ScottAstroNut
New member
I will re-flow the power supply and get back to you later.
Another BeePre 2 with Hum
- Thread starter ScottAstroNut
- Start date
ScottAstroNut
New member
I just re-flowed all of the solder joints in the HV supply. Absolutely no change at all.
Next idea?
Next idea?
ScottAstroNut
New member
Something new... comparing the "hum" to test signals, I don't think that the hum is 120Hz. Something closer to 200 to 240Hz. And more of a buzz than a hum.
Discerning the frequency of noise is not easy. With my current noise problem I listened to 60 and 120 Hz signals on two different online tone generators for an hour, going back and forth, and at the end I didn’t know what the hell I was hearing. I think it’s the kind of thing that requires a lot of experience to determine. It is said that 60Hz is hum and 120 Hz has a buzz but when I listened to the tone generator samples both frequencies had some buzz, 120 just had a slightly more buzzy character. The other thing that I wonder about is: is the AC current coming out of our plugs a perfect sine wave? If it isn’t, any anomalies in the wave shape could produce a buzzy sound at any frequency, I guess. Scott, I know you and PB will figure this out. I am watching with great interest.
Jamie
Jamie
Paul Joppa
Moderator
What you need is an oscilloscope or a spectrum analyzer, if you want to really know the character of the hum/buzz. PB has those tools, so we may know more soon.
ScottAstroNut
New member
I have access to test equipment in my lab at school. I will grab an oscilloscope and frequency generator to see what I can see. I can create a spectrum analyzer if needed using a voltage probe I can hook up to a computer. I had been holding off in the hopes that there might be an easy solution to this problem. I might bring in the help of a good friend who is a retired acoustic engineer who used to design speakers and amplifiers. My background is in electrical engineering and physics, but my area of expertise is digital logic systems. This analog, tube stuff is very different from that! I am having fun learning this new/old stuff.
ScottAstroNut
New member
Am l looking at the output using an oscilloscope and the signal is confusing. Not very clean at all. My friend the audio engineer is coming over in a little while. Perhaps he can make sense of what I am seeing.
ScottAstroNut
New member
My friend the audio engineer and I explored the BeePre in some more detail. With an oscilloscope on the output, some irregularities that would create a humming or buzzing noise were clearly seen. However, I did not get good photos of the traces. We then turned the BeePre over in its test stand to probe further the high voltage supply and the filament regulators. However, before we could take any measurements, and immediately upon powering up the BeePre 2, there was smoke and one of the 270 ohm resistors in the high voltage supply died. I've attached a photo showing which resistor blew. As far as we could tell, nothing we did would have resulted in the death of that resistor. Nothing about the circuit was modified in any way. All I did was turn the power on. That particular 270 ohm resistor was extremely hot to the touch, whereas none of the others were.
Perhaps just a bad component?
At this point I am bewildered.
Perhaps just a bad component?
At this point I am bewildered.
Attachments
No, that's not how this works unfortunately.ScottAstroNut said:
A resistor will smoke and blow up because you drew excessive current through it. I have circled in green one possible cause. If the lead circled and the middle terminal strip lug circled touch each other, you'll send that power supply dropping resistor up in smoke.
The leads circled in red should be bent straight up so there's no risk of them touching the chassis.
Attachments
ScottAstroNut
New member
Thanks, Paul,
It's possible that there could have been a short where you have circled the green, and I had thought of that, too, but upon close inspection I saw a gap between the grounded terminal and the lead off the 22 ohm resistor. I also didn't see a spark, just saw smoke. But again, I could be mistaken.
Regardless, the 270 ohm resistor is toast.
Will do on the other circled red items. That was my mistake. Being careless.
What would you recommend at this point? Make the adjustments you suggested and replace the 270 ohm resistor?
It's possible that there could have been a short where you have circled the green, and I had thought of that, too, but upon close inspection I saw a gap between the grounded terminal and the lead off the 22 ohm resistor. I also didn't see a spark, just saw smoke. But again, I could be mistaken.
Regardless, the 270 ohm resistor is toast.
Will do on the other circled red items. That was my mistake. Being careless.
