Mark V Effects Loop Not Working

[BoogieBoek]

Hey Folks! I hope you can help me out here. My Mark V effects loop is just not working. I've had the unit for several years, bought used. I never used to use much effects before but now I'm building a pedal board and I tested the effects loop with a brand new Timeline Delay and a Boss Chorus pedal. I used the exact same pedals in another amp and everything worked. Back to the Mark V, I had the "send" going to pedal input, pedal output going to "return". I tried the Loop Assign to all possible settings and nothing worked. I tried all possible settings on the Channel Select, but I usually use the foot switch so I keep it on FT SW.

What could be wrong with the effects loop? Like I said, I bought this used, supposedly in mint condition. I'm attaching pictures, including one that had a sticker covering the effects loop "EFX" lettering. Ok, so maybe the effects loop was broken and someone identified it with the sticker. Silly me, I thought it was a sticker from the manufacturer, it looks like some type of quality check sticker, but without a signature on it, lol.

Any ideas would be greatly appreciated.

Also, is it reasonable to get an effects loop repaired for a reasonable cost or is it better to offload this amp and start fresh?

Thank you!

 

[Fcal124]

Possibly the FX Loop tube is bad.

 

[mace]

If you haven't already, connect a short cable directly from SEND to RETURN and see if it works? Set the rotary dial to ON ALL. If nothing then maybe try replacing the tubes which send/return to the loop. V6 is the RETURN, not sure which tube(s) is SEND.

 

[BoogieBoek]

If you haven't already, connect a short cable directly from SEND to RETURN and see if it works? Set the rotary dial to ON ALL. If nothing then maybe try replacing the tubes which send/return to the loop. V6 is the RETURN, not sure which tube(s) is SEND.

If you haven't already, connect a short cable directly from SEND to RETURN and see if it works? Set the rotary dial to ON ALL. If nothing then maybe try replacing the tubes which send/return to the loop. V6 is the RETURN, not sure which tube(s) is SEND.

I tried this and it played just fine. I'm learning here! So I assume that means V6 Effects Return Stage vacuum tube is functional. What's next for me to test out?

Also, when I had the effects pedals in the loop the red light indicating the pedal is activated on the Boss pedal would not come on. That same pedal worked in the effects loop of another amp.

 

[Fcal124]

The Loop shouldn't have any effect on the pedal's function. What's powering your pedals? Battery or pedal board? Maybe there's a short in your pedal and it might be stopping the loop from functioning. Try taking the pedal out of the chain and see if the loop works.

 

[BoogieBoek]

I don't know exactly what was different but it just worked! I think it might be a tube that was a little loose and reconnected. Thank you very much for your help!

 

[Eevil]

As a side note - some pedals may not work correctly in the loop (namely the boss chorus in this case).. as the loop signal level is line level and boss is designed to handle instrument levels.

Just for heads-up if there’s some strange things happening while certain pedals are in fx loop…

 

[Rbh]

It might have just been a dirty jack connector, or the jack connector itself was bent a little and the weight of the cable or whatever caused a disconnection. good news is - it's possibly a simple mechanical fix and not internal circuit related.

 

[mace]

Glad you got it working!

 

[bandit2013]

If you haven't already, connect a short cable directly from SEND to RETURN and see if it works? Set the rotary dial to ON ALL. If nothing then maybe try replacing the tubes which send/return to the loop. V6 is the RETURN, not sure which tube(s) is SEND.

The mark V90 does not use a preamp tube for the signal to the FX send jack. It is managed by the GEQ mixing circuit. GEQ does not need to be active as the mixing circuit is always present in the signal chain.

First, does it work when there is nothing plugged into the send/return jacks? If you get no sound, it may be the return preamp tube. Turn on the FX loop (check if the LED is on, it is right above the power switch). the global master should be dialed up. If you cannot get sound, try working the rotary switches on the back panel. With the amp on, rotate each knob a few times. You should see the channels change and if the FX loop LED turns on or not.

It could be related to something else and not the return preamp tube. The footswitch cable can cause some weird issues.

If that does not work, try unplugging the footswitch at both ends and reconnect it. Sometimes there is a ground issue between the connections. Even if the rotary knob is set to all on. I have found that bad connections in the footswitch cable can lead to issues.

If you have been using the amp in the bypass mode for some time, it is possible there is some contamination or excess lubrication on the toggle switch. With the amp on, change the bypass switch a few times. Movement will help to clean the contacts.

Deoxit is a last resort when it comes to sealed switches. Using the switch actuator (lever or rotary knob) while the amp is turned on will create weak arcs when the switch contacts are changed. This aids in cleaning off the contacts.