Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: hesamadman on February 16, 2016, 06:49:33 pm
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Been tinkering around with a Lonestar. I was told that it had some issues but never had a problem with it while I played it. It has a hard bypass on the FX loop. Today I turned the bypass off and sure enough there was an extreme loss of volume. (This is with the slave and the output knob all the way up) This should have an increase in volume since it is now running through the buffered FX loop not a loss. Sometimes it will fizzle out while playing. Most of the time if it snapps out of it and comes back to normal volume, I can recreate the issue by simply flipping the FX bypass on and back off. Here's where it gets crazy. When I was having the issue I wanted to test voltage on V3 (FX loop 12ax7). All voltages checked out fine but when I have my multimeter black lead clamped on the chassis and I touch pin 3 (output since this is a cathode follower) it MAGICALLY regains consciousness! Ok maybe its a pad tube socket? Or maybe the tube pin isn't contacting correctly? Nope. If I touch this same pin with a wooden stick it does not change like this. If I touch it with anything else it doesn't change. It only comes back when my meter is connected to ground and I touch the other lead to pin 3. (unfortunately im not sure if this trick works on any other spots. I just know for sure pin 3)
Im assuming the issue is between the relay (if not the relay) and the cathode of the first stage. Which is only a few components. I have never dealt with relays before so im not sure if they can fizzle out or if they are one of those things that either work, or dont.
Schematic (fx loop on page 5): http://el34world.com/charts/Schematics/files/mesa_boogie/boogie_lonestar.pdf (http://el34world.com/charts/Schematics/files/mesa_boogie/boogie_lonestar.pdf)
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This must seem crazy to everyone else too :laugh:
Maybe someone could give a little info on what the purpose of a couple of these components are actually doing in the circuit. In the photo I have two things highlighted in grey with a red question mark. Two diodes in anti-series (i believe its called) and one transistor. The diodes look like they are placed for something similar to diode clipping, but I have not seen them facing each other like that in a clipping circuit. I know the transistor is part of the switching but not sure what it does exactly. I am still wondering if the problem may possibly be in the loop bypass switch, relay, or even send and return jack. Im not sure about the switch because it is a SPDT, but in the amp schematic it looks as if its just a single pole going to ground. Im assuming when the switch is on it just shorts out the relay?? I know nothing of relays so hopefully I pick up some knowledge from all this. I wouldn't think it would be in any of the caps or resistors in the circuit as none of those (with the exception of the cathode follower) have any voltage on them. I would think the problem would lie in something mechanical or something that has withstood an amount of heat for some time, and even that only leaves two .68 caps and maybe a couple resistors.
My suspicion of the relays are based upon when I touched my meter to pin 3 of v3a. With my black lead clipped to the chassis, I touched pin 3 with my red lead. It instantly takes the problem out of the amp. The only reason I can think of is it gives the signal a quicker path to ground through my meter and causes a quick jolt in current which makes the loose or weak component regain continuity. I dont know if this makes sense or is even possible in this scenario but its what comes to mind. Any advice would be appreciated.
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I don't fool with that kind of Mesa, but I can tell you what those components are. The one that looks like two diodes is a bi-directional transient voltage suppressor (TVS). That particular one is a Littlefuse SA20. The other one is a P-Channel jFET typically used as an analog switch.
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Have you cleaned the jacks? Including the switch contacts?
Cutting-in when audio is spiked is very typical of thin oxide (tarnish) on contacts.
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Have you cleaned the jacks? Including the switch contacts?
Cutting-in when audio is spiked is very typical of thin oxide (tarnish) on contacts.
im pretty sure this is what it is. I havent messed with it in a few days but when i was having the issue i tapped the jack and it seemed to make a difference. Do you have any preferred method to clean these tips?