Mesa MarkV / Saturation 'mod'

[Markageddon]

Best of luck Wayno. Hope you get her up and running real soon. Fingers crossed for you.

 

[bandit2013]

The black part is the same as the yellow part. Tantalum 0.22uF capacitor. Black ones may be a military grade as I have a few of those from a prior life (college student).

For those who may wonder why there is a 3.3M resistor on the board, it is probably the 3.3M grid leak resistor that is connected to V4A grid and the pair of Zener diodes. It may be missing from the board in its entirety and if so Wayno has an original or first run on the amp (assuming the part was not added later as a result of repair). If I am not mistaken, the original location for R? (it is on the schematic but does not have a reference designation so that may be apparent on the 2009 build with the resistor mounted as shown in the picture of Wayno's amp). I will have to look to see where the part was actually moved too and what the reference designation is (unless it is under the resistor) as this is not on my amp in this fashion but I am certain it is there on the board someplace.

I also have seen a network of back to back zener diode clamps on the input section to the EQ. I would assume that is there to prevent over driving the first transistor on in the circuit (used as an attenuator Emitter follower, along with a differential input circuit ).

 

[Markageddon]

This guy is highly recommended Wayne, should you get stuck. You need to replace that cap at most, said a friend who gave me this guys details. http://www.ampguy.co.uk/ My friend is solid and used to work for a store who were a Mesa dealership and he vouched for this guy in your neck of the woods, if you're past warranty.

I tried to send this via PM but it merely outboxed.

No harm in having a good contingency plan.

 

[Wayno]

This guy is highly recommended Wayne, should you get stuck. You need to replace that cap at most, said a friend who gave me this guys details. http://www.ampguy.co.uk/ My friend is solid and used to work for a store who were a Mesa dealership and he vouched for this guy in your neck of the woods, if you're past warranty.

I tried to send this via PM but it merely outboxed.

No harm in having a good contingency plan.

Thanks mate,

Funny you should mention Ampguy, i actually know him! Not personally but his band used to play my local a lot and they played one of my best mates mums wedding! Small world hey. Got chatting to him and he's a really nice guy, definately trust him with my baby. Gotta feeling i'm gonna have to give him a call. Thanks for reminding me of him.

 

[Wayno]

The plot thickens...

Had another go this evening. Still no joy but made a few discoveries that might help diagnose the issue.

I checked all connectors etc and all seem to be ok.

If i run a really hot audio signal, and i mean mp3 into a mixing desk ramped up to full on clipping hot, run that into the guitar input of the Mark V, i can hear a very faint signal out of the speaker. Its very quiet and a bit distorted but its definately there. It mutes when mute switch is selected by either pedal or pull solo. Makes no difference on 10w, 45w or 90w, no volume or any other noticable changes. Even with both channel and master volume up full bore. No volume change, only change is if i drop channel master below 8 o'clock the signal disappears.

If i run same signal into the effects return, including a plug in the send jack to activate the loop, no noticeable change. Would have thought if the power amp is ok i'd get a loud normal power amplified signal now?

If i run effects send to another power amp and speaker i get the same result, all the channel mode switches have their normal effect, for example on ch2 edge is thin and sharp, crunch is more beefy, Mk1 is very deep etc etc, TMB all have their usual effect, as does the 5BEQ.

I get same weak signal at the tuner out and the slave out. If i run a guitar into the input and effect send to another power amp i get all the functions of preamp channels, with all tone controls working etc, just a very weak signal. Using a very powerful PA amp into 15" PA speakers i can barely get above bedroom volume levels. Sounds like running direct from the amp to the PA would normally, only quieter. Crap quality, reminds me of running a guitar > distortion pedal > straight to desk setup, very dry and cold.

All this is leading me to believe there is a problem somewhere between the preamp and power amp of the Mark V. The signal is getting from the guitar input to the power amp therefore i'm guessing the preamp is ok. It is just very weak at the poweramp for some unknown reason? Mark 2C beyond bizarre...

Here's a few more gut shots of the area around V4 just incase any spots anything and to show the differences we've noticed to other peoples pics.

https://drive.google.com/open?id=0B_NBQ4NZ9lM4RUt1NHdieTl1OWc

Thanks for your help guys.

