Hi guys, I recently picked up a early 80's solid state Marshall 5010 Master Lead Combo. It sounded great at low volumes, but after a 2 hour band rehearsal at louder volumes, I am now getting an intermittent hum and some crackle on the output of the speaker as chords are allowed to ring out. My initial thought was that there might be something wrong with the opamp in the pre, but after some thinking and reading I checked for DC on the speakers. With nothing plugged in or played through it, I'm getting -.039V on the speaker output.
Here's the schematic.
http://www.drtube.com/schematics/marshall/5010.gif
Any ideas?
Here's the schematic.
http://www.drtube.com/schematics/marshall/5010.gif
Any ideas?
1. Get to that state and then plug in some headphones and see if the sound is still messed up.
This will eliminate or select the speaker as a culprit.
2. Carefully touch the output transistors to see if they are excessively hot. They might need to be re-mounted with new thermal paste.
Worth a try.
This will eliminate or select the speaker as a culprit.
2. Carefully touch the output transistors to see if they are excessively hot. They might need to be re-mounted with new thermal paste.
Worth a try.
When you come back tomorrow looking for your thread, look for it down in the instrument amp section, they will be moving it when they spot it.
As far as I know there is no thermal protection on this amp. Well unless there is a thermal fuse in the PT.
You could have something as simple as cracked solder under the input jack or even on the main filter caps, in fact try them first.
Once the amp warms up and is ready to do this, ball up your fist and whack the top of the amp. That will jolt any loose connections. If the amp reacts to the whack, there is a loose connection somewhere.
DUG's headphone test will tell you if the speaker is the problem, or its wiring, or the vibration it makes.
As far as I know there is no thermal protection on this amp. Well unless there is a thermal fuse in the PT.
You could have something as simple as cracked solder under the input jack or even on the main filter caps, in fact try them first.
Once the amp warms up and is ready to do this, ball up your fist and whack the top of the amp. That will jolt any loose connections. If the amp reacts to the whack, there is a loose connection somewhere.
DUG's headphone test will tell you if the speaker is the problem, or its wiring, or the vibration it makes.
Hi DUG and Enzo, all excellent ideas. Here's some additional info
1) I also suspected the speaker at first. Since the amp has no speaker jack, I tested it by hooking one of my other amps to the speaker cabinet via the speaker connects on the speaker. There were no issues running the speaker that way. Not sure if it makes a difference for what we are testing, but I used a tube amp to test the speaker. This version of the amp has no headphone or DI output either.
2) I'll try that today to see of they get hot. The output transistors are mounted with large heat sinks in this amp so I'm guessing that they're expected to get hot. I don't see any thermal paste around these transistors, but perhaps it's underneath? They sit directly on the PCB. I can provide a picture if that would help.
3) Enzo, you are correct - I don't think there is any thermal protection unless you count the heat sinks. Since the input jacks were pretty shot, I actually changed them last night to rule out a bad connection or weak contacts. That unfortunately didn't fix the problem. I will give it the whack test, though I'm sure I've already hit it out of frustration a few times to no avail.
1) I also suspected the speaker at first. Since the amp has no speaker jack, I tested it by hooking one of my other amps to the speaker cabinet via the speaker connects on the speaker. There were no issues running the speaker that way. Not sure if it makes a difference for what we are testing, but I used a tube amp to test the speaker. This version of the amp has no headphone or DI output either.
2) I'll try that today to see of they get hot. The output transistors are mounted with large heat sinks in this amp so I'm guessing that they're expected to get hot. I don't see any thermal paste around these transistors, but perhaps it's underneath? They sit directly on the PCB. I can provide a picture if that would help.
3) Enzo, you are correct - I don't think there is any thermal protection unless you count the heat sinks. Since the input jacks were pretty shot, I actually changed them last night to rule out a bad connection or weak contacts. That unfortunately didn't fix the problem. I will give it the whack test, though I'm sure I've already hit it out of frustration a few times to no avail.
Here's a sample of the hum and static I'm hearing.
Marshall Master Lead Combo - Static and Hum - YouTube
Marshall Master Lead Combo - Static and Hum - YouTube
OK, I took the PCB out completely and I remembered finding some lock washers that were in the chassis - I think these were meant to sit between the ground screw and the ground point. I put them in there, but unfortunately that didn't help.
The hum is now constant and the static has an almost strange tremolo effect to it. It's a weird sort of throbbing I'm having a hard time describing.
The hum is now constant and the static has an almost strange tremolo effect to it. It's a weird sort of throbbing I'm having a hard time describing.
Hi Enzo,
I went back to the cliff jacks and they seem to be working fine without oscillation. From what I've read, the cliff jacks on these short the high input to ground when it is unplugged, but that doesn't work for standard switchcrafts.
The only issue I'm having now is that there seems to be a very quiet distorted sound that is present all the time. So even if I set the amp clean, there is a little distorted sound in the background. It almost sounds like cabinet vibration - but playing other amps through the cabinet and speaker had no issues.
Honestly, I actually can't remember if the amp was doing this before or not.
I went back to the cliff jacks and they seem to be working fine without oscillation. From what I've read, the cliff jacks on these short the high input to ground when it is unplugged, but that doesn't work for standard switchcrafts.
The only issue I'm having now is that there seems to be a very quiet distorted sound that is present all the time. So even if I set the amp clean, there is a little distorted sound in the background. It almost sounds like cabinet vibration - but playing other amps through the cabinet and speaker had no issues.
Honestly, I actually can't remember if the amp was doing this before or not.
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