Customer comes in with a Marshall Origin 50 that once played, but then one day the sound just "turned to sh*t" as he put it.
I found out that the amplifier was stuck in low power mode (3 power settings via toggle, controls screen grid voltages of power tubes).
I discover that the power is being limited by the circuitry related to TR11/12. If I remove TR11, the amplifier can be set to all three power levels and works normally.
If you look at the 4R 8R 16R jacks (speaker outs), you see there are three diodes. Their job is to pull base of TR11 low and enable full power operation.
The way it's supposed to work is that when the barrel of 1/4" plug is inserted to any jack, the cathode of one of the 1N4148 diodes gets pulled to ground.
However, that is not happening. It's the opposite. When the plug is inserted, the contacts open up, preventing the diodes from being grounded. The other side (terminal NR) seems to be the one that SHOULD be connected to the diode, but it's floating on this schematic.
I tested all three jacks with a plug into it and they all behave the same. Plug inserted, D201/202/203 lose ground. Yet the amp supposedly worked, according to customer.
I have removed TR11 to test my hunch and the amp can be set to all three power levels now. The way this is wired, I don't see how this ever could have worked.
I found out that the amplifier was stuck in low power mode (3 power settings via toggle, controls screen grid voltages of power tubes).
I discover that the power is being limited by the circuitry related to TR11/12. If I remove TR11, the amplifier can be set to all three power levels and works normally.
If you look at the 4R 8R 16R jacks (speaker outs), you see there are three diodes. Their job is to pull base of TR11 low and enable full power operation.
The way it's supposed to work is that when the barrel of 1/4" plug is inserted to any jack, the cathode of one of the 1N4148 diodes gets pulled to ground.
However, that is not happening. It's the opposite. When the plug is inserted, the contacts open up, preventing the diodes from being grounded. The other side (terminal NR) seems to be the one that SHOULD be connected to the diode, but it's floating on this schematic.
I tested all three jacks with a plug into it and they all behave the same. Plug inserted, D201/202/203 lose ground. Yet the amp supposedly worked, according to customer.
I have removed TR11 to test my hunch and the amp can be set to all three power levels now. The way this is wired, I don't see how this ever could have worked.
The way I read the schematic, each TRS jack has 3 NC switches on each contact. Did you verify they still are NC? These can fail open over time or just get mangled by unknowing hands.
According to the schematic, inserting a TS plug should ground the diodes. Are you testing with a TS (mono) plug and NOT a TRS (stereo) plug?
According to the schematic, inserting a TS plug should ground the diodes. Are you testing with a TS (mono) plug and NOT a TRS (stereo) plug?
Yes, that is how I understand it. The idea being that if the speaker is unplugged the amplifier power is limited to a safe level.
The jacks contacts make closure when the plug is removed. That seems opposite of what it should be for this to work as a protection feature.
The plug I'm using is a standard T/S (mono) 1/4" phone plug for the dummy load connection.
Looking at the schematic, it WOULD work if R and NR connections were swapped. But the contacts open when the plug is inserted and any possible ground path is disconnected. (scratching head)
The jacks contacts make closure when the plug is removed. That seems opposite of what it should be for this to work as a protection feature.
The plug I'm using is a standard T/S (mono) 1/4" phone plug for the dummy load connection.
Looking at the schematic, it WOULD work if R and NR connections were swapped. But the contacts open when the plug is inserted and any possible ground path is disconnected. (scratching head)
That's what I am trying to get across here.. it behaves as if they are swapped. There's no continuity on ANY of the three plugs to ground from any of the three diodes when a plug is inserted in any of the three jacks. The odds of all three traces burning out are slim. It seems to be a simple circuit almost foolproof, but it's failed on all three jacks. I have continuity to the non-moving contact (the one that is stationary, not the one that moves) which makes no sense given how it is supposed to work. The moving contact has no continuity to the diodes once the plug is inserted due to the contact being broken. Baffling.
Each diode has continuity to a center terminal (marked by the red arrow), This terminal becomes floating when a plug is inserted into the jack.
IOW, the diode is connected to the "armature" when nothing is plugged in.
I was thinking that I could try grounding the 3 center armatures and that would probably fix the problem.
How this got into this state is still unknown.
I jumpered the upper armature to ground and it works as it should. It's as if the factory made a mistake and wired the shunt backwards. I can correct this by adding small jumper wires from ring to sleeve on each of these jacks on the moving contact armature side.
IOW, the diode is connected to the "armature" when nothing is plugged in.
I was thinking that I could try grounding the 3 center armatures and that would probably fix the problem.
How this got into this state is still unknown.
I jumpered the upper armature to ground and it works as it should. It's as if the factory made a mistake and wired the shunt backwards. I can correct this by adding small jumper wires from ring to sleeve on each of these jacks on the moving contact armature side.
