Yamaha YST-MS25 amp board recap: replace small electrolytics with different type?

[Exim]

Hi there, I'm working on the 25W amp board of a Yamaha YST-MS25. It's 20yrs old.
Since a few months the volume on the left channel is way too low. It turned out that - compared to the intended use and the former price tag of this thing - they put solid stuff into it. So I decided it's worth a recap.

The board uses two amp ICs on opposite sites of the board. Both ICs are connected to the same heatsink. The heatsink unfortunately is a bridge-like construction that covers almost all other components, including the majority of the caps (pictures and schematics below).

All 30+ electrolytics were Nichicon VX, old radial series, general purpose, 85°C, now out of production. Most of them measured ok.
The two 470 uF on the satellite outputs (C20 in H3, C41 in J4) both had only 420 uF. Maybe they came like that from the factory.
Some of the 10 uF/16 V had an ESR 2.5 times higher than other 10 uF/16 V. They are probably dry.

I'm going to replace all electrolytics with Panasonic FC. Unfortunately Panasonic and others stopped production of very small values for some variants. Currently smallest FC is 2.2 uF. But smaller ones are required as well.
As 1uF I can get my hands on Panasonic NHG, Chemi-Con KMG or Rubycon PX.
For the 0.22uF ones I couldn't get hold of anything suitable, only found crap caps or old stock that's out of production since years and sold at exorbitant boutique prices.

Four 0.22uF electrolytics are in the Yamaha schematics, red circles:
C24: sub amp IC6/LA4485 (runs in bridge mode) channel 1 input capacitor
C30: sub amp IC6/LA4485 unused channel 2 to ground
C39: sat amp IC5/LA4613 standby supply capacitor
C40: sub amp IC6/LA4485 standby supply capacitor
According to the LA4485/LA4613 datasheets increasing these caps isn't a good idea. So I'd like to keep them at 0.22uF.

Should it / is it possible to replace these 0.22uF electrolytics with another type, like ceramics or <...>? If so, which ones?
The LA4485/LA4613 datasheets do not mention any cap types, however, in all example circuit drawings they used the sign for polarized caps.

[Khron]

Re: Yamaha YST-MS25 amp board recap: replace small electrolytics with different type?

For values up to 1uF or so, assuming there's enough physical room on the board, i'd go with film caps.

It might take a bit of (creative?) leg-bending to get them to match the pitches of the originals though.

[momaka]

Re: Yamaha YST-MS25 amp board recap: replace small electrolytics with different type?

All 30+ electrolytics were Nichicon VX, old radial series, general purpose, 85°C, now out of production. Most of them measured ok.

Not surprised. Those older solvent-based electrolytic caps last a very long time. I rarely replace them unless they have been exposed to excessive heat for a long time. (Think: darkened/burned PCB and such.)

The two 470 uF on the satellite outputs (C20 in H3, C41 in J4) both had only 420 uF.

That's fine.
Electrolytic caps have a 20% tolerance rating usually. So for a 470 uF cap, anything in the range of 376-564 uF is still in spec, technically.

Some of the 10 uF/16 V had an ESR 2.5 times higher than other 10 uF/16 V. They are probably dry.

For 5x11 mm caps, it's not uncommon for them to read 1-3 Ohms ESR, but check the datasheet anyways. Only when their ESR goes higher than that I consider replacing them. I have 10's of different 100 uF 16V GP 105C caps reading about 1-2 Ohms. So it's more or less normal. For low-ESR type caps, of course, the ESR should be lower.

I'm going to replace all electrolytics with Panasonic FC.

While Panasonic FC is great, I don't think you need to limit yourself to just that. Amplifier don't usually require low-ESR caps and don't usually benefit from them either (exception to that, of course, is any amplifier with a switch-mode power supply, which should be recapped with low-ESR caps on the PSU output).

With that said, Panasonic NHG, Chemi-Con KMG, Nichicon VZ, and Rubycon PX are all fine choices.

In addition to that, Nichicon (U)KT and (U)KA are another choice. These are actually supposedly designed for audio circuits (though I don't find them to be that much different in terms of specs to regular GP caps). Just note they are rated for only 85C.

For values up to 1uF or so, assuming there's enough physical room on the board, i'd go with film caps.

+10

1 uF is indeed the smallest I'd source for an electrolytic. Below that, it's film type.

