Mark IV Footswitch Issue

[BradGone]

Hello,

I own a Mark IV with the standard footswitch for the 3 channels with the effects loop and the EQ, EQ-Lead options. The footswitch had been operating totally fine up until just recently when activating the lead channel became an issue.

Switching to or from Rhythm 1, Rhythm 2, and Lead-EQ is no problem. Once on either of those channels I can switch back to R1, R2, or Lead-EQ without issue. Switching the FX loop and EQ on and off is no problem, either. However, switching to the lead channel has proven to be not consistently possible.

For example, while playing on R1 (or R2), hitting the Lead switch button will cause a different channel such as R2 or Lead-EQ to be selected instead of Lead. The Lead channel LED will not light up. If I repeatedly tap the lead button I am switched, seemingly randomly, to and from R1, R2, and Lead-EQ (everything except Lead).

Occasionally, after maybe 10 times (give or take, varies) pressing the lead push button/switch I am eventually able to active the lead channel. After finally getting the lead channel to be selected, the randomness continues if I select another channel: I can change from lead to other channels fine, but re-selecting lead becomes a circus of hitting the lead button but then turning on R1, R2, or Lead-EQ... Anything but lead (unless I hit it a dozen times or so and get 'lucky' one of out those times.)

Has anyone else had this problem? If so, were you able to find a solution or repair the footswitch? (I hope it is not an issue with the amp itself.)

I suspect maybe the problem is a bad SPST momentary switch for the lead channel, but I really can't tell. The random effects of hitting the lead channel button is confusing, to say the least. And, when I took the footswitch apart to look for any obvious defects I found none, although I discovered that it is more complex than I had thought. I've included a picture for reference.

I tried searching for more info on this topic but could not find any, so this is my first post to this forum. I appreciate the quality of the majority of the posts on this forum - an uncommon characteristic for most online forums. Thanks to whoever posted tips about setting up reverb on the Mark iib 60/100!

Thank you,
B Gone

 

[BradGone]

Addendum -
Maybe these links for images will work better:

https://drive.google.com/open?id=1gCkjK4GfW4HBP0c9DmKeRe4BoUdc8LFq
https://drive.google.com/open?id=1ThKmB8o4jYwfYFB25jon3Ya8OeFu26U-
https://drive.google.com/open?id=1rKSH16CMuDFSaOc_7YTEND7hcO5Hsjly

 

[The_Nuge]

Hi!

I recently acquired a MK IV A myself and was also having issue with the channel switching. It turned out my problem was the low-voltage power supply (feeds 15V to the pedal among other things - have you measured that?), but I'm not convinced that's your problem. Can you switch the channels with the rotary switch on the back of the amp?

Cheers

Es

 

[BradGone]

Hi Es,

Thanks for the insight. I tried switching from the knob on the back. It had no effect...

I can check if the voltage going to the pedal board is 15v, but I'm not sure if I know where the 15v source is, though, in the event it turns out it is not supplying ~15v. I'll have to look for a diagram or schematic somewhere. If you happened to have any links to helpful info I wouldn't mind checking them out.

How did your Mark IV work out in the end? Was it a minor fix? Or, something more involved?

Thanks,
BG

Hi!

I recently acquired a MK IV A myself and was also having issue with the channel switching. It turned out my problem was the low-voltage power supply (feeds 15V to the pedal among other things - have you measured that?), but I'm not convinced that's your problem. Can you switch the channels with the rotary switch on the back of the amp?

Cheers

Es

 

[The_Nuge]

Hi!
I recapped the amp and also replaced the diodes and some resistors in the low-voltage supply - not all that easy, but doable....
I have schematics for both variants - which do you have?

Cheers

Es

 

[BradGone]

I believe it's the type "A" variant? It was made in '93 if I remember correctly.

Did you replace all the caps..?

I agree.. Doesn't sound especially easy, but seems doable.

Hi!
I recapped the amp and also replaced the diodes and some resistors in the low-voltage supply - not all that easy, but doable....
I have schematics for both variants - which do you have?

Cheers

Es

 

[The_Nuge]

Hi!

I replaced the two caps near the power trans, and also the three 500V caps down the other end of the board. This made the whole amp somewhat less noisy.
Essential to my problem however, was replacing the three 8200uF 16V caps.

Let me know if you want the diagrams for the amp!

Cheers

Es

 

[BradGone]

That doesn't sound too difficult all in all, at least in theory...

The three 500v caps are the big blue ones, right? I'm pretty sure I know exactly where those are.

That'd be great if you could send a copy of the diagrams. PM?

Thanks,
BG