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!
