Changing filter caps on '78 Acoustic 220?

[agedhorse]

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.

That's NOT what he is describing, and the opposite of what you are postulating.

 

[beans-on-toast]

That's NOT what he is describing, and the opposite of what you are postulating.

You start with something that has a grain of truth and it morphs into a cure all action that should be taken. That seems to be the case here.

Unfortunately, tech service all too often works as follows. A tech takes in an amp and without thinking, changes tubes or transistors and recaps the power supply. After all, that's their bread and butter. But it turns out that isn't the right solution so the amp's owner escalates the ticket to a more qualified tech. If they can't fix the amp, it becomes an amp that nobody can fix. [There's an expression, If I can't fix it, I can fix it so nobody else can. ].

Eventually it winds up on the bench of someone who knows how to troubleshoot and in short order, they effect repairs.

 

[Jeffrous]

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.

Have my '77 Acoustic 220 in the shop now. Was producing the scratchy sound effect (kind of different than dirty pots) only when playing a low B, coincidently. Turns out it was the caps after talking with the tech and we're replacing.

 

[agedhorse]

Good luck, my hunch is that you are going to need it.

IF it turns out that the new caps don't fix the problem, will the tech credit you back that cost towards the repair that it ends up needing? If not, run away fast.

 

[lug]

When I was a tech many many moons ago, I worked at a place where we ran hundreds and hundreds of Lambda power amps in our equipment. On occation a power cap would actually let loose. The troubleshooting was actually pretty easy....if you smelled something that resembled a pallet of dead fish left out in the sun for 3 days, you could reasonably be sure it was one of these caps.

 

[Short4since64]

Cannot understand this fixation on "bad caps" In most cases, if a filter cap fails you will hear a hum pitched between Bb and B, not a crackle, and on visual inspection, the end will be swelled and leaking/crusted. (had one go on the TV sound amp in a sports bar, loud fizzle from the amp followed by sound like a car horn in the speakers - what mental giant thinks it's a good idea to put a warming cabinet under electronics gear?). A scratchy sound triggered by vibration is more likely a bad connection. If amp has an effects loop, the contact on the return jack is often the cause.

 

[Jim C]

My guess about the fixation of filter cap replacement is possibly due to old wives tales, advertisements about caps that "sound better", and ease of installation. Seems like other components don't get nearly the attention and abuse like the poor audio capacitor does.

 

[JacklegBass]

I've worked on a lot of Acoustic amps over the years, and if memory serves I've only had one of the computer grade screw terminal caps deteriorate to the point where replacing it audibly helped anything. In that case it was the 3900uF output cap, not a power supply filter cap, and it was causing distortion not static.

The 220 is about as simple as it gets to troubleshoot. Since the power supply is common to both you can isolate the preamp and power amp individually and determine if the static is present in one or the other or both. If not in both it's unlikely to be a power supply capacitor. I've seen a flakey power boost switch cause static when its engaged (ON). I'd keep that switch turned off and also use a passive footswitch with a 1/4" plug to turn off the graphic EQ section. Work the BRT switch a few times.

I think the only components I've ever found causing static on any vintage Acoustic amp were maybe one bad board-level bypass electrolytic and a thermally-challenged flakey semiconductor or two. Pretty rare. This should be simple for a tech to run down, and hopefully before replacing an expensive filter cap he at least substituted a temporary outboard capacitor to verify a new cap isn't going to be a waste of time and money.

 

[larrysb]

Kind of an old thread and looked at the schematic. This is a solid state amp with quasi-complimentary power amp section. That means there's a big blocking capacitor between the power transistors and the speaker.

On general principals, I'd just replace that 3900 µf/75v capacitor after 46 years of service in that particular position. It has a tough life. A long-life high-ripple current part would be my preference.

That would be in addition to troubleshooting whatever other problems are involved.

It doesn't look like this amp has any sort of muting circuit, so power-on/power-off are going to come with whatever artifacts of thumps or noises.

The power supply is a simple affair, just a full wave bridge and a large filter capacitor (7800µf at 100v). Schematic has Vcc at 87v. The filter capacitor would be expensive and possibly hard to source. It's likely OK though, assuming it was a quality component to begin with.

 

[JacklegBass]

On general principals, I'd just replace that 3900 µf/75v capacitor after 46 years of service in that particular position. It has a tough life. A long-life high-ripple current part would be my preference.

Yes, that's the one I referred to being the only one of the two chassis-mounted caps in that amp that in my experience ever made an audible improvement by changing.

 

[Short4since64]

Yes, that's the one I referred to being the only one of the two chassis-mounted caps in that amp that in my experience ever made an audible improvement by changing.

Test it. 50Hz or so sine wave, dummy load. Probe both sides with scope, AC probe. Both should be identical. If not, cap has deteriorated.

 

[agedhorse]

Test it. 50Hz or so sine wave, dummy load. Probe both sides with scope, AC probe. Both should be identical. If not, cap has deteriorated.

They won't be identical at 50Hz, there will be phase shift between the two as well as ~1dB of attenuation at 50Hz. This is entirely normal.

 

[Jim C]

I remember as a kid looking at the guts of an Acoustic 150 and thinking that they hardly used any parts compared to a Fender Bassman head.