Mark V Design Defect - Channel Switching Noise

[steve Rhenius01]

May as well share this with others. If I am wrong on this, please respond with some corrections or comments. I do not want to misguide anyone having the similar issue.

I was able to get the Mark V90 chassis out. I did not realize I already mapped out the strobe mute circuit as the ribbon cable was marked with a black stripe. Here is a different view. At the top of the image below is the strobe mute marked with black and highlighted in yellow. It is the 5th wire from the right side of the ribbon cable. (4th wire from the left). This wire connects to the collector of the 2N6426 NPN Darlington Transistor.

Here is a closer look at the strobe mute circuit. The diodes in this image may be part of the trigger circuit connected to the base. None of those components are connected to the collector. the resistors above the transistor are part of the trigger circuit tied to the base. The tantalum cap or yellow bullet cap may be one of the timing caps. I did not probe it to see if it was connected to the collector.

I traced the strobe signal line from the 5th post on the pin header to a resistor close to the V7 mute circuit. That was a bit difficult as the J175 was pushed down so close to the PCB it was hard to get the test leads in there. The gate has a 330k resistor as marked. The strobe connects to the other side. The black blob next to the plastic board support between V6 and V7 is the J175M1 transistor. You probably found this part already.

The other mute for the signal path that is in front of the GEQ is actually tucked right next to the row of inductors. It is easy to find with the array of Zener diodes. J175M4 is sort of hidden by some wires. I could not find the gate feed circuit. I only focused on the V7 mute (phase inverter mute). Note that the front of the amp will be at the top of the image.

I also found a diode where the cathode was connected to the strobe mute bus. I could not find where the anode was connected too.

The strobe mute should at least have +12V dc on it most of the time. As for the actual voltage on the gates of each of the J175 FETs I did not measure. The only time the two JFETS will be shorting out the signal is when a change occurs on the channel. the signal will get pulled low and remain low for a timing period based on an RC time constant. I could not locate the pull up resistor on the collector of the strobe transistor. Just for S&G's if you want to find the chip used to decipher the voltage from the footswitch, it is located under the ribbon cable that connects to the two rotary switches on the back panel. it is on the same board as the Strobe mute circuit. Good luck if you needed to remove it. This layout is a mess, Almost like the RA100 as things are not where you would expect them to be located.

Reply


So if you are looking at voltages while the amp is powered up, measure the G side of the 330k resistor above in reference to ground. You should have a voltage above the pinch off voltage of 3V to 6Vdc. It may be higher than that as the resistor is probably just a current limit. On the strobe mute side it may be closer to the +12V. Sorry if I did not take this any farther. Not doing so well at the moment.

Thanks - this is really useful - will explore. From the schematic I have it looks as if the dc line derives from a 50V tap on the mains transformer rectified with a diode IN4007 then smoothed with electrolytic and tantalum filter and two zeners dropping the dc to 24V. I think I found the electrolytic which measures 2 V on the positive side. Thinking that either the IN4007 has gone or perhaps the zener(s). Trouble is I can't locate the IN4007 - any ideas about where to find it ? It must be somewhere near the electrolytic but can't find it at the moment. If I run the amp with sig gen at input I can hear the preamp valves and the signal,, low level but no speaker sound. This could be explained by mute circuit failure or the failure of the 24V line. Will explore as above but any pointers on location of the diode would be welcome. Thanks so much, Steve

 

[bandit2013]

The 24VDC circuit is only associated with the GEQ. If the 24VDC circuit is not working, the GEQ will be also be out of commission and no signal will pass through it. The diode for that circuit is probably next to one of the blue power caps on the main board. The kicker is they may be hidden by the caps. If you can find the capacitor for the 24V supply and the two zener diodes, Check to see if the zener diodes are shorted first. You can do a forward bias diode test on a zener diode. Not sure how to guide you with this.

 

[bandit2013]

I have to admit I was completely wrong on the +24V associated with just the GEQ. I was not aware it was also used to source the strobe mute circuits. My bad. I had assumed it was like the Rectifier using the +12V source for that purpose since the schematics are not complete. Also, the measurements I took on the PCB did not match the voltage I would expect from a linear regulator. 12.84V was a bit high. Also noticed the collector was fed by a 330K resistor as well. Based on tolerance differences and the 330k resistor on the gate of the J175M1 circuit, it would appear it is sourced from +24VDC. Go figure. The +12V supply is not used for the strobe mute after all.

