Peavey Mk IV Circuit diagram

[Ross Gibson]

Hi folks,
Firstly, I apologise in advance if I am in the wrong part of the forum or this has already been asked...I'm at work and shouldn't be on this forum.
I have a Peavey MK IV from 1983. On powering up it defaults to both channels. When using the footswitch to select either of the channels, the amp shuts down.
I've searched high and low for a circuit diagram to have my soldering iron guy do his bit, but I can only find the diagrams for almost every other Peavey device, except for the MK IV.
So here I am, asking you good 'well-mannered' folks for some assistance about sourcing the circuit diagram.
Cheers

 

[teemuk]

Bass model here:
[Invalid or Expired Link Removed]

 

[Passinwind]

Hi folks,
Firstly, I apologise in advance if I am in the wrong part of the forum or this has already been asked...I'm at work and shouldn't be on this forum.
I have a Peavey MK IV from 1983. On powering up it defaults to both channels. When using the footswitch to select either of the channels, the amp shuts down.
I've searched high and low for a circuit diagram to have my soldering iron guy do his bit, but I can only find the diagrams for almost every other Peavey device, except for the MK IV.
So here I am, asking you good 'well-mannered' folks for some assistance about sourcing the circuit diagram.
Cheers

Go straight to Peavey tech support, don't mess around with unofficial linked content.

 

[agedhorse]

Are you using the correct footswitch type?

 

[BogeyBass]

Are you using the correct footswitch type?

Peavey Used a 5 or 6 pin Din Plug for the channel switch, I forget which one. So you cant use tip/sleeve or tip/ring/sleeve type universal footswitches.
You have to use the Peavey footswitch.


For the OP

I believe the Mark3 and Mark4 both used the same 400BH power amp.
That info/schematic is easily found. Seems difficult to find the PreAmp for the Mark 4

Mark 3 was almost the same channel wise. The Mark 4 i believe added the crossover, and effect sends for each channel.
So the channel switching might be similar on the Mark 3

Could be a simple issue with the panel mounted Din Plug or footswitch switches.
Think this power amp only had a thermal shutoff. Why the channel switch would cause a false trigger
baffles me lol

 

[BogeyBass]

Schematic Attached

 

[BogeyBass]

Channel switching is with S1 and it grounds the switch.

I dont see why the thermal fuse would shut the amp off by a channel switch issue its just wired inline with the fuse.

Only possible issue is that amp has a older type power switch.
Toggling to one side of the other changes the ground reference.

So might be someone has been in there and grounding wiring has been done wrong, or a actual
power switch has issues. Toggling the switch changes the ground reference. Channel switching goes to ground.
Only place i could see a issue. Might be actual mechanical issues with the power switch. Or wiring issues around the switch.

Unless you mean the fuse just blows, then there is a lot more possibilities

 

[agedhorse]

I doubt the footswitch has anything to do with the power amp.

Do your amp just don't pass audio, rather than the amp shutting down?

I would be focusing on the footswitch and related circuitry.

 

[Killing Floor]

Ha! My first amp/cab rig was a Mark IV with the 2X15 cab. That thing was really hard to kill. It was as loud as I ever needed it to be. Only fault was the compression kicked in kinda hard so some notes turned into Pffffffff. But I loved that thing so much! I hope you get yours running. Good luck.

p.s. you can wad aluminum foil in the fuse cylinder and it will last for years. @agedhorse , don't yell, I was a teenage boy at the time. But it worked and kept the fuse from blowing. Sorry.

 

[Tim Skaggs]

Ha! My first amp/cab rig was a Mark IV with the 2X15 cab. That thing was really hard to kill. It was as loud as I ever needed it to be. Only fault was the compression kicked in kinda hard so some notes turned into Pffffffff. But I loved that thing so much! I hope you get yours running. Good luck.

p.s. you can wad aluminum foil in the fuse cylinder and it will last for years. @agedhorse , don't yell, I was a teenage boy at the time. But it worked and kept the fuse from blowing. Sorry.