Wayno.

 

[bandit2013]

The plot thickens...

If i run effects send to another power amp and speaker i get the same result, all the channel mode switches have their normal effect, for example on ch2 edge is thin and sharp, crunch is more beefy, Mk1 is very deep etc etc, TMB all have their usual effect, as does the 5BEQ.

I get same weak signal at the tuner out and the slave out. If i run a guitar into the input and effect send to another power amp i get all the functions of preamp channels, with all tone controls working etc, just a very weak signal.

Mark 2C beyond bizarre...

Wayno.

Wayno,
you are getting a weak signal regardless (send to other amp is same as from preamp to power amp on V).
The issue you are having is commutation by mutation.... (that was intended to be funny). If the global mute signal is stuck in active mode. I can assume two possibilities : there is a short causing the gates on the JFETS used for global channel muting during channel change (etc) to allow current flow thus shunting the signal paths to ground preventing preamp or power amp in supplying signal. Or there is an open circuit on the ribbon cable if the logic controller supplies a stable +5V or +12V dc signal to prevent the global mute JFETS from turning on, and the wire is open circuit on the Ribbon cable. Your amp is stuck on mute and not referring to the "MUTE" function on the pull pot or the footswitch. This is a channel mute that occurs when channels are changed from 1,2,3. Unfortunately my schematic does not go into the switching logic or other features of the amp beyond the preamp and power amp.

There are two main global mute circuits on the preamp. One is located just before the EQ input (all channel signals pass through this point ) J175M3 (also has 4 zener diode clamps in series) this circuit is on part 3 of the schematic. The Mute will effectively shunt the input to the EQ circuit. This would explain why only a weak signal is present on the SEND jack of the FX loop. The second mute circuit (not including the mute JFETS on the reverb circuit) occurs at the input to the Phase inverter by J175M1 thus preventing any signal entering the Return jack from the FX loop or from the bypass path around V6B circuit. This is not the same as the "MUTE" pull or foot switch function which is a relay but may be associated with it.

All of this is tied to one of the ribbon cables. It has noting to do with cutting one leg on the C39 but has everything to do with it since the Ribbon cable was removed to perform the intended task. Where the ribbon cable was resting when you powered the amp back up may have blown the gait on the JFETS causing them to short the signal paths. The P channel FETS are not as easily damaged as a MOSFET so it may not be component damage.

There are other JFETS but their function is tied to the toggle switches and others may be controlled by the logic controller responsible for changing channels (along with other things like the various JFETS to enable or disable bypass caps on some of the cathode resistors.

 

[IronSean]

It doesn't sound like your Mute is stuck on, since you said the Mute button is still functional to silence what signal there still is. If the signal from the Effects Send is good (Which it sounded like you said it is) Then the issue is either around V6B (effects return), the Output/Solo control, V7 (the Phase inverter), the presence controls, or the power amp. You could try replacing those tubes, and swapping the Left and Right power tubes to see if 10w mode comes back and it's a bum power tube?

 

[Wayno]

Right, done a few more tests with a multi meter, with the amp both on and off i get no voltage or continuity between all the legs of J175M1, that means thats working correctly yeah? J175M4 however, regardless of whether or not the amp is turned on, has continuity and a voltage of around 3.0mV when the amps on. Could this be the culprit? Shunting the signal to earth just before the EQ section? If thats the case gonna give Ampguy a ring as thats beyond my capability or tools to replace.

 

[Wayno]

It doesn't sound like your Mute is stuck on, since you said the Mute button is still functional to silence what signal there still is. If the signal from the Effects Send is good (Which it sounded like you said it is) Then the issue is either around V6B (effects return), the Output/Solo control, V7 (the Phase inverter), the presence controls, or the power amp. You could try replacing those tubes, and swapping the Left and Right power tubes to see if 10w mode comes back and it's a bum power tube?

Hi Ironsean,

If I understood Bandit correctly I think there's two different Mute circuits. One is an automated circuit designed to mute the amp momentarily during channel switching using Jfets. This when muted would allow a very small signal to pass due to them shunting the signal to ground, some signal would pass by however small it would be. The other mute circuit is the user controlled one, footswitch and pull mute on the solo control. This one is a relay which would kill the signal dead. That would explain it still being noticeable on the very small signal my amp is passing. Pretty sure I've got that right.

