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Thread: CRATE GX-15R Help

  1. #1
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    CRATE GX-15R Help

    I've been removing and replacing a lot of items on my Crate circuit board and still have no luck. I was going to remove and replace the Op-Amp chip but wasn't sure how to remove that large heat sink that it's attached too? Can someone chime in was some suggestions cause I am kinda bumbed out now.

    Thanks
    Joe

  2. #2
    g1
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    They are usually held on by a small screw.
    This one looks like it may be a special security type screw or maybe a rivet. If it's a rivet, you can cut or grind it off and replace with a screw and nut.
    gx15r.jpg
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    Hi g1,
    Thanks for your feedback but mine is a slight bit different than the picture. On my Heat Sink, there is a small clip that bonds the chip to the heat sink. Once I popped the clip off, the I.C. circuit separates from the heat sink. Looking down at the heat sink, I see a vertical post on each end and it looks like it's suppose to slip off with a little force. I don't want to break it, so I am asking for other people to help with advice.

    Thanks for answering my plea
    Joe

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    Senior Member Enzo's Avatar
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    Don't remove the heat sink. You have freed the 5-leg IC from the heat sink. Unsolder the leads and remove it. Solder a new one in its place, and return the clamp. If your old IC had a thin insulator between it and the heat sink, that insulator MUST be there for the new one.
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    What's wrong with the amp? If you've been replacing stuff with no luck, then stop and go down a more methodical process of troubleshooting. Random parts replacement isn't a good idea, as the parts you've replaced have little or no relationship to the fault. A bit like having a misfire with your car and replacing suspension components, and still having a misfire.
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    Mike,
    It will not produce any sound at all despite the power light illuminates..

    Joe

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    Thanks Enzo,
    I'll have to rebuild my strength and enthusiasm before I try taking it all apart again. I appreciate that advice. The heat sink looks like it's pressed down onto the circuitboard and is guided by the two support pins soldered to the board.


    Joe

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    Senior Member Enzo's Avatar
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    Yes, it is not meant to come off.
    Education is what you're left with after you have forgotten what you have learned.

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    Seems like this is a revisit to this thread;

    Crate GX-15R Schematic

    The indicator light doesn't necessarily mean that power is going anywhere useful. Have you checked the DC voltages as per the schematic test points? You have to be sure the amp IC (and the rest of the circuit) has power.

    Also, when the amp appears to be dead, if you touch a meter probe onto TP6 do you get a hum?

    Is the speaker OK?

    I don't mean for this to be the Spanish Inquisition, just trying to make sure you're on the right track.
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    O.K. Thanks for verifying that.

    Joe

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    Yes Mike This is a continuation from a while back. I just got around to motivating myself to remove and replace some suspected parts. I guess the power light is a deceptive indicator that the unit is getting power?

    I'll check to make sure the speaker is good.. Also when your referring to the "Amp I.C." exactly which computer chip are we talking about? The 5 legged chip attached to the Heat Sink? or another one? I'll have to retrace my test points to see if I get the humming sound whenever I probe the TP6 test point.

    I really do appreciate your help because it's given me a second wind and now a bit more hope to press on with the investigation. I was bumbed out yesterday after doing a lot of work with no positive results. I will take a little time off and regain my strength and knowledge, thanks to you.

    Sincerely
    Joe

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    Yes, I'm referring to the 5-leg IC bolted to the heatsink. If you look at the schematic, TP6 is a convenient point to connect to pin 1 (the input) of the device. Touching it with a meter probe should give you enough noise injection for a hum to come through the speaker with the amp powered up.

    TP8 and TP9 are the test points for +/- 15v with respect to ground.

    A quick check of the speaker is to disconnect it from the amp and momentarily use a 9v battery across the terminals to see if it makes a noise.

    When troubleshooting a dead amp with a working power light it's best to work backwards - speaker, then power amp, then preamp. But you always first need to make sure the DC voltages are correct.

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    Wow, I did not know this. On the five legged IC circuit bolted to the heat sink, would it do harm to touch the wrong leg if I was testing it? I am assuming the Negative lead of the multimeter (Blk) will be grounded to the chassis? Pin # 1 will be one out of the three legs exposed, "Correct"?