The aluminum foil doesn’t keep the fuse from blowing, it replaces the fuse and will carry about triple the current of the fuse. Don’t do this. You will burn something down; the amp, and probably your house.

 

[Ross Gibson]

Are you using the correct footswitch type?

Yes, the one with which it came. Cheers

 

[Ross Gibson]

Thanks everyone. I knew I had come to the right place. Your boundless enthusiasm for each others' issues is something many people in society could learn from.
Agedhorse. The whole amp shuts down; no lights, no sound, nothing.
Passinwind - that is sound advice (no pun intended).
Bogeybass - thank you for your attachment and thoughts.
KillingFloor - I agree, the compression complaint is a common one
Tim Skaggs - Yes, the fuse is rated for a reason. I hope everyone takes this advice on board.
Cheers - RG

 

[agedhorse]

Does the amp work normally without the footswitch attached?

I don't think I have the schematics for the Mk IV that uses the DIN plug footswitch, this is the one I know about and I think the linked schematic refers to this:

 

[Ross Gibson]

Does the amp work normally without the footswitch attached?

yes it does - two channels at once.
cheers

 

[Killing Floor]

The aluminum foil doesn’t keep the fuse from blowing, it replaces the fuse and will carry about triple the current of the fuse. Don’t do this. You will burn something down; the amp, and probably your house.

Ha! Where were you in 1985? Too late for me to use that advise. Like I said, don't yell now, the ship has sailed. But I played that way for at least 2 years. Now I know better and I also care more about my gear.

 

[AudioTaper]

I had a few Peavey amps come through the shop with footswitch issues. It was almost always a bad spot in the cable at the DIN connector (open conductor caused by years of flexing the cable) I would cut about 8" off and install a new connector.

But you have confirmed that BOTH channels come on. This might be a dumb question.... is it supposed to do that with no footswitch?

A quick search of TB has a number of posts about the footswitch. Seems the consensus is to turn one channel down with a footswitch is not being used.
Eliminating the foot pedal on Peavey Mark IV??
Peavey Mark 4 , no footswitch. Is my work around safe for the amp?

 

[agedhorse]

I recall something to do with the automix circuit where both channels are on without the foot switch but it’s been a very long time since I worked on one.

 

[Passinwind]

I recall something to do with the automix circuit where both channels are on without the foot switch but it’s been a very long time since I worked on one.

My recollection is that both channels should be working that way, but it's been quite a while for me too, maybe 15 years os so since the last MKIII I worked on.

 

[fig]

A minor thing, but as drawn the schematic indicates S1 (COMbiner) and S2 (SELector) as integral to the amp and not a separate outside footswitch. I'd probably have drawn it with dashed lines to show these switches as outside the amp (I removed some non-essential stuff from the screen capture):

Anyway, to me, it's possible U2 is faulty and shorting out the + and/or - 15 volt supply voltages at U2 pins 4 and 8 when the S1 ground is applied thru S2 to U2 A or B. This would remove all + and/or - 15 volts in the entire amp, disabling most everything. May account for the OP saying "The whole amp shuts down; no lights, no sound, nothing.".

Fortunately, that chip sits in adapter socket. Pluck it out by hand, put in a new one if needed.

First, I'd open the footswitch and its DIN plug to do a visual inspection and then check for proper readings to the DIN plug with an ohmmeter, wiggling the wiring during each operational check. There's enough comments in here of bad cabling to warrant the wiggle check. I can't find a dedicated schematic for the footswitch to show which DIN pins the wires are connected to. Should be easy to determine.

If footswitch checks good, I'd remove U2 and do some visual and meter checks in that area.

If it checks good, I'd contact Peavey and see if they'd send you the U2 chip (4558 Dual Op Amp). They used to send me low cost things for free when I owned Peavy stuff. You might get lucky. If you have to buy, a quick check shows they're under $1 each.

Just some thoughts in my now plentiful spare time, but I've had a few beers LOL.

 

[agedhorse]

IF that's the footswitch, there's nothing in that footswitch that can cause those symptoms.

I have a suspicion that there may be a later version with a DIN that is wired differently. THAT could explain the symptoms.