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Thread: TRS-80 Model 4P needs "Preheating"

  1. #11
    Join Date
    Sep 2011
    Location
    Melbourne, Australia
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    1,019

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    On the 4P the light in the power switch is wired before the mains connection to the PSU, so the upshot of that is the red light on the power switch comes on even with a dead PSU or blown fuse on the PSU, making it confusing. Conversely the red light in the power switch can go out, but power still flows to the PSU (the light is in parallel not series) so the computer would otherwise work fine just with no red power light illuminated.

    When I get a 4 or 4P in my workshop which seems slower to get going that it should be I immediately try a known good PSU to make sure that I eliminate problems with the rest of the computer from the slow start. Like the others have said, check your capacitors, especially with an ESR meter if you have one on hand. It sounds to me like the "ramp up" is not working properly.
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  2. #12
    Join Date
    Mar 2013
    Location
    Chaffee, MO
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    1,445

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    But not even the fan is spinning on power up.
    If that is the case, I'd suggest starting at Section 5.4.3 Theory of Operation in the Model 4P Service
    Manual (1984) (Tandy).pdf. Check the 160 VDC being created by BR1 (No Open or Shorted Diodes)
    and if Caps C29 are good, I'd move along to the Kick Start Latch Circuit. This gets the Power Supply
    Kick Started (Soft Start) until it's running off the Pulse Generator, as described in the Control Section
    on p.150.

    Suspect's could be R26, C14, or C1. That's where you will likely find the problem, if it's not in the
    160 VDC circuit.

    Larry

  3. Default

    Quote Originally Posted by TRS-Ian View Post
    On the 4P the light in the power switch is wired before the mains connection to the PSU
    There was also a problem with the power cord ... after replacing the red light glowed again ...

    Quote Originally Posted by ldkraemer View Post
    Suspect's could be R26, C14, or C1. That's where you will likely find the problem, if it's not in the 160 VDC circuit.
    C1 was replaced (I had found one), but to solder on old stuff you need more experience than I have, so the whole power supply will be replaced.

    The 4P was switched on for the last time in 2010 to check the german keyboard driver for LS-DOS ... since then it has been properly stored ... seems you should let some things rest better ...

  4. Default

    After replacing the PSU the 4P booted: fan and floppy drives worked, Montezuma CP/M "beeped", but the CRT display remained dark: no power on the electron gun ...
    Two dry solder joins on the PSU (TB2 connector COM/V2) were the reason for this ... now the 4P works again:

    m4p.jpg

    Thanks to all who've helped!

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