Marshall MG 100HDFX-Clean switching?

[retrorod]

Have a MG100HDFX for repair. It won,t switch to clean channel. The channel switch indicator does not go on/off when pushing switch button. I do not have the footswitch for this amp either. I am not up to par on this solid state stuff. I AM striving to learn though! Amp plays,but does not have the 'balls' that I would expect from a Marshall. I have read and re-read prior posts on this amp and it seems to me that it has a lot of problems....heat-related,chips,etc. Possibly a bad design to start with!
What is the function of the green/red led's mounted on the PCB as far as diagnostics?
The heat sink fan on this amp makes a whining noise for a minute or so on start-up, and then smooths out and quiets. Normal? I have cleaned some dust from around it,although it was not extremely dirty.
Hopefully, someone can call out some part #'s that would be suspect on the board. Thanks and I hope I am not wearing out the subject on this amp. Rod

[booj]

The heat sink fan on this amp makes a whining noise for a minute or so on start-up, and then smooths out and quiets. Normal? I have cleaned some dust from around it,although it was not extremely dirty.

That unfortunately is normal from what I've experienced
mg100hdfx.pdf
Here's the scat. Hope that helps.

[retrorod]

Thanks booj, for that answer and the schem. This thread had sunk faster than the Titanic. I still have the amp on the bench but have not devoted much time to it lately. When activating the channel switch button, the panel light stays on and no change in tone. I have not seen any of the red/green pcb led,s go on or off either when activating any switches. I will do some checking of voltages by the schem.
From what I have read about these SS marshalls it is OK to test powered up with no load connecected?? I know I would never do that with tube gear..Rod

[Jazz P Bass]

Marshall 100 HDFX

The two LEDs, green & red, on the PCB are your OD channel clipping diodes.
Look on the schematic, right underneath the Gain control. Led3 & LED 2.
They should illuminate when you are playing the OD channel.

The channel switching is provided by IC2. #DG212CJ.
You will have to verify that the IC is getting the correct input signals.
If that is true, then you can proceed to test whether or not the switches are indeed normally open and that they close open input command.

[retrorod]

Thanks! I have located IC2 on the board,but not sure how to verify that it is getting proper input signals? Could I just replace it, or potentally is the problem ahead of IC2? I will keep checking the schematic to see if it starts 'sinking in'
Thanks for the help and patience!

[Jazz P Bass]

IC 2 DC212

The DG212 is a logic controlled switch. There are four of them in the IC.
Look at IC 2 at the OD gain control on the schematic.
Right above the green & red LEDs.
This switch uses pin 9 (input) & pins 10 and 11 are "contacts".
The data sheet for DG212 indicates a logic low on pin 9 = switch off.
Logic level high on pin 9= switch on.
The footswitch or the front panel channel select switch should be sending the logic signals.
So, if pin 9 has a high voltage on it, pins 10 & 11 should be connected (closed).
There are 2 switches over on the right.
The top one is IC2 pin 1,2 & 3. When on this enables the OD channel signal.
Pin 1 is logic. Pin 2 & 3 are the switch "contacts"
The one below this one enables the clean channel.
Someone forgot to label the pins.
Your on your own there, trace it out.

[retrorod]

Thanks Jazz P Bass, You explained that very well and I can now "see what you are sayin" !
Since I don,t have the footswitch for this amp, I guess I'm screwed since it is used to send the logic signals. I just assumed that the functions could be accessed manually with switch buttons on the control panel. Possibly the footswitch is an integral part of this particular amp? Rod

[Jazz P Bass]

Marshall MG100 channel select

Sorry for the misinformation.
The footswitch or the channel select switch on the front panel will send the logic signal to IC2.
Carry on.

[Jazz P Bass]

Marshall MG100

I just noticed in the instruction manual that if the footswitch is connected the Channel Select switch on the front panel is nonfunctional.
You may want to check the footswitch part of the circuit for a short.
If the circuit "sees" a FtSw input , that will disable the front panel switch.

[retrorod]

Thanks for the clarification. I did some voltage testing on the DG212 this evening. This is what I have determined...+15v on#13, -15v on#4. I understand that logic control inputs are #1, 8, 9, 16
The readings in volts when switched for clean 3 1.1 1.1 4.9
The readings when switched to overdrive chan. 4.9 .004 .004 4.9
The reading of 3v on pin #1 stands out as being odd. When activating the push button for channel switching the LED dims slightly(from full bright) when it should go out when operating the clean channel.
I could not detect any shorts around the footswitch jack.
Please advise, if you get the time. Thanks, Rod

[Jazz P Bass]

Marshall MG100

Rehash:
You have OD channel & it works fine.
Correct?

When you select Clean, the LED dims (should go out?)
When you select Clean no output is heard.
Correct?

O/k. Can you check the 8 pin header that goes to the effects/ footswitch board.
If the FtSw jack is good, then pin #2 & pin #3 on the header should have continuity.

The 4.9 Vdc on pin 16 does not look good.
With voltage applied check the contact pins 14 & 15 for a closed condition.
This should be your Clean channel switch.
It has to close to pass the Clean signal.
Likewise, the OD signal uses pin 2 & 3 as contacts.
They should open when selecting Clean.

[retrorod]

RE: Rehash

Rehash:
You have OD channel & it works fine.
Correct? *correct!

When you select Clean, the LED dims (should go out?) -*Yes
When you select Clean no output is heard.
Correct? *No-Still get overdrive sound when switched to clean

O/k. Can you check the 8 pin header that goes to the effects/ footswitch board.
If the FtSw jack is good, then pin #2 & pin #3 on the header should have continuity. * YES-have continuity

The 4.9 Vdc on pin 16 does not look good.
With voltage applied check the contact pins 14 & 15 for a closed condition.
This should be your Clean channel switch. *14 &15 show continuity(closed?)
It has to close to pass the Clean signal.
Likewise, the OD signal uses pin 2 & 3 as contacts. *2&3 show Continuity(closed?), either way the channel switch is operated
They should open when selecting Clean.

Thanks again, please advise!

[Jazz P Bass]

Marshall MG100

Do you have +5Vdc on pin #12?
I think IC2 is bad.

[retrorod]

Yep, +5vdc on pin 12. Good or Bad?

[Jazz P Bass]

Marshall MG100

+5Vdc on pin 12 is according to schematic.
Last ditch effort, check the zener diodes on the FtSw board.
I cannot see how the voltage is not changing on the inputs of IC2.
If you can successfully remove IC2, then you may check the input voltages unteathered.
Sorry, but without the amp in front of me, I am now shooting from the hip.