    Thanks Again
    Joe
    P.S. Sorry for messing up your name in the previous reply. I wrote, "Mike" instead of "Mick"

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    No harm touching the wrong leg, but very likely damage if you short two legs. When doing this type of thing I look where the leg is connected to and use a point a little less crowded. Look at the schematic - you have R31 and C20 attached to that point. Not that if P7 is fully clockwise the input will be more sensitive.

    You're not taking a measurement when doing this check. It's referred to as a 'disturbance test'. The other probe doesn't matter and can be left unconnected. Sometimes in a low-voltage circuit I'll make sure the probe is in position and dab a finger on the plug end to get more noise. A more sophisticated way is to use a simple oscillator and inject a signal at the input of the device.

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    Mick, So what your saying it's kinda like grabbing the end of the guitar audio cable when it's hooked up to the amp but not the guitar? When you touch the tip you immediately
    hear the distortion. I looked for the Crate GX-15R Schematic but don't seem to have it in printed form. I believe the last time I tried printing it from this forum there was a compatibility issue with my lap top. You wouldn't happen to have the diagram available would you? You also mentioned a gasket between the IC Chip and the Heat Sink. Where can I get one? and or can I substitute thermal grease in place of the gasket?

    Thanks Always
    Joe

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    Exactly the same as your guitar cable. I opened the PDF fine from the link in the other thread. Enzo mentioned the insulator between the IC and the heatsink; if it had one originally you need to replace it. You cannot use thermal grease on its own if it needs insulating. The thermal grease just helps to make contact between two surfaces by filling any voids. Note that if there's an insulator, there's usually an insulating bush so that the screw is insulated from the device tab, otherwise it will short out. Best to check between the tab and heatsink with your DMM set to resistance when you reassemble it (if it needs to be insulated). Do this before you switch on.

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    Mick, Thanks for that information, On mine the I.C. Chip looks like it marry's the Heat Sink with a small circular indentation that clips onto the Heat Sink and then a small metal clip secures the two together. I never noticed any insulator or adhesive grease. I appreciate all of the caution notes and warnings as well.

    Joe

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    Mick,
    I tried it this afternoon and It worked. I hooked up a known working Speaker from another amp and did exactly what you described as a disturbance test. The speaker made a crackling sound (In a Good Way) So now I am Gung Ho to remove and replace the TDA 2030A chip. Wish me good luck and I'll let you know how it goes.

    Thanks Again
    Joe

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    That check tells you that the amp IC is amplifying the noise and is probably good, and shouldn't be replaced. You say that you amp is not making any sound. If the preamp was getting a signal up to that power amp chip then it would make a sound - just like touching the pin makes a sound. We don't know what the quality would be like, but it would produce something.

    The fault lies further back.

    Did you check the voltages are present on TP10/TP11? (+/-15v) ?

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    Mick,
    Yes I did check the test points 10 and 11 with the (power on) and the negative probe of the ohm meter grounded to the chassis and (without) a speaker connected. It did produce Voltage at those test points. What's the next culprit?

    Joe

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    Simplest way is to diagnose the amp in clean mode - this way you cut down the signal path by half. Firstly, make sure the switch is operating correctly, as a faulty switch means the amp will not work. In Clean mode the signal gets amplified by 2 sections of IC1 which is a quad opamp and each section is shown as a triangular functional block. So the signal is fed into the first opamp and appears on the output pin 1. It then goes via C1 and gets picked off the wiper of volume pot P3 and is routed via the slider switch to the second opamp (actually the 4th one in the package if you look at the pinout for a TL074) and outputs to the tone stack on pin 14 and at the same point splits off to the reverb section.

    In the absence of a scope and signal generator you can use the 'disturbance test' to introduce noise into the amp at various points. With the controls turned up and a plug inserted into the input socket to open the grounding contacts, carefully touch a probe onto IC1 pin 3. Anything? then try pin 1, pin 13 and pin 14. You should get a loud hum through the speaker at every point.

    Edit; Or you could use your DMM set to AC volts and use a sine wave source plugged into the input. 400Hz/1KHz will do it. You should 'see' the signal on the opamp outputs. Also worthwhile checking the outputs for DC to see if the opamp is shorted, or there's unwanted DC on the input.
    Last edited by Mick Bailey; 08-25-2017 at 11:12 AM.

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