Since I was not happy with the Redback the day after I installed it, Still a good speaker but does not seem to tailor the boxy tone out. It sounded really good before I installed the chassis. I will change the speaker yet again to the Jensen Jet Blackbird as I have already painstakingly broke that speaker in. It was one of the few speakers in the Mark V90 combo that sounded good. 3-6 months of clean driving with limited gain or distorted channels. ALNICO speakers do some wonky stuff during the break-in period. I never had that issue with the California Tweed, it was one of the first runs on the amp and it came with the 100W Blackbird without the bell cover. Same for the Extension cab.

The magnet is much smaller in diameter than the ceramics. It is odd why the amp sounds terrible when the chassis is installed vs while it is out of the combo. Not just with the speaker in the combo, I get same problem with the extension cab (Mesa Vertical 212). My Mark V90 was originally a head but converted it to a combo to improve on chassis cooling. Amp was overheating in the head shell. While I have it out, I will check those two diodes under the one 220uF cap where the red/blue-stripe wire is soldered. I believe those to diodes are used for the +24VDC and negative bias voltage since they are arranged in a manner that would suggest those are the parts in question.

The two diodes in question are not in an easy access area, One of the death caps needs to be removed go gain access. Wonder what I did with my bleeder resistor? It is around somewhere. I need to ensure no stored potential for sudden heart attack. Bleed resistor connects to the two cap ends closer to the PT. Then hit the standby switch to connect the rest of the power supply so I can drain any stored energy through the choke and then through the bleed resistor. I will test for voltages before sticking my hand into the circuit. Just looking for the diodes. I will be able to see more once I get the chassis out again.

 

[KRichter]

20 years of MKIV's and the last 10 years with the MKV. Never had an issue with the popping, I do however cycle thru each channel after I power up. I don't know, but heard cycling thru each switch on the pedal controller, dissipates static electricity. Whether of not that is true, I don't know. But, I do it every time.

 

[shredgd]

Hi everyone,

I don’t know if this might be helpful to anyone to identify the problem, but my Mesa Mark V does the loud pop when all channels are in 10W mode only when going to channel 2. I can switch between channel 1 and 3 without pops, and I can switch between all three channels in 45W mode without pops. A mystery!

Giulio

 

[MarkV User]

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[DECEMBER]

My Mark V:90, made in 2012, with the all-black footswitch, that I bought used this year, makes a substantial pop when any footswitch is pressed, and when switches on the amp are flipped. The 90/45/10W switches are the loudest.
My apartment is plagued with EM/RF noise, mostly coming from the pickups, but this amp does hum a tad louder than others. Tubes are all new. Everything is plugged into a Tripp Lite line conditioner and amps also into Morley Hum Exterminators.
I don't play live and don't switch channels while recording, so it hasn't been a problem for me, but it would be if I did either of those things. It's the best sounding amp I've ever had, but it doesn't seem right that the most expensive amp I've ever bought is one of the noisiest.

 

[skoora]

Hi everyone,

I don’t know if this might be helpful to anyone to identify the problem, but my Mesa Mark V does the loud pop when all channels are in 10W mode only when going to channel 2. I can switch between channel 1 and 3 without pops, and I can switch between all three channels in 45W mode without pops. A mystery!

Giulio

That is normal to get a loud pop if switching from a channel set to 10 watts to another channel set to 45 or 90. the amp wasn't designed to be used like that live. If recording or playing at home you need to go to standby to switch silently. Or put all channels at 10 watts for no popping.

from the manual...

"NOTE: The 10 Watt Mode is not really intended as a viable part of the Footswitching matrix of the MARK V due to its limited volume capabilities. While it is plenty loud to play with others, there are huge internal voltage changes that occur when you switch to - and back out of – the 10 Watt setting from the other two power settings (90W & 45W).
These voltage changes across the power tubes and related power supply create noise (pop) during the instant that the voltage is chang- ing and though there is an entire circuit that mutes many places in the signal path, this pop is still audible. If you insist on switching between some Channels that are in 90 Watt or 45 Watt and others that are in the 10 Watt Single-Ended mode, you will have to overlook the “pop” in favor of achieving the exact sound you need. This is unavoidable, normal and will in no way hurt your amplifier."

 

[Eevil]

My current Mark V doesn’t make pops while switching - except to/from the 10W mode.

Used to have a Roadster and after swapping some powertubes it started popping. Even after the mode cycle -trick. Finally bought properly matched power tube set from local vendor and the popping went away

 

[tbonesullivan]

My Mark V is relatively new, and the only pops it makes are for the 10 watt mode, and the first time I switch channels around after power up, which is minor. The 10 watt pop is a bunch more.

