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  • Crest FA1201 Protect Relays

    I have a Crest FA1201 rack power amp that I briefly loaned to a friend about six months ago. By his account it worked fine for him and it's been sitting idle since then. I tried using it again recently and it won't go out of protect mode after powering it on. Normally when turning the unit on the "protect" LED's are lit for a couple of seconds, then you would hear the protect relays click and the LED's would turn off and there would be signal at the output. The relay designation is T90N5D12-12. If you look at the schematic there is a connector J2 with one wire for each channel labelled "relay". My understanding is that I should see the nominal 12V on each of those wires, or at least the turn on voltage of 9V. The actual voltage is 1.6V. According to the user manual the relays will remain open if the amp is overheated (which is not the case here, although I suppose it could a sensor problem?) or if the amp senses DC at the output. I measured 0V DC at the output. But if I remove the "signal ground lift" jumper I measure -0.2V at the output and also on the J2 relay wires. So I'm pretty confused as how to move forward. If the problem is in the protection circuit itself it seems odd to me that the circuit for both channels became problematic at the same time. As far as I can tell the protection circuit is discrete for each channel. Any insight is greatly appreciated, thanks.

    hfe_crest_audio_fa601_901_1201_schematics.pdf

  • #2
    The relay disconnects the amp circuits from the connectors on the rear panel, so if the relay does not click on, you will always measure zero AT THE SPEAKER POSTS. But that is not the circuit. Look at the circuit itself. On the power amp board page on the right it says OUT through L1. All those 0.33 ohm 5w resistors for each power transistor are on this line. WHen your relay stays off, measure for DC in those places.

    The amp may not be overheated, but that doesn't mean the heat sense circuit isn't being tripped.


    I see J9 rather than J2 labelled RELAY.?

    Page 1, Q1, Q2 control the relay. They are fed a +20v supply and are controlled by signals MUTEA and MUTEB. Those MUTE signals come from page 4, and transistors Q1, Q2 on that page. Note each board has its own series of numbers. They do appear to be independent per channel, but that circuit shares the LM614 IC

    But I might check that +20, If it isn't getting to the control circuit, then the relays cannot be powered by it.
    Education is what you're left with after you have forgotten what you have learned.

    Comment


    • #3
      Originally posted by Enzo View Post
      I see J9 rather than J2 labelled RELAY.?
      I'm referring to J2 as shown on page 1. The top two pins connect to J9 on page 2

      Originally posted by Enzo View Post
      The relay disconnects the amp circuits from the connectors on the rear panel, so if the relay does not click on, you will always measure zero AT THE SPEAKER POSTS. But that is not the circuit. Look at the circuit itself. On the power amp board page on the right it says OUT through L1. All those 0.33 ohm 5w resistors for each power transistor are on this line. WHen your relay stays off, measure for DC in those places.

      The amp may not be overheated, but that doesn't mean the heat sense circuit isn't being tripped.

      Page 1, Q1, Q2 control the relay. They are fed a +20v supply and are controlled by signals MUTEA and MUTEB. Those MUTE signals come from page 4, and transistors Q1, Q2 on that page. Note each board has its own series of numbers. They
      do appear to be independent per channel, but that circuit shares the LM614 IC

      But I might check that +20, If it isn't getting to the control circuit, then the relays cannot be powered by it.
      I decided to focus on the control circuit to begin with. I'm attaching page 1 with some measurements. I'm not sure what to make of it. Note that I currently have the +85 and -85 supply to the power amp modules disconnected, FWIW.

      Click image for larger version  Name:	Display.jpg Views:	0 Size:	1.10 MB ID:	908485

      Comment


      • #4
        Well, that is a whole sea of numbers, but what I see:
        You do have the 20v supply, and the two MUTE signals are not going low to turn on the relays. SO go to page 4 and find out why Q1, Q2 are not turning on.
        Education is what you're left with after you have forgotten what you have learned.

        Comment


        • #5
          Originally posted by Enzo View Post
          Well, that is a whole sea of numbers, but what I see:
          You do have the 20v supply, and the two MUTE signals are not going low to turn on the relays. SO go to page 4 and find out why Q1, Q2 are not turning on.
          Weeell, ain't that a kick in the head. I started looking at the main PCB to get acquainted with the page 4 components and I discovered that the LM614 chip was backwards in its socket. All good now!

          I read through my text messages from a year and half ago when I lent it out and it shows my friend thought one of the channels wasn't working, so maybe one of his cronies was messing with it? It's a mystery to me. I'm waiting to hear back from him.

          Anyway, thanks Enzo, much appreciated.

          Comment


          • #6
            Almost like in post #2 where LM614 was the common element to the two channels...

            ANyway, I am glad you found it.
            Education is what you're left with after you have forgotten what you have learned.

            Comment

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