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:
(http://i238.photobucket.com/albums/ff309/txflood/crateHeakSinkTopView_zps61d79b8d.jpg)
and from the side:
(http://i238.photobucket.com/albums/ff309/txflood/crateHeatSinkSideProfile_zpsc99cda2a.jpg)
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.
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?
Wow that's a lot of Locktite!! I have been inside many Crate amps and have never encountered one having that much Loctite.
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.
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.
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.
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: 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:
(http://i238.photobucket.com/albums/ff309/txflood/crateCableFromEffectsBoard_zps8e9a4228.jpg)
and here is the same connector on the effects board itself (the one that is all the way on the right):
(http://i238.photobucket.com/albums/ff309/txflood/crateEffectsBoard_zpsfbcab64c.jpg)
Sorry for the blurry pictures it was the best I could get it to focus last night.
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 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?
Quote from: txflood on February 13, 2013, 08:41:58 AM
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.
Then there is a fair chance Nothing is wrong or broken.
Modern High gainy Amps can indeed be very hummy in a small quite room but Just because the other amp is quite does not mean something is wrong with the Crate Amp.
I'd lay money on this being a simple design mistake.
Google *ground loops* for clues about the subject, I have a hunch it has been discussed here also some time back.
If it is a design flaw in grounding and layout I very much doubt it would be worth trying to fix it.
Grounding issues are a nightmare for designers and close to impossible for the novice to fix.
Having said that it could be as simple as a bad connection to the chassis but finding it is the big Q.
This is not something that can be seen just by looking at a schematic you really need someone who understands how to trouble shoot wiring and layout issues.
I've often found ground loops just by a jumper lead with alligator clamps and jumper all around The signal common, PSU ground plane and chassis can give you a hint as to where the issue might be hiding.
But for gods sake don't try it unless you know damn sure what ground, common and case means as you could blow it up in the blink of an eye.
Yes it could be a dodgy efx loop issue,,, may not be using shielded wire to those sockets.
So up to you to decide? :-X
I'm from the school of thought; "If it aint broke, then hardly worth fixing" 8|
Phil.
Here is an oddball ground issue waiting to catch the unwary.
Grounding issues pop up everywhere in this game and this one happened recently causing the lfo in a chorus pedal to bleed back through the amplifier with a horrible tick.
The player asked Me if I'd take a look at his problem as it was driving him nuts and he was about to get up on stage. :o
I noticed his mains power stripboard had the Amp plugged into the far end socket while the plug pak was plugged into the first, Closest to the entry of the mains cord. (see pic)
Without hardly a word I just swapped the plugs and problem gone. It got some WTF looks from those around looking on. lol
Phil.
@Phatt>>> If I understand correctly "Plug pack" could be an AC to DC adapter for foot pedals? Perhaps that adapter had a reversed negative type adapter??? To me that might do something later down the line on ground line for the power strip...>>>???
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.
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: 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: 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.
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.
Maybe my day was just crazy and my brain is mush.... Maybe that is everyday for me... Sounds like your referencing a schematic here and it would be nice to follow along in better detail. Can you send us copies through messages/email or post a copy on the thread? It will help greatly to follow along with you, thanks.
Edit: Sorry I thought was looking a schematic that might be the right one but was wrong. However, I think this was the right one?? Let us know if this schematic is the same as the one you are referencing...?
Double Helix Edit: Yeah that looks like the right to me... Look at the link to MEF website for the exact problem you described.
http://music-electronics-forum.com/t13806/
Read the last post for a possible fix.
Well spotted Dr G, :tu:
Yep my pet hate, connector blocks designed for computer crap used in Audio circuits are a bad idea and asking for trouble.
I recall reading way way back many years a comment from a Teck chap who worked in radio stating if you want it to work reliably,,, SOLDER EVERYTHING.
My other favorite rule of thumb; 90% of all electrical problems will be found in connections.
Why just this week I revived a dead 22 inch computer monitor (about to go in land fill) by finding the dud connector which was hidden deep inside the harness.
One tiny bent pin in the connector to the Screen was causing the colour to drop out at random.
Works perfect now and I always wanted a big screen. :dbtu:
Phil.
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:
(http://i238.photobucket.com/albums/ff309/txflood/crateBoards_zpsc52001a1.jpg)
And here is picture of this multi-plane very dense SMD digital board:
(http://i238.photobucket.com/albums/ff309/txflood/crateDspBoard_zps36538908.jpg)
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?
Maybe i am just dense, but I can't find what you are looking at. Can you link the schematic or tell me exactly what drawing numbers you have? You mentioned RFX65, but Gonz linked GFX65. And I cannot fins half the parts you mention. For example I see C64, but the DSP cards have 5v regulators. And C64 only goes to teh DSP conector on this print. I can find Q16 as a mute, but cannot see a Q31. And Q16 doesnl;t appear to conect to C64.