 

[bandit2013]

With the Mark V90 and its many different power amp configurations from pentode/triode on CH3, tube tracking/silicon diode at 45W for CH1 and CH2 (has no effect on CH3) and then for the 10W class A power mode, it runs the rectifier tube. Not sure if the CH3 is set to triode for that but I think it is.

Sorry for the length here. Not trying to write a book, just to express some observations.

The switching network is rather complex in how the power tubes are managed. What is going on when changing channels and such. Not only are the preamp tubes reconfigured for each channel (some are bypassed, but some circuits also get re-routed and so on. The strobe mute circuit is almost as complex considering it is based on a fixed time delay for the quick strobe and one that operates on a slow delay for the reverb. Strobe mute only works when using the footswitch or changing the channel on the back panel with the rotary knob. Chance of hearing a pop during the channel transition is possible. One amp that comes to mind that makes pops that are very loud is the Roadster. Similar method of how channels are changed, sure it is different but there is one thing that both have in common, mute mode. I found that after taking the Roadster out of standby to cycle all modes including mute. Once that is done no more pops. The Mark V is also similar in some ways, I always cycle the amp through mute at least once after changing all of the channels. Demons out, it sort of resets the strobe mute circuits and the other mute to remove the stored energy or gate charge associated with the JFETS in the circuits. It became a ritual every time at start up. Lets just say it helps. I still can get pops even after cycling everything. Amp needs to be active and not in standby when cycling the channels and mute circuits. Why cycling the mute helps, not sure but if I do not cycle the mute function, I will have pops all the time. If you do not use the footswitch with the Mark V90, I believe the solo/boost is a pull-pot to activate the mute. I barely ever used it so I could be wrong on that.

Amps that I found do not have the pops, or if they do it is very subtle: JP2C, Mark VII, Triple Crown. I thought perhaps since there was a major change in how the channel configurations were managed compared to the old school method it is better as the midi controller CPU could apply different delay times compared to an RC circuit that may trap the pop or pass it depending on the state of charge on the capacitor used to delay the release of the strobe mute circuit. For all I know, the same method is probably still used as I have not seen the schematics for these three amps (JP2C, Mark VII, TC). They are practically dead silent when switching, no pops. JP2C does have a notable reverb wash in effect more so than the Mark VII. Also present in the Roadster. Mark V90, it did that but then it stopped since the JFET used to mute the reverb circuit failed and had to be replaced if I wanted to hear the reverb again.

Amps that do not have midi controller, Royal Atlantic, Badlander, and the MWDR are also quite tame when it comes to pops when changing channels. The BAD and MWDR do not have reverb so there will be some differences. RA100 does. Never took notice with any pops or reverb wash. If you are not familiar with the reverb wash, it is a quick fade-out and then a gradual fade-in to prevent any noises from channel changes making it into the tank as that will have a slower decay rate than the strobe mute delay.

Considering the amps are running at a moderately high plate voltage, this includes the preamp tubes, there will be some noises associated with the channel change, it is more notable when changing power modes with the toggle switches or the mode switch on each channel since the toggle switches that define the channel characteristics are not included in the trigger for the strobe mute. Not all amps are created equal, component tolerances may have some effect on hearing pops vs not hearing them. Some amps of a different brand may have a silent change of channels. Hard to compare such as the bulk of the preamp may not change much in signal routing. They may also make use of a noise gate with the circuit muting feature if so equipped.

I would not necessarily call it a defect, more of a minimized circuit that needs to be improved upon considering the level of locations where the strobe mute takes effect. There are a total of 6 circuits associated with the strobe mute. I do believe the circuit used for the general mute is also triggered by the strobe mute circuit (that may be incorrect). Reverb mute (J175M2 and J175E), phase inverter mute (J175M1) GEQ-FX send mute (J175M4), CH1 and CH2 mute (J175M3) and the last one is associated with the speaker mute (J175SM) as it is also triggered by the pulse bus that causes the J175 JFETS to short out the circuits in an attempt to hide the electrical transients that occur with the configuration relays and such.

 

[MarkV User]

My memory of it is vague now but I think only had enough channel switching noise to bother me when the low voltage diodes were failing. But it was a long time between when I started to have pops and dropouts with the foot controller and when the whole amp stopped working. It seemed that several annoyance problems went away at once when I put in the new diodes. For what it's worth. It doesn't mean that other people's similar problems would necessarily be fixed by changing the diodes. But it is possible to check them in circuit with a meter against the possibility.