New Mark IV problem

[AJ_Newkirk]

I plugged in the amp, let the tubes warm up for a few minutes with the amp on standby, then proceeded to take the amp off standby and play thru it...but it sounded very strange. The amp had very low volume .. no matter how high I turned up the volume it didn't seem to change much , and it sounded really raspy & broken up on the clean channel ...almost like how an amp sounds if someone hits the standby or power switch while you're in the middle of playing something - the raspy sound you hear just before the amp goes totally off.

I initially thought one or more of the tubes may have gotten jarred around in transport, so I swapped all of them with some brand new Boogie tubes..but no change. All of the tubes appear to be lighting up fine.

To make things a bit more strange, I left the amp on for 5 - 10 minutes and it still sounded odd. Powered it off then back on again and it sounded fine - with the exception of a low volume "rumbling noise coming from the speaker. I Played thru it for an hour and it was great, aside from the rumbling noise. BUT once I hooked the amp back up again the next day it started all over again.

I tried disconnecting the speaker and using my known-good 2x12 cabinet - no change. I also tried a few known good guitar cables - no change. Tried using Class A and Simul-Class , Triode and Pentode, Full power and Tweed power and the problem was always present.

Any ideas on what could cause this? Cold solder joint? Bad capacitor?

 

[Boltino]

Is the low volume only on the clean channel or on all channels?
Is the raspy sound on the clean channel only, or on all channels?

 

[xarkon]

Since you mention it gets better after awhile, I would suspect bad cap, but hard to say for certain. It might be that something is getting warm enough thermally during that hour for a joint to fully close. Again, just guessing.

Does the problem exist on all channels? With EQ in or out?

If on all channels, and EQ doesn't matter, then it's power supply, phase inverter, or output stage.

Dave

 

[Boogiebabies]

I would pull the chassis and look for the obvious signs like a smashed in pot or a wire/component that has come off. You can also look for a burnt spot just in case. If you cannot find anything visual, and the power tubes check out, it should go back to Mesa. The circuit board may expand and contract, but finding a cold joint that uses flow soldering may require more than the naked eye. I'm shocked that most of these amps work as well as they do after taking a few twenty foot vertical drops courtesy of UPS.

IIRC, the MK IV warranty is not transferrable. The owner would have to send/take it in for warranty work.

 

[AJ_Newkirk]

Unfortunately a warranty repair is not possible as he hasn't sent me his reciept. I may have to drop it off at the service center, but they charge a pretty steep fee for non-warranty work, including estimates.

I took the chassis out and looked at it. I can't see anything that looks "obviously wrong" such as a wire or component which has came loose. The only are that even looks questionable is the one I took this pic of. in the center of this pic it appears to have a small spot that gets hot, or at least did at one time? Also the PCB trace just under the letters BLU on the left side appears to be about 1/2 gone. I will say that as a whole..the solder work in this amp is absolute crap.

 

[AJ_Newkirk]

Since you mention it gets better after awhile, I would suspect bad cap, but hard to say for certain. It might be that something is getting warm enough thermally during that hour for a joint to fully close. Again, just guessing.

Does the problem exist on all channels? With EQ in or out?

If on all channels, and EQ doesn't matter, then it's power supply, phase inverter, or output stage.

Dave

It does it on all channels, and the Eq does not have an effect on the "noise"

I just noticed that when it does come on there is an almost constant rumbling-like noise coming from the speaker. This rumbling is pretty low in volume but I can visibly see the speaker move out when the rubmle "hits". The rumble does not get any lower/higher in volume when the master is turned up or down.

 

[stokes]

Sounds like you have a low frequency oscillation,probably related to the power supply.Cant tell for sure if there is a problem with that burnt looking spot on the board you posted,but check the neg. end of each of those 220uf caps,you should see about -23v's on one and a little more on the other.

 

[Boogiebabies]

Those 220uf/63V caps have been out. The silicone is cut and the soldering is atrocious. You might want to do the chopstick test on that circuit. Fire the amp up, plug you guitar in and while making some noise poke around the area with a chopstick. With that solder job and the obvious signs of overheating with the iron, that spot may very well be the source of the problem. That does not look like a factory repair.

If you could, post a pic of the whole board so we can see what part of the circuit it is. I can only assume now that it is the GEQ section judging from the part values.

It might not be bad to PM Restless Rocks. He's North Carolina's resident MK IV Guru and a super nice person. If we get that amp fixed, you should make an offer that benefits you now, and in the future just in case anything happens.

 

[Boogiebabies]

Sounds like you have a low frequency oscillation,probably related to the power supply.Cant tell for sure if there is a problem with that burnt looking spot on the board you posted,but check the neg. end of each of those 220uf caps,you should see about -23v's on one and a little more on the other.

100%. Who knows how bad the caps were heated up. It seems they never heard of flux, so naturally a heat sink is a foreign object as well. It's an ugly repair.

 

[stokes]

Just noticed that silicon cut,I thought we were looking at a brand new amp.I would also question the 15 ohm resistor there,the schem shows a 1.5k.