Raw voltage like C64 is normal to have some ripple. What matters is how much ripple is there after the regulator. The DSP doesn;t run on the raw voltage.
I think Roly touched on this. It may help to over filter at C64, but I suspoect you have a ground issue, and the filtering ground return currents are getting onto the DSP signal. If I recall, the DSP card has four corner poip-in standoffs. Is there a grounding screw on any corner?
Yeah I think I might have screwed the pooch there... xP
I thought the file I had loaded was right but just had the wrong named file....
I think this file is correct and maybe the OP can confirm this schematic is correct or post a copy?
Edit: Enzo is right on with these amps having a grounding screw on one of the DSP board standoffs. That needs to be looked at too.
Sorry I didn't look more closely at the previous schematic that was posted. Dr Gonz has posted the correct one now.
I believe on this amp there were 4 plastic standoffs in each corner of the DSP board so I don't remember any kind of chassis ground connection or screw. I will take another look this evening.
Q16 Q31 are just a power amp mute.
Quote from: EnzoI think Roly touched on this. It may help to over filter at C64, but I suspect you have a ground issue, and the filtering ground return currents are getting onto the DSP signal.
I did? I don't recall; but I was a bit surprised that the recto is on the same board, and I certainly agree that it could be a grounding issue. This digital stuff is much much more like RF than audio and grounding digital earths and ground planes can be much more critical.
Quote from: Enzo on March 26, 2013, 10:08:01 AM
Q16 Q31 are just a power amp mute.
So this circuit is there for power on muting then, correct? Right now even with the standard C64 1000uf cap in place there is a pretty good speaker thump at power on time.
It is a mute, it shunts the INPUT signal to the power amp. It can do absolutely nothing about the power amp's power up thump.
Roly, I THOUGHT it was you who mentioned something along those lines, but I see i was mistaken, I forget now which post was in my thinking.
Quote from: Enzo on March 26, 2013, 06:52:29 PM
It is a mute, it shunts the INPUT signal to the power amp. It can do absolutely nothing about the power amp's power up thump.
Right, so the idea is that the transistor switch grounds the input (preamp output) going to the power amp until some time after the power amp is fully powered up. This would be governed by the RC time constants of the transistor circuit, correct? Just trying to learn.
Also I confirmed that the DSP board has 4 plastic standoffs. The only ground comes from pin 3 of the connector.
Sorry if it is bad form to revive a year old thread, but all the info is here and my findings directly build on the previous posts.
My GTX-65 has the same terrible "hum with dsp attached" problem as txflood and many others. I attempted perform the "switch J10's pin 1 to ground" as pondered in this thread, and while some high freq noise was cut (from dsp I assume) the main hum was still there.
I have since found that simply disconnecting J10's pin 4 (the power pin) removes the hum completely (as well as dsp air noise).
My current plan is to add a switch to break/join pin 4, however this creates a loud pop when disconnecting, which I suppose is not good. I do not mind only using this switch when the amp is off, but perhaps this gives some people in here some insight into the nature of the problem/a better fix?
Reference schematic: http://music-electronics-forum.com/attachments/6109d1251661835-195sch_2.pdf
Edit: This may also explain why over filtering at C64 also reduces the hum significantly, perhaps the dsp board is not well-behaved at its power input?
Very welcome by me. :dbtu:
Ah HA! Comparing this .pdf with GFX-120a circuit (<gx 212 gfx.pdf>, here already apparently), there is an interesting difference at pin 4 where the DSP power feed is.
In one circuit it is simply a 10 ohm series resistor, but in the other circuit they have lifted the resistor to 47 ohms and added an 0.1uF bypass cap! They have had a problem here. I'd try adding this extra decoupling of the DSP power feed and see if it helps.
Thank you for the quick reply! Exciting to think some headway may be made on this. I am new to this forum and indeed just getting into electronics hacking! :)
Please bear with my noobness if I am overlooking where the pdf is.. Could you please refer to the pdf with the 47ohm R and bypass cap? They do not appear in this thread, and the closest I can find elsewhere is http://elektrotanya.com/crate_gx_120,_gx_212_sch.pdf/download.html but that does not appear to have j10 dsp connection.
I assume you are referring to R107 being increased 10 > 47 ohms? Is the bypass cap an additional 0.1uf cap put in parallel to R107? (I assume you are not referring to C63 as this is in my schematic/amp as well).
