Mark v volume loss
- Thread starter Ckocher
- Start date
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Sent you a message. I hope that helps you out.
tbonesullivan
Well-known member
Are all the tubes lit? How long have you had the amp? How much mileage is on the tubes?
I assume you did not see the message I sent you. This was based on what you posted on my profile page. Sorry I could not answer in detail. 450 characters is just a few sentences.
You say the amp just silently faded out? No preceding noises like a loud hum?
Anyhow, the tubes may look ok as the heaters should be on. As long as the silver coating on each of the tubes has not turned white. If that happened, it lost vacuum and should be replaced.
Does the silence cover all channels? If yes, try moving the channel selector knob on the back panel while the amp is on and out of standby as if you are ready to play through it. The only time I had a fade out issue with the Mark V, it corrected itself after I moved the channel selector switch a few times. Cycle it though all positions, no need to go fast with it. Just confirm it changes the channels. Try playing the guitar again. No sound. Does the guitar use active pickups? if yes, check the battery.
Assuming you have gone through the basic checks, like removing everything from the FX loop. Simplify the signal path by removal of all devices. That would be the best way to figure out what is up with the amp.
Does it work in 90W mode? This may not do anything but try it first.
First, plug the guitar into the FX loop return jack. The amp needs to be set to FX loop active, move the rotary selector to "on all", This will turn on the FX loop and bypass the footswitch. However, confirm the LED is turned on the front of the amp. If not you may need to press the FX button on the footswitch. Adjust the output volume above the solo boost a bit higher than normal. Play the guitar plugged into the FX return and listen for sound from the speaker. If you get sound, great, that overs at least one and a half preamp tubes, V7 and V6B, and rules out a strobe mute fault. If no sound at all, V6, V7 may need to be replaced. However, if there was a defect in the strobe mute circuit, you may not get any sound at all. If it is not one of the two tubes, the amp may need to be serviced. That generally does not sound like a fade out thing. I suspect a preamp tube failed. If you have any spare 12AX7 tubes, does not matter what brand they are for this analysis. We just want to get the amp working. We can also swap tubes if you have none available. Remove V7 and set aside. Then remove V6 and set that in a different spot so you do not mix them up. Move the tube in V1 to the V7 position, move the tube in V2 to the V6 position and try playing the guitar though the FX return again. If you do not get sound after that, chances are there is an internal issue and that could be many things. Trying to isolate what component could be time consuming. It could be a JFET on the strobe mute, there are a few of those. It could be a DC blocking cap, there are many of those. If a DC blocking cap shorted out, that will cause the control grid voltage to exceed the cutoff point and not pass any signal. It could also be a short somewhere on a control grid. For now, lets assume it is not internal issue. If you do get sound, that is great. So that would mean V6 or V7 you removed are no good. There is a trick to find out which one. Power down and return the V1 tube you inserted into V7 and the V2 tube inserted into V6.
Put the V7 tube you set aside back into V7 socket. And return V6 to its socket. Change the mini toggle switch on the back panel to hard bypass. Just remember to drop the channel volumes on each channel before you power it up. Plug the guitar cable into the front input jack. Try playing and listen for sound. If you get no sound but did with the FX test, it would mean the tube that was in V7 is no longer functional. If you do get sound, that would mean the V6 tube needs to be replaced.
Sure this is great and all but say you get no sound at all. Is the fan running? If the answer is no, and the fan is turned on with the toggle switch located next to the bias selector switch. If the fan is not running with the switch in the on position, that would indicate the +12V regulator has failed or one or more of the rectifier diodes on that circuit has failed. You may get some function of the channels but if the regulator is not maintaining 12V, there is a good chance three of the preamp tubes are running cold, no heater current means a cold tube, no heat, no electron flow, results in no sound. V1, V2 and V5 run a DC heater voltage supplied by the 12V regulator. Say you lost one of the diodes in the bridge circuit, the 5V part may still be working ok but the 12V circuit will be reduced. Fan would not be running like normal and the heaters in the three tubes may not be glowing hot enough to spit off electrons from the cathode. This is one of the common faults in the Mark V. Voltage regulator or a diode failed on the bridge.
If that is not the case, but no sound in hard bypass, power the amp down, let cool. If you got sound with the FX loop test using V1 and V2 as described. that means V1 and V2 are probably OK, that would narrow it down to V3 as it runs in all channels. Set the channel to CH1. Swap V2 with V3 and check for sound. no sound. that is a bummer. If you did the steps from start to finish and did not determine if you have a bad tube. It may be possible you do but it could be more than one. If that is the case, go back to the fx return test. Just keep the V7 tube in a place to prevent mixing up with the others as it may have matched triodes. Use any of the other tubes in V6 and V7 position to confirm you get sound or no sound. So what if they all work with the FX loop test trick? Well, it may mean your amp will need service. If you cannot get the FX loop trick to work at all, that means internal problem. amp will need service.
Trying to find out what JFET is damaged assuming that is the issue, very tedious task. Checking and replacing diodes and voltage regulator, not easy either. May require PCB removal. Tring to figure out if it is a dc blocking cap, another tedious task. I am not the Mark V expert but have seen inside the amp. May have also messed around with a few circuits and such. Getting a good look at the JFETS and other components, not something I would want to work on unless the amp board is removed from the chassis. Not something you want to tackle on a project unless you absolutely know what you are doing. Best for tech service. If they are certified Mesa tech, they have the backing of the engineers or technical staff to supply them with information not common to us.
