Traded a 3ch Dual Rectifier for a Mark IV, got a question

[pabloguitar]

Between 9 and 1= 0 volts (same with buttons either in or out)
Between 9 and 5= 2.5 volts (buttons in and out)
9 and 2= 0 volts (same with button either in or out)
9 and 3= 0 volts (same with button either in or out)
9 and 4= 0 volts (same with button either in or out)

 

[Blueracer]

Ok there is some good news and some bad news with these measurements. First the voltage on pin 5 at 2.5 volts is correct. The bad news is that the voltages on pins 1, 2, 3 and 4 are not right. Internal to the IC each of these inputs is pulled up (i.e. a positive voltage is impressed on the pin) through a resistor and each should have a voltage of about 4.5 volts without any input. Further pins 2, 3 and 4 are not connected so regardless of whether your channel buttons are pushed or not they should not change and should have a voltage on them. If you look on page 3 of the Fairchild data sheet I emailed you you will see a block diagram of this IC. The block labeled Encoding Logic and 2 Key Rollover has a bunch of inputs labeled Y1, Y2, etc. There are a bunch of squiggly lines label internal that all attach to a label Vcc. The squiggly lines are resistors. I think this Vcc, which is internal to the IC, has opened up. Without this voltage the IC thinks it is getting inputs all the time and locks up. The alternate situation that could cause this is an external short to ground on all these pins. I don't believe this as it would mean four different pins would need to have a short which is not likely. Usually in these situations the simplest explanation is the correct one which would be the point failure of the Vcc internal connection. The external short hypothesis can be tested by disconnecting the foot switch and checking for continuity between pin 9 of the IC and pins 1, 2, 3 and 4. My guess would be that you will see some resistance but not zero. Do try this just to convince yourself but I doubt we will find any direct shorts here.

So what can we do to fix this. There are several options all of which assume that the Vcc connection to the pull up resistors internal to the IC has failed.

1. You can wire in a couple of external resistors to these inputs. This can be a little tricky to do but is doable. I would want to confirm that putting in these resistors really would pull these inputs up before committing to this. This fix also assumes that there are no other problems in the IC. This may not be a good assumption.

2. Replace the IC. This is a good option however replacing these ICs is a bit tricky. As you notice there are lots of leads to the IC and desoldering them is not easy. I personally would cut the device out then remove each pin one at a time. The pads would need to be cleaned of solder probably using a solder wick then a new device installed. The pads for the device are close together and good soldering skills and proper equipment are required to do this successfully. Finding a good tech willing to take this on would be a good option here.

3. Board level replacement is a viable option. Call Mesa and find out if you can still get the entire board. There is a very good chance that you can and hopefully the cost wouldn't be too high. If you like I can make the call and find out the ins and outs of this. One of the potential gotchas on this is getting the board out with all those LEDs. I don't know if those snap into the lens or not and how hard it is going to be to remove the board.

4. Get a new foot switch. These do show up on evil bay at around $100. If Mesa is still selling these I think they charge about $279 which would really kind of hurt.

It is hard for me to make a recommendation as I don't know your skill level in all this. So let me know how you would like to proceed. Hope this is helpful.

 

[pabloguitar]

I called Mesa today but I didn't ask about a new board. One thing that still really has me confused is that this footswitch doesn't work on my Quad (a good footswitch should) which points me into the obvious direction that my Mark IV footswitch is bad. But then when I try the Quad footswitch (which does work on the Quad) it doesn't work on the Mark IV, which would make it look like it's a problem with the Mark IV. Who knows, maybe there is a problem in each, the amp and footswitch. When I originally opened the footswitch, there was one wire that was loose. Who knows, maybe the previous owner took it apart, and if he took the 4 wire harness off, then put it back on but backwards, the red wire would be going to the wrong place and possibly causing a problem with an IC in the switching part of the head. Talking to the guy from Mesa today, he did say that would do it. Anyway, he is having me do some other tests. I'll report back after Thanksgiving.

