RM4 hum problem

[eedude]

just got a used RM4. It is in great shape inside and out, but it has a terrible hum issue. It's not tubes, and I have ran several experiments that lead me to believe (at this time) that it is not a ground loop issue. Has the the problem with several power amps that all work great. Inspected all the cable connections, everything looks good. Lift switch does change the tone of the hum a little. Power Supply board is a Rev1.03 and the backplane is Rev1.02.

does anyone know of any issues, or has anyone had a serious hum problem with an Rm4 and resolved it?

thanks

 

[eedude]

i'll give you all an update as to where I am with this since I know you're all dying to know what is goin on with my rm4.

I have traced the hum back to the B+ supply. In the RM4 there is a NPN transistor and a few zeners used to make a simple 300V high-voltage regulator. It looks like the regulator is not functioning. The main purpose for the regulator, it seems, is for ripple rejection. So without it functioning there is a lot of 120Hz on the B+.

At this point I need to rip the power supply board out...

 

[eedude]

If it's not too much of an inconvenience, could you snap a few photos while you're performing the surgery?

yeah, no prob, anything in particular you wanna see?

 

[eedude]

so i finally got the time to finish this project. So it was the little home grown B+ regulator. The NPN transistor failed shorted. The purpose of the regulator is to get good 120Hz rejection without having to use huge (uf) caps. With the regulator failed there was a lot 120Hz ripple on the supply. The ripple is picked up by each of the preamps and amplified, thus loud audible hum.

It seems to me that to make the design more bulletproof there needs to be a diode backwards across the transistor (very common in linear voltage regulators). This prevents the Base-Emitter junction from reverse biasing (indirectly through the collector) while the caps discharge ofter power off. Should the collectors side discharge more quickly that the emitter side the diode forces the collector and emitter to track in voltage. I installed a new transistor, a different one that actually has the diode in it. The unit is now silent! So yeah...

 

[Daryl]

Holy crap, eedude!!!! :shock: :shock: :shock:
You are the fucking man.
I want an RM4 now... :shock:
Once again, I'm impressed. Bravo...

 

[warlok]

Wow eedude.. You use your tongue prettier than a twenty dol... Ah nevermind.. :lol:
That's pretty impressive! Now I know where I'm sending my stuff to be fixed. Most techs would replace the board and be on with it. Good job man!

 

[SacredGroove]

so i finally got the time to finish this project. So it was the little home grown B+ regulator. The NPN transistor failed shorted. The purpose of the regulator is to get good 120Hz rejection without having to use huge (uf) caps. With the regulator failed there was a lot 120Hz ripple on the supply. The ripple is picked up by each of the preamps and amplified, thus loud audible hum.

It seems to me that to make the design more bulletproof there needs to be a diode backwards across the transistor (very common in linear voltage regulators). This prevents the Base-Emitter junction from reverse biasing (indirectly through the collector) while the caps discharge ofter power off. Should the collectors side discharge more quickly that the emitter side the diode forces the collector and emitter to track in voltage. I installed a new transistor, a different one that actually has the diode in it. The unit is now silent! So yeah...

Love the shop-talk you bring. In the new transistor, is the diode across the C-E?
Fortunately, I haven't had any issues with my RM4. Thankfully because I hate unracking it. :x

 

[m0jo]

Hey eedude, I'm wondering, could you describe te noise a bit more?
In terms of level and type of noise?

Because I'm having some noise but I'm not sure wether it's just because of the gain levels and my setup or actually something wrong with the thing.
A noise gate easily kills it, but I'd rather have no or almost no gating.

 

[eedude]

In the new transistor, is the diode across the C-E?

Yes, so the emitter can never ever be more than ~0.8V higher in voltage (and the base as well since the base will track the collector by design). A transistor, even a high voltage one, can only tolerate about 5V reverse across the base-emitter junction an then they die.

 

[SacredGroove]

In the new transistor, is the diode across the C-E?

