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Thread: Where to start?

  1. #21
    Join Date
    Feb 2015
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    Cleveland, OH, USA
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    Quote Originally Posted by clh333 View Post
    Well, we released the magic smoke this morning. I'm glad you warned me of the possibility as I was wearing clean underwear.

    Been a long time since I heard a capacitor let go. Once upon a time I was a commercial photographer and we all used "strobes",
    i.e. electronic flash, for illumination. Those had giant capacitors charged to thousands of volts. When one of those failed it was
    newsworthy.

    Haven't popped the top yet but I assume it was the filter caps. Depending on their values I may have to order from DigiKey.

    But first to see what has happened. I'll address the drive service and the clock timing issues while I'm in there.

    Thanks,

    -CH-
    Update: Removed Astec PS; note blown cap in foreground near T1. How many and which type are recommended for replacement? Pics attached.

    -CH-

    PS1.JPG PS2.JPG PS3.JPG PS4.JPG PS5.JPG

  2. #22
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    Mar 2013
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    Chaffee, MO
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    CH,
    Somewhere on the Forum there is a listing of parts. I'll look to see if I have copied the information.

    Also, for that type power Supply, be sure to use a magnifying glass to check the solder joint of all
    those pins. They may have a ring around them where they have a broken solder joint.

    Larry

  3. #23
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  4. #24
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    Larry, I'm a little puzzled:

    The PS is an Astec AA12090. It was C2 that blew, and there are two others shaped like it on the board, at C1 and C13. The PS.PDF you were kind enough to furnish shows on P.116 a board component layout that is attributed to Power Supply Assembly #8790043 (Astec AA12090). In that diagram are C32 and C33.

    The problem is that this layout is not the same as my board, which has capacitors C1 through C25. On page 118 of the PS.PDF there is a Parts List, Power Supply #8790043, (Astec AA12090) with only C1-C25 listed, and C2 is listed as a 0.1 ufd +/- 20% 250VAC, MPR (which I think stands for metallized paper + resin). C1 and C13 are the same values except for their capacitance which is 0.01 ufd. The documentation appears (to me) to conflict itself.

    In one of the bug-boxes in my collection of useless electronic components I happened to find an exact replacement for C2, but since it was acquired about the same time as the faulty cap was manufactured and since I had no 0.01 ufd values on hand I ordered replacements from DigiKey. I wasn't sure whether to get metal-paper or metal-poly so I got both.

    The remaining question is whether to replace any of the ceramics or electrolytics as a matter of course. I will inspect the solder joints for cracks.

    In the meantime I took the drives apart for cleaning and lubrication. Nothing remarkable there except that the A / 0 drive was a little tighter than usual moving the carriage on the rails, even after lubrication.

    I also examined the motherboard. I could not find a revision number on the top side, but this is an early one, Rev A at the latest, according to the docs. There are two crystals, Y1 and Y2, and the PAL at U3 is labeled 8075166. There are no pins bent outward on the PAL, so this setup is unmodified. There is some ad-hoc wiring which apparently was common on the early boards as well. All of this points to the clock-timing issues (3.x MHz vs 4 MHz operation) noted in the documentation.

    There is one bank of RAM occupied with 8 Tandy 8040665 and one bank empty. The 8040665 is equivalent to 4164s, my research tells me. I have enough 4164-15 RAM on hand to fill all 16 positions; do you think that would be a good idea?

    There is indeed a shunt at U72, and J10 looks as if it were once occupied but someone pried something off hastily. What the significance of all of this may be is still under investigation.

    Thanks again for your assistance. Pics attached.

    -CH-

    Mobo1.JPG Mobo2.JPG Mobo3.JPG Mobo4.JPG

  5. #25
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    Chaffee, MO
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    CH,
    If you add the additional Memory you must have the PAL that goes in U72 to support it. I have my M4
    upgraded and use the extra RAM as drive M. It works good. I sure there was information on setting
    the jumpers for the extra banks and several of the bypass caps that were to be removed. Once again
    I'll have to search my Hard Drive to see if I brought a copy of that information with me. U72 is going to be
    hard to source. I've got some code (from Internet) that is supposed to work, but I've not tried it.
    Ian (TRS80) is supposed to have those available.

    Typically ONLY the RIFA's need replaced, unless you see signs of leaking or rounded tops.

    When I replaced my Rifa's I'm sure I bought them from Mouser Electronics, but I can find any saved information
    on my Drive. Maybe I'm not remembering correctly.

    Larry
    Last edited by ldkraemer; Today at 06:19 AM.

  6. #26
    Join Date
    Jun 2010
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    Vancouver, BC, Canada
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    Here's a good reference for RIFA cap replacement:

    http://www.akhara.com/trs-80/psrepair/index.html

  7. #27
    Join Date
    Feb 2015
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    Cleveland, OH, USA
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    Drive questions:

    The cable has no "twist", and neither drive has a jumper for drive select. Is this accomplished by the cable itself therefore? I think I remember reading this ...

    Neither drive has a terminating resistor although each drive has space for one. Shouldn't there be one, in the 150-300 Ohm range, on the farthest drive?

    These drives are SSDD, about 150K formatted. Later model 4s (D?) had DSDD drives. Can these be swapped one-for one, or will a different FDC be required, or not possible?

    Display question:

    Apparently J10 is a port for a graphics card adapter. There is no such card with the machine, but I see there is aftermarket availability. Will attachment of a graphics card limit or prohibit other video abilities of the machine?
    In other words, is it "A OR B" video mode or "A PLUS B" video mode or "ONLY B" video mode after installing the graphics card?

    Thanks again for your responses,

    -CH-

  8. #28
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    Mar 2013
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    Chaffee, MO
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    CH,
    No there should not be Terminator Resistors (Pullup Resistors) on Internal Drives. Yes terminator
    for the last External Floppy Drive on the end of cable. REF: The FDC Schematic

    Absolutely NO twisted Conductors in the Cable.

    Yes, you can install DS/DD Floppy Drives with existing FDC.

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