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Thread: Crate GX-212 power amp puzzle

  1. #36
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    I have a similar issue with a crate GX-212. The power amp section is similar to the GTX 212 unit. i'll attach an schematic that will be used for the references of this post

    The design is slightly different from the old GX-212 design with the NE5532 op-amp driving the power stage.

    I've replaced most of the components that were burn or shorted. including diodes (D13) and burnt resistors (R63)

    I've measured the voltage across R72 - R74 and it gives me around 5,5 mv which is under the 7 mv specified in the schematic .I kept this under the "standard"
    value until i verify that the power stage is stable and it has no issues.

    The issue is that R61 & R64 seem to dissipate more heat than the other 2 emiter resistors. The voltage across these are average 5 mv too
    the collectors on each power transistors show around +/- 32-33 volts DC being 35V the unloaded voltage i get from the power supply.

    If i measure the voltage referenced to ground the 2 TIP142 transistors have around 155 mv on each emiter
    and the 2 TIP147 have around 80-88 mv on each emiter

    The DC voltage i can measure on the speaker output is around -11,3 mv

    also the voltage measured on the base of each pair of transistors is 1,31 votls for the TIP142 and -1.26 volts for the TIP147.

    R61 and R64 are ok and not open resistors. in fact they are brand new.

    The amplifier plays fine with plenty of bass but this amp had lots of power amp failures.

    Any suggestions?

    Thanks in advance
    Attached Files Attached Files

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  2. #37
    Old Timer J M Fahey's Avatar
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    The issue is that R61 & R64 seem to dissipate more heat than the other 2 emiter resistors. The voltage across these are average 5 mv too
    Why do you say so?
    Voltage across/current through them is the same, also ohmic value and rated dissipation

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    Juan Manuel Fahey

  3. #38
    Supporting Member Jazz P Bass's Avatar
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    TP12 is too high. (TIP147 base voltage)
    The chart states : -0.06Vdc.
    Your reading is -1.26.

    Have you changed the bias transistor Q9?
    Inspect the bias pot as it may be dodgy.

    You have not explained the failure in any detail, but I find that it helps to highlight the failed components to track the failure path.
    This may provide clues to what may or may not have gotten stressed.

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  4. #39
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    yea this amp just cooked transistors like nothing. I replaced them all and set the bias according to the schematic and totally toasted them again after a while.

    I replaced the following componentes

    Q8,Q10,Q11 and power transistors
    D13,D21
    R63
    C58,C29 (100uf instead of 220uf according to the schematic).

    Everything else has been checked and they are within spec except R73 that was around 230 Ohms. I replaced it and still get a high base voltage on both TIP147's
    compared to what is shown on the schematic. I was measuring the power amp without a load and no signal being fed into it. I'll put a signal and probe it with my scope and see how that goes.

    I know it seems weird that R61 and R64 are slightly warmer than the other 2 emiter resistors since they have around the same voltaje measured across them.

    Thanks guys for your replies. i'll keep this updated.

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  5. #40
    Supporting Member Jazz P Bass's Avatar
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    About the only thing that can fry both sides (short of massive abuse) is if they both turn on at the same time.

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  6. #41
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    i'll replace the transistors on the differential pair and see what happens. also i plugged a load and still get -1,24 volts but goes down to 0.95 when i insert the signal through the insert jack

    at 3 mv average on R72 and R74. I Check the scope and i see a perfect sine wave on the output. with no artifacts.

    Also the bases reach +/- 32 volts ac peak to peak on their bases

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  7. #42
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    Hi enzo
    I have the same amp on the bench.customer replaced 2 5W 270ohm power resistors that come off of the bridge.poor solder job= no output and DC hum.cleaned up this mess.amp powers up and plays fine...for a few minutes.snap,crackle and pop but still plays.occasionally clears up but always returns to noisy state. at one point right in the middle of monitoring, the audio quits and I get DC. all components seem fine except the output of the bridge is .032. not good.I have had issues with Crate bridge failure in the past. the schematic isn't clear on the part.can you recommend a replacement bridge? thanks. gordo (ogeecheeman)

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  8. #43
    Lifetime Member Enzo's Avatar
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    SOunds to me more like you have a bad solder connection somewhere.

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  9. #44
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    sorry I never got back to you on this one but you were right on the solder joints... Q14 had broken foil trace & C31 was faulty.

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