 

[Boogiebabies]

Now I see why the owner does not have his receipt.

 

[Shep]

Thats a mess... i wonder how much more of the board is like this.. look how the 63V 10uf has been cut and re soldered right on the component it's self..There Would HAVE to be some kind of heat damage there.

 

[AJ_Newkirk]

This is supposed to be a new amp :?

To summarize for those who sent me PM's..I was initially getting a thin, raspy, distorted VERY low volume sound from the amp. After turning the amp off then back on the volume returned to normal, and the amp is now making a low pitched and low in volume "rumbling" sound. This rumbling sound is present no matter which amp options I turn on or off. I replaced all tubes with new Boogie tubes, replaced my guitar cable, tried different speakers and the noise is still present. The rumbling sound is also still present when the amp is in "silent recording mode" (master volume knob pulled out). When this rumbling noise happens (intermittently, from almost constant to a few second pause in between the noises) I can visibly see the speaker move outward.

I was able to scrap away the burnt spot as seen in the earlier picture very easily. I do not see any pcb damage under where the spot was.

I probed all over with a chopstick and it never did anything that registered to me at least.

The voltage on the 220's are -22.33v and -45.1v, and I labeled each of those 2 caps in the pic below with their respective voltages.

and here is the full board shot. The area in the above pic is in the middle of the far left side in this pic below.

 

[AJ_Newkirk]

It might not be bad to PM Restless Rocks. He's North Carolina's resident MK IV Guru and a super nice person. If we get that amp fixed, you should make an offer that benefits you now, and in the future just in case anything happens.

I PM'ed Restlses Rocks and he suggested that I first "use your DMM to look for the presence of power in the EQ section to eliminate this as a source of the problem". Where on the board would I check for this? The EQ functions just fine so I assume it is getting at least some voltage. But I cannot assume it is the correct amount. He also thinks the problem "seems to exist in the post Master volume FX loop section and may very well involve the EQ." Additionally, he spotted one of the 220uf caps has been slightly crushed and should be replaced.

 

[xarkon]

Yeah, I just got back on and am reading the updates - that cap is crushed, which could be very bad if the internal plates are touching each other. Looks like someone used pliers to insert it.

BTW, the brown marking around the solder joint is rosin flux. Sloppy soldering, so clearly the amp has had some problem, possibly in manufacturing but probably later and had some parts replaced. More than likely, Mesa's boards are soldered using wave solder machines, which when set correctly do an outstanding job. (And when set incorrectly turn out a pile of scrap in a very short time. :shock: )

Someone earlier suggested that the Mark IV warranty is not transferrable. I don't think that's correct; I believe that all Mesa warranties are 5 year transferrable; call Mesa and give them your serial number and they should be able to tell you whether it's still in warranty.

Dave

 

[Restless Rocks]

Looks like someone has reheated many of the joints around V4 (reverb/lead circuit) and the power supply to the EQ section. The rumbling noise with "silent recording" pulled would make me go to the phase inverter (V5) and check the plate load resistors. And tubes could still be the issue, another fellow just bought a brand new set of JJ's and received a bad 12AX7 right out of the box. I would also check the V5 (PI) socket joints closely as well as making sure the socket itself is not sprung out causing bad contact on the tube pins.While you hear the rumbling noise carefully wiggle the V5 tube to listen for an crackling sounds. (careful not to burn yourself on the output tubes.)With the output pulled (silent recording mode) the presence circuit is still active on the power amp section. Listen for the rumble and switch the channels to see if any change occurs to the rumble noise to determine if it is channel specific which would point to that channel's presence circuit, there are several LDR's and relays which also could be acting up. These intermittent gremlins are a pain in the speaker so to speak. Let us know how it goes from here
John

 

[AJ_Newkirk]

The noise is present equally in all 3 channels.

As for the tubes I tried 2 additional new tubes in each preamp socket this morning, this had no effect on the noise.

I checked V5 and it appears to have good contact with the tube pins. When I wiggle V5 around it has no effect on the rumbling noise and it produces no cracking sounds.

Plate voltage is not listed on the poor schematic I found for this amp...what should the voltage be on the plates of V5?

 

[Boogiebabies]

Pin 1 - 293V
Pin 6 - 295V

 

[Restless Rocks]

At this point I would have my occilloscope fired up and see if the noise was present on both outputs or just one side of the phase inverter. Also take a close look at all the solder joints on both sides of the plate load resistors, the coupling caps etc. These types of noise problems are most frequently attributed to very old and drifted carbon resistors in vintage age amps. To save a bunch of time I would at this time shut the amp down for a while to cool off then power it up, put a sine wave on the input and scope the signal path till I found the problem. You unfortunatly may be nearing the time to find your closest mesa service center since it could very well be a warranty repair anyway. Besides if you're looking to puchase the amp from that fellow this problem is a great way to talk the price down considerably 8)
John

 

[AJ_Newkirk]

pin 1 is fluctuating between 244 - 250v and pin 6 fluctuates from 279 - 285 or so.