What would you recommend at this point? Make the adjustments you suggested and replace the 270 ohm resistor?
I would make your amp look as much like the one in the manual as possible. I wouldn't leave that resistor lead like you have it, that may continue to smoke your power supply randomly if you don't address it.
I would replace the 270 ohm 5W resistor after you've made the adjustments and give it another shot.
I would replace the 270 ohm 5W resistor after you've made the adjustments and give it another shot.
Attachments
ScottAstroNut
New member
Will do.
Do you recommend I contact Eileen for a replacement or simply purchase one from one of the electrical supply companies?
Do you recommend I contact Eileen for a replacement or simply purchase one from one of the electrical supply companies?
Whichever one is more convenient for you. For a single part order, it tends to be far less expensive to just use us.
ScottAstroNut
New member
Great. Will contact Eileen.
Do you think that the proximity of the 22 ohm wire to that ground lug could have been a source of the hum?
Thanks for your help!
Do you think that the proximity of the 22 ohm wire to that ground lug could have been a source of the hum?
Thanks for your help!
I don't really know, it's certainly worth the effort to correct that area and see if that helps.
I was mulling this over while doing some other work, and that short wouldn't blow out that resistor, but shorting that connection to ground will remove one of the filtering elements in the power supply and increase the noise floor. This wouldn't be a huge increase in noise, but it would be audible, and it would come with ever so slightly higher than expected voltages.
ScottAstroNut
New member
Paul, I was thinking about that, too. The combined resistor/capacitor/diode assembly is essentially both a high pass/low pass filter, isn't it? So in one sense it is already a short to ground as seen at terminal 36? Shorting the 22 ohm resistor to terminal 35 therefore shouldn't be able to take out the 270 ohm resistor between terminals 36 and 39. In which case, what did? All I did was turn the power on to a circuit that was already working, albeit with a slight hum.
Also, the readings I measured at terminals 6 and 16 in the last voltage test measured lower than the expected 100VDC by 7%, not higher.
What I can say is that the oscilloscope trace of the output with no input signal was not clean. My friend the audio engineer helped me with the oscilloscope readings. What we saw was baffling enough to him that he wanted to trace the signals through the circuit. That was what we were getting ready to do when the 270 ohm resistor blew. We hadn't touched anything in the circuit, however, before the resistor went up in smoke, so it wasn't caused by a slipped probe. All I had done was invert the circuit in the building jig I use during construction and testing. Rather than use the wooden base provided as a building support, my jig allows me to invert the circuit with tubes and transformer attached without bottoming out. Just more convenient and secure.
Also, the readings I measured at terminals 6 and 16 in the last voltage test measured lower than the expected 100VDC by 7%, not higher.
What I can say is that the oscilloscope trace of the output with no input signal was not clean. My friend the audio engineer helped me with the oscilloscope readings. What we saw was baffling enough to him that he wanted to trace the signals through the circuit. That was what we were getting ready to do when the 270 ohm resistor blew. We hadn't touched anything in the circuit, however, before the resistor went up in smoke, so it wasn't caused by a slipped probe. All I had done was invert the circuit in the building jig I use during construction and testing. Rather than use the wooden base provided as a building support, my jig allows me to invert the circuit with tubes and transformer attached without bottoming out. Just more convenient and secure.
If you flip the amp over and a resistor goes up in smoke, something may have been really close to touching where it shouldn't have, or barely touching but then touching too much when you flipped it over, so in a way you have a more solid hint at what's going.
I repaired an old Pilot EL84 amp that relied on ground contacts built into the 9 pin sockets. It would only work upside-down, when you flipped it back over the sockets were loose enough that those connections would open up and the amp would do strange things. This isn't all that related to what you're experiencing, but similar in the sense that you have a hint now that something may be moving around more than it should.
I repaired an old Pilot EL84 amp that relied on ground contacts built into the 9 pin sockets. It would only work upside-down, when you flipped it back over the sockets were loose enough that those connections would open up and the amp would do strange things. This isn't all that related to what you're experiencing, but similar in the sense that you have a hint now that something may be moving around more than it should.
