{"id":10861,"date":"2026-02-14T10:24:24","date_gmt":"2026-02-14T09:24:24","guid":{"rendered":"https:\/\/retrofixer.it\/?p=10861"},"modified":"2026-02-14T13:39:57","modified_gmt":"2026-02-14T12:39:57","slug":"ti99-4a-black-screen","status":"publish","type":"post","link":"https:\/\/retrofixer.it\/en\/ti99-4a-black-screen\/","title":{"rendered":"Ti99\/4a Black screen"},"content":{"rendered":"
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The Texas Instruments TI-99\/4A<\/strong> is one of the most iconic home computers of the early \u201980s (and even earlier), known for being one of the first home systems with a 16-bit processor. Many enthusiasts still restore it today, but like all vintage machines it can suffer from faults due to age and component oxidation.<\/p>

This TI-99\/4A,<\/strong> powers on but emits a constant tone and the screen stays black, a fairly common problem that can have various hardware causes. As with the Commodore 64, a black screen can be caused by many faulty chips, but here we have a little help: the sound. If the sound chip isn’t initialised, it emits a noise similar to a constant beep, though slightly varying. After a certain sequence, this chip is correctly initialised and stops making noise, up to the point where the BIOS makes it beep and the system is ready for use.<\/p>

Here are some useful links for the repair:
TI994A_HC_Servicing_Manual_Oct_1983.pdf<\/a>
inerpedia.org – Troubleshooting<\/a>
Step Troubleshooting<\/a><\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t

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\n\t\t\t\t\n\t\t\t\t\t1.2 Console power up procedure.\n\nA. TMS9900 CPU resets and addresses low ROM locations.\n\nB. TMS9900 initializes.\n\nC. TMS9900 sets up workspace registers in MCM6810 RAM.\n\nD. TMS9900 begins GROM read.\n\nE. TMS9900 enters delay loop for about 1\/4 second.\n\nF. TMS9919 sound chip is disabled.\n\nG. TMS9918A VDP chip is initialized.\n\nH. 4116 VDP RAM is initialized (requires about 1 second).\n\nI. Title screen is loaded into VDP.\n\nJ. TMS9919 sound chip emits beep.\n\nK. TMS9900 CPU enters keyboard scan.\n\nL. System is ready for use.<\/xmp>\n\t\t\t\t<\/code>\n\t\t\t<\/pre>\n\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-caaaedb elementor-widget elementor-widget-text-editor\" data-id=\"caaaedb\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<p>With a continuous tone, the problem lies somewhere between step A and step E.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<div class=\"elementor-element elementor-element-eb72ec2 elementor-widget elementor-widget-text-editor\" data-id=\"eb72ec2\" data-element_type=\"widget\" data-e-type=\"widget\" data-widget_type=\"text-editor.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t\t\t<h3 data-start=\"1076\" data-end=\"1100\"><img decoding=\"async\" class=\"emoji\" role=\"img\" draggable=\"false\" src=\"https:\/\/s.w.org\/images\/core\/emoji\/17.0.2\/svg\/1f3af.svg\" alt=\"\ud83c\udfaf\" \/> Initial diagnosis<\/h3><p data-start=\"1102\" data-end=\"1431\">When the computer powers on and doesn&#8217;t get past the black screen, it often means that <strong data-start=\"1185\" data-end=\"1232\">the start-up routine can&#8217;t proceed<\/strong>; the constant tone indicates that the sound chip is active but the system isn&#8217;t progressing through the boot. This behaviour is also documented in other similar restorations and is often linked to problems with:<\/p><ul data-start=\"1433\" data-end=\"1638\"><li data-start=\"1433\" data-end=\"1496\"><p data-start=\"1435\" data-end=\"1496\"><strong data-start=\"1435\" data-end=\"1496\">Corroded contacts or chips not properly seated in their sockets<\/strong><\/p><\/li><li data-start=\"1497\" data-end=\"1522\"><p data-start=\"1499\" data-end=\"1522\"><strong data-start=\"1499\" data-end=\"1522\">Faulty video RAM<\/strong><\/p><\/li><li data-start=\"1523\" data-end=\"1562\"><p data-start=\"1525\" data-end=\"1562\"><strong data-start=\"1525\" data-end=\"1562\">Clock or power supply problems<\/strong><\/p><\/li><li data-start=\"1563\" data-end=\"1638\"><p data-start=\"1565\" data-end=\"1638\"><strong data-start=\"1565\" data-end=\"1600\">Faulty ROM modules or CPU<\/strong><\/p><\/li><\/ul><h3 data-start=\"1640\" data-end=\"1670\"><img decoding=\"async\" class=\"emoji\" role=\"img\" draggable=\"false\" src=\"https:\/\/s.w.org\/images\/core\/emoji\/17.0.2\/svg\/1f6e0.svg\" alt=\"\ud83d\udee0\ufe0f\" \/> Disassembly and inspection<\/h3><p data-start=\"1672\" data-end=\"1939\">The first step in the video is <strong data-start=\"1707\" data-end=\"1743\">disassembling the TI-99\/4A case<\/strong>, removing the bottom screws and separating the top panel.