Hey it's been a while since I have been able to get back to this problem, but I wanted to provide everyone an update on where things stand. In my case I don't believe that I have a connector problem but rather a noise problem due to the unregulated voltage being sent down to the DSP board.

One of the things that I've done since I last posted was to turn an unused laptop into a primitive scope using the software available on the net plus a circuit buffer board. I wouldn't want to try to take precise measurements with this rig, but it at least offers some insights as to what is going on.

If you look at the schematic, there is a rectifier circuit built around 1000uf C64 that sends unregulated DC down to pin 4 of the connector on the DSP board. Once it arrives there it goes to a LM117 voltage regulator that outputs 3.3 V for the digital circuitry. This regulator has a typical 65dB rejection ratio if no adjustment capacitor is used. I can't really tell what components they have tied to the regulator and haven't tried to figure it out yet.

To show you the big picture, the red wires that are about 10 inches long run from the main amp board to the DSP board:



And here is picture of this multi-plane very dense SMD digital board:




So what I am seeing on my "scope" is about 1 Vpp 120 Hz ripple arriving on the DSP board DC input and going to the voltage regulator. This is producing a noticeable 120 Hz hum being being present all the time coming out of the amp because of the DSP output always being mixed in. This equates to a flat low-B humming out of the amp and what has been driving me crazy enough to try to do something about it.

One thing that I found that noticeably reduced the hum was to add a 4700uf capacitor across C64. This gets the ripple down into the .200-.300 Vpp range and seems to get the hum down to a tolerable range where just the broadband type of noise coming from the DSP board is more present.

This looks like it might be a good solution in replacing C64 but one thing I'm not sure about is the purpose of the other circuit that C64 feeds. If you look at the schematic to Q31 and Q16, I think what is going on there is that this is supposed to effectively ground the input to the power amp when the power is turned off to kill wacky artifact sounds as everything fades out. I may be totally off on that so I was hoping someone else here could comment on the intent of that circuit.

One other thing that I tried was to physically separate the red wires going to pins 1 and 2 on J10 from the wire going to pin 4 as much as I could. I was thinking that perhaps there is some significant noise coupling going on along the 10 inch run of unshielded red wires, but I can't say that this made a noticeable difference. I was thinking that running shielded wire with the shield grounded on one end for the audio signals might help here, but I'm not convinced at this point that it would be worth the trouble.

The other idea I was toying with is to add either a pot or a switch between J10 pin 1 and R92 to ground the DSP output (maybe jumper wire JOW1 is the ideal spot) so that when I just want to get the amp as quiet as possible and use external effects, I kill the DSP board output signal. This would still leave the capability to turn the internal DSP back on when the situation allows it.

Years ago I had some experiences dealing with RF noise and personal computer digital communication equipment and even though we had lots of theories on suppressing things, it always seemed to turn into a long tedious trial and error process to find the right combination of things to pass the interference tests. I know this is an audio circuit with much lower frequencies but I tend to think this could also waste a lot of time with diminishing returns trying to do too much with what is probably just a noisy DSP board to begin with.


Any thoughts?

Well I think I've made some progress on this tonight. The ribbon cable that I was disconnecting goes to the FX control board and supplies power to it but no audio signal. That is J4 on the schematic.

When I unhooked J10 on the other hand, that stopped most of the annoying hum that I have been hearing. This connector has audio going to and from the effects DSP board along with DC power being sent to it that is rectified through diodes D31, D32, D33, and D34 on the main circuit board. The main filter cap for this DSP voltage supply is C64 1000uf.

The DSP board has lots of surface mount components and I don't have any schematics for it.

It looks like the output from the DSP board goes to pin 1 on J10 and from there is mixed with the selected preamp channel and CD input and then on to the power amp. Point D comes into play to have the signal inverted if there is a cable plugged into the input jack or it bypasses the inverting opamp IC4A if there is no cable. Either way it looks like the DSP output gets routed on to the power amp which would explain why I hear the noise even if there is no cable plugged in.

The wires running from J10 to the DSP board are not shielded which I'm sure is not helping but I'm not sure at this point if that really accounts for the noise. I'm stopping here for now.

Quote from: DrGonz78 on February 14, 2013, 04:49:04 AM
I would not replace those main filter caps on first check... Instead I would remove the solder and re-solder the existing caps to start. While I was in there I would solder up the Bridge or rectifier diodes, while also hitting solder joints on all the wire wound resistors to make sure those were solid. Also, hit the input jack solder joints as that might be the culprit right there to start. Let's see how many parts really need replacing and how many parts just need to be soldered w/ real leaded solder...

Edit: BTW what make model is this amp and what year do you think it was made?

I will give this a shot and see if it makes any difference.

It is a Crate RFX 65 that I think must have been made around 2005 from looking at the date codes on some of the components.

