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

[pabloguitar]

U1-16 to U2-13 = nothing
U1-17 to U2-1 = nothing
U2-4 to U3-2 = 0 ohms
U2-4 to pin 1 = nothing
U3-2 to pin 1 = nothing
U2-10 to U-31 = 0 ohms
U2-10 to pin 2 = nothing
U3-1 to pin 2 = nothing
U1-8 to U1-13 = nothing
U1-8 to U2-7 = nothing
U1-8 to U3-8 = nothing
U1-8 to U3-3 = nothing
U1-8 to U3-4 = nothing
U1-8 to U3-5 = nothing
Where it is written as nothing, I guess that would equal infinity.
U1-1 to U1-11 only showing continuity with Rhy1 switch = successful
U1-1 to U1-10 only showing continuity with Rhy2 switch = successful
U1-1 to U1-8 only showing continuity with Lead switch = successful
U1-1 to U1-7 only showing continuity with Ld+Eq switch = successful

 

[Blueracer]

Well this looks pretty grim but I did make some mistakes here. Let me correct these. I will copy what I sent before and highlight the changes in red.

U1-16 to U2-1

U1-17 to U2-13

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

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


U1-9 to U1-13 to U2-7 to U3-8 to U3-3 to U3-4 to U3-5

This should make things better. The first two connections U1-16 to U2-13 and U1-17 to U2-1 not being correct is due to the schematic being incorrect. I checked these connections on my foot switch and know that these are the now listed as the correct connections. The rest of these were my mistakes. Hope that this makes things appear better.

Let me know if this helps things.

 

[pabloguitar]

I think this is probably more like what you were looking for...

U1-16 to U2-1 = 0 ohms
U1-17 to U2-13 = 0 ohms
U2-4 to U3-2 = 0 ohms
U2-4 to pin 2 = 0 ohms
U3-2 to pin 2 = 0 ohms
U2-10 to U3-1 = 0 ohms
U2-10 to pin 1 = 0 ohms
U3-1 to pin 1 = 0 ohms
U1-9 to U1-13 = 0 ohms
U1-9 to U2-7 = 0 ohms
U1-9 to U3-8 = 0 ohms
U1-9 to U3-3 = 0 ohms
U1-9 to U3-4 = 0 ohms
U1-9 to U3-5 = 0 ohms

Another series of tests passed, I'm starting to get excited! I just ordered the needed parts, along with the extractor and two of the ones you suggested.
As always, thank you!

 

[Blueracer]

This is all good news. You are now in a good position to replace the ICs and have a fully functional foot switch. As noted in a previous post there is an issue or potentially multiple issues in the Mark IV. This is a logical place to venture while you are waiting for your parts. Let me know if you are up for this now, would like to wait until the foot switch is completed or resolve this some alternate way.

 

[pabloguitar]

You're right, this is a good time to start looking in the Mark IV. I'm ready, on a little time constraint this weekend, but I'll definitely get to it when I have an opportunity. I'll get the amp out of its shell.

 

[Blueracer]

Well pabloguitar and I continued this saga off line via email. Parts were received and the Mark IV side of things investigated. A faulty IC in the Mark IV was identified and replaced which rendered the amp working with the FU2A foot switch. This is a big step forward.

Next on the list of things to do is replace the three ICs in the FU3 foot switch.

If anyone is following this thread I can post the emails that went between pabloguitar and myself. If not I won't bother. Sufficed to say progress is being made.

 

[Mongo1]

This is a great thread.

The process you used could easily be drafted into a great troubleshooting manual. Logical and conclusive.
Combined with the schematics, this would be an awesome resource for MKIV owners.

I know I'd love a copy.

 

[pabloguitar]

Blueracer,
Just installed the 3rd IC into the foot switch.

 

[Blueracer]

Great. I am assuming this is the 74LS138 device in the U3 position. Hook up a 9 volt battery as shown in the Mesa test directions. At this point the Lead+Eq LED should be lit. Now with a wire make the following sequence of connections.

