have the amp in pieces

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Mike H
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Re: Checking capacitors

Post by Mike H » Sun Feb 16, 2020 4:03 pm

Dave the bass wrote:
Sat Feb 15, 2020 11:15 am
IME.... Measuring caps in situ can sometimes give false results I've found Max, if you can unsolder one leg to isolate each cap you'll be sure to be measuring only that one cap and not anything else in the cct.
I concur impossible to get sensible results measuring a component in situ (including resistors) without one end being isolated. Could be the perceived low value is caused by parallel leakage in this case.
 
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Paul Barker
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Re: Checking capacitors

Post by Paul Barker » Mon Feb 17, 2020 4:20 pm

I always lift a pin to test caps. Gluing caps is standard practice. Electrolytics that have failed push out the end opposite where it sits in circuit board. Should be level. A blown cap will have a resistance measurement to ground whereas a good condition will be off the scale infinity ohms. If u had a Megger which operates at 50v you could check for shorts accurately but the earth fault impedance testers for general use are required to test at twice mains voltage. Not any use for you.
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