B&K ST-202 Plus :: hum

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Chamberman

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Joined 2001
In general the film cap is better at passing higher frequencies than an electrolytic cap, this is true for both polar and bipolar e-caps. Combining the two gives you a good compromise, high capacity and extended frequency response in a small package. If you look at the schematic you'll notice the the yellow film caps are bypasses for the polar 100uF e-caps on the DC rails. So even here B&K is using them on polar e-caps.
 
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phase

Member
Joined 2004
After using some of the 1837 a few years ago when I was getting back into audio, I eventually replaced them after chasing a fizzly treble, have tried them since, and while they are somewhat better than cheap Mylar parts, they aren’t getting used by me on anything except control or display

I will use a bypass to an electrolytic on an input capacitor on equipment that I’m working on for other people, usually for free, but try to use a single large film cap on my stuff. The feedback position however usually can only make room for an non-polar electrolytic (muse es), with a small film/foil bypass.

Same with adding noise reduction on the A/C, always do them on gear that I will be using, since there is always a global benefit and low cost.
 
kenwood61

kenwood61

Member
Joined 2009
phase said:
if you can swing it, try and get a Wima FPK1, in both .01uf, and another MKP10 .1uf to make a snubber for the diode bridge. Those parts along with a 3 watt 18 ohm resistor will help to reduce the a/c noise as well as the noise from the diodes interacting with the transformer.

The .01uf goes across the two A/C terminals, while the .1uf is connected to the resistor, and then that assembly goes to the same points. If that makes sense.
Click to expand...

Not really, but the parts are cheap enough, I can order, and get squared away with you on this by the time the parts show up.

I do see that there are many voltage ratings on these. What am I looking for?
 
kenwood61

kenwood61

Member
Joined 2009
Chamberman said:
Most amps use a bipolar in the feedback position (C2) bypassed with a small value film. If it were mine I would use a Nichicon Muse ES bipolar for C2 and a small MKP film of .1uF in parallel across the pins.

For C1 I would do the same thing, use a Muse ES 47uF with a small value MKP film bypassing it.

That's my .02
Click to expand...

Just to be clear, connecting the small MKP film cap in parallel is the same thing as "bypassing", correct?
 
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phase

Member
Joined 2004
Yes, the additional smaller cap is in parallel performing as a bypass for the higher frequencies that the larger part can’t adequately supply without distortions.
With the input capacitor situation, I almost look at it like the the larger part is bypassing the lower frequencies only, and make sure to use say a .22uf, such as to provide a majority of the spectrum from a better (film) part.

Film capacitors will have lower distortions and resonances compared to electrolytic parts and it shows in the more critical, low level signal positions. Power supply is more forgiving.

The snubber caps should probably be at least the 250 or 400 volt versions. Huge bang for the buck with that modification, and won’t have much bearing on the other parts as far as interacting badly or anything, is isolated.

The parts I have recommended are well above average quality, fairly neutral sounding in all the applications where I have used them, are reasonably priced, and available through Mouser.
There may well be better parts available that will work better for this specific amplifier, may be worth doing a thorough search if you haven’t already to see what others have done maybe.
There were probably several amplifiers made in a series that utilized the same or similar circuit, so to first find that, and search for those models having been upgraded is what I would do.
If you don’t feel like obsessing that much, then the other parts should be more than adequate.
 
kenwood61

kenwood61

Member
Joined 2009
phase said:
Yes, the additional smaller cap is in parallel performing as a bypass for the higher frequencies that the larger part can’t adequately supply without distortions.
With the input capacitor situation, I almost look at it like the the larger part is bypassing the lower frequencies only, and make sure to use say a .22uf, such as to provide a majority of the spectrum from a better (film) part.

Film capacitors will have lower distortions and resonances compared to electrolytic parts and it shows in the more critical, low level signal positions. Power supply is more forgiving.

The snubber caps should probably be at least the 250 or 400 volt versions. Huge bang for the buck with that modification, and won’t have much bearing on the other parts as far as interacting badly or anything, is isolated.

The parts I have recommended are well above average quality, fairly neutral sounding in all the applications where I have used them, are reasonably priced, and available through Mouser.
There may well be better parts available that will work better for this specific amplifier, may be worth doing a thorough search if you haven’t already to see what others have done maybe.
There were probably several amplifiers made in a series that utilized the same or similar circuit, so to first find that, and search for those models having been upgraded is what I would do.
If you don’t feel like obsessing that much, then the other parts should be more than adequate.
Click to expand...

Thanks. The problem with this amp is I can't find much at all, including finding info about modifications. Every other component I've worked on in the past (for myself) is common enough that it's easy to find a variety of approaches to upgrades.

As I looked for snubber caps, I found Mouser has some in that category, but they don't have leads, they have tabs. I'm guessing you mentioned the series you did (FPK 1, and MKP10) because the leads are necessary for proper connection points in this application. I should also note that I don't have any scopes, etc. I have a DMM, variac, and DBT. I don't work on gear for anyone but me, for the simple reason that I don't know this well enough to do so. And I try to be realistic with not going beyond my capabilities. The only reason for my past success is due to the generosity of time and intellect from guys like you all, and for that, I sincerely appreciate it!

I'm going to work up a parts list offline and come back to update post #1 and a new post to make it easy for you guys to see. I'll have a couple of options, but to be honest, I don't yet know if this is an amp I'm going to want to keep for a long time or not. If it stays long enough, I can always make changes down the road. What I don't want to do is put too much stress on the output boards with multiple removal work. It was surprisingly hard to get one board off. I still need to pull the other one, but have been focused on arriving at a parts list so I can get this ordered.

I'll be back to post within 30 minutes.
 