U3-8 to U3-1 this should lite the R2 LED

U3-8 to U3-2 this should lite the Lead LED

U3-8 to U3-1 to U3-2 (all three connected) this should lite the R1 LED

If all these tests pass then install the second IC, the 74LS14, into the U2 position. Otherwise post back the problems encountered.

 

[pabloguitar]

Yep, with the battery hooked up, I have the Lead+Eq LED lit.
There is a little discrepancy on the next part...
U3-8 to U3-1 does not light R2 LED, it lights the Lead LED, and U3-8 to U3-2 doesn't light the Lead LED, it lights the R2 LED. These two are backwards, could this be the same situation as before, where the schematic is incorrect?
The next one was good, U3-8 to U3-1 to U3-2 did light R1 LED.

 

[Blueracer]

This was my mistake on this one. What you tested was correct and I had it backwards. So you are good to put in the next IC which we called U2. It is the 74LS14. I think you will have a working foot switch soon.

 

[Blueracer]

Once you have the second IC in place do the following test. Hook up the 9 volt battery as before. The Lead+Eq LED should be lit as before. Now with your wire make the following connections. Note this will be on the IC you haven't replaced yet.

U1-9 to U1-17 the Lead LED should lite

U1-9 to U1-16 the R2 LED should lite

U1-9 to U1-16 to U1-17 (all three connected) the R1 LED should lite

If these tests pass do the following. Remove the 9 volt batter. Hook up the foot switch to the amplifier. Turn on the amplifier but leave it in the standby off mode. Make sure the selector switch on the back of the Mark IV is in the foot switch position. The Lead+Eq LED on the foot switch should be lit as well as on the Mark IV. Now make the same connections as shown above and make sure that both the foot switch and the Mark IV are showing the same channels. If all this works time to replace the last IC. Remember to turn off the amplifier and disconnect the foot switch before replacing the last IC. Also use your wire to connect U1-9 and U1-18 before replacing the IC and after disconnecting the foot switch. This will discharge the capacitor hooked up to the IC. Note this capacitor acts as a memory to the foot switch. It retains a charge that keeps power to switch encoding IC and hence "remembers" what the last state of the IC was. Since this IC draws very little power the capacitor can retain this last state for a relatively long time. If there were any problems with the tests post your results and we will take it from there.

 

[pabloguitar]

After installing the 2nd IC, I did all the tests as you described. Everything checked out Ok. Went ahead and installed the 3rd IC. We now have a fully functional foot switch! Absolutely awesome, and I'm not talking about the foot switch, I'm referring to the help that you provided. Step by step, you explained everything perfectly clear and in a logical order. For me, this has been an extremely fun little project. I'm glad that the foot switch is finished, but a little sad because every time I was looking forward to the next step of testing and replacing what was found to be bad. I'm also obviously very grateful that you helped me get the amp and foot switch talking to each other again, so to speak, but even more grateful that you took the time to walk me through all this and all the time you spent troubleshooting. I'm sure no one wants to hear me keep going on and on, but it really says a lot when someone goes out of their way to this extent!!

Thank you,
Paul

 

[Blueracer]

Paul I couldn't be more pleased that we, and that is we as in you and me, were able to solve your foot switch problem. I do hope that the amplifier brings you years of pleasure and now it is even fully functional. I really did feel bad about someone taking advantage of you in a straight up trade and hopefully this kind of sets that right not to mention I always have fun solving these sorts of problems. Take care and if something else comes up with the amp feel free to contact me and of course thank you for the very kind words.

Gary

 

[RoadkillTCMC]

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.

Okay....I went back and reread a couple times and cannot find how Mesa says to hook up 9 volt battery.Could you please explain?

 

[Hollis]

Wow, what a great thread. That is what is so great about these forums. There are guys like Blueracer that go out of their way to help other folks. Keep up the good work ya'll!