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This one has me stumped. It seems to have 2 separate hum issues on the clean channel.
There is something causing a very loud hum before the tone stack. The tone controls affect it. With gain maxed the noise is LOUD, with tone controls dimed, including bass, it motorboats both channels with a slow low oscillation (why its affecting the gain channel...no idea, it shouldnt, and doesnt except the motorboating that occurs with bass dimed and gain on 10 from the clean channel). With mid/bass all the way down and treble dimed, it disappears with gain on 10 but gets much louder as the gain is turned down. All of this is with channel volume and master dimed.
This issue occurs even with V1 removed. Disconnecting the tone stack from the gain knob (same place as the "volume" would be in a fender clean circuit) eliminates the motor boating but it hums like no tomorrow; that may be normal for a live grid with no input though.
Things I've tested: No DC at the pots (less than a mv at least), resistors in that section test ok, some ac (3-13mv) showing at various times at the treble pot output even with v1 removed, thats with no input. Touched up solder connections. No joy.
Hum issue 2: Even With V1 removed and the clean channel gain all the way off, but V4 (other 2 stages for clean channel) in, there is a loud hum as the channel master comes up. With channel master and overall master dimed, the GAIN channel with the master dimed has to have its gain at about 6 before it matches the noise level of the clean channel with the gain at 0. You expect that kind of noise from a very high gain channel at that volume (i.e. wide open), but not from a clean channel with a grounded grid on one stage and only one stage really operating.
With the gain down the grid of V4b is grounded; so the issue is occuring somewhere between V4b and the master volume. V4a/b are reversed in order in the circuit; so V4b comes first, V4a second. Solder joints touched up, resistors tested.
Weird stuff: 28-50mv of AC at the grid of V4a which goes down as its traced back to the plate of V4b so it looks like the source is at the grid of V4a, but........10mv of ac at the plate of v4a........not sure how thats even possible since the ac at the grid should be larger at the plate but thats what I'm seeing.......No DC at the grid of V4a so the coupling cap is doing its job.
With V4 and V1 removed there is a minimal noise as the channel master comes up to 10 but nothing like exists with v1 removed and v4 in. i.e. removing all valves from that channel eliminates (almost) all the noise.
Channel works and makes guitar noise with all valves in and guitar plugged in, just hums like hell.
Looking at the schematic there just arent that many things it can be, given where its isolated to, but damned if I can find the problem. Its bizarre, I've never come across anything that seems so simple but can't be easily diagnosed, and appears to have 2 completely separate issues.
The gain channel appears to work as it should and noise levels are what I'd expect given the volume/gain levels set. Therefore, cant be filter caps since both channels share them.
Any ideas? Schematic can be viewed here: http://schematicheaven.com/boogieamp...aliber_dc5.pdf
Its the "a" version.
Oh yeah.....its not the tubes.
There is something causing a very loud hum before the tone stack. The tone controls affect it. With gain maxed the noise is LOUD, with tone controls dimed, including bass, it motorboats both channels with a slow low oscillation (why its affecting the gain channel...no idea, it shouldnt, and doesnt except the motorboating that occurs with bass dimed and gain on 10 from the clean channel). With mid/bass all the way down and treble dimed, it disappears with gain on 10 but gets much louder as the gain is turned down. All of this is with channel volume and master dimed.
This issue occurs even with V1 removed. Disconnecting the tone stack from the gain knob (same place as the "volume" would be in a fender clean circuit) eliminates the motor boating but it hums like no tomorrow; that may be normal for a live grid with no input though.
Things I've tested: No DC at the pots (less than a mv at least), resistors in that section test ok, some ac (3-13mv) showing at various times at the treble pot output even with v1 removed, thats with no input. Touched up solder connections. No joy.
Hum issue 2: Even With V1 removed and the clean channel gain all the way off, but V4 (other 2 stages for clean channel) in, there is a loud hum as the channel master comes up. With channel master and overall master dimed, the GAIN channel with the master dimed has to have its gain at about 6 before it matches the noise level of the clean channel with the gain at 0. You expect that kind of noise from a very high gain channel at that volume (i.e. wide open), but not from a clean channel with a grounded grid on one stage and only one stage really operating.
With the gain down the grid of V4b is grounded; so the issue is occuring somewhere between V4b and the master volume. V4a/b are reversed in order in the circuit; so V4b comes first, V4a second. Solder joints touched up, resistors tested.
Weird stuff: 28-50mv of AC at the grid of V4a which goes down as its traced back to the plate of V4b so it looks like the source is at the grid of V4a, but........10mv of ac at the plate of v4a........not sure how thats even possible since the ac at the grid should be larger at the plate but thats what I'm seeing.......No DC at the grid of V4a so the coupling cap is doing its job.
With V4 and V1 removed there is a minimal noise as the channel master comes up to 10 but nothing like exists with v1 removed and v4 in. i.e. removing all valves from that channel eliminates (almost) all the noise.
Channel works and makes guitar noise with all valves in and guitar plugged in, just hums like hell.
Looking at the schematic there just arent that many things it can be, given where its isolated to, but damned if I can find the problem. Its bizarre, I've never come across anything that seems so simple but can't be easily diagnosed, and appears to have 2 completely separate issues.
The gain channel appears to work as it should and noise levels are what I'd expect given the volume/gain levels set. Therefore, cant be filter caps since both channels share them.
Any ideas? Schematic can be viewed here: http://schematicheaven.com/boogieamp...aliber_dc5.pdf
Its the "a" version.
Oh yeah.....its not the tubes.
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