Effects Loop concerns! HELP!!!

[Deputy Dawg]

Hello again everyone,

Questions for all of you and Steve D if you can help. I have owned my VM combo for a couple of months. I have noticed that the effects loop is possibly causing problems.

When the loop is engaged with the switch on the back, the output level of the amp drops and there is a distinct rise in background noise. It is my unerstanding that even with the loop switch set to "engaged", if there is nothing plugged in the loop jacks, there should be absolutely NO drop in volume level. Is that correct?

I am finding that, as soon as I engage the effects loop, the volume drops and the background noise goes up. I tried bypassing the loop with a guitar cord - plugging just the cord from the send to the return loop jacks - and then engaged the loop. Even with a direct cable connection from send to return, the level dropped and the noise floor rises. A

Also of note is the fact that the selectable effects loop level control has no discernable affect on the signal level. For example, pushing it in and out with an effect in the loop does not change the volume level.

Question. Could this be a tube problem? It appears to be on the "return" side of the effects loop circuitry. Does the "effects return" signal path pass through a tube or is it solid state? If I can play around with a few tube swaps to test this out, no problem, but I don't really want to waste my time if not required.

Otherwise, if it looks like an effects loop circuit board problem, I will be raising a stink at my retailer for a new amp.

Suggestions?

Deputy Dawg

[SteveD]

When the loop is engaged with the switch on the back, the output level of the amp drops and there is a distinct rise in background noise. It is my unerstanding that even with the loop switch set to "engaged", if there is nothing plugged in the loop jacks, there should be absolutely NO drop in volume level. Is that correct?

That is correct.

I am finding that, as soon as I engage the effects loop, the volume drops and the background noise goes up. I tried bypassing the loop with a guitar cord - plugging just the cord from the send to the return loop jacks - and then engaged the loop. Even with a direct cable connection from send to return, the level dropped and the noise floor rises. A

You can't 'bypass the loop' with a cord, only the switching jacks, so that test only tells you that the switching contacts on the return socket is fine. The only way of bypassing the loop is to switch it out with the 'loop' button.

Also of note is the fact that the selectable effects loop level control has no discernable affect on the signal level. For example, pushing it in and out with an effect in the loop does not change the volume level.

Of course it won't, the boost at the send is compensated by an equal attenuation at the return. It is only the pedal or rack units that you have connected to the loop that require different levels, not the amp. If you measure the signal at the send lead or attach a rack unit with a level meter then you will see the difference in the levels when you toggle the -10/+4dB switch.

Question. Could this be a tube problem? It appears to be on the "return" side of the effects loop circuitry. Does the "effects return" signal path pass through a tube or is it solid state? If I can play around with a few tube swaps to test this out, no problem, but I don't really want to waste my time if not required.

There are no valves related specifically to the FX loop, it is solid state so don't try and resolve it by switching valves.

Your problem seems to be with the FX circuit, possibly a faulty FET or dry joint. A repair is covered under the warranty and if it were me, I would stick with the amp after the repair if you're happy with it in all other respects.

Has it just happened or is this the first time you have used the loop?

[Deputy Dawg]

Steve,

Thanks so much for the reply. I understand the loop's technical side better now.

It is the first time I have tried the loop. The main symptoms are the level drop and the noise when the loop is engaged.... (Whether or not anything is plugged into the loop) The noise is like a loud hiss with intermittent "spitting or crackling" sounds. What alarmed me is that these sounds occur only when the loop is engaged.

Am I correct in assuming that there should be NO level drop and NO increase in background noise when the loop is engaged? As I mentioned before, it sounds like an issue on the "return" side circuitry of the effects loop...

I live in Canada and am very good friends with the manager of the shop where I bought the VM so I will discuss this issue with him. If it comes to replace or repair, you suggest repair?

Thanks so much. I don LOVE the amp! Thanks for the wonderful sound you created!

Randy (Deputy Dawg, Canada)

[SteveD]

Regarding the loop there should be no appreciable volume difference but it's normal to detect a little more hiss when the loop is engaged because it has high frequency compensation to account for cable and pedal losses.

[Deputy Dawg]

Steve,

Thanks for the info. I will have the amp checked out at the shop and let you know what the story is. My guts tell me that there is something wrong.

Thanks,

Deputy Dawg (Randy in Canada.... very Randy!)

[Deputy Dawg]

Hi all,

I did post to the general VM "Discussion" board but thought that I should follow up here too. Kudos to Steve D for helping poiint me and the warranty repair techs to the source of the effects loop problem as described above.

Turns out Steve D was correct. A couple of FET's were replaced on the FX loop board and viola.... a silent, non-level compromising effects loop. Thanks again Steve.


Deputy Dawg (Canada)

[SteveD]

My pleasure, that's what I'm here for. :)

[Ex-Mesa]

Hey Deputy,
Mine also just started hissing loudly with the loop engaged!!!!!!! I also live in Canada - Montreal. Where did you get yours fixed? How long did it take? Would you consider replacing FETs is something one could do oneself? I bought mine used but mint.