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Heatsink questions on Crate

Started by txflood, February 12, 2013, 12:33:41 PM

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txflood

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.

phatt

#1
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?

DrGonz78

Wow that's a lot of Locktite!! I have been inside many Crate amps and have never encountered one having that much Loctite.
"A person who never made a mistake never tried anything new." -Albert Einstein

Enzo

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.

DrGonz78

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.
"A person who never made a mistake never tried anything new." -Albert Einstein

txflood

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.

txflood

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.

txflood

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.

g1

  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.

DrGonz78

#9
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?
"A person who never made a mistake never tried anything new." -Albert Einstein

phatt

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.


phatt

#11
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.

DrGonz78

@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.
"A person who never made a mistake never tried anything new." -Albert Einstein

txflood

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.

txflood

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.