 

[Blueracer]

The Mark IV foot switch not working with the Quad is not a mystery. There is an identified issue with the decoder IC. This does not mean that the foot switch doesn't have other issues. It also doesn't preclude there being issues on the Mark IV amplifier side. Trouble shooting is by its nature a binary process. You locate a problem and fix it then see if the issue is resollved. I hadn't forgot that your Quad footswitch didn't work with your Mark IV and it is a reasonable place to look. Just to be sure the Quad foot switch needs to be the FU-2A not the FU-2 to work with the Mark IV. Using the Quad foot switch were the symptoms the same or did it have different behavior. I am still willing to help on this if you like.

 

[pabloguitar]

Yep, the Quad foot switch is the FU-2A. The symptoms were slightly different. I'm sure you know this, but just in case anyone following doesn't... where on the FU-3 we have EQ and Loop, the FU-2A has EQ1 and EQ2. Those did activate the EQ and Loop on the Mark IV (different labeling on the FU-2A, but same configuration). However on the FU-3, where only the RHY1 light is lit and will not change, the FU-2A has a different behavior. Pressing either Rhythm 1 or Rhythm 2 buttons will light up only the Rhythm 1 light, and pressing either the Lead 1 or Lead 2 will only light up the Lead 1 light. So as an example, if Lead 1 light is on and you press Rhythm 2, the light for Rhythm 1 will light up. None of these changes however will change the channel on the Mark IV from the Rhythm 1 channel. Oh yes, I would still like your help on this. It's kicking my butt so far, but I haven't given up yet!
Thanks!

 

[pabloguitar]

Tried out the test that you mentioned for continuity with the footswitch disconnected. Between pin 9 and 2,3,4 they all showed about 6 ohms. But when I tested it between 9 and 1, it did go to 0. Also like you mentioned, I turned the amp on with the footswitch disconnected but the switch in the Ft. Sw. position, and yep, it stayed in the Rhy 1 channel.
As far as the test that Mesa suggested, it powered up, but still Rhy 1 light stayed on and would not switch to any other channel. Again, Loop and EQ would light up when pushed. The readings on pin 1 and pin 2 were about .5 volts, constant even if I pushed the individual channel buttons.
For the heck of it, I checked my FU-2A footswitch and here's what I got...
________Pin 1_____Pin2
Rhy1____0v________0v
Rhy2___3.5v_______0v
Ld1_____0v_______3.5v
Ld2____3.5v______3.5v
The 3.5 is where they were expecting 5v and on Ld1 Pin2 their diagram showed .5 volts, but either way you look at it, there's voltage where there should be, and not where there shouldn't be. From what I can see, this confirms that there is a problem with my FU-3 footswitch, probably the IC like you suspected, and also most likely something in the Mark IV switch since the FU-2A is working correctly, do you agree? I will probably order the IC switch and see if I'm able to change that out if you think that's the problem, or do you think there's more troubleshooting needed on the footswitch? I'll get to the Mark IV switch later.
Thanks!

 

[Blueracer]

Ok what you have are multiple issues. Your FU-2A is working fine. That the output is 3.5 v verses 5 v is not a big deal. 3.5 v is more than enough to drive the Mark IV logic plus it may have something to do with using a battery at 9 v rather than having 15 v coming in which is the norm for the foot switch.

I would go ahead and change out that IC on your FU-3 foot switch. Again there may be multiple issues but these can be tracked once the IC is out. In fact we can test the rest of the logic once the offending IC is removed so if you do plan on doing this replacement be prepared to do some tests with the IC removed.

There is definately an issue with your Mark IV. This is illustrated by two factors. First your FU-2A foot switch doesn't work correctly with your amp and with the foot switch disconnected the amp is not going to the logical default channel which would be the lead channel. The good news is that most of the logic involved in channel switching the Mark IV is ok as you are able to use the channel switch in the back of the amplifier to change channels. If you look at page 12 of the schematic I sent you will see a device in the lower left corner called a 74LS138. I suspect that there may be a problem there or possiblily in the 74LS14 IC above it. All the rest of the circuitry to the right of these devices is probably ok which is a good thing.

We can get into trouble shooting the amp and do this in parallel with fixing the foot switch if you like. Your call. One question I have is using your FU-2A foot switch you said the foot switch would go into either the Rhy 1 mode pushing either the Rhy 1 or Rhy 2 switches or go into the Lead mode push either the Lead or the Lead+Eq switches. The question I have is what are the amp lights doing. When you have the Lead light on the foot switch is the amp also in the Lead mode. This is kind of important. I would suspect that the amp would not go into the Lead mode since it didn't with the foot switch disconnected but I want to make sure of this.