Besides, film caps are generally superior to anything else in audio circuits - especially polypropylene type. If you can, always try to use PP (polypropylene) caps in your audio circuits. They have very good frequency linearity. Ceramics are not that great, due to changing capacity with temperature, frequency, and voltage. And electrolytic caps have relatively higher leakage current, in addition to higher ESR.

So for small caps, this is how I'd look for replacement from best to worst:
PP film > film (other) > electrolytic > ceramic

[Exim]

Re: Yamaha YST-MS25 amp board recap: replace small electrolytics with different type?

For values up to 1uF or so, assuming there's enough physical room on the board, i'd go with film caps.

It might take a bit of (creative?) leg-bending to get them to match the pitches of the originals though.

Lead spacing is 5mm. Found some film caps that will fit.

Those older solvent-based electrolytic caps last a very long time. I rarely replace them unless they have been exposed to excessive heat for a long time. (Think: darkened/burned PCB and such.

The area below the large heatsink - almost the whole board - is darkened, it's not really visible on the pictures. Airflow in the box is the bare minimum, maybe less, there were small 85°C caps next to hot ICs and resistors. But yeah, seems at least replacing the 4700u and 1000u PSU filter caps was probably not necessary.

For 5x11 mm caps, it's not uncommon for them to read 1-3 Ohms ESR, but check the datasheet anyways. Only when their ESR goes higher than that I consider replacing them.

Couldn't find ESR in the datasheet. The 10/16 "ok" ones measured 1.5 Ohms, the ones closer to heatsink/resistors had 4 Ohms. Didn't check if they were in the signal path or not, just decided to throw them all out.

While Panasonic FC is great, I don't think you need to limit yourself to just that. Amplifier don't usually require low-ESR caps and don't usually benefit from them either (exception to that, of course, is any amplifier with a switch-mode power supply, which should be recapped with low-ESR caps on the PSU output).

The idea behind using FCs is the sound. I read a little about using standard electrolytics in audio equipment. A few people praise the lower ESR FCs, also if used in the signal path. Others says they don't like the FC in the signal path because compared to standard ESR and dedicated audio caps it changes the sound to a little less bass and little more "sparkling" treble, even more with the Panasonic FM.
With the bass knob at 2/3 this amp already pumps out more deep bass than the decent but small subwoofer speaker chassis can handle. But the satellites miss a little brighness. If the FC really moves the sound a little more to the bright side it would be exactly what it needs.
In case it doesn't work out I'm fine with it, primary reason for recapping is fixing it, not adjusting the sound.

So for small caps, this is how I'd look for replacement from best to worst:
PP film > film (other) > electrolytic > ceramic

The PP films I could get won't fit onto the board, except the more expensive ones from Wima. Wima - of course - recommends their PP ones, but they also say PET (polyester) ones are way better than dedicated audio electrolytics. So for the low values I'll get PET films, Wima MKS2.

Just learned from Wikipedia that "Mylar" caps actually are PET films. So the film caps already on the board are PET films. *thumbs up*

Thanks for clarifying. Will order parts.

[momaka]

Re: Yamaha YST-MS25 amp board recap: replace small electrolytics with different type?

But yeah, seems at least replacing the 4700u and 1000u PSU filter caps was probably not necessary.

Well, if you already did, that's fine. Just keep the old ones as spares in case you run into another amplifier (or other device) that needs a recap. In a pinch, they will do.

Couldn't find ESR in the datasheet. The 10/16 "ok" ones measured 1.5 Ohms, the ones closer to heatsink/resistors had 4 Ohms. Didn't check if they were in the signal path or not, just decided to throw them all out.

Oh yes, that was a silly suggestion on my part - general purpose (GP) caps won't have ESR mentioned anywhere in the datasheet. So what I usually do is I pick an entry-level low-ESR caps like Nichicon PS or PM and check what ESR those have. Then go back to the GP caps and see if they have ESR somewhere in the ballpark (2-5x higher ESR is not uncommon).

That said, those small (5 x 11 mm and 6.3 x 11 mm) caps were more or less okay at 1-3 Ohms. I consider changing them only after they are 5 Ohms or more, usually (unless of course it's a low-ESR series cap).

The idea behind using FCs is the sound. I read a little about using standard electrolytics in audio equipment. A few people praise the lower ESR FCs, also if used in the signal path.