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Final update:
Looks like it was some dodgy solder connections under the jacks. All three of them looked like micro cracking on the center row of pins.
Resoldered everything and installed TR11 and now it works properly. That jack strip was a royal PITA to R&R though! Had to do it twice, due to the washers falling off the jacks.
Looks like it was some dodgy solder connections under the jacks. All three of them looked like micro cracking on the center row of pins.
Resoldered everything and installed TR11 and now it works properly. That jack strip was a royal PITA to R&R though! Had to do it twice, due to the washers falling off the jacks.
Hello, so sorry for posting here even if the post is some months old.
I'm having an issue with my Origin 50C which might be related to this: when I use it in 50W power mode, at a certain point it starts "losing power" sonically speaking, I can't really tell but it feels like it drops to the 20W (which is the Mid mode) power mode. Once I was letting it warm up while in the 50W power mode, and I started hearing internal noises much similar to the ones I hear when manually switching power modes. I don't know enough about electronics to understand the schematic you posted and neither to put my hands inside an amp like this one, but at least I could point to a technician where to start looking.
I'm having an issue with my Origin 50C which might be related to this: when I use it in 50W power mode, at a certain point it starts "losing power" sonically speaking, I can't really tell but it feels like it drops to the 20W (which is the Mid mode) power mode. Once I was letting it warm up while in the 50W power mode, and I started hearing internal noises much similar to the ones I hear when manually switching power modes. I don't know enough about electronics to understand the schematic you posted and neither to put my hands inside an amp like this one, but at least I could point to a technician where to start looking.
Marcello,
Failing tubes (or other components) or dodgy solder connections under the jacks like Ampexperts found could cause a loss of power.
That said, "losing power sonically speaking" may not be due to the amp function.
The switch should reduce power from 50W to 10W (not 20w) in the middle and 5W at the lowest setting.
The 12" celestion speaker can do as much as 105dB SPL (sound pressure level) at one meter with one watt, over 120dB in the ears most sensitive region at 50 watts.
Exposure to that SPL level can cause a TTS (temporary threshold shift) in hearing after a short time, you could hear the same level as "less loud".
The difference between 50 and 5 watts is -10dB, and sounds about half as loud to hearing that has not experienced TTS.
The level difference between 50 and 10 watts is a little bit more than -6dB.
TTS can drop your hearing sensitivity more than -6dB.
When speakers get hot, the voice coil impedance rises, and the amp puts out less power into the higher impedance, could be a loss of -2dB in addition to TTS.
Art
Failing tubes (or other components) or dodgy solder connections under the jacks like Ampexperts found could cause a loss of power.
That said, "losing power sonically speaking" may not be due to the amp function.
The switch should reduce power from 50W to 10W (not 20w) in the middle and 5W at the lowest setting.
The 12" celestion speaker can do as much as 105dB SPL (sound pressure level) at one meter with one watt, over 120dB in the ears most sensitive region at 50 watts.
Exposure to that SPL level can cause a TTS (temporary threshold shift) in hearing after a short time, you could hear the same level as "less loud".
The difference between 50 and 5 watts is -10dB, and sounds about half as loud to hearing that has not experienced TTS.
The level difference between 50 and 10 watts is a little bit more than -6dB.
TTS can drop your hearing sensitivity more than -6dB.
When speakers get hot, the voice coil impedance rises, and the amp puts out less power into the higher impedance, could be a loss of -2dB in addition to TTS.
Art
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Art thanks for the thorough answer which clarified some stuff I vaguely knew and introduced some new insight to me.
However I have to exclude that this issue is related to TTS, since when it happens there's a time frame of some minutes in which the volume suddenly drops and comes back to level. Operating the amp at 10W gives no such issues so I'm also excluding that it's related to pre-amp tubes failure (I've that in the 12AX7s, one of the two triodes may fail before the other and result in a drop of volume even if the whole amp still works).
Might it still be that the power tubes have issues at 50W that don't show up when they're at lower power settings?
But still this wouldn't explain the "switching"-like noises I hear when this thing happens (the noises are from something mechanical moving, since they don't come from the speaker and happen even with both master and gain set to 0).
However I have to exclude that this issue is related to TTS, since when it happens there's a time frame of some minutes in which the volume suddenly drops and comes back to level. Operating the amp at 10W gives no such issues so I'm also excluding that it's related to pre-amp tubes failure (I've that in the 12AX7s, one of the two triodes may fail before the other and result in a drop of volume even if the whole amp still works).
Might it still be that the power tubes have issues at 50W that don't show up when they're at lower power settings?
But still this wouldn't explain the "switching"-like noises I hear when this thing happens (the noises are from something mechanical moving, since they don't come from the speaker and happen even with both master and gain set to 0).