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Changing filter caps on '78 Acoustic 220?

donalddemon

donalddemon

Feb 26, 2010
47
16
Asbury, NJ
www.reverbnation.com
#1
I have an old 220 amp that I love the sound of but it's developing some static like the caps are going bad. I don't think they have ever been replaced so it's long overdue.

Anyone done this before and are there any tips to ordering the right parts? I'm handy with the soldering iron but never have done a cap job, this will be a good chance to learn. I have done some work before inside amps so I know about the harmful voltages and such.

I found a schematic here: http://www.ampix.org/albums/userpics/10075/Acoustic_220_service_manual.pdf

It looks like there are several (8) electrolytic filter caps, not sure which ones to replace or where to source the parts. Any help would be greatly appreciated!
 
#2
donalddemon said:
but it's developing some static like the caps are going bad.
Click to expand...
What if it is a jack, or a pot that is in need of cleaning or something else out of spec?

donalddemon said:
I don't think they have ever been replaced so it's long overdue.
Click to expand...
Caps do not have an expiration date and only NEED to be changed IF they are bad.
Find the true cause of the noise before swapping parts that may not do anything.
 
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#6
Quit blaming the poor caps for something that's quite likely something else.

I see more damage from folks swapping random parts than just about anything else (with the exception of spraying "magic cure-all miracle spray" (tm) in pots and other areas where there should be nothing contaminating.
 
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#7
Don't replace anything without proper troubleshooting. If you don't know how to troubleshoot an old amp, take it to someone who is experienced in amp troubleshooting and repair. You may save your amp, some $ and possibly your life if you aren't knowledgeable about high voltage safety.

Good luck.
 
#8
donalddemon said:
I have an old 220 amp that I love the sound of but it's developing some static like the caps are going bad. I don't think they have ever been replaced so it's long overdue.

Anyone done this before and are there any tips to ordering the right parts? I'm handy with the soldering iron but never have done a cap job, this will be a good chance to learn. I have done some work before inside amps so I know about the harmful voltages and such.

I found a schematic here: http://www.ampix.org/albums/userpics/10075/Acoustic_220_service_manual.pdf

It looks like there are several (8) electrolytic filter caps, not sure which ones to replace or where to source the parts. Any help would be greatly appreciated!
Click to expand...

One question further than the good advice you've already received....

What about the problem leads you to believe that the caps need replacing?
 
#9
Plus new caps contain 87% less mojo. You will however likely get 26% more spank out of them.

Pro tip: If in the unlikely event the caps are really going, you will sometimes notice an odor like dead fish coming from them.
 
#10
lug said:
Plus new caps contain 87% less mojo. You will however likely get 26% more spank out of them.

Pro tip: If in the unlikely event the caps are really going, you will sometimes notice an odor like dead fish coming from them.
Click to expand...
I wonder when the Vintage Capacitors will be a big seller like Vintage Tubes? :greedy:

This is funny... Like how people think a tube amp means you always need to replace tubes, yet tubes that are over 30 years old are very sought after. :cautious:
 
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#14
Geri O said:
One question further than the good advice you've already received....

What about the problem leads you to believe that the caps need replacing?
Click to expand...
When powering down it takes almost a minute to fully shut off, a symptom I've seen on amps that need new caps. It's also the same static sound my Peavey made before replacing the caps. Beyond that, I don't know. Finding a tech has been difficult right now because of the virus and in in the middle of recording so I need it done ASAP.
 
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#16
donalddemon said:
When powering down it takes almost a minute to fully shut off, a symptom I've seen on amps that need new caps. It's also the same static sound my Peavey made before replacing the caps. Beyond that, I don't know. Finding a tech has been difficult right now because of the virus and in in the middle of recording so I need it done ASAP.
Click to expand...
Those are not symptoms suggesting that it needs new caps (no matter what the internet might say)
 
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#18
donalddemon said:
When powering down it takes almost a minute to fully shut off, a symptom I've seen on amps that need new caps. It's also the same static sound my Peavey made before replacing the caps. Beyond that, I don't know. Finding a tech has been difficult right now because of the virus and in in the middle of recording so I need it done ASAP.
Click to expand...

I bought a brand new Acoustic 450 back in the day, & out of the box, from the very first time I powered it down (after the first time I'd powered it up), it did that.
 
#19
donalddemon said:
Ok, care to share any further thoughts on what it might be?
Click to expand...
Any of a hundred possible causes.

With proper troubleshooting of the circuit, you will be able to rule thing out (like filter caps) to narrow down the likely causes step by step.

Repairing isn't about replacing random parts that somebody on the internet who hasn't tested the amp says. It's about correctly identifying the cause of the symptoms so that the correct part(s) are replaced (IF it is even due to a bad part)
 
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#20
donalddemon said:
When powering down it takes almost a minute to fully shut off, a symptom I've seen on amps that need new caps. It's also the same static sound my Peavey made before replacing the caps. Beyond that, I don't know. Finding a tech has been difficult right now because of the virus and in in the middle of recording so I need it done ASAP.
Click to expand...

What you are describing is a higher internal resistance inside the electrolytic capacitor that can be seen with age. This can cause the cap to operate hotter and eventually bulge and fail. I would carefully inspect the caps, looking for signs of bulging. Some caps have a small rubber covered port at one end that can bulge. Look for that. Any bulging, change the cap.

As these power supply caps age, the amp can loose headroom and have an earlier onset of distortion. Static as a cap discharges can be heard of you pull the plug and hit a low note. This is normal. The amp doing this while playing could be a sign of this early onset of distortion due to a bad cap.

As others have said, these symptoms could be due to issues unrelated to the caps. There are many possibilities. It could be as simple as a bad resistor for example. With the aid of test equipment, the problem can be identified and fixed.

It is important to note that a cap is not a cap. They are not all alike, specs are different. Low ESR is usually good. Working voltage is important. Long life is good. A high temperature rating is good. Even size is important. A higher performing cap may change the character of the amp. For example, some amps have a certain characteristic distortion. Upgrading the power supply to perform better can change that. Sometimes in a positive way, making it cleaner. Sometimes not though, leaving people thinking that the magic is gone.
 
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