The loop effect of mesa mark v cannot be turned off.

[xpy621]

hi，everyone。 I bought the mark v for a while and almost never used the loop.

Recently, I connected the loop and found that I couldn't turn it off. Neither the knob behind the mark v nor the Fx switch on the foot pedal could turn it off. When I turn the rear knob to ft sw and press the fx button on the foot pedal, the light will turn on or off, but the loop effect cannot be turned off.

This problem has some impact on the live performance. If you have a solution, please let me know. Thank you very much.

 

[bandit2013]

I would suspect the relay used to bypass the send/return is stuck on. It is labeled as RYFXJ. Not sure what controls this relay as the general schematic does not have the switching portion. It could be a shorted JFET keeping the relay active. I assume that if the Send/Return loop is open circuit (just two cables inserted in the SEND/RETURN with nothing connected at the other end you get no sound? I would try that and see if that is occurring.

Send an email to customer service and see how they respond. Just say you suspect the relay is always active preventing the FX loop to be turned off. What JFET or device is controlling the relay?

 

[xpy621]

yes. I tried just two cables inserted in the SEND/RETURN with nothing connected, the cabinet makes no sound.

If I unplug the send/retuen line, or connect a Pedal, there will be sound.

I only connected the original mark v pedals。

 

[bandit2013]

With the patch cable in place and nothing at the other end, it should be an open circuit. If the footswitch did nothing or if the selector on the back panel did not allow for footswitch control of the FX loop, it is a latched relay.

When you unplug the patch cables, there is an internal switch in the Return that couples the send to return path. When the amp is placed in hard bypass mode, relay RYFXL switches to the NO contacts and it fully bypasses the FX loop completely. When in active mode, the relay RHYXJ turns on when the FX loop is set with the footswitch and off when the fx loop is turned off with the footswitch. this just bypasses the Send/Return jacks and sends the signal to the recovery stage V6B. This way you get to use the master output and solo boost. Unfortunate that I do not have the part of the schematic that shows what is used to manage the RYFXJ relay. The mini toggle on the back of the amp under the send level control is what controls the RYFXL relay. Since this is a partial based on the first release of the Mark V back in 2009, it does not include the changes made in 2010 or have the switching circuits used for the channels, modes, and peripheral features. Sort of a bummer. The JFET175 are used in may locations on the circuits. Finding out which one is used to pull in that relay would be easy if it was identified on the schematics. Best to email Mesa Customer Service on the issue at hand. They should be able to indicate what device pulls in the FX loop relay. Trying to replace it is the harder thing as it is well packed high density assembly.

 

[xpy621]

Thank you very much for your answer. I'll try to fix it, and if it doesn't work, I'll find someone to fix it. Thank you again!

 

[bandit2013]

Not sure if the relays on the back panel are used with the send/return part of the Mark V90 ( I assume you have the 90W). I keep forgetting about the V:25 and V:35 models.

For the V90, the relays and transistor drivers may be closer to the V6 socket than located on the rear panel. Hard to say, sometimes parts are not located where you would think they are.

the SEND/RETRUN signal is passed along one of the ribbon cables. I believe the wires are marked as S and R on the PCB next to the pin header. Associated parts are more than likely near the V6 socket as there are many relays in that general location. The relays are not marked for easy identification on the PCB as they are on the schematic.

 

[xpy621]

My mark v is the 90w version, I also forgot that it has 25 watt and 30 watt versions. . . . .Sorry for not making it clear before.

 

[bandit2013]

That was always the confusion when the V:25 and V:35 came out. I always forget about them and assume every Mark V thread is on the V90. After it became apparent nobody was making it clear, I adopted the V90 for reference to the 90W as the amps are different. Sorry I did not mention it earlier.

Based on some Schematics of amps made at or during the same time the Mark V90 came out, Mesa has used J175 FETS for many years, to mute circuits, enable bypass caps on cathodes of triode circuits and such. If the culprit is a J175 a thing to understand that it will appear like a short circuit when measuring across the D to S channel. That does not mean it is bad. When the J175 is off or not conducting current, the gate has a +12V signal. When there is 0V or no voltage, the channel will be in conduction mode. It is a P-channel JFET.

However, when it comes to pulling in relays, Mesa has also used 2N6426 NPN transistors. There is a 2N6426 very close to the relay in question but I do not believe that part is used for the relay driver. I believe that is actually the strobe mute circuit. As I mentioned before, I believe the transistor is closer to the V6 tube (assumed). Since it is to turn on or off a relay, it could be anywhere on the PCB.

I wish I had the actual switching section of the schematic as that would be a big help in finding what component is possibly damaged. Assuming it is a transistor could be the wrong approach. The relay could just be stuck mechanically. Try tapping on the relay next to the send/return jacks. If that works, more than likely the relay needs to be replaced. Not much of a tap, perhaps with a marker as a tool. You do not want to tap it too hard just in case that is not the problem.

 

[xpy621]

I also think the switch is stuck due to not being used for too long. I turned the loop control knob on the back repeatedly, and just two cables inserted in the send/return with nothing connected, the cabinet make a little sound. Before there was no sound at all. I will try tapping on the relay next to the send/return jacks. See if that works, thanks for the help.