Yes, so the emitter can never ever be more than ~0.8V higher in voltage (and the base as well since the base will track the collector by design). A transistor, even a high voltage one, can only tolerate about 5V reverse across the base-emitter junction an then they die.

That's cool. I wasn't aware of such a thing, but it looks like it will serve well. I may do this to mine too. :wink:

 

[eedude]

Hey eedude, I'm wondering, could you describe te noise a bit more? In terms of level and type of noise?

hi m0jo

It was raw hum. So it sounds fairly pure in tone. The volume was quite loud, you would still be able to use the amp (sort a) but it was louse enough to know that something was not right and would not do well for live or recording. The sound is similar to a grounding problem (which is what I thought I had initially). It was loud enough for sure to open my properly set gate in normal operation.

Separate the ideas of noise and hum. They are two different things. So noise is going to be more like a hiss. Hum is going to be a tone based on your AC line frequency, or some harmonic of it. if you have a lot of hiss, you have a noise issue. Noise can be attributed to:
1) very high gain preamp (all the noise in the first stage gets amplified by all the later stages, thus loud hiss. This is why the first stage of a high-gain preamp is so important)
2) faulty tube
3) other stuff that I'm probably unaware of

the thing about noise in a high gain preamp is that it can be a hard problem to solve (during design), because tone comes first. So some high gain modules are just going to be noisy. But, that said, bad design can also lead to an unnecessarily noisy module or amp!

So do you have a hum or a hiss issue?

 

[m0jo]

Hey eedude, I'm wondering, could you describe te noise a bit more? In terms of level and type of noise?

hi m0jo

It was raw hum. So it sounds fairly pure in tone. The volume was quite loud, you would still be able to use the amp (sort a) but it was louse enough to know that something was not right and would not do well for live or recording. The sound is similar to a grounding problem (which is what I thought I had initially). It was loud enough for sure to open my properly set gate in normal operation.

Separate the ideas of noise and hum. They are two different things. So noise is going to be more like a hiss. Hum is going to be a tone based on your AC line frequency, or some harmonic of it. if you have a lot of hiss, you have a noise issue. Noise can be attributed to:
1) very high gain preamp (all the noise in the first stage gets amplified by all the later stages, thus loud hiss. This is why the first stage of a high-gain preamp is so important)
2) faulty tube
3) other stuff that I'm probably unaware of

the thing about noise in a high gain preamp is that it can be a hard problem to solve (during design), because tone comes first. So some high gain modules are just going to be noisy. But, that said, bad design can also lead to an unnecessarily noisy module or amp!

So do you have a hum or a hiss issue?

I'm going to listen to it closer next weekend. I didn't quite listen to it that way.
Didn't have time for troubleshooting so I turned up the gate and kept rocking
Thanks for helping out!

 

[eedude]

That's cool. I wasn't aware of such a thing, but it looks like it will serve well. I may do this to mine too.

I try not to over do the "Beware the voltages in a tube amp can kill you". But, in this case I must say proceed with caution!! If you are experienced then working in the power supply should be fine, but novices should stay away from power supplies and power amp sections. The components in you amp will be punished for your mistakes..as too you. So caution please....

Also, should you do this mod you just need to add the diode, you don't need to use the part I used. The diode would fit nicely on the back of the board. I changed the transistor because mine was dead...and i had an idea for additional hum reduction with the Darlington, but from the sound of the amp i don't think i need to do anything more.

 

[BenMesaBoog]

Hi all,

I'm a noob to the forum. I think my RM4 is noisy (RT2/50 has NO HUM at all)- it has a hum that gets much quieter when I touch the metal part of my guitar cable. The ground lift switch doesn't seem to impact it. Is this just a noisy preamp, or is there a common issue w/hum and these units?

