Marshall 6100LM fx loop problems

[simo_slash]

Hi Guys. The first time for me in this forum. In Italy we haven't this kind of forum, so detailed in technical matters regarding guitar amps. A great community.
I recently buy from France a used Marshall 6100LM. New tubes, great sound, a new world respect my old Valvestate VS100. My Gibson LPS is so happy :-)
But.... today I have tried to connect my multieffect pedalboard (Digitech GNX1) to the fx loop and ... nothing.
In serial mode no sound at all, in parallel mode only the direct sound without any effect.
I'm sure that the pedalboard work correctly because I used it on the old amp in the same way.
I'm also sure that the send line works because if I use the padalboard for tune the guitar the signal arrives correctly through the send line.
So probably the signal arrives back in the return port but is not processed correctly.
Can someone suggest what can I check more? Or what can I verify also internally (I have a few of electronic competencies)? Or any kind of suggestion.
Thanks for your help.
Ivan
P.S. I'ld like to use the fx parallel mode.

[simo_slash]

Up!
Please, a Marshall guru can help me!

[oc disorder]

Hi Simo_Slash and welcome to the forum.
The hand book for the amp can be found here
http://www.marshallamps.com/archive/...niversary.html
http://www.marshallamps.com/handbooks/pdf/30thanniv.pdf

You may have to adjust the level trims at the back

Operational Note D: Series effects option
Items 38 & 40 can also be used as a 2nd series effects loop (See operational note F). The selection switch (Item 39) will
switch the loop in or out of the circuit, but will not mute the power amp.
41. Effects Send Trim Channel 1 Provides -10 to +4db level control of the effects send
from Channel 1 only.
42. Effects Send Trim Channel 2 Provides -10 to +4db level control of the effects send
from Channel 2.
Operational Note E: Effects level balancing
On 41 & 42 - When setting up the effects loop, the effects processor’s level controls should be set up against the lead
channel -3 and matched using switch (44), then channels 1 & 2 set up via the trim controls for an effects send balance.
If desired, more or less sound level can be set for differing amounts of effected signal.

I'm not familiar with the Digitech GNX1 but I presume it was designed to plug in at the input (at the front) to an amp rather than be placed in a loop.

It is expecting a tiny electrical signal from your guitar not a boosted level from a preamp.
The outputs will be also at a low level expecting to feed a sensitive guitar input whereas the 6100 can accept high line level +4 signals that a pro studio rack mount processer can put out .

They need to be more towards the -10 area than the + 4.

I would guess in parallel mode the output from the GNX1 was too small and insignificant compared to the "beefy" internal signal.

Can you increase the overall output vo;ume from the unit..master out ?

The other possibly slight problem is the GNX1 is in stereo I hope it reverts to mono if only one output is connected.

A lead plugged into the return should hum when you put your finger on the tip of the jack.

Hmmm just checked the manual myself and it appears the trims only affect the send.

It may mean you need to boost the output of the GNX1 to more of a line level.

This problem occurs also in studios matching semi-pro gear (-10) to pro gear +4.
There are some commercially available units to do this.

Perhaps someone else may be able to comment on this.

Hope thats a start for you.

[simo_slash]

Hi Simo_Slash and welcome to the forum.
The hand book for the amp can be found here
http://www.marshallamps.com/archive/...niversary.html
http://www.marshallamps.com/handbooks/pdf/30thanniv.pdf

You may have to adjust the level trims at the back



I'm not familiar with the Digitech GNX1 but I presume it was designed to plug in at the input (at the front) to an amp rather than be placed in a loop.

It is expecting a tiny electrical signal from your guitar not a boosted level from a preamp.
The outputs will be also at a low level expecting to feed a sensitive guitar input whereas the 6100 can accept high line level +4 signals that a pro studio rack mount processer can put out .

They need to be more towards the -10 area than the + 4.

I would guess in parallel mode the output from the GNX1 was too small and insignificant compared to the "beefy" internal signal.

Can you increase the overall output vo;ume from the unit..master out ?

The other possibly slight problem is the GNX1 is in stereo I hope it reverts to mono if only one output is connected.

A lead plugged into the return should hum when you put your finger on the tip of the jack.

Hmmm just checked the manual myself and it appears the trims only affect the send.

It may mean you need to boost the output of the GNX1 to more of a line level.

This problem occurs also in studios matching semi-pro gear (-10) to pro gear +4.
There are some commercially available units to do this.

Perhaps someone else may be able to comment on this.

Hope thats a start for you.

Hi OC Disorder,
and thanks for your help.
I believe that the problem is inside the 6100LM because I used the same GNX1 with my old VS100, and worked well.
The GNX1 have a parameter to set the "effect loop" working mode and I tried the output level al maximum.
I tried also the serial mode of the 6100 but I heard really nothing (I suppose that also in case of low signal I should hear something at low volume).
What do you think about? Do you think I can check something inside?
However, Marshall forever!

[simo_slash]

Up!!!!!
Please, more suggestion. I need your help, gurus.
Is there someone that can suggest me what can I check internally before spent a lot of money for a professional.

[oc disorder]

Well the schematics are here http://www.drtube.com/marshall.htm#AN30
although it is the version with 4 x 5881 the Fx loop should be the same.
IC 206 a TLO 71 IC is the first "port of call" for the FX return and we need to see if a signal is applied there, if it is present at pin 6 (the output)or at C225 or even Pin 3 of connector 206.
You could try feeding a signal from a cd or mp3 player , need a mini jack to a standard 6.5 jack.Make sure the level is low at the player and you should hear it . It is a difficult amp to work on I would recommend taking it to someone used to this work.
If the IC is in a socket it would be easy enough to plug in a new one but I havn't seen one for a while and can't remember.
If you are going to explore it yourself remember there are leathal voltages inthere so be extra careful.
Worth noting where all the connectors go maybe also worth taking a few photos of the inside before any work is done.