So next steps as I would see it are 1) remove bad IC in the foot switch and be prepared to do some tests and 2) do the test above using the FU-2A foot switch.

 

[pabloguitar]

Blueracer,
You are correct, when I tried it with the FU-2A, it did the same thing... even though the foot switch did change from Rhy1 to Ld1, the amp stayed on Rhy1.
Took the IC out, I'm ready to do some more testing. Also, I'll order that new IC tomorrow.
Thanks!

 

[Blueracer]

Ok the next thing to do is verify the rest of the circuitry on the foot switch. Hook up the 9 volt battery the way Mesa showed you. Now you will measure the same pins that Mesa had you measure. If all is well both pins will measure in the 3 to 5 volt range. Also the foot switch should be showing the Lead + Eq LED being lit. If all this passes then we go to the next step.

You will need a short piece of wire. Use the wire to connect pins 9, 16 and 17 of the IC you removed. You should be able to bridge pins 16 and 17 with one end of the wire since they are next to each other. If all is still going to plan your foot switch should now be in Rhy1 mode (Rhy1 LED lit) and the pins Mesa has you measuring should be around zero volts. Be carefull with the wire and do not touch pin 18. That would be a short circuit of the power supply. A bad thing.

Repeat the above except only use the wire to connect pins 9 and 16 of the IC this time. In this case the foot switch will be in the Lead mode (Lead LED lit). Pin 1 of the pins Mesa has you checking will be in the 3 to 5 volt region and pin 2 will be around zero volts.

Last case is to connect pins 9 and 17 on the IC. The foot switch should go to the Rhy2 mode (Rhy2 LED lit). Pin 1 of the pins Mesa has you checking will be in the zero volt region and pin 2 will be in the 3 to 5 volt region.

If any of the above doesn't work post back (actually post back no matter what) and we can do more investigation. Good luck.

 

[pabloguitar]

Testing the same pins that Mesa had me test, it didn't go to the Lead + Eq channel, it stayed on the Rhy1 channel. Pin 1 showed 1 volt, Pin 2 showed 1.5 volts.

Doing the test with pins 9, 16, and 17 connected, the foot switch showed it was on the Rhy1 channel. Pin 1 showed about .4 volts, pin 2 showed 0 volts.

Doing the test with only 9 and 16 connected, it didn't go to the Lead channel, it stayed on Rhy1. Pin 1 showed about .4 volts, pin 2 showed 0 volts.

Doing the test with only 9 and 17 connected, it didn't go to Rhy2 channel, it stayed on Rhy1. Pin 1 showed about .5 volts, pin 2 showed about .5 volts also.

Interesting thing, while doing the first test, when I first hook up the battery Rhy1 comes on right away as it has been all along, but Rhy2 comes on for a split second and then goes off.

 

[Blueracer]

Well that didn't go well. First check that there is voltage on the 74LS14 IC which is the middle IC on both the circuit board as well as on the schematic I sent you. The pin orientation is the same as in the picture I sent you for IC you removed except this IC has 14 rather than 18 pins. Pin 1 will be on the lower right of the picture I sent you; pin 7 lower left; pin 8 upper left; and pin 14 will be upper right. Check the voltage between pin 7 (black probe) and pin 14 (red probe). This is power to the device. It should be about 5 volts. Use the battery for these measurements.

Next check the voltage between pin 7 and pin 2,3 (they are connected) on the 74LS14 IC. Also check the voltage between pin 7 and pin 11,12 (also connected). Next with your wire again connect pin 9 and pin 16 on the IC you removed. Again check the voltage between pin 7 and pin 11,12. And last connect pin 9 and pin 17 on the IC you removed with the wire then check the voltage between pin 7 and pin 2,3 on the 74LS14 (middle) IC.

Without the wire connection the voltage between pin 7 and any of the other pins (2,3,11,12) should be around zero volts. With the wire these voltages should be up in the 3 to 5 volt region.

What I am thinking may have happened is that your former owner may have miss wired something on the Mark IV side of things and taken out the 74LS14 outputs which are what hooks into the Mark IV. This doesn't explain the bad 74C922 (the IC you removed) though. At any rate the 74LS14 is not operating properly and may need to be removed. But lets do the tests and we will take it from there.

