[Pipina]

Hallo , I don't speak german so I'll write in English.
I bought a pair of serial FX-loop from Dirk and installed one into a Marshall Origin 50 with good results and the other into a Marshall DSL40CR but I'm not complitely satisfacted with this second one.
My friend Andrea (which is a REAL repairman and much more!) simply removed the SEND and RETURN jack from the amp and replaced them with the corresponding jacks from the loop's PCB and then connected the jacks to the PCB with a beautiful and clean job; as for the imput we decide to connect "input loop" on the amp (see the attached schematic) on the low input B of the loop, so the input voltage divider resistors were not populated.
Levels are ok, no noise at all, everything works as it should except for the frequencies.
I have an emphasis on high frequencies, which is not a con and can be addressed probably removing or lowering C7 (es 250pF) but I also have a CUT OFF on the bass area (switching on and off the loop) and I don't know HOW to address this issue.
Could someone of you help me with this?

Thanks

Carlo

[Dirk]

Please provide an exact schematic of how and where you installed the problem. I guess there are a few issues with ground, power supply or impedance. Without taking a deeper look into the amp it won't be easy to give good hints. I recommend to install the loop somewhere in the signal path and not as a replacement for the original loop to make sure that the loop itself works, than you can go on and move the loop to the position you want.

Best regards, Dirk

[Pipina]

Dirk, please, help me; I'm very close to fix the problem.
I don't wanna modify the amp, so I installed the loop when Marshall did originally; in this way I can restore the original configuration once I decide to sell the amp.
Today I tried swapping R13 (stock 100K) in favour of 470K because the normal range of external effects impedance vary from 470K to 1 Mega; I restored all the bass frequencies! but now I have a CUT OFF on the extreme high frequencies!!
The input B (low) is connected to the input of the SEND jack on the main PCB (the Marshall PCB), as well as the output is connected to the RETURN jack of the main PCB (again, the Marshall PCB).
Everything works as it shound....no noise at all, same volume, no ground loop, no unpleasant distirtion.....just a loss in the EXTREME HIGH FREQUENCIES which I would like to restore.
I just want to let it pass through the TT loop ALL THE FREQUENCIES SPECTRUM, no less, no more.
You suggest to remove C7 if the loop sounds too bright; I notice to opposite problem: a bit emphasis on mid-bass frequencies, a bit emphasis on mid-high frequencies and a loss on the extreme high frequencies.
Can you help me?
Thanks

[Dirk]

What is "EXTREME HIGH FREQUENCIES" ?

Sounds like that there is a problem with the impedance / load maybe ? Anyway, if the C7 is the key in your case then just try different values like 220pF or 100pF. You can also try a potentiometer with 500k instead of 100k for the send level or maybe a combination of both.

Best regards, Dirk

[Pipina]

EXTREME HIGH FREQUENCIES are the crystal glasses shattering for which Marshall is well famous
We can say the 6,4 kHz if you consider a Boss GE7 equalizer.
The TT loop - in my case - boosts the low-mid and high-mid frequencies and cut those high frequencies.
It's barely audible but enough for my ears!
And YES, were exactely a problem on impedance/load with the 100K in R13 (if you come in in the B input).
Now I restored all the bass frequencies with 470K load in R13.
In order to restore brigtness, I'll try what you suggest and report back.
Thanks

Carlo

[Dirk]

6,4 kHz is really not what I want to hear and regarding the load I was talking about the potentiometer for the send level and not the resistors at the input.
Just give it a try with a different potentiometer and the bright-cap and report the results.

Best regards, Dirk