rm100kh fx loop problem (resolved)

[MegaGoo]

i got a used rm100kh about 3 weeks ago. it has a manufacture date of 10/2008 if im reading the serial # right.

just went to use the fx loop at its acting funny

summary after a day of fiddling:

with the guitar plugged into the input jack of the amp, my rackmount EQ unit in the SERIES loop. no sound change. nothing

if i switch to parallel and turn the input gain on the eq unit down a little bit and play, then gradually turn the input gain up, once i hit about 0db, i start getting this squeeling like a sine-wave or something. as i turn it up more it gets higher pitched. as if i were going through a whammy pedal you know? the same thing happens when i plug in a distortion stomp box pedal. as i turn the gain up it starts giving me that strange whammy pedal effect


other scenerio:

if i plug my guitar into my rackmount preamp distortion/fx unit (digitech gsp2101) into amps FX return, it sounds wonderful through my 4x12 speaker cab. as it should since im bypassing the amps preamp section. if i go one step further and run a line from the FX send to say, my computer to record, it sounds fine again. perfect on BOTH the SERIAL and PARALLEL

so from what im observing, the fx loop send/return works when you feed from an external preamp, but not from the internal preamps

there are 3 preamp tubes. one as a buffer for the preamp modules. one for a phase inverter, and one for the FX loop. ive changed all of them out

original post:

1) serial fx loop: doesnt do anything. no matter what i plugged in or did with the fx mix knob

2) parallel fx loop: fx mix knob works; when i plug a distortion pedal i get a strange sound like a sine-wave. as i adjust knobs the pitch gets higher and lower. similar strangeness happens when i use my typical fx rack unit in the loop.

if i run my guitar through a modeler preamp, into my parallel FX return (uses the head as simply a power amp) i get an expected result which sounds great but obviously bypasses the entire preamp section of the rm100

swapped all 3 preamp tubes in the back. swapped what i believe to be the fx loop's preamp tube multiple times (the tube closest to the back panel, farthest away from the poweramp tubes)

should i bother taking it apart to see if i can find anything bad? will this void my warranty? could it be a fuse?

eddie

 

[Mister Joshua]

Bear in mind that the FX loops are not designed for pedals. I used to use my delay and chorus pedals in mine and had to turn the master on my modules down to about 2 or 3 or they would overdrive. Most Multi FX are designed for that. The weird sine wave thing could be caused by severe overdriving, especially using a distortion pedal.

That being said you may have to either put your pedals up front or find a way to attenuate your FX loop send to a much lower level.

As for the series loop it is not designed for the knob to do anything. The effects knob only affects the parallel loop. It should still make some sound though.

Peace, Joshua

 

[MegaGoo]

should the series fx loop do SOMETHING though?

i just tried the distortion pedal as an easy test. in my old engl i did this once and it worked perfectly as if the pedal were up front. at least at a low volume.

i guess the easiest way to tell is to try sending my preamp modeler in to the series return and see if i got the same good results as my parallel return. dont recall if i tried that

then take the fx send and send it to my computer's recording inputs and see if that works. they should theoretically be the same signal as the "slave out" as far as i know

took the chassis out of the box. fuses appear ok but my multimeter's continuity check doesnt work right. will have to visually inspect. without taking pcb's out, things look ok..


have to leave for a show. going to use my line6 vetta. yuck

eddie

 

[MegaGoo]

guitar>external preamp>fx return>fx send>computer = signal

this is on both serial and parallel

so im even more confused.

 

[MegaGoo]

might go to guitar center and buy a rackmount fx unit then return it. im using an old digitech gsp2101. manual says how to hook it up in a fx loop. worked fine in my old engl. dunno. need to rule **** out.

it'd be nice if i could just use a delay or chorus pedal in the loop but manual says floor pedals dont work well with it

 

[Julia]

You could try a chorus pedal in the loop as a test. Just turn the channel volume down to between 8:00 and 9:00 to be on the safe side. You should get a signal through.

 

[galao]

unfortunetly chorus pedals don`t work properly in most fx loops..

 

[crankyrayhanky]

line level pedals will work; i.e. Digitech Hardwire

 

[Julia]

The suggestion for the chorus pedal at low signal level was just to see if anything was getting through.. I know it would have sounded like crap, but the OP has a problem determining whether the loop works at all. This wasn't a good test and....

Probably not as good as this one:

Get a short instrument cable (like a pedal connector cable). Plug one end into the send and the other into the return. Try this on both loops. Do you get sound through?

Yes? then it's your effects unit/pedal/whatever.

No? then it's your amp.


This is the cheapest way to test. It won't harm your loop. It's like having an effects unit set on bypass.

 

[MegaGoo]

ok used a guitar cable as a jumper between. this had no effect. as i expected because when i run FX processors/whatever through the loop, nothing happens until i am using parallel loop and turn the gain way up on the fx unit. series never did anything to the tone


i forgot i had a rackmount behringer 15 band EQ so i tried that. same thing

series: no sound change
parallel: with the input gain set to 0 (12 o'clock) on the EQ unit, i starting getting the sine-wave thing again. as i fiddle with the input gain of the eq unit, the sound goes up and down. similar to if you had a whammy pedal and were adjusting the pedal back and forth. actually its exactly like that. depending on the signal going through the pitch changes, it seems

plugging in to my vetta's fx loop, the digitech gsp2101 (original fx loop i tried) and the behringer eq work perfectly

so without even goign to guitar center and getting a known good fx unit, i've decided that this amp just has issues. will be contacting tech support tomorrow but i dont expect anything but an RMA

Eddie

 

[Julia]

I run my FX unit in the serial loop. I don't use the parallel loop at all. There can be phasing issues with parallel loops especially with time based effects.