<br \/>Then you need to remove the keyboard ribbon cable, the power supply screws and 3 of the motherboard screws. Once that&#8217;s done, all that&#8217;s left to remove is the shield, which is held by 3 screws with their nuts and two metal clips on the edges. Then we check:<\/p><ul data-start=\"1941\" data-end=\"2154\"><li data-start=\"1941\" data-end=\"1993\"><p data-start=\"1943\" data-end=\"1993\"><strong data-start=\"1943\" data-end=\"1993\">The +5V, +12V and -5V supplies<\/strong><\/p><\/li><li data-start=\"1994\" data-end=\"2074\"><p data-start=\"1996\" data-end=\"2074\"><strong data-start=\"1996\" data-end=\"2074\">Visual check for any corrosion, oxidation or burnt components<\/strong><\/p><\/li><li data-start=\"2075\" data-end=\"2154\"><p data-start=\"2077\" data-end=\"2154\"><strong>Check that no chips are getting very hot; the sound chip should be warm but not scorching.<\/strong><\/p><\/li><\/ul><p data-start=\"2156\" data-end=\"2391\">These basic operations, though simple, can often solve many problems.<\/p><h3 data-start=\"2393\" data-end=\"2428\"><img decoding=\"async\" class=\"emoji\" role=\"img\" draggable=\"false\" src=\"https:\/\/s.w.org\/images\/core\/emoji\/17.0.2\/svg\/1f50d.svg\" alt=\"\ud83d\udd0d\" \/> Focus on the critical components<\/h3><p data-start=\"2430\" data-end=\"2503\">The <strong data-start=\"2433\" data-end=\"2445\">TI-99\/4A<\/strong> uses several integrated circuits that are crucial for booting:<\/p><ul data-start=\"2505\" data-end=\"2666\"><li data-start=\"2505\" data-end=\"2522\"><p data-start=\"2507\" data-end=\"2522\"><strong data-start=\"2507\" data-end=\"2522\">CPU TMS9900<\/strong><\/p><\/li><li data-start=\"2523\" data-end=\"2558\"><p data-start=\"2525\" data-end=\"2558\"><strong data-start=\"2525\" data-end=\"2558\">VDP (Video Display Processor)<\/strong><\/p><\/li><li data-start=\"2559\" data-end=\"2579\"><p data-start=\"2561\" data-end=\"2579\"><strong data-start=\"2561\" data-end=\"2579\">System ROM<\/strong><\/p><\/li><li data-start=\"2580\" data-end=\"2600\"><p data-start=\"2582\" data-end=\"2600\"><strong data-start=\"2582\" data-end=\"2600\">4116 video RAM<\/strong><\/p><\/li><li data-start=\"2601\" data-end=\"2666\"><p data-start=\"2603\" data-end=\"2666\"><strong>CPU RAM<\/strong><\/p><\/li><\/ul><p data-start=\"2668\" data-end=\"2938\">These are the first components to come into play during the early stages of initialisation; if the computer emits a constant tone it means it has hung before the sound chip was initialised, and the causes are often to be found among the ROM (2 chips), the clock and the CPU RAM (two chips near the ROMs).<br \/>Unfortunately there&#8217;s no silkscreen on this board, so you need to pay attention to the orientation of the chips and consult a guide to identify the ICs if needed.<\/p><p data-start=\"2668\" data-end=\"2938\">You also need to be careful when resoldering components, because there are traces running between the IC pins that have no solder mask, and if the solder is a bit generous it can easily touch the adjacent trace, causing an accidental short.<\/p><p data-start=\"2668\" data-end=\"2938\">In my case the problem was indeed the system ROM, the low one, but to find it I followed the guide and it led me somewhat astray, because a signal was missing on the VDP and it pointed me to check a 74LS138 that was actually fine.<\/p><h3 data-start=\"2940\" data-end=\"2984\"><img decoding=\"async\" class=\"emoji\" role=\"img\" draggable=\"false\" src=\"https:\/\/s.w.org\/images\/core\/emoji\/17.0.2\/svg\/1f9f0.svg\" alt=\"\ud83e\uddf0\" \/> Cleaning and servicing the keyboard<\/h3><p data-start=\"2986\" data-end=\"3111\">In my case the keyboard wasn&#8217;t the classic one with square switches, but uses plungers similar to those on the C64 that press onto a membrane. Really over the top: the slightest problem is enough to stop a key working. Someone has made a PCB that uses mechanical switches instead of this strange cascade system.<br \/>Another version of this keyboard uses square switches that close a contact inside them, and this solution seems more reliable.