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B&K ST-202 Plus :: hum
B&K ST-202 Plus :: hum
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Old 8th December 2018, 07:53 PM   #11
kenwood61 is offline kenwood61  United States
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I just found a video where it appears that the boards were repositioned at 90 degrees from back panel, from the stock parallel position. Allows more space around.

YouTube

I can't see where he put the rca jacks, I think in line with each board ... I can see the original jacks abandoned. If anyone can vouch for what is done here, I might consider using this hum as an opportunity to recap, and make changes that are known and generally accepted as just enough to maintain the integrity of the sound character, by addressing weak points, replacing items that could fail, or maybe be out of spec.

Thoughts?
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Old 8th December 2018, 10:04 PM   #12
Duke58 is offline Duke58  United States
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My thoughts- no need to reinvent the wheel. I'm not familiar with the B&K ST-202, but it looks very familiar to a Marantz amplifier from the 1970s.

The story about welding and soldering reminds me of the first time my father helped me assemble a model airplane and the instructions said to "cement" part X to part Y and my dad commenting about cementing vs gluing the parts together.

Back to your amp. I looked at the video and noticed that the RCA jacks are wired to the PCBs on each channel, so I can't verify the jacks have been abandoned and don't know of any other way the input signal would connect to the amp?
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Old 8th December 2018, 10:10 PM   #13
kenwood61 is offline kenwood61  United States
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I remember the days of using plastic cement (glue) on model planes, tanks, and cars ... those were fun!

Looking back at it, I see that what appear to be two jack holes in chassis, about an inch apart are not the stock positions. Makes me think maybe at some point, someone put those holes there?

I remember having a similar hum, but through speakers, not from the amp itself, from a Sansui BA-3000. Turned out to be the molex connectors on a board. Removed connectors and hardwired. Hum gone.
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Old 8th December 2018, 10:17 PM   #14
Duke58 is offline Duke58  United States
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You're saying the hum isn't heard through the speakers, but you hear it when you're close to the amp? That most likely is the transformer. You can try to tighten the fasteners, but if you can't hear the noise when you're playing music, then why worry?

It's like people who say they don't like vinyl LPs because they hear noise BETWEEN the tracks. I personally only care if you can hear the noise while playing the music and don't generally listen in between tracks.
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Old 8th December 2018, 10:39 PM   #15
kenwood61 is offline kenwood61  United States
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Yes, sorry for not being clear on that very important aspect. It's not coming through the speakers, but the transformer.

Confirmed every nut is tight on transformer, as well as everything on the chassis. Powered up and hum may be a little less, or could be my imagination.

Good news ... it's not going through output to speakers!

Now to figure out what the problem is. There are a lot of wires around the transformer, but even carefully moving these, I'm not hearing any variation in the hum.

Last edited by kenwood61; 8th December 2018 at 10:42 PM.
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Old 10th December 2018, 01:30 AM   #16
kenwood61 is offline kenwood61  United States
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Duke, I get what you are saying ... but the hum is loud enough, that I'm expecting something to happen. I'm a big vinyl lover and don't let too much bother me with surface noise unless it is distracting during music playback. I've had enough amps, and know that some have a very low transformer hum ... that doesn't bother me. This is louder than I've heard any other "healthy" amp. On the other hand, like you say, since the amp does sound really nice, it might not be a concern.

This does bring up one thing. If I keep this amp, I'll want to update electrolytic caps, and anything else that makes sense on this amp. I've read about Musical Concepts / Musical Designs and looked at their site to see they have kits for Hafler amps as well as BK. Anybody have experience with these kits, specifically for ST-202 amps?
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Old 10th December 2018, 07:26 PM   #17
sgrossklass is offline sgrossklass  Germany
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There are about 3 things that can cause excessive xmfr hum:
1. Loose laminations or windings.
2. Excess loading in idle, e.g. from excessive bias current or badly leaky filter caps.
3. A DC component on the mains. This particularly affects high-power toroidals (500+ VA) as they have low primary winding resistance and are quite easily pushed into (or at least towards) saturation. If that should be the problem, this amp would seem to have plenty of space to squeeze in a DC filter. Here's an article covering this topic.
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Old 10th December 2018, 08:20 PM   #18
kenwood61 is offline kenwood61  United States
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I appreciate the ideas sgrossklass, thanks!

1. I'm guessing there is no fix for loose laminations/windings other than replacing the transformer, which is a no-go for me.

2. Any commonly accepted quality modern filter caps (brand)? Screw top limit options. I was thinking about recapping the boards anyway, but if I go this route, I need a plan to make changes that are smart long-term updates.

3. Plenty of space for adding the kitchen sink ... but I would want to know this is the problem before doing.

I think I need to deal with bias first. I already mentioned I don't have what I need to do this on my own, so might find someone local who can.

I'll report back once I know more.
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Old 11th December 2018, 07:27 AM   #19
RiLoWa is offline RiLoWa  United States
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Since you mentioned that you have the pot that adjusts the DC offset this schematic might be the one you're looking for. I wish I had a better copy of it.
Attached Images
File Type: gif B&K ST202 Schematic.gif (59.3 KB, 117 views)
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Old 11th December 2018, 03:59 PM   #20
kenwood61 is offline kenwood61  United States
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I only have an instruction sheet from HiFi Engine, so that schematic should be helpful, thanks!
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