You say the amp just silently faded out? No preceding noises like a loud hum?
Anyhow, the tubes may look ok as the heaters should be on. As long as the silver coating on each of the tubes has not turned white. If that happened, it lost vacuum and should be replaced.
Does the silence cover all channels? If yes, try moving the channel selector knob on the back panel while the amp is on and out of standby as if you are ready to play through it. The only time I had a fade out issue with the Mark V, it corrected itself after I moved the channel selector switch a few times. Cycle it though all positions, no need to go fast with it. Just confirm it changes the channels. Try playing the guitar again. No sound. Does the guitar use active pickups? if yes, check the battery.
Assuming you have gone through the basic checks, like removing everything from the FX loop. Simplify the signal path by removal of all devices. That would be the best way to figure out what is up with the amp.
Does it work in 90W mode? This may not do anything but try it first.
First, plug the guitar into the FX loop return jack. The amp needs to be set to FX loop active, move the rotary selector to "on all", This will turn on the FX loop and bypass the footswitch. However, confirm the LED is turned on the front of the amp. If not you may need to press the FX button on the footswitch. Adjust the output volume above the solo boost a bit higher than normal. Play the guitar plugged into the FX return and listen for sound from the speaker. If you get sound, great, that overs at least one and a half preamp tubes, V7 and V6B, and rules out a strobe mute fault. If no sound at all, V6, V7 may need to be replaced. However, if there was a defect in the strobe mute circuit, you may not get any sound at all. If it is not one of the two tubes, the amp may need to be serviced. That generally does not sound like a fade out thing. I suspect a preamp tube failed. If you have any spare 12AX7 tubes, does not matter what brand they are for this analysis. We just want to get the amp working. We can also swap tubes if you have none available. Remove V7 and set aside. Then remove V6 and set that in a different spot so you do not mix them up. Move the tube in V1 to the V7 position, move the tube in V2 to the V6 position and try playing the guitar though the FX return again. If you do not get sound after that, chances are there is an internal issue and that could be many things. Trying to isolate what component could be time consuming. It could be a JFET on the strobe mute, there are a few of those. It could be a DC blocking cap, there are many of those. If a DC blocking cap shorted out, that will cause the control grid voltage to exceed the cutoff point and not pass any signal. It could also be a short somewhere on a control grid. For now, lets assume it is not internal issue. If you do get sound, that is great. So that would mean V6 or V7 you removed are no good. There is a trick to find out which one. Power down and return the V1 tube you inserted into V7 and the V2 tube inserted into V6.
Put the V7 tube you set aside back into V7 socket. And return V6 to its socket. Change the mini toggle switch on the back panel to hard bypass. Just remember to drop the channel volumes on each channel before you power it up. Plug the guitar cable into the front input jack. Try playing and listen for sound. If you get no sound but did with the FX test, it would mean the tube that was in V7 is no longer functional. If you do get sound, that would mean the V6 tube needs to be replaced.
Sure this is great and all but say you get no sound at all. Is the fan running? If the answer is no, and the fan is turned on with the toggle switch located next to the bias selector switch. If the fan is not running with the switch in the on position, that would indicate the +12V regulator has failed or one or more of the rectifier diodes on that circuit has failed. You may get some function of the channels but if the regulator is not maintaining 12V, there is a good chance three of the preamp tubes are running cold, no heater current means a cold tube, no heat, no electron flow, results in no sound. V1, V2 and V5 run a DC heater voltage supplied by the 12V regulator. Say you lost one of the diodes in the bridge circuit, the 5V part may still be working ok but the 12V circuit will be reduced. Fan would not be running like normal and the heaters in the three tubes may not be glowing hot enough to spit off electrons from the cathode. This is one of the common faults in the Mark V. Voltage regulator or a diode failed on the bridge.
If that is not the case, but no sound in hard bypass, power the amp down, let cool. If you got sound with the FX loop test using V1 and V2 as described. that means V1 and V2 are probably OK, that would narrow it down to V3 as it runs in all channels. Set the channel to CH1. Swap V2 with V3 and check for sound. no sound. that is a bummer. If you did the steps from start to finish and did not determine if you have a bad tube. It may be possible you do but it could be more than one. If that is the case, go back to the fx return test. Just keep the V7 tube in a place to prevent mixing up with the others as it may have matched triodes. Use any of the other tubes in V6 and V7 position to confirm you get sound or no sound. So what if they all work with the FX loop test trick? Well, it may mean your amp will need service. If you cannot get the FX loop trick to work at all, that means internal problem. amp will need service.
Trying to find out what JFET is damaged assuming that is the issue, very tedious task. Checking and replacing diodes and voltage regulator, not easy either. May require PCB removal. Tring to figure out if it is a dc blocking cap, another tedious task. I am not the Mark V expert but have seen inside the amp. May have also messed around with a few circuits and such. Getting a good look at the JFETS and other components, not something I would want to work on unless the amp board is removed from the chassis. Not something you want to tackle on a project unless you absolutely know what you are doing. Best for tech service. If they are certified Mesa tech, they have the backing of the engineers or technical staff to supply them with information not common to us.
Well that was something different. Hope you get it up an running soon.
Yes. I bought brand new in 2010. I’ll keep you posted.