Quote from: DrGonz78 on February 14, 2013, 09:56:31 AM
Try one trick to get rid of hum... Guaranteed not to blow up your amp too... Try clipping a ground wire from the input jacks ground to the chassis. I know this violates star grounding schemes etc etc etc... But it won't cause your amp any harm at all I promise. Let us know if connecting the input jack ground to the chassis makes any noticeable difference, as I am curious on this front.

Since this was easy I gave it a quick try. In my case it did nothing.

Quote from: g1 on February 13, 2013, 01:54:08 PM
  Those ribbon wires are pushed into those connectors.  The connector is spring loaded, you push down on the spring loaded side and pull the ribbon cable out.  The insulation is stripped back a bit so it is just bare wires pushed into the connector.

I finally had a chance get back to this today. Thanks for the tip, it seems obvious now that you explained it.     Unplugging the audio ribbon to the FX board made no difference in my case.

Quote from: phatt on February 12, 2013, 04:14:51 PM
There is likely a fault (which is why the hum) and you would need to track that first before replacing parts that may not be broken. That approach often leads to more problems.
Phil.

ed; Try to isolate the source of hum,,try another guitar, or different leads.
maybe send a signal via efx loop out to another amp. still hummy?

This is a good point and I will try to describe what I know so far. The amp hums from the moment you turn it on even with no input at all. It isn't what I would call very loud, so for example if it was setup in a noisy club no one would hear it. At home however it gets to be annoying and is definitely noticeable compared to my silent  SS Peavey sitting right next to it if they are both on.

The hum does not change with increases or decreases in overall volume level.

I tried running into the power amp return of the Insert jack to try to isolate preamp from power amp. The hum still remains so that would seem to indicate that its not coming from the preamp to me.

Unfortunately I don't have a scope handy to look at the ripple on the power supply. I did try tacking on some extra capacitors in parallel and it did NOT seem to help, hum was still there. That is as far as I got last night.

I have also been reading of others who complained of hums on their same model amp and ended up disconnecting the effects board as a fix. I thought that would be an easy thing to check, but for the life of me I can't seem to figure out how these connectors plug in and didn't want to break them.

Here is a view where the ribbon cable from the effects board connects to the main board:



and here is the same connector on the effects board itself  (the one that is all the way on the right):



Sorry for the blurry pictures it was the best I could get it to focus last night.

Quote from: DrGonz78 on February 12, 2013, 07:45:37 PM
Personally I never really put Loctite back on the screw heads. It sure can't hurt unless you put on way too much. Add in loctite once your 100% happy with the amp and you are done trouble shooting. Looking at this example in the pics it looks like an excessive amount of loctite, so much that it dripped down on one of the transistors. Looks very sloppy... A silicon based heat sink compound is easily found at Radio Shack. I bought a big tube of it on mouser and it will last a long time.

This is my first time working on a Crate so I didn't know if this was par for the course with these builds or not. Anyway I was leaning toward not bothering with it when putting it back together. I have a Peavey that just has screws going into the heat sink with no loctite in sight. Also I don't believe it actually has heat sink compound on it either come to think of it.

Quote from: Enzo on February 12, 2013, 06:47:05 PM
Um... flip the chassis over, is there not a row of screws holding the heatsink down from underneath?   In my experience, all the top screws remain in place on the Crates, the heat sink is not detached from the board in the process.   ALmost all of them come apart the same:  pull the hrdware from the panel controls and jacks, pull the row of screws from underneath, and typically a few screws through the board from the top.  The board screws typically along teh edge or corners, and some of them also servfe to ground teh thing to chasis.

Yep I ended up finding the same thing you described. I took a good screwdriver and worked the loctite screws out from underneath with some effort.

I've got a Crate amp that I plan to replace the main power supply filter caps to stop a hum that it has developed. Looking at the circuit board, I had some questions about putting it back together when done. It looks like I am going to have to unscrew the heatsink with the power transistors from the chassis and then that will stay together with the main circuit board.

The heat sink looks like this from the top:


and from the side:



It looks like there are 4 screws attaching the heat sink to the chassis and I'm guessing its some sort of a green Loctite that has been put on them. The screws may look black in the photo but the stuff is pretty green. (It appears that they were very serious when putting this amp together about making sure that nothing moved due to vibration. I have a heck of a time just unplugging the speaker wires and transformer wires as the plugs are extremely tight.) So my first question is do you guys normally use some sort of Loctite when putting screws like this back into place?

Also there looks to be some thermal paste in use under the heat sink. Again just looking for recommendations here as to what you use.

Hey, my first post here. I picked up a Crate RFX65 the other day and really like the clean channel on this amp. I've been playing it for a while now and all seemed ok. The other day when I had left it running and was in another room, I thought I heard some sputtering sounds coming from the amp. Of course when I returned to the room, all seemed normal - some low level hiss from just being turned but no sputtering coming from the speaker.

I am concerned that it might be a sign that there is trouble coming with this amp. I was wondering if anyone has any techniques they use when trying to determine if there is an issue in cases like this. I thought about just leaving it on for a long period of time to see if there is a possible heat related issue going on. Any other ideas?

Thanks