Re: Mackie CR4 amplifier no sound
Yes, both ESR and capacitance look normal.
However, the problem with many of the cheap crap brands is that their leakage current can go high once voltage is applied. Well, that can happen to any electrolytic capacitor, really. But it just happens more to the cheap crap brands.
No, the cap doesn't have to be 1 uF. The higher the capacitance, the slower the amplifier will switch between STB, Mute, and Play operation. So with around 2x higher capacity, you'll probably get 2x slower transition between these states... which you should not notice at all, because even with the 1 uF cap, the switch between the states is almost instant. So with 2.2 uF, it will be "slightly less instant" (that's a highly technical term, OK.
) I'll give an explanation of what C39 does a little further bellow.
Any brand and series will do.
However, because these caps are close to the heatsink and are exposed to heat, I'd suggest going with 105C -rated caps. GP 105C caps will probably be fine, but the Panny FR you have should be a nice treat, since those are quite reliable and long life.
+1000
In fact, I suggest getting as many of the caps replaced as possible, especially the ones in the hot areas. All the caps look like no-name cheap brands, and sooner or later they will fail. So probably a good idea to recap the working unit, too.
So, the function of this cap is kind of similar to C74, except instead of providing a delay (by taking time to charge through a resistor) for the power-on mute, it provides a delay when the amp is switched between STB/Mute and play when the headphones are plugged or unplugged. Otherwise, going abruptly between these states could cause crackle to be heard through the speaker, as you noticed already.
Just remember this: capacitors act sort of similar to very tiny tiny batteries in the circuit and as such, they don't like to allow sudden voltage changes across their terminals. So whenever something in the circuit tries to change voltage abruptly, the capacitor will smooth that voltage change into a more gradual one. When you add a resistor in series (in the case of C39, it's resistor R91 and C-E junction of Q2), the change of voltage across the terminals of C39 becomes even slower/"smoother". So this allows the amp to go more slowly between STB, Mute, and Play operation.
Yes, no-name / crap brands and heat don't mix well. I find that in a lot of 90's and 2000's budget/mid-tier audio gear, if there are any no-name or crap cap brands, the ones located closest to any heat sources are usually the first ones to fail. And even the ones that aren't can sometimes fail too. Like you noted, it kind of varies with the batch. So you can have the same cheap brand of caps last a long time in one unit and fail rather quickly in the next. For this reason, I always recommend to use only Japanese capacitors from the big 5 (Panasonic, Rubycon, Nichicon, United Chemicon, and Sanyo/Suncon) - at least if you don't want to have to do this repair anytime soon in the future again.
How about replace the rest of the crap caps with good ones then? That's one way to get a more "uniform" look. 
In all honesty, though, I don't know why you should care about that. Unless the unit has a clear window with lights inside pretty much begging people to stare... you're just not going to see the insides again. So IMO, you should put whatever cap brand actually does the job reliably rather than focus on the looks.
Otherwise, if you want some really cool-looking caps, I have some bright-yellow CapXon KM caps from an Apevia PSU.
They look kind of catchy with their yellow sleeves... but no telling when they will fail (well, some of them have failed already, which is why I removed them all and replaced with good Japanese brands.)
Yes, both ESR and capacitance look normal.
However, the problem with many of the cheap crap brands is that their leakage current can go high once voltage is applied. Well, that can happen to any electrolytic capacitor, really. But it just happens more to the cheap crap brands.
No, the cap doesn't have to be 1 uF. The higher the capacitance, the slower the amplifier will switch between STB, Mute, and Play operation. So with around 2x higher capacity, you'll probably get 2x slower transition between these states... which you should not notice at all, because even with the 1 uF cap, the switch between the states is almost instant. So with 2.2 uF, it will be "slightly less instant" (that's a highly technical term, OK.
) I'll give an explanation of what C39 does a little further bellow.Any brand and series will do.
However, because these caps are close to the heatsink and are exposed to heat, I'd suggest going with 105C -rated caps. GP 105C caps will probably be fine, but the Panny FR you have should be a nice treat, since those are quite reliable and long life.
+1000
In fact, I suggest getting as many of the caps replaced as possible, especially the ones in the hot areas. All the caps look like no-name cheap brands, and sooner or later they will fail. So probably a good idea to recap the working unit, too.
So, the function of this cap is kind of similar to C74, except instead of providing a delay (by taking time to charge through a resistor) for the power-on mute, it provides a delay when the amp is switched between STB/Mute and play when the headphones are plugged or unplugged. Otherwise, going abruptly between these states could cause crackle to be heard through the speaker, as you noticed already.
Just remember this: capacitors act sort of similar to very tiny tiny batteries in the circuit and as such, they don't like to allow sudden voltage changes across their terminals. So whenever something in the circuit tries to change voltage abruptly, the capacitor will smooth that voltage change into a more gradual one. When you add a resistor in series (in the case of C39, it's resistor R91 and C-E junction of Q2), the change of voltage across the terminals of C39 becomes even slower/"smoother". So this allows the amp to go more slowly between STB, Mute, and Play operation.
Yes, no-name / crap brands and heat don't mix well. I find that in a lot of 90's and 2000's budget/mid-tier audio gear, if there are any no-name or crap cap brands, the ones located closest to any heat sources are usually the first ones to fail. And even the ones that aren't can sometimes fail too. Like you noted, it kind of varies with the batch. So you can have the same cheap brand of caps last a long time in one unit and fail rather quickly in the next. For this reason, I always recommend to use only Japanese capacitors from the big 5 (Panasonic, Rubycon, Nichicon, United Chemicon, and Sanyo/Suncon) - at least if you don't want to have to do this repair anytime soon in the future again.
That's one way to get a more "uniform" look. In all honesty, though, I don't know why you should care about that. Unless the unit has a clear window with lights inside pretty much begging people to stare... you're just not going to see the insides again. So IMO, you should put whatever cap brand actually does the job reliably rather than focus on the looks.
Otherwise, if you want some really cool-looking caps, I have some bright-yellow CapXon KM caps from an Apevia PSU.
They look kind of catchy with their yellow sleeves... but no telling when they will fail (well, some of them have failed already, which is why I removed them all and replaced with good Japanese brands.)
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