I am the Proud owner (what?) of a Crate DT50C that I bought mostly because it came with 2 nice MIE Celestion 10s. It hummed badly and one of the two inputs was inoperable. I replaced the power filter caps and it had little to no effect on hum. Then I did as others have suggested, thorough inspection and re-flowed the solder joints on everything I could get my solder tip close enough to especially around the input jacks, potentiometers and power supply. I also had a Native American medicine man perform an exorcism on the demon spirits that typically haunt Crate equip. Wala, it only hums 1/3 as much and I use it as a backup at church. One great thing about this amp is that I don't worry that someone will steal it and I can torture it with any combination of pedals and gain settings without worrying if it blows up. :loco
Well, you did the right thing.
Strong hum and unoperable input jacks point straight at cracking in that area.
Which is very common, it´s very easy to yank a guitar cable in a bad way, or have the amp fall on its front breaking the plug or simply stepping on the cable.
Any/all of these will either break the jack itself or make it twist strongly and crack/break whatever they are soldered to, in this case the PCB.
Quote from: domohawk on May 20, 2014, 07:14:42 AM
Please bear with my noobness if I am overlooking where the pdf is.. Could you please refer to the pdf with the 47ohm R and bypass cap? They do not appear in this thread, and the closest I can find elsewhere is http://elektrotanya.com/crate_gx_120,_gx_212_sch.pdf/download.html but that does not appear to have j10 dsp connection.
I assume you are referring to R107 being increased 10 > 47 ohms? Is the bypass cap an additional 0.1uf cap put in parallel to R107? (I assume you are not referring to C63 as this is in my schematic/amp as well).
The 47 ohm is the new value for the 10 ohm resistor in series with the supply to the Fx DSP. The new 0.1uF cap goes between the DSP supply pin and ground at, or near as possible to, the DSP connector. This should reduce the sing-song digital crud getting out.
Earlier posts suggest that beefing up the earlier electrolytic filter cap for this supply may help the hum problem.
I've had a good search for the previous post of this circuit so I could just link to it, but I can't find it; so...
Roly, the schem. with the 10 ohm has its own supply, not running off the 40V line.
The gfx212 schem. shows a different supply and 470, not 47 ohm. Are we looking at the same schematics?
Quite likely not;
Title: GFX120a
No: 07S252-XX
Sheet: 1 of 2
Date: 6/19/98
The basic point is that one has extra bypassing the other doesn't, and this change in DSP supply arrangement may be a clue they were having some problems in this area.
Roly, I am also seeing the 470 ohm.
How exactly would this 'bypass filter' work? Wouldn't the 0.1uF cap just roll off the very high frequencies? The hum is pretty low, I'd guess around 240hz.
Let me be the first to say that this is a bit of a shot in the dark.
Quote from: domohawkHow exactly would this 'bypass filter' work?
Bypass filtering of supply lines normally consists of a resistor in series, then a cap to ground. This forms a First Order low pass filter which has a cutoff frequency where the reactance,
Xc = 1/2 Pi f C, of the cap C is equal to the series resistance, Xc = R. The bigger either C or R are, the lower the frequency.
For 470 ohm and 0.1uF this is;
Xc = 1/ 2 Pi f C
f = 1/ 2 Pi Xc C
1/(2*Pi*470*(0.1*10^-6)) = 3,386.3Hz or 3.4kHz.
This would help prevent high frequency noise/signals getting in to the DSP, and would help to reduce DSP noise getting out, but have no effect on hum on the supply lines.
If we were to place, say, a 100uF of suitable voltage rating across the 0.1uF we would get;
1/(2*Pi*470*(100*10^-6)) = 3.4Hz, well below any mains hum.
"Hum" in amplifiers is by convention
power mains related, either 50/60Hz sinewave by direct pickup, what you get when you touch the tip of your guitar lead, or a sawtooth at double that frequency as the result of full wave rectification in the power supply getting into the audio path somehow, generally through insufficient filtering, either due to design error or more commonly due to aging/failing filter caps. 4th harmonic 200/240Hz is pretty unlikely but it is easy to be fooled by 2nd harmonic sawtooth because the waveshape itself has a strong 2nd harmonic 200/240Hz content 'tho its fundamental is actually 100/120Hz.
Yes, 470 ohms and 0.1uf has a high cutoff and is intended to keep the digital noise in the DSP board, but if the DSP board is suffering from hum related to its supply then this suggests that this supply needs close examination. Assuming we can discount a faulty part* and an earth loop, does this supply need extra filtering, or even regulation in the form of a zener or three-pin regulator?
(* have a very close look at the DSP module with a bright light and lens for any evidence of on-board electros leaking or swelling)
The amp either didn't come out of the factory with a hum-on-DSP problem, in which case a component has failed (e.g. filter electro gone low-C); or it did, in which case we have a basic design problem that can only be fixed by doing the design work that was skipped over, and modifying the amp accordingly.
