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?
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[ UPDATE ] The amp is fully functional now! The culprit turned out to be a loose end of the R channel fuse holder on the PS board. I deeply appreciate the help of forum members to help, especially Chamberman!
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PARTS LIST
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Output Board Manufacturing Info
B&K Components Ltd.
EM/3/12/88
PROSDG REV A
Support info: SUPPORT — B & K
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Trimmers
Bourns 3296P 3/8" through hole 500 ohm side adjust 3296P-1-501LF Bourns | Mouser
Electrolytic capacitors
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C1 Nichicon Muse ES 47uF, 25vdc UES1E470MPM Nichicon | Mouser
C2 Nichicon Muse KZ 100uF, 50vdc UKZ1H101MHM Nichicon | Mouser
C4,C5 Nichicon Muse KZ 100uF, 100vdc UKZ2A101MHM Nichicon | Mouser
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?
--------------------------------------------------------
[ UPDATE ] The amp is fully functional now! The culprit turned out to be a loose end of the R channel fuse holder on the PS board. I deeply appreciate the help of forum members to help, especially Chamberman!
--------------------------------------------------------
PARTS LIST
--------------------------------------------------------
Output Board Manufacturing Info
B&K Components Ltd.
EM/3/12/88
PROSDG REV A
Support info: SUPPORT — B & K
---------------------------------------
Trimmers
Bourns 3296P 3/8" through hole 500 ohm side adjust 3296P-1-501LF Bourns | Mouser
Electrolytic capacitors
---------------------------
C1 Nichicon Muse ES 47uF, 25vdc UES1E470MPM Nichicon | Mouser
C2 Nichicon Muse KZ 100uF, 50vdc UKZ1H101MHM Nichicon | Mouser
C4,C5 Nichicon Muse KZ 100uF, 100vdc UKZ2A101MHM Nichicon | Mouser
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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
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
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.
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.
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.
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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.
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.
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?
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?
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?
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?
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?
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?
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.
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.
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.
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.
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.
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.
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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?
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?
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.
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.
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.
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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