<\/p><p data-start=\"2986\" data-end=\"3111\">You can find the PCB for the new keyboard here:<br \/><a href=\"https:\/\/www.pcbway.com\/project\/shareproject\/KEYBOARD_4A_99_0361cd65.html\" target=\"_blank\" rel=\"noopener\">KEYBOARD_4A_PCBWAY<\/a><br \/><a href=\"https:\/\/hackaday.io\/project\/195508-keyboard4a-99\" target=\"_blank\" rel=\"noopener\">hackaday.io<\/a><\/p><h3 data-start=\"3470\" data-end=\"3520\"><img decoding=\"async\" class=\"emoji\" role=\"img\" draggable=\"false\" src=\"https:\/\/s.w.org\/images\/core\/emoji\/17.0.2\/svg\/1f680.svg\" alt=\"\ud83d\ude80\" \/> Conclusion: what to expect from a restoration<\/h3><p data-start=\"3522\" data-end=\"3646\">Repairing a <strong data-start=\"3543\" data-end=\"3563\">TI-99\/4A\u00a0<\/strong>isn&#8217;t always straightforward, but by following a methodical approach as shown in the video:<\/p><ol data-start=\"3648\" data-end=\"3814\"><li data-start=\"3648\" data-end=\"3684\"><p data-start=\"3651\" data-end=\"3684\"><strong data-start=\"3651\" data-end=\"3684\">Disassembly and visual inspection<\/strong><\/p><\/li><li data-start=\"3685\" data-end=\"3733\"><p data-start=\"3688\" data-end=\"3733\"><strong data-start=\"3688\" data-end=\"3733\">Testing power supplies and clock signals<\/strong><\/p><\/li><li data-start=\"3734\" data-end=\"3776\"><p data-start=\"3737\" data-end=\"3776\"><strong data-start=\"3737\" data-end=\"3776\">Checking memory and critical chips<\/strong><\/p><\/li><li data-start=\"3777\" data-end=\"3814\"><p data-start=\"3780\" data-end=\"3814\"><strong data-start=\"3780\" data-end=\"3814\">Progressive testing at each step<\/strong><\/p><\/li><\/ol><p data-start=\"3816\" data-end=\"4124\">you can solve many of the most common problems, including the black screen at boot. Don&#8217;t be discouraged: this computer has somewhat convoluted logic and it isn&#8217;t easy to find your way around without a silkscreen, but luckily spare parts are available and they aren&#8217;t expensive.<\/p>\t\t\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-7ec586d e-flex e-con-boxed e-con e-parent\" data-id=\"7ec586d\" data-element_type=\"container\" data-e-type=\"container\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t<div class=\"elementor-element elementor-element-5f1bbde elementor-widget elementor-widget-video\" data-id=\"5f1bbde\" data-element_type=\"widget\" data-e-type=\"widget\" data-settings=\"{&quot;youtube_url&quot;:&quot;https:\\\/\\\/www.youtube.com\\\/watch?v=kXYxb8LZNSI&amp;lc=Ugzd2-WqYT3vrdezspV4AaABAg&quot;,&quot;video_type&quot;:&quot;youtube&quot;,&quot;controls&quot;:&quot;yes&quot;}\" data-widget_type=\"video.default\">\n\t\t\t\t<div class=\"elementor-widget-container\">\n\t\t\t\t\t\t\t<div class=\"elementor-wrapper elementor-open-inline\">\n\t\t\t<div class=\"elementor-video\"><\/div>\t\t<\/div>\n\t\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t<div class=\"elementor-element elementor-element-16d0861 e-flex e-con-boxed e-con e-parent\" data-id=\"16d0861\" data-element_type=\"container\" data-e-type=\"container\">\n\t\t\t\t\t<div class=\"e-con-inner\">\n\t\t\t\t\t<\/div>\n\t\t\t\t<\/div>\n\t\t\t\t<\/div>","protected":false},"excerpt":{"rendered":"<p>Il Texas Instruments TI-99\/4A \u00e8 uno dei home computer pi\u00f9 iconici dei primi anni \u201980 (anche prima), noto per essere stato uno dei primi sistemi domestici<span class=\"excerpt-hellip\"> [\u2026]<\/span><\/p>\n","protected":false},"author":1,"featured_media":10866,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[19,158],"tags":[575,166,167,309,147,496,574],"class_list":["post-10861","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-computer","category-texas-instrument","tag-black","tag-fix","tag-riparazione","tag-rom","tag-schermo-nero","tag-tastiera","tag-ti99"],"_links":{"self":[{"href":"https:\/\/retrofixer.it\/en\/wp-json\/wp\/v2\/posts\/10861","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/retrofixer.it\/en\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/retrofixer.it\/en\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/retrofixer.it\/en\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/retrofixer.it\/en\/wp-json\/wp\/v2\/comments?post=10861"}],"version-history":[{"count":7,"href":"https:\/\/retrofixer.it\/en\/wp-json\/wp\/v2\/posts\/10861\/revisions"}],"predecessor-version":[{"id":10870,"href":"https:\/\/retrofixer.it\/en\/wp-json\/wp\/v2\/posts\/10861\/revisions\/10870"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/retrofixer.it\/en\/wp-json\/wp\/v2\/media\/10866"}],"wp:attachment":[{"href":"https:\/\/retrofixer.it\/en\/wp-json\/wp\/v2\/media?parent=10861"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/retrofixer.it\/en\/wp-json\/wp\/v2\/categories?post=10861"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/retrofixer.it\/en\/wp-json\/wp\/v2\/tags?post=10861"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}