 

[pabloguitar]

Between pins 7 and 14, 5 v.
Between pins 7 and 2/3, 3.7v.
Betwee pins 7 and 11/12, 3.7v.
Between pins 7 and 11/12 with bridge between 9 and 16 on right IC, 3.7v.
Between pins 7 and 2/3 with bridge between 9 and 17 on right IC, 3.7v.

Per our other discussion:
Between pins 7 and 11/12 with bridge between 16 and 18 of first IC, 3.7v.
Also during this last test, both the Rhy1 and Rhy2 lights were on.
For anyone following this... Blueracer said it looks like the middle IC is bad too. I'll be removing that later tonight and then more tests.

 

[pabloguitar]

Middle IC is out.

 

[Blueracer]

The next thing to do is set up the 9 volt battery as Mesa showed you. Without the second IC in the Lead + Eq LED should be lit. With a wire connect pin 8 and pin 1 on the remaining IC. This should lite the Rhy2 LED. Now connect pin 8 and pin 2 on this same IC. This should lite the Lead LED. And finally connect pin 8 and pin 1 and pin 2. This should lite the Rhy1 LED. If all is working properly with the remaining IC this is what should happen. Let us know how this goes.

 

[pabloguitar]

With the battery hooked up, only Rhy1 was lit.
Pin 8 and 1 connected, Rhy1 lit.
Pin 8 and 2 connected, Rhy1 lit.
Pin 8, 1, and 2 connected, Rhy1 lit.
That Rhy1 light is stubborn! Darn thing doesn't want to go out.

 

[Blueracer]

Well that was predicatable. Time to yank the last IC. Still some more tests to run once it is out. Sorry about this but somehow this foot switch really got hosed. All three ICs NG or NFG depending on your mood. Hang in there. Things will get better.

 

[pabloguitar]

Last ones out.

 

[Blueracer]

Next test as follows. Hook up the 9 volt battery. Use the wire to connect pin 8 and pin 15 of the IC you just removed (the 74LS138). This should lite up the RHY1 LED. Next connect pin 8 and pin 14 of the same IC. This should lite up the RHY2 LED. Next connect pin 8 and pin 13 of the same IC. This should lite up the Lead LED. And finally connect pin 8 and pin 12 of this IC. This should lite up the Lead + Eq LED. Once you do this I do want to verify a few connections doing some continuity tests. More on that in the next post.

 

[pabloguitar]

Hey, alright! Finally passed some tests. All of the last ones worked as you described.

 

[Blueracer]

Yes the bottom has been found now we should be on an up swing. The next tests should go fine also. They are a series of continuity tests just to verify that the trace integrity of the circuit board is intact. I want to introduce a nomenclature for the ICs. The first one you removed we will call U1, the second one you pulled U2 and the last U3. A pin associated with that IC will be hyphenated to that you number. So for instance pin 5 of the first device you removed would be U1-5. Now I want to string together a bunch of these IC-pin combinations. The idea is to check for continuity in these strings. So here we go.

U1-16 to U2-13

U1-17 to U2-1

U2-4 to U3-2 also to pin 1 of the connector (the one that mates into the Mark IV)

U2-10 to U3-1 also to pin 2 of the connector (the one that mates into the Mark IV)


U1-8 to U1-13 to U2-7 to U3-8 to U3-3 to U3-4 to U3-5

The last tests are to check the integrity of the switching system. The idea is to check for continuty between two pins then sequencially push the 4 channel changing switches. Only one of the switches should make contact and give an indication of continuity. So here they are.

U1-1 to U1-11 works with RH1 switch

U1-1 to U1-10 works with RH2 switch

U1-1 to U1-8 works with Lead switch

U1-1 to U1-7 works with Lead + Eq switch

You should be able to find all this on the last page of the Mark IV schematic I sent you. I am incline to think all this will be fine but there have been so many problems with this board that I want to make sure. If you haven't ordered the parts go ahead and do this as that is where we will be headed next. The best way to go about the IC replacement is to install one at a time and then do some of the testing we did at each stage of removing them. It should be instructive this way as well as verifying everything in a step wise manner. Let me know how things go and we can continue.