The changes in that DSP supply area suggest to me that they found a problem after they started their initial production run and modified it on later runs. While this mod looks like it was to address higher frequencies getting into/out of the DSP it is a bit of a smoking gun now that these amps seem to also have a problem with hum going in via the same route.
So I'd try adding a suitable electro cap across the supply at the DSP board socket.
This will have one of three outcomes;
- Hum eliminated - we are right on the right track.
- Hum reduced - we
may be on the right track (try a bigger cap).
- Hum the same - it's something else (it's a red herring).
Roly, thank you for your in depth reply! It helped me learn a bit more about filter circuits.
I suspect adding a higher F cap will provide much better results in getting the 120hz out, but I will also give the higher R and 0.1uF a go. Just waiting on digi-key now.. I even picked up an 8200uF cap in addition to the 100uF for fun, since it was less than 2 bucks :)
For the interested, a bit of backstory I think leads to an explanation at why I think this "crate dsp hum" problem is so prevalent on the internet:
In my case I did not notice hum when I bought the amp many years ago. Then one day it got a REALLY LOUD hum, out of the blue. After many hours googling I found a solution was to disconnect the dsp and it was dead quiet.
Fast forward a few years to last week when I decide to take a look at it again and see if I can salvage the dsp use. More relentless googling leads me to the answer, in a 2011 forum post (last: http://music-electronics-forum.com/t13806/). The problem is the crappy pc power connector (J10). The J10 on the dsp is mounted horizontal, so after hours of play the connector rattles loose and causes a huge hum. Crimping the connector or better yet soldering it removed the LOUD hum.
So if the LOUD hum is fixed why am I posting this? Well because the dsp still adds hum (I'd say 20% of the LOUD hum) over no dsp, but since I never knew the crate could be ninja silent with the dsp disconnected, I didn't notice the problem when I first used it. My ears have probably gotten more picky as well.
So in final, if txfloods 'over filter' cap does indeed decrease this 20% hum by 2/3 I will indeed be a happy hacker! Of course I'm still adding the true bypass switch because I'm persnickety :)
Quote from: Roly on May 20, 2014, 02:30:39 PM
Quite likely not;
Title: GFX120a
No: 07S252-XX
Sheet: 1 of 2
Date: 6/19/98
The basic point is that one has extra bypassing the other doesn't, and this change in DSP supply arrangement may be a clue they were having some problems in this area.
Yes, that is the same drawing I have. It's a 470 ohm (R88), not 47.
It is used as a voltage dropper because they are tapping off the +40V supply. The other circuit has it's own winding and rectifier for a separate supply and has a 10R resistor.
I'm not arguing that the .1uf bypass cap may help, I just wanted to point out that the supply circuits are quite different and the 470R is not just for filtering.
Agree to try the .1 bypass as close to the DSP chip as you can and see if it helps!
Don't change the resistor. As g1 points out it is also a voltage dropper for the DSP and should remain at that value.
I think that 8000uF is too big. This will give a low frequency rolloff far lower than required, and may slow the rise of the supply to the DSP which may have implications for its internal power on reset function.
The fact that you remember the hum starting suddenly suggests to me that a power supply filtering electro on the DSP board has gone low/open. One reason I have for suspecting this is that there was a period when makers of these small board-mounted electros were having some troubles with their electrolytes, and for some time a lot of these caps were turning up that were leaking onto the PCB. I had a video projector brought in one time where the PCB was simply covered by goo that had leaked out of several of the caps, and I'm not known for throwing the towel in easily but this was clearly a lost cause.
I'm hoping that in this case simply putting back 20, 50 or 100uF across the supply will fix the problem.
Copy on leaving the resistor alone.
To be clear the sudden hum was the loose J10. The hum now is back in it's "normal" state, however since I noticed this thing can really whisper with the dsp disconnected, trying to see if I can improve on the original "with dsp" circuit with some more filtering.
Sorry for delay on results. Better late than never I suppose..
So building the caps into the J10 connected cable (by bridging power and gnd pins/wires) seemed to have little if any effect. However putting it directly across the diode bridge worked pretty well and cut out a lot of hum. I went with a 4100uF cap, since 8200uF seemed to have no added benefit, but was noticeably better than 100uF and even 1000uF.
I also wired in a switch in series with the J10 cable to simply break the power pin, and JB welded it to a cutout on the back panel. It's pretty dumb but works to completely kill DSP+associated noise (both hum and the slight digital "air" noise). Just make sure the amp is powered off when toggling this one or you get a great pop.
Thanks for all the help! I think I have the best of all reasonably possible worlds now with this amp.
Thank you for going to the trouble to close the circle on this one. Too often people seem to have a successful repair, then don't bother telling us about it.
So from what you say it looks like the basic problem has always been in the actual supply - not enough filtering microfarads (and that would get worse with time).
Very well done, and thanks again. :dbtu: