Bottlehead Forum
Bottlehead Kits => Crack => Topic started by: TravAndAlex on February 15, 2018, 08:51:55 AM
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Hi good folks of the BH forums. Hoping you can help, because I'm rather stumped. I purchased an assembled Crack a year or so ago, and the previous owner was clearly new to assembly and soldering. I discovered and fixed a few unsoldered connections and purchased a Speedball upgrade and installed it. That led me down the upgrade path with film caps, bypass caps, choke and Alps blue volume pot - and of course a plethora of tube options. It has worked great for a year or so.
Last week, after it has sat for a couple weeks unused, I plugged my trusty Sennheiser HD650's into it and found a hiss noise mainly in the left channel. This hiss is affected by the volume knob and becomes noticeable at just about listening level (9:00 or so on the volume control) and continues to get louder as you turn up the volume knob, until it disappears (at a completely too loud listening level) around 3:00 on the volume control. Prior to this, it was completely silent at regular volume and only had an audible hiss near full volume.
I've searched and read through the forums and haven't found anything to address my issue. Here's what I've tried so far:
Tried different tubes. Swapped both tubes for several different known working tubes.
Moved to a different room and ensured no phones nearby.
Changed power cord.
Used input short jacks (shorted RCA connectors) to verify the problem is within the Crack and not from the source (typically a Dragonfly Black DAC).
Replaced the original volume pot with an Alps Blue (that I've had lying around waiting for it). I found this actually lowered the sensitivity (not quite as loud at the same knob position as the old pot) but had no impact on the hiss noise other than to move the "noticeable range" higher in line with the music volume.
Carefully gone through the solder connections and resoldered anything that looked in any way dodgy. And then resoldered anything I could reach.
Unsoldered bypass caps on the power supply.
I have a new kit on hand waiting for me to put it together and it has the new v1.1 Speedball boards (WOW what an improved design!). I assembled it and swapped it in place of my existing old version Speedball. No change in hiss.
Tested voltages (using RCA red base 6080WA and CBS tubes):
Terminal Expected Measured (DC v - at line voltage AC 120.6v)
1 75 73.1
2 170 174
3 0 0
4 170 174.2
5 75 73.9
6 0 0
7 100 107.1
8 0 0
9 100 105.3
10 0 0
11 0 0
12 0 0
13 170 174.3
14 0 0
15 185 193.8
20 0 0
A1 75 72.9
A2 0 0
A3 1.56 1.53
A4 0 0.4-0.6 FLUCTUATING
A5 0 0.4-0.6 FLUCTUATING
A7 0 0
A8 1.56 1.536
A9 0 0.001
B1 75 73.1
B2 170 173.9
B3 100 107.5
B4 75 73.7
B5 170 174
B6 100 106.3
B7 0 0.6-0.7 FLUCTUATING
B8 0 0.35-0.45 FLUCTUATING
Are those fluctuating voltages above a concern? Anything else there too far out of range?
What do I do now? Thanks in advance!
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Picture in case it helps.
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Are you hearing hiss or hum? The choke could be introducing hum. When you change the layout as much as you have it gets quite difficult for us to give you a definitive answer of what could be causing the problem. Returning the amp to the stock layout might be a good starting point, then add each mod back one at a time and listen to see what is causing the problem.
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I'd definitely call it a "hiss" rather than a "humm".
The thing is, it's worked in this configuration for the better part of a year. Then just decided one-day last week to start the hiss noise.
I was afraid of the "take it back to stock" suggestion was coming! :)
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I'm not quite the expert on these things, but I have some reasons to believe that your volume pot may not be authentic.
Our recommendations will always be to take things back to the stock configuration for debugging.
-PB
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I'm sorry if I wasn't clear. It actually happened with the stock volume pot in there. I changed to the (perhaps counterfeit) Alps as a troubleshooting step after the issue started.
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I absolutely hear you about easier troubleshooting if it is in stock form (and I'm sure you can appreciate why I'm reluctant to start with that step). I was hoping to avoid that labour if the issue pointed to a certain item - hiss controlled by volume control but disappearing at 90-100% volume. Or if any of my voltages point towards any issue?
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your voltages are all just fine. This is most likely a flaky solder joint in the amp somewhere, though I have seen loose hardware also cause random noise in one channel.
I have also seen this happen once in a while when the jumper between the two ground lugs on the volume pot is missing or not well connected.
-PB
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OK.... thanks! I'll take another run at the volume pot.
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I've decided to do something a bit more radical and commit to the 'back to basics' approach. I've never been thrilled with the original paint job on the top plate (nor the original assembly job), so I'm going to strip it all the way back, repaint and rebuild. New components (resistors, capacitors, diodes, LEDs) ordered. Since I'm going to need to back up to troubleshoot, I figure I'll just go all the way.