Mark IV A channel switch problem (pic inside)

[DFGeneer]

Hello guys, my first post on this very nice forum.

I just got my first MarkIV. It's a MkIVA combo, ser.n. IV3641 and it's... defect. When on, the pedal works, but the amp doesn't seem to. The tubes are warmed, but no sound comes out of it.
The guy who sold it to me sad he had the problem before, then a tech guy repaired it and in 2 months the problem appeared again.
I opened the amp and had a look at it. I noticed the only thing that seemed to have been changed is one of the two integrated circuit that seem to trig the muting of the channel switching. Here's a pic:

The one on the left on the pic is a Motorola SN74LS138N decoder/demultiplexer and the one on the right that seem not to be the original one is a Portugal SN74LS14N dual gate/hex inverter. I really suspect that's the one that causes the trouble.

Now, I'll try to replace it with whatever compatible IC I can get, but my question is, whats the original integrated circuit used in MarkIVA? Is it again a Motorola?

Of course, other opinions of what might cause the trouble are welcome!

 

[Restless Rocks]

The 74LS14 is the correct chip. Set the rotary selector on the rear to FW and see if the channel indicator LED's on front light and change as you press the footswitch buttons. If so this is probably not your problem. The LS14 sends control to the J175 gates to its right which ground the selected rail of the switching matrix. If the foot switch does not function try rotating the channel selector switch in the rear and check for proper channel change. If all this works try putting a short patch cable from your FX send to the FX return to check for a bad jack (a common problem). Next I would check the pull silent switch on the Output Master. Of course you've already checked the Preamp tubes especially V1.

 

[DFGeneer]

Regardless of the position of the rotary selector, no channel on the amp is active. The LEDs on the front panel are always off.
Only the LEDs on the foot-switch act according to the button pressed.
So, I have the feeling that the amp is in permanent muted mode which should appear for a short time during channel switching, but somehow the triggering signal to stop the muted mode never appears.
Thanks for the suggestions!

 

[Restless Rocks]

Okay sounds like you have power to the foot switch, which is working. If you can get hold of a schematic you'll see that the rotary switch does not rely on the LS14 or the J175's to ground the selected matrix rail activating one of the three channels. There is no muting function. An individual channel rail is either grounded and active or not and this can only happen one at a time. If LS14 were faulty the symtom you'd likely see would be trigger being sent to more than one of the J175's simultaneously. You would have a situation where probably two matrix rails would be grounded at the same time and two of the channel LED's would be lit. A very noisy situation. Check for +5volts at pin 14 of LS14. I suspect you have a loss of power to the LDR/Relay rails at the other side where you should have a +12.6V regulated supply (When the power switch is is full power position). The low voltage supply is rectified and filtered off of the filament supply and supplies 15v to the footswitch , 12.6V to the matrix and 5V to the logic chips.

 

[DFGeneer]

Yes, it seems the problem is not in the LS14.
The 5V at pin14 are actually 2.6V. The 15V to the foot-switch are 4.6V. It's surprising the foot-switch works at all. The 12.6V I still havent figured out where to measure it.
I got the schematics recently, so things are getting clear and, as you said, the problem comes from a power loss on the low voltage circuits.
Oh, one more thing: tubes in positions V1 and V2 are not glowing. Even after a tube swap with V3 or V4.

So, how should I proceed?

 

[Restless Rocks]

You have a short on your 6.3volt AC filament supply. (The two green wires coming out of your power tranny) This could be a tube. If not a tube locate and test Zenor diodes D8 and D9 IN5392. If either of these shorted you would loose power to your filaments and the low voltage supply to the footswitch and switching matrix. Locate R301 and 302 the 100ohm filament load resistors a short in one of these is also possible. I'd say they are probably burned and possibly open from the other symtoms you're having. It might be a good idea to start by removing one of the green wires and checking for your 6 volt AC to make sure it's not the power tranny tap itself. By the way remember to look for these voltages when your NOT in the TWEED MODE.

 

[DFGeneer]

Thanks, man! You were straight to the point!
It turned out that this amp was modded. The 12V DC between pin 5 and 4 of V1 were applied in parallel to V2. That was the reason why both of them were missing the warming current. And probably the reason why the 12V DC circuit got overloaded and damaged..
I changed both VR12 and VR5 stabilizers with new ones, but the result remained the same. So it seems the problem comes from the diode rectifiers. I'll be able to verify that as soon as I get replacements for them.
The R381 and R382 (I guess you mean those instead of R301 and R302) seem to be fine as well.

