Effects loop ... epic fail

[VMGravy]

i dont use any effects, i like to plug straight in, simple is better, right?
So i dust out the old delay out of the closet and run it out front, then i try the effects loop and no volume and right tinny. Read back on the forum and it said something about FET's. The amp is two years old and this is the first time ive tried to use the loop. Its not a big deal as i just play at home and dont use effects, but i would like an explanation on what FET's are. Much appreciated, thanks.

[DeanM]

its a type of transistor! FET stands for "field effect transistor" can either be jfet or mosfet. the ones in question (in the fx loop) are jfets!

im in work at the mo so cant really go in depth! google jfet and maybe that will tell you more!

[Ex-Mesa]

Do you mean no volume or a lower volume? I thought I read that the loop jfet issue only caused a volume drop. Some fellow here was going to DIY replace the jfets. results?

[MKB]

You might have several issues at play here. First, plug a short shielded cable between the Send and Return jacks on the VM, and then enable the effects loop (press in the LOOP switch on the back panel). You should hear very little change in the tone and volume if everything is working OK in the amp. If you hear a volume drop, you may have the FET problem.

Now if the amp works OK with the jumper and the effects loop enabled, there may be a problem either with the delay or level settings in the delay or VM effects loop. All the levels should be set correctly in the delay to match the level setting of the VM effects loop (-10dB or +4dB switch).

[DeanM]

@MKB, he said he tried it out front first so it cant be the delay.

[SteveD]

Actually, no need to plug the patch cable in as the signal will still find its way through the switching contact on the return jack. Simply push the 'Loop' button in and out and see if there is a significant difference.

[VMGravy]

Ya im a fucking idiot. Faulty lead. Thanks for all the info tho, learned a bit anyway.

[Cr4nK]

Hi everyone,
I actually have this problem. When I got my guitar str8 in with effect loop disengaged everything work good, but then when I enable it there is a big drop in volume (nothing plugged into effect loop). I got to turn the master all the way to 10 to hear any playing.

What do I have to replace to fix this problem?

[SteveD]

Hi everyone,
I actually have this problem. When I got my guitar str8 in with effect loop disengaged everything work good, but then when I enable it there is a big drop in volume (nothing plugged into effect loop). I got to turn the master all the way to 10 to hear any playing.

What do I have to replace to fix this problem?

One or more of the fets on the FX PCB probably.

[Cr4nK]

Thanks, this shouldn't be a problem for me. Just to make sure whats the value for the jfets there, so I replace it with the same one.

[SteveD]

Thanks, this shouldn't be a problem for me. Just to make sure whats the value for the jfets there, so I replace it with the same one.

Fet 1 = j202
Fets 2 and 3 = TN2404

Be aware that fets are static sensitive devices.

[DeanM]

ESD, electro static discharge.

when you can feel it ~2000V
When yu can hear it ~4000V
Enough to damage a component is as little as 20V!!

-- Sat May 15, 2010 10:37 pm --

So like steve said, keep that in mind! touch a radiator or something first before handling!

[SteveD]

... touch a radiator or something first before handling!

If only it were that simple.

[MKB]

... touch a radiator or something first before handling!

If only it were that simple.

Indeed. At work we have to use static dissipative jackets, dual heel straps, and static protective gloves. The workbenches have to be outfitted with extensive grounding mats and wiring. You have to keep all paper away from these parts, as paper can hold a static charge. You can't even use regular cellophane tape near a static controlled workstation, pulling a 8" length of tape off the roll can generate thousands of volts. Even our tools have to be static controlled. It's complicated and expensive.

The trick with ESD protection is not to get everything grounded, but to get and keep all pins and the installer and tools at the same voltage level.

The suggestions by some companies to discharge yourself against a ground or chassis (like computer board manufacturers as you to do when installing boards) might work if you are using a board assembly, as the high impedance paths on the boards are somewhat protected, but for individual components it may not be good enough. Parts like fets can be easily damaged, or the parameters weakened so that they fail prematurely in the future.

[DeanM]

... touch a radiator or something first before handling!

If only it were that simple.

well theres not a whole lot else the average diy'er can do at home!

or maybe wrap a wire around the earth plug connector on an unused power chord and plug into an outlet and connect the other end to the amps chassis and also wear a wrist strap connected to amps chassis.

well its better than nothin anyway!

-- Sun May 16, 2010 10:33 pm --

... touch a radiator or something first before handling!

If only it were that simple.

Indeed. At work we have to use static dissipative jackets, dual heel straps, and static protective gloves. The workbenches have to be outfitted with extensive grounding mats and wiring. You have to keep all paper away from these parts, as paper can hold a static charge. You can't even use regular cellophane tape near a static controlled workstation, pulling a 8" length of tape off the roll can generate thousands of volts. Even our tools have to be static controlled. It's complicated and expensive.

The trick with ESD protection is not to get everything grounded, but to get and keep all pins and the installer and tools at the same voltage level.

I'm doin a student co-op workplacement for college at the moment with EMC in cork, ireland. its exactly the same in there too! theres jackets, mats, ankle straps, esd boots, even the FLOOR is esd compliant! theres test stations at regular intervals that you stand on to test your gear is doing its job etc. and everyone working there has to do an ESD course EVERY year. its pretty strict.

I dont know about keeping everything at the same voltage level tho. where i work the mats are wired to a plug that connects to earth thru the mains socket. the idea of the mats (and all the other esd gear) is that they dont fully conduct electricity cos that would cause a rapid discharge and they dont insulate cos that would build up the charge. but what they do is allow any charge that may build up to discharge BUT at a slow safe rate so that damage doesnt occur. if it didnt discharge it would build up.

actually, if everything was kept at the same voltage level and a part happened to build up a charge much higher than that level, it could still cause an ESD from the higer potential to the lower, where as if it was allowed to discharge but at a slow rate it might avoid damage. but anyway diff companies prob have diff practices anyway!