I don't know about that - sounds a bit like some audio-foolery there (if I can call it that). When people start telling me how things sound instead of actually picking up some measuring equipment (oscope, spectrum analyzer, etc.) to see if there is a difference in the actual sound itself, I can't help but call their "experiment" subjective.

That said, I don't think you'll hear any difference. The only reason I sometimes consider audio-grade caps for the signal paths is due to lower leakage current - which I read in some audio books is desirable, because on certain designs it can lower the noise.

In case it doesn't work out I'm fine with it, primary reason for recapping is fixing it, not adjusting the sound.

YESSIR, Panasonic FC is good for that. Won't need to recap it anytime soon after this.

The PP films I could get won't fit onto the board, except the more expensive ones from Wima. Wima - of course - recommends their PP ones, but they also say PET (polyester) ones are way better than dedicated audio electrolytics. So for the low values I'll get PET films, Wima MKS2.

Should be fine.

[Exim]

Re: Yamaha YST-MS25 amp board recap: replace small electrolytics with different type?

Replaced all electrolytics up to 10uF with PET films, the others with Pana FC.

It is (well, it was ) fixed. Left channel working fine again.
The signal path between input and power amps now contains PET films only, the remaining signal-path-electrolytics - now FCs - are between the power amps and the speakers.

My impression is that actually the sound is a little different, less bass, treble is about the same. All subjective, no science. If this comes from the now-films-only-in-the-pre-amp-section or the now-FCs-after-the-power-amps or old caps getting replaced with new ones or my ears getting older or the current phase of the moon - I have no idea. And I don't care. After all it's not a boutique hifi device. But it's fixed. And the new caps are rated 105°C.

Unfortunately it worked for the 10 minutes test only. After putting it back together the sub channel was gone. It turned out the "click" noise when tightening the screws was not the sound of the PCB snapping into the front panel holder. It was the sound of legs being ripped off a cap.



While getting creative with leg-bending to fit the bigger film caps I forgot about the space required to mount the PCB. The red cap (arrow) was in the way of the mounting slot, it was pushed against the second red one, both legs of the second one snapped.
But the PCB is ok, nothing else was damaged, will get two new caps and put one to the bottom side of the PCB. It will fit. Also when mounting the PCB. I checked. Twice.

This was my first amp repair. Thanks for the comments. Much appreciated.

[Khron]

Re: Yamaha YST-MS25 amp board recap: replace small electrolytics with different type?

I guess size does matter after all, eh?

[sam_sam_sam]

Re: Yamaha YST-MS25 amp board recap: replace small electrolytics with different type?

Replaced all electrolytics up to 10uF with PET films, the others with Pana FC.

It is (well, it was ) fixed. Left channel working fine again.
The signal path between input and power amps now contains PET films only, the remaining signal-path-electrolytics - now FCs - are between the power amps and the speakers.

My impression is that actually the sound is a little different, less bass, treble is about the same. All subjective, no science. If this comes from the now-films-only-in-the-pre-amp-section or the now-FCs-after-the-power-amps or old caps getting replaced with new ones or my ears getting older or the current phase of the moon - I have no idea. And I don't care. After all it's not a boutique hifi device. But it's fixed. And the new caps are rated 105°C.

Unfortunately it worked for the 10 minutes test only. After putting it back together the sub channel was gone. It turned out the "click" noise when tightening the screws was not the sound of the PCB snapping into the front panel holder. It was the sound of legs being ripped off a cap.



While getting creative with leg-bending to fit the bigger film caps I forgot about the space required to mount the PCB. The red cap (arrow) was in the way of the mounting slot, it was pushed against the second red one, both legs of the second one snapped.
But the PCB is ok, nothing else was damaged, will get two new caps and put one to the bottom side of the PCB. It will fit. Also when mounting the PCB. I checked. Twice.

This was my first amp repair. Thanks for the comments. Much appreciated.

So did you get it working again

[Exim]

Re: Yamaha YST-MS25 amp board recap: replace small electrolytics with different type?

I guess size does matter after all, eh?

Yeah, but you need to know how handle it.

So did you get it working again

Yes, since replacing two replacement caps with replacement caps it's working fine again.
The initial reason for the left channel volume drop was probably one of the very small old caps around the pre-amp. Couldn't measure all of them, my ESR meter only handles caps >= 1uF.

The new solder joints are ok and shiny, the camera is just crap.