I did change my power tubes early on after the problem first happened. Didn't make no difference unfortunately.

The signal from the effects send is there, but being a line level signal it should have been able to drive my external power amp to full volume if it was at its normal level. As it is I had to crank the send control to full and the external amp attenuator to full and still only got a barely above bedroom level signal. This should have been deafening if everything was normal, it's a tapco juice 2500 power amp capable of running a full pa pretty much on its own.

Thanks for your input, think I might have isolated the problem but my soldering skills and tools would do more damage than good on a circuit board this tightly populated. I can piggy back components and can repair the leg of the capacitor I clipped if needed but I'm not comfortable working directly on the board with components as small as the Jfets. Best left to the experts that I think.

 

[Wayno]

Shame there's no schematic publicly available for the switching circuit as that would help me here.

 

[dlpasco]

Hi there. I had some work stuff come up in a big way and sort of dropped off the face of the planet.

I had some really interesting results with my own A/Bing of the stock and modded configurations, as well as comparisons to my IIC+.

One thing that I did was get a decibel meter and dial in everything at exactly the same volume when I recorded.

I was not able to distinguish much difference between the recorded sound of the modded or stock Mark V *at volume* - I think I was recording at about 94db, which is pretty full-throated.

It might be ear fatigue, or maybe using the AT7s pushes the amp harder at lower volumes. I can't say for sure. I know that at lower volumes I've immediately been able to tell that the stock configuration was in play - the amp sounded dead to me. I've kept the AT7s and I love it.

The acquisition of a gen-u-ine Mark IIC+ also ended up consuming a lot of my time.

IMO - and I have sat down and A/B/C'd the amps and configurations in awhile, at a higher volume the stock and modded Mark V and the Mark IIC+ all sound epic. The modded V and the IIC+ also sound great at lower volumes and I've really not bothered to go back and play with the stock Mark V configuration at low volumes since my last big testing spree a few months ago.

I think I've reached to point where, if I had to get rid of everything but one amp, I'd keep the IIC+, but it's because it sounds good and it's rarer. The V sounds awesome and I get a bigger change in sound switching guitars and pickups than I do switching amps, for sure.

These are just my experiences. Your's (not anyone specific, I mean all y'all) may be different.

 

[bandit2013]

Shame there's no schematic publicly available for the switching circuit as that would help me here.

That would help more than one. I did spend some time poking around the amp (no voltage applied) as I was trying to determine what signal line would control the "strobe mute" as that is how it was labeled on the TC-50. The term mute may be misleading and I do not mean to reference the user controlled function called mute.
There are several control signals that govern the strobe or silencer circuits. As is the case with my TC-50 when everything when silent (no signal from input to send, no signal from return to output, there was a hint of preamp signal that could be heard with all masters cranked but to no avail it seemed dead silent. I do not believe it is related to the same component that latched on in the TC-50. My assumptions on the channel mute, strobe, or silencer function (not relating to the mute or tuner out on the footswitch or control panel on the pull switch) is limited in believing the signal is sourced from one wire on the ribbon cable. I too had powered up the amp and forgot to hook up the cable. Along with silence, I was also getting a pulsed sound though the speaker. That may have been a bad tube I had in V1 as the pulse continued after I reconnected the wide ribbon cable.

I also decided to look at every JFET on the schematic. There seems to be a pattern in the nomenclature. So far there are 4 mute circuits (J175M1 - J175M4), eight function circuits (mainly on toggle switches or other) , labeled J175A through J175H and three EQ functions (J175SL, J175PS, J175EQ). If you remove the J175 and look at the legends on the ribbon cable connectors you may find the wire associated with that task. Wait, not that easy as it is a bit cryptic or it appears that way. Need to think about this... Also need to look at the other connector header legends and see if there is a pattern in that too.

