Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: jj4001 on February 15, 2024, 04:14:56 PM
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After instal of the Speedball, I now have a low volume hum (110 -120 hz approx). Mostly in left channel. It is there regardless of volume position, inputs connected or not, tried different location, etc.
It is pretty low level but noticable between tracks and very quiet music passages.
I've done the chopstick test, reflowed a few joints, and tightened all screws/bolts. Any ideas for next things to check? Odd to me that the volume control has no impact at all.
Thank you!!
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What are the DC voltages on each terminal of each Speedball board?
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Well I did something wrong for sure. When I went to test the voltages (black lead on 12U) I got a loud spark when I went to measure at 1A.
The fuse is not blown, but it won't power up now so I could have caused a big problem.
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If you got a big spark, perhaps your meter wasn't set to read DC voltages?
If the amp won't power up, the fuse likely is blown and you'd want to pull it out and test it for continuity with your meter.
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Ok - the fuse was good and amp powers up fine.. Whew... Here are the voltages
OA 77
IA 180
BAB 0
IB 180
OB 72
OA 106
OB 106
G 0.1
B+ 180
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Those are working voltages.
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Good to hear. Any thoughts on what might be causing the low volume hum I described?
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Do you get this noise with nothing plugged into the RCA jacks and the volume pot turned all the way down?
Is there a wifi router sitting next to the amp by chance?
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Position of the volume control has no impact on the Hum. Rca's connected or not also has no impact. I've also moved it into a different room and still the same.
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Can you post some build photos?
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https://photos.app.goo.gl/JRRznj9BZFjryLPz8
Here's a link to 5 build
pics in my Google photos
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I would give this a try:
https://forum.bottlehead.com/index.php?topic=11676.0 (https://forum.bottlehead.com/index.php?topic=11676.0)
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Thanks, I will give this a try. Parts have been ordered.
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I inserted the 2 opposing diodes today and removed the bare wire per the instructions. The left channel low hum is still there. No change in the level at all.
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The next step in isolating is to pop off the front C4S board, then reinstall it 180 degrees from how it was installed previously. You won't be able to screw it down in this orientation, but you should be able it leave it flipped up and get the wires reattached. Once you've done this, observe if the noise has switched sides or stayed put.
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Ok. So that's the small board between the volume Pot and the output jack, is that correct?
And if I understand you correctly, the white wires at OA and OB would reverse places. Red wires at 1A and 1B would reverse places. And black wire would stay in BAB. Do I have that right?
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Yes, that's all correct.
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Thank you for your help.
I will try this today or tomorrow and report back.
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I flipped the board, rewired, and the hum has stayed in the left channel.
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Now try doing the same thing with the big board (which is a bit easier.
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Ok. And should I leave the small board flipped, or put it back to normal?
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That won't really matter for this test.
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Ok. And for the bigger board do I exchange positions of the white wires with each other? And what about the black and red wires?
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The white wires switch, the red and black need to stay where they are.
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Thanks again for your help! After this switch the hum is still in the left channel.
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I think you may need to take the small PC board out and put the 22.1K resistors back and see if the issue goes away. If it doesn't, tack in the 3K resistors and see if the noise goes away. If it doesn't then something was disturbed while installing the Speedball that was present in the underlying build.
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Anything that was taken out is long gone unfortunately. Are there any areas or solder joints i should look carefully at if you had to take a guess on what the issue might be?
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One interesting thing I just noticed. While I had the amp upside down, when I applied pressure (pressing down) in any area of the main chassis between the large tube and the transformer the hum greatly reduced.
When I flipped it upright and pushed down in the same area it didn't have the same result. It was only when pushing down on the under side while it was upside down.
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Tighten all your screws before doing anything else.
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Oddly, all screws were very tight from previous troubleshooting. Last night I tried experimenting with the tightness of the transformer screws while listening and loose or tight it didn't change the hum.
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If you just touch the chassis plate, does the nature of the hum change? Often pressure will make slightly loose hardware appear to be tighter (electrically at least), while just touching the chassis plate can mess with what appears to be ground. The key two screws are the one on the front 5 lug terminal strip and the ground lug by the power entry module. It's also pretty critical that the volume pot nut is relatively tight, as the volume pot body needs to be making good contact with the back of the chassis plate.