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B&K ST-202 Plus :: hum
B&K ST-202 Plus :: hum
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Old 8th December 2018, 05:45 AM   #1
kenwood61 is offline kenwood61  United States
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Default B&K ST-202 Plus :: hum

I have a really clean wonderful sounding ST-202 Plus. Only problem is there is a low level hum in the transformer ... not through the speakers.

When I first got the amp, I powered it up (unconnected) to check dc offset. DC offset was around 160mV on one side and close to 240mV on the other. Didn't take much to dial P1 trimpots down within spec. At this point, it had the hum.

Then I hook up to DBT and variac, power up, no hum. What??

I haven't checked bias. But when I hook up the amp to preamp and speakers, I get the hum again. So it is only when the amp is connected.

Visually, components look good, with no bulging caps or signs of overheating. As I mentioned, the sound is really nice.

Any ideas where to check, if not bias first?

Last edited by kenwood61; 8th December 2018 at 10:41 PM. Reason: clarifying problem
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Old 8th December 2018, 07:20 AM   #2
huggygood is offline huggygood  France
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and have you adjusted the bias?
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Old 8th December 2018, 07:41 AM   #3
kenwood61 is offline kenwood61  United States
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No. B&K instructions include some items I don't have. For instance, they say to place a current meter across the fuse clips, which I don't have. Then connect an 8 ohm load to each channel and supply an input signal of 100 mV RMS @ 20 kHz ... I don't have the setup.
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Old 8th December 2018, 07:49 AM   #4
huggygood is offline huggygood  France
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you can weld a 10 ohm resistor on each positive rail fuse dc of the power supply and then measure the voltage drop to transfer this resitance and then deduce the current.
or weld a 10 ohm resistor on the bias adjustment potentiometer (between the base and the brush) and measure the voltage drop across this resistance
or, if you have an ampere meter, you remove a power supply fuse and then replace the fuse with the dots on your multimeter to Ampere dc
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Old 8th December 2018, 05:36 PM   #5
sgrossklass is offline sgrossklass  Germany
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That's soldering, not welding (which is the stuff with sparks flying and all).

Anyway, from pictures it would appear that this amp has an inherent ground loop problem that would manifest itself the very moment both left and right input are connected to the same source (similar to Quad 405 but less severe). Left and right channel share a common power supply but the RCA input jacks are spaced a reasonable distance apart. While apparently care has been taken to reduce internal loop area, it is decidedly nonzero. Does it still hum when merely connecting the RCA cables at the source end with a female to female adapter?

I suggest keeping input cables together / twisted as far as possible. If that doesn't reduce the hum far enough, I would suggest modifying the amp - move at least one RCA jack so that both are next to each other, and extend the ground returns from the input boards so that they first run to the back panel (alongside input audio connections) before being twisted and going to star ground. The idea always is reducing loop area on the star ground - left input board - left RCA cable - right RCA cable - right input board - star ground round trip.
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Old 8th December 2018, 06:28 PM   #6
PRR is offline PRR  United States
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Quote:
Originally Posted by sgrossklass View Post
That's soldering, not welding (which is the stuff with sparks flying and all). ...
As I am sure you know: English makes this distinction, many other languages use their word for "welding" for "all" melted-metal processes.

English has Soldering for lead-type alloys, Brazing for copper-alloy (Cu, Al, Ag, Au, Sn...) joints on high-temp metals (usually iron, often copper), and Welding for melting the base metal (usually iron).

Thanks for pointing it out. I would not like to see the Lincoln (225 Amp welder) inside the B&K.

Last edited by PRR; 8th December 2018 at 06:33 PM.
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Old 8th December 2018, 06:30 PM   #7
huggygood is offline huggygood  France
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Quote:
Originally Posted by PRR View Post
As I am sure you know: English makes this distinction, many other languages use their word for "welding" for "all" melted-metal processes.

English has Soldering for lead-type alloys, Brazing for copper-alloy joints on high-temp metals (usually iron), and Welding for melting the base metal (usually iron).

Thanks for pointing it out. I would not like to see the Lincoln (225 Amp welder) inside the B&K.
+1
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Old 8th December 2018, 07:40 PM   #8
kenwood61 is offline kenwood61  United States
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I knew what he was saying

Here's the reason I don't think bias is at fault here, and speaks to the inherent ground loop problems sgrossklass mentions.

The power cable on the amp is two prong. My DBT and variac are three prong. When the amp is plugged into those, there was no hum.

Second of all, the heat output on the amp is good, not what I would expect if the bias were too high. And as I mentioned in my first post, the sound I'm hearing is blowing my mind ... I didn't expect it to sound this good. I ran the amp playing a few cds with different types of music from low to high volume levels. No distortion of any kind. Just clean sound.

I'll use a different set of interconnects (Canare LV-61S coaxial) from the preamp, that I can twist.

I don't have a female to female adapter, so I can try sgrossklass' other test.

If I can't get the hum down enough, I'll consider the last suggested modification of moving one or both rca jacks, and rerouting ground returns.

I appreciate the input guys! I'll report back as I am able to get updates.
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Old 8th December 2018, 07:44 PM   #9
kenwood61 is offline kenwood61  United States
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I forgot to ask. Anybody know where I can find the schematic? Hifiengine doesn't have it. Only a sheet on setting bias and dc guide.
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Old 8th December 2018, 07:52 PM   #10
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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