My effects unit has parallel, series, stereo, and dual mono options within each preset. I set my parallel mixes in the unit -- say run a reverb at 15% wet in parallel with a full dry signal and have it mix before going out of the unit.

I don't know much about the parallel loop on the Randall. I know about the parallel loop on the Marshall JVM. We were told to run the Parallel loop 100% wet otherwise there's those comb filtering and phasing issues that sound like a whammy pedal.

According to the Randall manual the Parallel Loop is only for "time based effects". They tell you not to use it with Equalizers, noise gates, tremolo, compressors. Then they go and design the whole thing for rack units.

It's really kind of superfluous IMO. Most half way decent rack units do their own internal effects routing and mixing. Personally I don't like parallel effects loops. I just prefer to use the serial loop and adjust my effects unit accordingly. I don't even know if mine does the same thing that yours does. I've never used it.

 

[MegaGoo]

Well my serial does nothing :/

 

[MegaGoo]

sending in to randall tomorrow. Ryan says they're 3 - 4 weeks on turnaround. arg.

will update this thread with any details i get

i'm actually a little excited because while doing some research on an FX unit to buy to test this (way past that now obviously), i came across the boss gt-8. i might be able to just about completely eliminate my rack with this. fcb1010 midi foot pedal, 2 space digitech gsp2101 artist, tuner, 2 space pedal drawer. only thing i could possibly see myself keeping is one of my two 4-space racks with a furman power conditioner, wireless, stereo 15-band eq, and might keep the tuner to fill space. or might get rid of EQ and tuner and keep the drawer i dunno..

 

[Julia]

Make sure the GT-8 can handle +4 dbv. I think it's instrument level or -10 dbu.

This is my biggest gripe about this amp ==> Marshall made their's switchable +4 dbv/-10 dbu. Why didn't Randall? How hard can this be?

You might want to see if they can mod it for you while it's back there.

 

[MegaGoo]

yea i found out about a product that converts to and from +4 and -10. i dont really understand whats going on exactly but have an idea. but the Ebtech Line Shifter for 75 bucks new should do the trick.

heres a thread about using the gt-8 and the
http://mtsforum.grailtone.com/viewtopic.php?p=13421&sid=4df702c9cb837a60ed8a90254d0a452f

 

[VitaminG]

plenty of amps don't have the capability to switch the level in the loop. Randall aren't alone there. If we wanted to be really fussy, we could ask why there aren't 2 series & 2 parallel loops, with each switchable? Because I might want to run a mix of pedals & rack gear in either loop.

So many amps have substandard or flatout crappy loops, I'm just happy to find one that does a decent job, even if it doesn't do everything I would like it to

 

[Julia]

yea i found out about a product that converts to and from +4 and -10. i dont really understand whats going on exactly but have an idea. but the Ebtech Line Shifter for 75 bucks new should do the trick.

heres a thread about using the gt-8 and the
http://mtsforum.grailtone.com/viewtopic.php?p=13421&sid=4df702c9cb837a60ed8a90254d0a452f

I'd try a Radial Pro DI instead. It's a little more but has a ground lift. It's a better product. All you need is something that has a 15 db Pad on it. This does. It's $99.

http://www.sweetwater.com/store/detail/ProDI/

You will need a XLR to 1/4" cable to go to the GT-8 or whatever you get though.

 

[MegaGoo]

yea i found out about a product that converts to and from +4 and -10. i dont really understand whats going on exactly but have an idea. but the Ebtech Line Shifter for 75 bucks new should do the trick.

heres a thread about using the gt-8 and the
http://mtsforum.grailtone.com/viewtopic.php?p=13421&sid=4df702c9cb837a60ed8a90254d0a452f

I'd try a Radial Pro DI instead. It's a little more but has a ground lift. It's a better product. All you need is something that has a 15 db Pad on it. This does. It's $99.

http://www.sweetwater.com/store/detail/ProDI/

You will need a XLR to 1/4" cable to go to the GT-8 or whatever you get though.

the PAD will get the signal down between the FX send (+4) and the gt-8 (-10) right? well what gets the signal back up from the -10 output of the gt-8 to the fx return of the rm100? i guess i'd just compensate by turning the volume up on the gt-8? but wouldnt that introduce more noise? i suppose the ebtech line shifter does this all without introducing noise?

i actually have a direct box (behringer ULTRA-G GI100) with a -20 pad i suppose i could use between the fx send and gt-8

eddie

 

[Julia]

You don't bring the signal back up.

My personal experience with the EBTech is that it uses noise cancelling. This means an out of phase noise. If you have no noise then you get some.

But a -20 db pad is going to sound about 4 times softer than a -15 db pad. It will work though.

 

[JKD]

Nice job...I really like that song.