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phase

Member
Joined 2004
No problem, just trying to pass along some of what I have picked up over the past few years.
I don’t use a scope, have based my hobby on upgrading decent equipment that’s been well designed but may be lacking a bit where the production cost cutting measures have infringed a bit.

Hard to have the time to really dive in, maybe someday.

If you aren’t sure about keeping this amp, then I would definitely hold off on the main filter caps.

That’s $100 you will not get back if you sell.
 
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kenwood61

kenwood61

Member
Joined 2009
My current parts list. I'll update the first post once I place an order ... at least maybe that will be helpful to someone else one of these days.

Trimmers
Bourns 3266P trimmers (500 ohm, horiz adjust, 1/4” through hole)
3266P-1-501LF Bourns | Mouser

Electrolytic Capacitors
KEMET 20,000uF, 100v
ALS70A203KF100 KEMET | Mouser

C1
Nichicon Muse ES 47uF, 25v
UES1E470MPM Nichicon | Mouser
— plus —
WIMA MPK10 0.1uF, 100v (film bypass)
https://www.mouser.com/ProductDetail/505-MKP1D031003G00KI

C2
Nichicon Muse ES 100uF, 50v ***changing from polar to bipolar cap***
https://www.mouser.com/ProductDetail/647-UES1H101MHM | Mouser
— plus —
WIMA MPK10 0.1uF, 100v (film bypass)
https://www.mouser.com/ProductDetail/505-MKP1D031003G00KI

C4,5
Nichicon Muse KZ 100uF, 100v
https://www.mouser.com/ProductDetail/647-UKZ2A101MHM
 
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kenwood61

kenwood61

Member
Joined 2009
Considering a local tech told me one channel was shorted and one or more MOSFETs on that channel were bad ... I'm a happy camper. It allows me to make these updates.

Last two questions of the day ...

#1 - can I get two more more thumbs up on my parts list in post #331 so I can place my order? Thanks guys!!

#2 - can someone suggest where I can find 8A, 125v or 250v fuses for the AC panel? I can't find any. I'm guessing they should be AGC Bussman type ... but Mouser has 8A ceramic fuses.
 
kenwood61

kenwood61

Member
Joined 2009
So I'm thinking about the power supply caps ... the KEMET's I have on my list are pretty cheap. That's got me wondering if there is a better option.

I see Epcos/TDK, Cornell Dubilier, and United Chemi-Con options that could work. Not all post ESR, but the Epcos first on my list below, has the lowest posted ESR of 6mOhms.

Epcos 22,000uF B41560A9229M EPCOS / TDK | Mouser this costs TWICE as much as the KEMET 20,000uF I'm planning on buying.
 
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phase

Member
Joined 2004
Well maybe even the ones that are in there would actually measure better than the lower priced replacements even?

Do you have part numbers from your existing parts that you might be able to look up the data sheets and get a better idea perhaps?

It isn’t quite the parts list I would have, not too far away, but again it’s not my project either.
It should be fine however with the what you have listed, and is much better parts than what were in there before.

Might want to snag a .01, and .1uf in a decent variety to use as a snubber, along with an 18ohm, 3watt resistor.
 
kenwood61

kenwood61

Member
Joined 2009
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phase

Member
Joined 2004
I would rather keep the capacitors that came in it before using those from the surplus place, and besides aren’t they are only good for 50v?
I’m sure if they’ve lasted this long that they are fine with the voltage, is probably still within the tested window I suspect. Is your line/wall voltage over 120 by very much by chance?

There have been several audio amplifiers that I have seen with marginal or slightly over voltage to the main filter capacitors, so that’s not the only one. The large caps are pretty easy to swap later, no white knuckle electronic surgery or anything, pretty mechanical with only fasteners, I have ones in use that are 40 years old and working fine, have correct ratings/voltage values however...


I might go a bit bigger on the input bypass, if going with the muse es there, go up to .22uf.
It would be better still if you can squeeze in the 4.7uf film cap there however and leave the muse out. Might search around and get some impressions from others as well.

You have decent solder right? No radio shack stuff?
 
kenwood61

kenwood61

Member
Joined 2009
Oh, I wasn't going to use the Mepco at the surplus place ... that was just to show the only place I could find that carry the same brand/type caps that are in my amp now. I was still planning on going with the KEMET or Epcos. But as you say, it's not a big deal to swap those out later.

My mains at the wall, and surge protector where I connect are 123.5v on my killawatt, and I have a Cyberpower UPS Backup for my office computer that shows 122v. So even if neither is completely accurate, we could say it's slightly over 120v. This is a new build home, and we've noticed that if you run a laser printer, it causes the lights on the same circuit to flicker a little. I've had the electrician come out twice and they think all is well.

Anyway, I'll place the order for all the other parts. I don't mind having extra parts at these prices. Maybe I'll hear from KC by tomorrow. It would be nice for me if I had at least two people confirm the idea of getting new ps caps.

I have four WIMA .22uf, 100v caps in my parts list at Mouser.

MKP1D032204D00KI00 WIMA | Mouser
 
Mark Whitney

Mark Whitney

Member
Joined 2013
kenwood61 said:
I don't mind buying new power caps, especially since the voltage level is just over at 77v. Plus, I'd like to see if I can hear a difference. I only have $83 into this amp right now, and my parts list is under $100
Click to expand...

In reality, every part you replace will reduce the value of the amp. Having components hanging on a Board will not improve reliability or its resale value.

To improve a circuit, you need to understand it and how the components work.
What does bypassing an input capacitor do? Why do we need to bypass power supplies? Are those bright green UES capacitors really any good? Is it wise to run this amp above its intended voltage?
 
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