Once again: Thanks a lot!
I am glad I decided to fix my amp myself, because this way I'll get to know it better. And it turns out it will be faster than bringing it to a local tube amp tech, cause they said they have too many customers in line, so it would have taken about 2 weeks.

 

[Restless Rocks]

Thanks DF, I also get a lot of satisfaction out of tinkering with and modding my own amps, then taking them out to a gig and hearing my work. It's also very rewarding to see someone get their amp back from repair and see that big smile on their face when they turn it on and it works like new again. Best of Luck
John

 

[DFGeneer]

Aarghh! There's the devil in this amp!

I removed the D8 and D9, tested them and they work fine. So I place em back and the amp works just like new.
That's all prefect up to now, but after 30 min of tonal bliss it stops again. Same symptoms..
Now it's clear though that the pin5 of V1, where 12V DC should be, is grounded. The problem is that the physical connection between the output of VR12 and pin5 of V1 is on the back side of the board.
So, I really have to disassemble the whole amp.

Is there any trick to that? To remove the board without unsoldering all those hundreds wires connecting the controls on the front and rear panel?
And is there an electronic version of the lower side of the board layout?

 

[Restless Rocks]

I would undo the mod. I have to guess that the filament tracks to V2 were severed and jumper wires were placed to V1. I would avoid trying to remove the main board. After having undone the jumper mod remove V1 from it's socket and power the amp. Look for 12V DC at pin 4 with pin 5 being ground. There is still the possibility that V1 or V2 12AX7 tube could be causing this problem as it acts like heat has something to do with it. Remember that V2 is using all three filament pins 4, 5, and 9 while V1 only uses pins 4 and 5. I'm not sure that you would see or hear any bennifit from running V2 on DC anyway. Again try removing anything that does not look original and repair any cut traces to V2. Again you would be opening an enormous can of worms to try and remove the board. Rely on your DMM to check for continuity and shorts to ground.

 

[Restless Rocks]

Sorry I've got too many things going on here. I just made an error in telling you that V1 pin 5 was ground. You should see +12V DC at pin 5 with pin 4 being grounded. I still keep wondering if you shouldn't try new preamp tubes if the problem persists.

 

[DFGeneer]

Sure, first thing I did when I found out about that mod was to undo it.
Anyway, I removed the board, not that this helped me anyway... However I discovered that the connections to pins 4, 5 and 9 of V2 were cut.
I noticed as well the area under the D8 and D9 being blackened.
So, after putting back everything together and switching on the amp I noticed the new welding of D8 boiling after a while. Amp not working again.
Now I am pretty sure there's something fishy around D8+D9+C442+C443 part. I think the best bet is to replace them all.

 

[Restless Rocks]

Yes replace D8, D9 and be sure they are properly oriented. Since you have the board up replacing C441, 2, and 3 as well would eliminate another possibility for a shorted component. This is what we call a "pop and swap" type of repair and at this point not being there to check for current draws and a more methodical trouble shoot I can't be of the best assistance. Sounds like you have the determination to exorcise the gremlin.

 

[DFGeneer]

Yeah, well.. I had enough too...
It works just fine with external power, so I think I'll keep it like that for a while.
I removed the board though, it was not that difficult. Only had to cut the connection of the front 3 LEDs, the reverb, the EQ and the 2 connections to one of the smaller transformers. 13 connections all together.
However, it's far not pleasant way to spend your time.

Once again, thanks for the hints.

 

[Mystic-Creations]

Instead of starting a new thread it looks like I am also having similiar problems, the shitty part is that it's a brand new head, manufactured in March of 07, bought it in June, non display model.

Here goes: The mode select switch in the back seems to have a short of some kind but only when selecting the FT.SW, it puts the amp on Rhy 1 regardless of what the ft switch is set to. I let it sit for a while as I was checking to see that everything looked ok. All of a sudden it's working as it should be, worked great for an hour and then all of a sudden lost my lead and back into clean, even though the ft sw was set for lead. When I touched the mode select it went back to the lead, I held it there and it stayed, when I let go it went back to the clean....FACK! Faulty Pot?

I hope someone might be able to help, if not, could someone give me the number to Mesa Customer service, I'm really disappointed, I didn't spend this kind of money to have to deal with **** like this. I've owned far cheaper amps and nerver had a single issue with them.........

thanks guys

 

[DFGeneer]

Mystic, from the experience I got with my problem I would say your problem is with the switching matrix or the pedal.
Since you amp is quite new, the warranty must be valid, so I would advise you to bring it to an authorized tech for repair. There is a list of the authorized repair companies on the Mesa official website.