Checking capacitors
#1 Checking capacitors
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' 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.....
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' 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.....
- IslandPink
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#2 Re: Checking capacitors
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.
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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#3 Re: Checking capacitors
cheesewire or top E guitar string should defeat the glue.
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#4 Re: Checking capacitors
I use this - possibly recommended by someone on here many moons ago? 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 thoughIslandPink 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.
- Dave the bass
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#5 Re: Checking capacitors
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.
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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#6 Re: Checking capacitors
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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#7 Re: Checking capacitors
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. CheersDave 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.
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#10 Re: Checking capacitors
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.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.
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#11 Re: Checking capacitors
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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