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Audio and Video Talk
Reel-to-Reel & Cassette Tapes
Revox B77 Record switch repair
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<blockquote data-quote="marcochezzi" data-source="post: 959208" data-attributes="member: 21001"><p>Thanks Steerpike & Mygoggie.</p><p></p><p>Sandblaster, why did I not think of that..!!</p><p></p><p>Sadly this story does not end well, machine is faulty, worst part of the fault is that it?s directly in the PROM chip on the logic board. I have traced and traced and traced, but no two things about it, on turn on the pressure roller solenoid activates, and stays that way, no response from any other inputs thanks to a faulty Pin 2 PROM output locked in a permanent high state.</p><p></p><p>I desoldered the EPROM pin feeding the solenoid drive circuit and the solenoid releases and the machine goes out of play mode. Desoldered the PROM 4 input lines and forced the chip input pins to ground, output 2 remains high, so something has gone belly up on the PROM, which is a pity, gate chips are easy to replace, a chip with a dedicated program written to it 40 years ago not so easy.... </p><p></p><p>Probably related but apart from the pressure roller solenoid signal, pin 2 of the PROM drives a TTL nand gate integrated circuit, it?s a small chip with four 2 input nand gates, that chip was also damaged and it too on the same pin that connects to pin 2 of the PROM chip, pity, I replaced all the TTL chips as well today but no difference...</p><p></p><p>If I do not find a solution either by virtue of a scrap machine or a replacement board I am going to design my own logic interface and be done with it..., guess it will be unique..</p></blockquote><p></p>
[QUOTE="marcochezzi, post: 959208, member: 21001"] Thanks Steerpike & Mygoggie. Sandblaster, why did I not think of that..!! Sadly this story does not end well, machine is faulty, worst part of the fault is that it?s directly in the PROM chip on the logic board. I have traced and traced and traced, but no two things about it, on turn on the pressure roller solenoid activates, and stays that way, no response from any other inputs thanks to a faulty Pin 2 PROM output locked in a permanent high state. I desoldered the EPROM pin feeding the solenoid drive circuit and the solenoid releases and the machine goes out of play mode. Desoldered the PROM 4 input lines and forced the chip input pins to ground, output 2 remains high, so something has gone belly up on the PROM, which is a pity, gate chips are easy to replace, a chip with a dedicated program written to it 40 years ago not so easy.... Probably related but apart from the pressure roller solenoid signal, pin 2 of the PROM drives a TTL nand gate integrated circuit, it?s a small chip with four 2 input nand gates, that chip was also damaged and it too on the same pin that connects to pin 2 of the PROM chip, pity, I replaced all the TTL chips as well today but no difference... If I do not find a solution either by virtue of a scrap machine or a replacement board I am going to design my own logic interface and be done with it..., guess it will be unique.. [/QUOTE]
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Audio and Video Talk
Reel-to-Reel & Cassette Tapes
Revox B77 Record switch repair
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