 

[bandit2013]

I did some poking around this evening and found that my assumptions were correct. The two mute circuits in front of the EQ and in front of the Phase inverter are controlled by a single signal. Both JFETS have a gate resistor of 330k (current limit and provides isolation between the gates). The signal is identified on the rear control board at the pin header, wire is 4th from end without blue stripe (assuming that would be pin 1). Also note that without power applied to the amplifier, the resistance from drain to source should measure anywhere from 50 ohms up to 80 ohms. It will appear as a low voltage due to the lack of energy on the gate. This is normal for a P channel JFET. Keep in mind the JFET will be conducing with zero volts in respect to the source on the gate. It requires +3 to +5 volts to cut off current flow (turns the JFET switch to off). The cable that has the MU signal on it is the long one that stretches from the rear of the chassis almost to the front and terminates on a header next to the inductor array for the EQ.

Here are some pictures I took with areas highlighted to indicate what resistor ties in the gate to the common signal MU. I also marked the position on the 8 wire cable where the MU signal is (4th from the end farthest from the inductor array). At this point it is uncertain if this is related to the Mute function on the control panel or footswitch. The ribbon cable near V4 does not appear to have any association to the mute circuit but with it unplugged there will be no signal from input to output. I found this out the hard way by forgetting to plug it back in. Opps.

https://www.flickr.com/photos/159449305@N05/albums/72157687769651654

 

[Markageddon]

Hi there. I had some work stuff come up in a big way and sort of dropped off the face of the planet.

I had some really interesting results with my own A/Bing of the stock and modded configurations, as well as comparisons to my IIC+.

One thing that I did was get a decibel meter and dial in everything at exactly the same volume when I recorded.

I was not able to distinguish much difference between the recorded sound of the modded or stock Mark V *at volume* - I think I was recording at about 94db, which is pretty full-throated.

It might be ear fatigue, or maybe using the AT7s pushes the amp harder at lower volumes. I can't say for sure. I know that at lower volumes I've immediately been able to tell that the stock configuration was in play - the amp sounded dead to me. I've kept the AT7s and I love it.

The acquisition of a gen-u-ine Mark IIC+ also ended up consuming a lot of my time.

IMO - and I have sat down and A/B/C'd the amps and configurations in awhile, at a higher volume the stock and modded Mark V and the Mark IIC+ all sound epic. The modded V and the IIC+ also sound great at lower volumes and I've really not bothered to go back and play with the stock Mark V configuration at low volumes since my last big testing spree a few months ago.

I think I've reached to point where, if I had to get rid of everything but one amp, I'd keep the IIC+, but it's because it sounds good and it's rarer. The V sounds awesome and I get a bigger change in sound switching guitars and pickups than I do switching amps, for sure.

These are just my experiences. Your's (not anyone specific, I mean all y'all) may be different.

Nice one. 8)
You have a great collection man.
Bet that was a lot of fun to experiment with...!!

 

[bandit2013]

I doubt it is the mute circuitry that is causing the issue with Wayno's amp. Since the ribbon cables are basically bridge connections from one board to the other, the Send and Return signals have to get to the back panel somewhere. Since the hard bypass for the FX loop is on the back panel along with the FX loop jacks, I believe the large ribbon cable next to V4 is the root cause of the issue. I too had forgotten to reconnect the cable and had the same issue but all was restored when I plugged it back in. So if the assumption is correct there may be some disconnect between the signal path for the FX loop which would affect both hard bypass and FX loop active modes. No signal will be passed from Send or Return and the sound will be muted. The low signal levels heard though the amp may just be the feedback circuits. Even if there is continuity found, it may just be a strand of wire making the connection or wire could be loose in the plug such that placing a meter probe onto the wire if used against the internal pin may be enough force to complete the circuit. It is either that or the small board along the back panel has something wrong with it. Removal of C39 should not be the cause for the amp to go silent as it only affects one tube that ties the grid to cathode on V4B. Besides that it should only affect CH3 and is not related to channels 1 and 2.

Also if there is another problem it was coincidental at best. Once can easily remove V4 and you would still have CH1 and CH2 (without reverb).

 

[bandit2013]

I was not able to distinguish much difference between the recorded sound of the modded or stock Mark V *at volume* - I think I was recording at about 94db, which is pretty full-throated.

dlpasco,

You are not alone on this. I took things one step farther and removed a capacitor. I can hear the difference when in the room but what seemed to be more apparent was the feel on pick attack as this shifted the response to a more forward attack (if that makes any sense). Feel and response cannot be captured with any microphones or video cameras. I wonder if this could be measured on an oscilloscope but that would be overkill and not necessary.