 

[bandit2013]

If you set the FX selector to the FS position and disconnect the footswitch from the amp, the relay that disconnects the send/return path should be turned off and the internal bypass will be in effect.

I wonder what happens with hard bypass is used. Forgot about that. This should bypass the entire FX loop and its recovery tube stage.

It could also be the ribbon cable connecting the small PCB that has the rotary switches. Either a loose connection or one of the pins has oxidized. Not sure if the switch is sealed or open frame. If it is open frame it may be possible to clean the contacts. I have had issues with the rotary switches before. The amp got stuck in mute for some odd reason. Exercising the spirits did the trick. Ok, sounds funny, but when you rotate the control a few times back and forth, the connecting parts will aid in cleaning off any corrosion or sulfidation on the contacts. Best to do this with the amp powered up. My Mark V90 has a few issues. My amp did not age well but it had many issues to start with. I rarely ever fire it up anymore.

 

[xpy621]

I tried just about everything ,including tapping the knob, disconnecting the footswitch, rotating the control many times with the amp powered up, and vacuuming. . . . I gave up. haha, just use the pedal to turn it off.

 

[bandit2013]

Still odd. but you have a temporary solution.

 

[xpy621]

I found that the fx light above the power switch never light up. Whether it's adjusting the knob on the back or using a foot switch. Because it wasn't bright, I didn't notice there was a light there before. But the effect of fx loop is always there.

 

[bandit2013]

Just for curiosity, have you ever tried the amp in hard bypass? This should eliminate the FX loop in its entirety (you will also loose the output and solo controls, channel masters will be the output control so be prepared for that if you try this).

 

[xpy621]

Yes . I've tried bypass mode. In this mode, the fx loop effect is gone. It would be really weird if the fx loop effect is still there in this mode，haha.

 

[bandit2013]

Yes it would be weird. At least you know the relay RYFXL is working properly.

I would suspect that the RYFXJ has broken its mechanical connections to the contact armature and the relay is stuck in the on or NO position. However if the yellow LED is on indicating the loop is active even with the footswitch disconnected and the rotary knob set to FS position, there should be no circuit path to ground to pull in the relay. The only other explanation would be a bad rotary switch where the contacts are not moving with the rotation of the knob. It is possible there may be some contamination on the rear board housing the switches. I can see the rotary knob switch board. I never tried to remove it but if there is an issue it would be there. If the yellow LED does not turn on and off when trying to enable/disable the relay you have a bad switch that is stuck in the on position. If the yellow FX LED does turn on and off on the front panel near the 5BGEQ, then the relay is stuck in the active position. You may have to remove the ribbon cable from the lower board, remove the switch knobs and then remove the nuts that hold it into place. Not sure if there are other fasteners that secure this small board other than the solder on the pins. I never took a close look at the area between the board and the rotary switch body. If there is some contamination or perhaps some of the foil from the heat shield on the shell fell off and is in there making contact. Hard to say.

The only other possible issue is that the 15uF has shorted out. the LED will probably burn out as it will have 12V across it. They are not tolerant for overvoltage. So if the LED does not turn on, and the FX loop is stuck in being on, the rotary switch in the FS position and the footswitch disconnected from the amp, it should be an open circuit. If that 15uF cap is shorted out, the relay will turn on and stay on.

Wonder if this will work, while the power is off, plug in two cables into the FX loop jacks and measure for continuity between the tips on the patch cables. If you can measure low resistance and not get an OL or high resistance measurement in the Mega Ohm range, the relay RFFXJ is probably damaged. No power to the coil it should be in the NC position which bypasses the two jacks. If you measure open circuit or high resistance, then we know the relay is ok, it is more of a mechanical fault with the rotary switch or something causing a sort to ground on board or perhaps the switch is broken and the moving contacts to make the circuit is shorted to ground or stuck in one position.

 

[xpy621]

The rotary switch is probably not broken because turning it the mark v's foot pedal's fx light will come on or off. It is estimated that the circuit that broke the effect loop internally or a component failed. But this is beyond the scope of what I can repair, so I can only do this for the time being, using the effect pedal to switch the loop. Sorry my English may be a bit weird, I use translation software because it is a bit difficult for me to type out the professional terms about guitar amp. Thank you for your help, I learned a lot.

 

[bandit2013]

I see. I can understand what was translated quite well.

The Mark V90 FX loop relay is controlled by that simple circuit diagram I attached. The rotary knob on the back panel is what pulls in the coil of the relay that connects the FX loop to the send/return jacks or bypasses the signal to V6B fx recovery triode. Since you can see the FX led on the footswitch change while it is connected when you rotate the rotary switch, you should also see the FX led light up on the front panel of the amp. The din cable pin 8 will tie into the footswitch if it is connected. If you pull the footswitch cable from the amp and you cannot get the relay turn on or off the FX signal, it would be a relay fault or a damaged ribbon cable, rotary knob contact. At least that would narrow things down a bit.

I tried to help the best way I can. Other than going into the circuit at the board level with power applied to debug the circuit, tying to keep it high level approach to eliminate unnecessary circuit analysis or component removal. If this must be fixed, you will need a technician skilled for such repair.