 

[MarcoR]

Hum as in a 60 cycle hum? Or hiss as in typical high gain hiss?
Here is my check list:
Is it just with very high gain or every module even clean/mid gain? Do you have anything in the loops? Have you tried going out pre vs post loop or even out serial send to the power amp? Do you have florescent lighting near you? Have you tried electric from a different room/source? Is it a single coil guitar? Are your effects in front of the amp plugged into the same electric source as the preamp, power amp and post effect? Are your effect powered from transformer isolated power sources.
I don?t have an RM4 anymore but when I did, if everything was hooked up correctly, I had no noise issue. That said, tracking down weird ground loops in a complex system is challenging. Currently, I have an M4 and in my rehearsal room, it is dead silent. Recently I brought my rig to Jaded Faith Mods and the florescent lights in Rob?s studio made my rig hum on the higher gain modules. I actually had to turn on my Decimator (which I never do).

it has a hum that gets much quieter when I touch the metal part of my guitar cable.

It should also get quieter when you touch your bridge or strings. Check that the pickup wiring internals are grounded to your bridge (may not be applicable to your guitar).

 

[BenMesaBoog]

Thanks for responding so quickly, and for your suggestions! I'll go through 'em one by one this evening. Will circle back. Thanks again!

 

[BenMesaBoog]

Oh, and I should've mentioned that I've removed all but on mts module, replaced the 12ax7s, unplugged everything from the loops, and only the RM4 and RT2/50 are going into the furman which goes into the wall. You've provided some awesome ideas that I didn't even think about, like plugging in somewhere else, using the loop outs to the power amp from the RM, etc.; I'll be trying all of those tonight and I'll circle back in the next couple o days. Thanks again!

 

[BenMesaBoog]

NOOOOO. I just wrote a huge response and it got lost and told me to log in again! Another coming this AM.

 

[BenMesaBoog]

MarcoR, Thanks again for your help. Here are the results of my testing against your checklist.

Hum as in a 60 cycle hum? Or hiss as in typical high gain hiss?
-----more of a ground style hum. in fact, it gets quieter when I touch the metal part of my guitar cable (where it's plugged in), but it gets louder when I touch the strings or when I put my hand over the bridge humbucker.

Here is my check list:
Is it just with very high gain or every module even clean/mid gain?
-----happening with every module including blackface (which is cleaner, I'm sure you know), I also tried w/top boost, recto, and SL+

Do you have anything in the loops? Have you tried going out pre vs post loop or even out serial send to the power amp?
----no effects plugged in or hooked up. for testing's sake, guitar to rm4 (one module), rm4 to rt2/50, 1 channel rt2/50 to one channel in cab. tried sending from each different out, but the hum was still there.


Do you have florescent lighting near you?
----nope, all incandescent

Have you tried electric from a different room/source?
----yes, but same noise.

Is it a single coil guitar?
----nope, dual humbuckers w/a push/pull coil split. Noise changes w/push/pull, but still there/mostly the same.

Are your effects in front of the amp plugged into the same electric source as the preamp, power amp and post effect? Are your effect powered from transformer isolated power sources.
--ironically, in our studio, it's all dedicated circuits and furman line conditioners....also, no effects even plugged in or connected for testing's sake


I reached out to eedude from the beginning of this post, since he did that elaborate fix. I'm wondering if that's what needs to happen, but I'm considering selling the RM4, RM2/50, and the 4 modules in a rack case as is.

What do you think? would it make sense to sell as is, or get the RM4 fixed first? I did see the "please call us first" post from Randall support. I just don't want to spend a ton of money, and I'm not married to the sounds I get from it, even when it's too loud to hear any humming.


Thanks again! I really appreciate your helpfulness and time!

-Ben

 

[MarcoR]

You mentioned that you replaced the 12ax7s, was that all three in the RM4 and in the module? Did you swap the positions or replace with known good working tubes?

I'd hate to see you sell it when it's something that could be fixed. I would contact Rob at Jaded Faith [email protected]. It would be worth it to send the RM4 to him for a going over and while it's there, see if he can do any of his magic to improve it overall. He will likely be happy to work with you over the phone to further toubleshoot before you send it. If Rob hasn't already seen this thread, point him to it as eedude's findings may be useful.

Good luck!