Mark IIb Effects loop mod

[Rob Lockwood]

Hello all. I recently bought a 1983 late model Mark IIb HRGX. I have been after one of these for ages and am very happy with it. I took the chassis out recently to change some tubes and saw a separate small board about 5x2 inches on the side of the chassis. That got me thinking whether this board is the effects loop mod I have read about on here? If not, how would I know whether I have the loop mod or not? I would really like to hear anyone's experience of how the loop mod affects the tone of these amps (without the loop in use), as I understand that it affects more than just the loop. At the moment I am using the 'clean' channel with pedals all in front. Thanks in advance.

 

[dodger916]

My C+ has a "side board" like that; I don't think it has the lop mod. I thought it had to do with the graphic eq. Does it look like this?

 

[Rob Lockwood]

Hi, thank you for your reply. Yes that's the board. Mine has the same 2 capacitors on the left. The other components look a little different from memory, but its basically the same size board in the same place so I guess it is doing a similar job. I will have to pull the chassis out again and take some photos. From what I've read here the IIC and C+ had the loop mod as standard but it was a factory mod. on the IIb. If that's correct then this board still could be related to the loop mod - but I am only guessing really at this stage because it looks like a add-on. I will download a schematic and see if it shows this separate board. Thanks again.

 

[dodger916]

Although it's been a couple of years since I was deeply into this stuff, IIRC, that board is for the graphic eq, not the loop mod. I had my C+ "blueprinted" by Boogie Board member Boogiebabies, and we spoke quite a bit before, during and after the work. I'm sure he did not mention seeing (or doing) the loop mod, and I'm pretty sure we spoke about that little board being for the graphic eq. Plus which, there's waaay more electronics on that board than I've seen on-line for any loop mod, but I'm no expert by far. A loop is fairly simple: the preamp circuit is either a) interrupted with the loop signal inserted in series (100% signal) or b) the loop signal is run parallel to the preamp signal, with a pot to blend the amount of loop signal that gets added to the preamp. The only mods I've seen are to change the loop to or from serial to parallel, and that does not require anywhere near the gaggle of electronic hardware shown in my photo.

Do you know the purpose of the loop mod you mention?

 

[kippiejr]

Hello,
its for the EQ, trace wires. NON EQ models do not have this "side" circuit board.
Peace
#AmpFairy