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Help with Hartke LH500 Protection circuit

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  • #1

    Help with Hartke LH500 Protection circuit

    Hi

    I've been researching what appears to be a common issue with Hartke LH500 amps, that being the protection circuit.

    The amp I have belongs to a friend. He's asked me to have a look at it as it's dropping out. The amp may or may not produce sound on start up, and if it does come 'on' on power up (after a short delay as expected) it doesn't last long. Its hit and miss whether it comes on or not. You can hear the relay actuate when it does decide to allow the output to the speakers.
    Voltages to the power amp board are good on measurement.
    I can tell from my limited knowledge and following the signal through the amp with my CRO that its the protection circuit causing the issue as I can see the amplified signal at R336. Having seen a video from another thread showing how to change the 'differential transistors' Q067 and Q301, and reading another thread commenting that the caps in the protection circuit C315-318 should be replaced, I've gone ahead and ordered the parts.
    Not knowing much about how this bit of the circuit/board works I'd like to understand it a little more so if the daughter board is bad I can diagnose it.

    The schematic is v3.8 Attached for reference

    Any help gratefully received.
    Attached Files
    Tags: None
  • #2
    Maybe this will help.
    Attached Files
    Last edited by g1; 04-26-2017, 07:09 PM. Reason: fixed attachment, thanks nickb
    Originally posted by Enzo
    I have a sign in my shop that says, "Never think up reasons not to check something."


    ​

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    • #3
      Please don't start throwing parts at it. If you have good signal on the inboard side of the relay, then the amp is working. The relay driver g1 posted has several small electrolytics that dry out more often than they should. usually if there is no DC offset on the amp circuit but the relay stil won't come on, that is the issue. Look at those 1uf and 0.47uf lytics.
      Education is what you're left with after you have forgotten what you have learned.

      Comment

      • #4
        Originally posted by g1 View Post
        Maybe this will help.
        Sadly that one looked like it got sucked into the vortex.


        Try this: TA7317_P.pdf
        Experience is something you get, just after you really needed it.

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        • #5
          Originally posted by Enzo View Post
          Please don't start throwing parts at it. If you have good signal on the inboard side of the relay, then the amp is working. The relay driver g1 posted has several small electrolytics that dry out more often than they should. usually if there is no DC offset on the amp circuit but the relay stil won't come on, that is the issue. Look at those 1uf and 0.47uf lytics.
          OK got those caps on the way. How do I measure the dc offset - and please explain what that is. Is it just a DC voltage where there isnt supposed to be one?

          Cheers for you help guys much appreciated.

          Comment

          • #6
            The DC offset in question is the voltage that appears on the output of the power amp with no signal applied. You can measure the DCV across R342 (56K) on sheet 2.
            Experience is something you get, just after you really needed it.

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            • #7
              That^^^

              The output of the amp SHOULD settle to zero volts. If the output has some DC voltage sitting on it - it is offset by some DC amount - that is one thing the 7317 detects and keeps the speakers off. So we check for it.
              Education is what you're left with after you have forgotten what you have learned.

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              • #8
                According to datasheet, TA7317 triggers output relay OFF (or refrains from turning it ON), IF it reads +/-1.1V DC atbthe speaker out, no signal applied of course.
                Sensitivity may vary between +/- 0.9V and 1.4V DC .

                *Usually* we expect no more than 100 to 150mV DC at amp speaker out rail so check that first.

                And yes, the voice of experience says it might be dried caps, so after checking for abnormal DC (in which case protector would just be doing its job), that should be the next step.
                Juan Manuel Fahey

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                • #9
                  Yes, the idea is to first determine if the IC is just doing its job or if it is the problem itself.
                  Education is what you're left with after you have forgotten what you have learned.

                  Comment

                  • #10
                    Originally posted by nickb View Post
                    The DC offset in question is the voltage that appears on the output of the power amp with no signal applied. You can measure the DCV across R342 (56K) on sheet 2.
                    R342 isnt 56K, R341 is 56K and seems to be in the signalchain for the speaker relay. I'll measure it there shortly. been busy with real life but will spend some time on it tonight now I have all the potential spares I need.

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                    • #11
                      Sorry Andy, R341 is the one -it was a typo.
                      Experience is something you get, just after you really needed it.

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                      • #12
                        Right in the garage - measuring 0.5v with a variance of up to +/- 40mV with the speaker signal path interrupted by the relay. It's just clicked on and I measured 0.43v. Remeber this is not an IC but a daughter board with discrete components.

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                        • #13
                          That's really much bigger than is acceptable and the TA7317 is behaving as it should. I'd like see under 100mV. My guess for now is the offset varies and sometimes it big enough to release the protect relay. Fix that and all will be solved.

                          Try measuring the voltage across R316, it should be zero, to see if C306 has gone leaky. There a quite few components in this design that could cause this. There is an offset adjustment control SVR301 that is used to zero the offset, it might be bad. Or it could be a leaky C304 or C302, or one or both of the transistors Q301 and Q067 going bad.

                          Try turning SVR301 while monitoring the offset voltage. If the voltage doesn't vary smoothly then the trimmer is bad. If OK, then the mentioned capacitors are the next target.
                          Experience is something you get, just after you really needed it.

                          Comment

                          • #14
                            Just replaced c315-318. Now time to re-assemble and test.

                            Comment

                            • #15
                              Well that didn't go well...Q315 has just melted down, lots of sparks, something has shorted. Do I have to replace all 8 of them? Are they matched?

                              Comment

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