Randall RH200 strange noise??????

[sweatyk]

What I have is about a 2003 Randall RH200 G3 head. This one has the toroidal transformer.

I have this strange noise when the gain2 control is turned up past 8. I know any hi gain amp is going to be somewhat noisy however this is different. It almost sounds like what you would hear thru a phone if you dialed a fax number. I have some decent electronic skills so I’m no stranger to it but do lack a full or complete understanding of how some of it works. So if you get too technical with me I would look at you as if you were wearing a Viking helmet….OO

Here is what I have done so far. I have cleaned and lubed all of the pots and switches. I cleaned and checked all ground connections, checked them for continuity. I have replaced the input, line out, and return jack (that was necessary). I plugged my guitar into the return jack and all was perfectly quiet so I do not believe there is an issue on that board. I also was getting the noise thru headphones.

I realize most of you guys frown upon these amps and I don’t blame you. After seeing what lack of support there is out there it will be the last for me. All I want to do is repair it if possible and give it to my son.

I hope someone here can offer some insight and help guide me thru this.

[g1]

In the schematic section you asked for the schematic for a G2. So is it a G2 or G3? Does it have stereo chorus or not?
Here is a preamp schematic that may be close, depending on what you have.

[sweatyk]

In the schematic section you asked for the schematic for a G2. So is it a G2 or G3? Does it have stereo chorus or not?
Here is a preamp schematic that may be close, depending on what you have.

Sorry for the typo.....it is a G2. And thanks for that schematic. Just what little I glanced at it ...it seems to be more accurate than any others that i have found.


The server here seems really slow today. Took forever to post this.

[g1]

Maybe you could do some checks, when the fault is occurring what other controls affect the noise (tones, master, etc.).

[sweatyk]

Maybe you could do some checks, when the fault is occurring what other controls affect the noise (tones, master, etc.).

The only control that truly affects it is the "Gain2" control. Going past the 3 o'clock position starts the noise. The treble, level, and master will only adjust the level of the noise but it only disappears when you back off of Gain2. I tested it hooked up with my guitar but I had all of the volumes turned completely down on the guitar. I took a video of it with my camera but not sure how to convert the MOV format to post it to this site. If I could you can really hear what it's doing.

[Jazz P Bass]

My money is on IC2 being bad.

[sweatyk]

http://i449.photobucket.com/albums/q...psfb4cd149.mp4

[sweatyk]

Not sure but I don't think I got the video upload correct.

[sweatyk]

Wow the timing was really off on that video. Sorry about that. Here's where I'm a little green on things. If IC2 is the culprit as Jazz P Bass suggested, could a TL072CP be used in place of the JRC072D? Based on a datasheet comparison they are pretty close except for the slew rate, but not sure if that's a factor here. I'm asking because I have a few of those laying around.

[Jazz P Bass]

Try it.

As far as a guitar amp goes, they should be the same IC.

[sweatyk]

**UPDATE**.........the used 072CP I had was obviously not good. When I installed it the Gain2 channel was non functional. So I reinstall the original and it put me back to square one. On a whim I decided to swap VR5 and VR10 being they are of the same value. The nasty squeal has seemed to vanish and left it with the usual hi gain hiss which is to be expected. VR10 was the treble pot for the OD channel and does not seem to be effected even turned to 10. I had previously cleaned and lubed all of the pots but maybe this one has a touchy area when it comes to the gain circuit. And BTW when I took both pots out and tested them with my DMM they read identical. Not sure what to make of it but glad it is resolved.

[sweatyk]

**CANCEL UPDATE**......spoke too soon...........DANGIT!!!!

[g1]

Did the problem go away temporarily and return? If it has become intermittent, you may have a bad solder connection issue.

[sweatyk]

G-one all of the solder connections are rock solid. It's the first thing I look at. I happened to have found a replacement for IC2 locally which is next to impossible in this town. The factory one was a JRC072D, what I replaced it with is a TL072CP. Close enough that it should have worked glancing at the datasheets but here is what happened. The gain2 channel was completely silent.....no signal. If I put the 072D back in it works but with the quirky squeal. Either the 072CP will not replace it or aliens have invaded this amp. I'm too tired to play with it any more tonight so tomorrow will have to do.

[g1]

the used 072CP I had was obviously not good.

I happened to have found a replacement for IC2 locally which is next to impossible in this town. The factory one was a JRC072D, what I replaced it with is a TL072CP.

Not clear from this whether you installed a new or used IC. Regardless, any 072 properly installed in there that does not work is defective, be it new, used, or counterfeit. So you have not really eliminated the IC as the possible source of the problem. Try another new one.

Still wondering if there was a time when the problem disappeared, even temporarily.