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B&K ST-202 Plus :: hum
B&K ST-202 Plus :: hum
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Old 12th December 2018, 02:54 PM   #21
Chamberman is offline Chamberman  United States
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You can easily determine if excessive loading is an issue. Excessive bias should show up as high heat sink temps. The heatsinks on those B&K models weren't all that large so it wouldn't take much excess bias to really heat them up. Either way get online and buy a Kil A Watt meter they're cheap and a great tool to have around the house. I've had mine for about 13 years now and use it all the time. You can use the Kil A Watt to see how much power your amp is drawing from the wall.

Based on the age of the amp my guess is its a mechanical problem, in other words loose windings.
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Old 12th December 2018, 04:59 PM   #22
kenwood61 is offline kenwood61  United States
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Thanks KC.

How will this meter show excess bias? My DMM has a temp wire to measure heat, but I don't have info on what the heat should be.

At this point, I would agree (in my limited knowledge) that the loose windings diagnosis is most likely ... since the heat sinks do not feel hot, but warm, and not at a level above what I'm used to with other amps of the era.

Is there a test for loose winding?
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Old 12th December 2018, 05:19 PM   #23
Ylli is offline Ylli  United States
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From the pictures I see online, it appears this unit uses a toroidal transformer. Perhaps you could add rubber pads between the transformer and its mounting.
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Old 12th December 2018, 05:24 PM   #24
Chamberman is offline Chamberman  United States
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The current draw will be quite high.

When we're talking about excess bias being an issue its not +10% we're referring to. It takes a lot of extra bias to load a large amp transformer to the point it begins to hum, I would say like 40 - 50% more or double or higher. This shows up in the power draw from the wall which the killawatt can show you. Based on the heatsink size I'd be surprised if the AC power draw at idle exceeds 50 - 60 watts without overheating the sinks on that amp.

You also do not need a temp probe to check heatsink temps. Your hand will suffice. I know its not that scientific but it works. These heatsinks are not huge and do not have a lot of excess capacity so theyll heat up fast. I owned a pair of M200 monos for years that I bought brand new in 93. Without a signal their heatsinks were barely above room temp. I also owned an ST140 for a period of time. Once again barely warm to the touch when idling with no signal. If your heatsinks are getting to the point they are uncomfortably hot when the amp is just idling then your bias certainly is too high.
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Old 12th December 2018, 05:50 PM   #25
kenwood61 is offline kenwood61  United States
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Ylli ... yes, it is toroidal.

Chamberman, I'll hook it up again today for a run, but last time, I could keep my hands on the heatsinks. I ordered a Kill O Watt that should arrive in two days.
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Old 12th December 2018, 07:40 PM   #26
kenwood61 is offline kenwood61  United States
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After an hour of playing ... I can't keep my hand on the heat sinks for long ... to the count of 1 ... so maybe, just maybe, the bias is a little high ... sure would be a "good" thing if it just needs that. If the hum cut in half, I'd probably be happy.
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Old 12th December 2018, 07:45 PM   #27
Chamberman is offline Chamberman  United States
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Now that you've warmed it up good turn the music (input signal) off and leaving the the amp powered on let it sit with no signal for an hour then go see how hot the sinks are.

Its not uncommon for them to heat up playing music at a reasonably high output level and then cool down while powered on with no signal.

But yeah that seems like the bias may be too high.
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Old 12th December 2018, 07:51 PM   #28
kenwood61 is offline kenwood61  United States
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Ok ... be back in an hour.
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Old 12th December 2018, 08:01 PM   #29
Nico Ras is offline Nico Ras  South Africa
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I think you should also take cognizance of sgrossklass statement (3) few people realize that even a slight DC bias on the mains causes a lot of buzzing noises in transformers.
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Old 12th December 2018, 08:30 PM   #30
kenwood61 is offline kenwood61  United States
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Ok. So after a little while, no signal, heat sinks are warm. I can hold my hand on them indefinitely.

DC offset is as close as I can get to spec (5mVDC) ... under 4mVDC ... but as I go to set each channel, I'm getting some readings that swing up into the teens. The hum didn't change from what I could hear.

DC Bias, I need to find someone local who might be setup for checking that ... I don't feel comfortable with the process outlined, and know all it takes is one mis-step to cause damage.

Last edited by kenwood61; 12th December 2018 at 08:35 PM.
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