Sounds like you have had an interesting encounter with the IIC+. Sorry for my ignorance but is it a head or combo?

I would agree with the Mark V comments. The basic or fundamental tone will be there regardless of the tubes you are using. Some slight changes will be had with different tubes but that will not shift the frequency response of each stage enough to make a huge difference. With microphones you will capture the fundamental tone and miss out on the additional harmonics embedded in the output of the amp. Perhaps it could be done with several different microphones with small to large diaphragms, mass also plays a role in regeneration of the sound waves. Not something I am willing to purchase as some of the reference mics are quite expensive and cost as much as an amp.

Enjoy the IIC+ as for what amp to let go.... that is difficult to answer. If my house was on fire I would go down in flames while grabbing all of my guitars, perhaps I would grab the JP-2C and let the rest burn but would also want to take the TC-50... They are all replaceable. Guitars may be more difficult to replace depending on what it is. As it were I do not have any difficult to replace items, except for the dog and he will be rescued first bar none.

 

[dlpasco]

Sounds like you have had an interesting encounter with the IIC+. Sorry for my ignorance but is it a head or combo?

It's an SG (60 watt with graphic eq, no reverb) head that Mike B modded for me: pull deep now activate the Hetfield/++ mod, and the mid knob serves the regular pull deep function.

It's a very nice little beast. Although all of these amps sound awesome, it can nail album tones exactly - the differences of the other amps isn't too noticeable at all until you turn this one on, then it's "OMG, that's the sound from Nevermind/Puppets/etc"

I would agree with the Mark V comments. The basic or fundamental tone will be there regardless of the tubes you are using. Some slight changes will be had with different tubes but that will not shift the frequency response of each stage enough to make a huge difference. With microphones you will capture the fundamental tone and miss out on the additional harmonics embedded in the output of the amp. Perhaps it could be done with several different microphones with small to large diaphragms, mass also plays a role in regeneration of the sound waves. Not something I am willing to purchase as some of the reference mics are quite expensive and cost as much as an amp.

Yeah, I think that capturing the actual room sound of one of these amps would take about five microphones. Actually recording a great sound isn't heard: reproducing what you're *HEARING* is much more problematic.

If my house was on fire I would go down in flames while grabbing all of my guitars

OMG that is the best answer I've ever heard. And yes, doggos into the lifeboat first

 

[bandit2013]

Besides, it would take too long to tear down the drum set.... I love that thing too. I may toss a bass or two into the flames just so I can watch them burn (only kidding I like my bass gear too but nothing to write home about).

If I did have to face a home fire, the dog would be a happy pup since that would mean no more loud guitars, drums and bass. Acoustic is okay but the electric or loud stuff is not. Well then again it could be my playing so I have to take that into consideration.

 

[Wayno]

Hi guys,

Been stuck at work again finally got a Sunday off and no V to rock out with, gutted.

Good to hear from dlpasco, great to read about your findings. Very jealous of your iiC+ :mrgreen:

Waiting on tech to check out my V as he's pretty busy. Will definitely post up his findings/fixes for you lot. You never know, it may help someone in the future if they have the same problem. Maybe start a new thread for it as I feel I've hijacked APEMAN's thread here a bit. Still, all relevant to the mods though I guess.

Massive thank you to bandit and you all for all your help with my V. Really appreciate the time you've put into helping me out and i've learnt a lot about the workings of the amp which I love. Even if I haven't been able to fix it myself, you've probably saved me a lot of tech time/expense by enabling me to give him as much info as possible as to what the problem may be. Really do appreciate this.

Thanks guys,

Wayno.

 

[bandit2013]

Hey, no problem, how else can one figure this thing out without a full schematic. I believe your issue is related to the ribbon cable you disconnected when you cut the leg to the cap. The ribbon cables also carry signals too. I have not decoded the other ribbon cables yet but at least the mute thing may not be the problem.

I have gone back to 12AX7s then back to 12AT7 in V4, now have both in V4 and V6. The tube swap is a good fix for the ice pick.

I think I have been the one hogging the thread. sorry. :roll: