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Thread: TRS 80 Model 1 - 5 volts problem

  1. #1

    Default TRS 80 Model 1 - 5 volts problem

    Hi All,

    I been trying to diagnose a power problem with a spare TRS 80 model 1 16K level 2 (UK model) and wondered if anyone could offer some advice.
    12 volts looks fine but I'm not getting 5v at pin 3 on regulator Z1 (as per Sams Computerfacts manual). I've tried replacing the 723 IC at Z1 with a known good one, this didn't make any difference. The power supply is good which I've tested with a working Model 1.
    Is there anyone that could offer some advice on how to diagnose what part(s) may at fault?

    Any help would be appreciated,

  2. #2
    Join Date
    Mar 2013
    Chaffee, MO


    Maybe this will help.

    Start by verifying the 11.9 VDC, then 5.0 VDC, then the -5 VDC, because the +5 VDC Circuit
    requires 11.9 VDC.

    If the 11.9V is missing, check the voltages and components associated with Regulator Output
    Transistor (Q6) and pins 2 thru 7, 10, 11, and 13 of IC Z2. Pin 6 (Vref) should be 7.15 VDC.
    If the 11.9V is incorrect, check the adjustment of the 12V Adjust Control (R10).

    If 5.0V is missing, check for 10.6 VDC across 10,000 MFD Cap C9. This voltage is generated
    by Bridge Rectfier CR8. Also check the voltages and components associated with Regulator
    Output Transistor (Q4) (Tandy schematic shows it incorrectly as Q2), Regulator Drive
    Transistor (Q3), Error Amp Transistor (Q5) and pins 2 thru 7, 10, 11, and 13 of IC Z1.
    Pin 6 (Vref) should be 7.15 VDC. If the 5.0V is incorrect, check the adjustment of the 5V
    Adjust Control (R5).

    If the -5.0V is missing, check Resistor (R19), Zener Diode CR2, and check for possible short
    to ground.

    System Power Supply

    The TRS-80 needs three voltage levels: +12 volts at about 350 milliamps; +5 volts
    at about 1.2 amps: and -5 volts at 1 milliamp. The +12 and -5 volts are needed by
    system RAM and everything needs +5 volts. The +12 volt and +5 volt supplies are
    regulated and current-protected against shorts. The -5 volts supply is not as
    critical as the other two supplies, and it uses a single zener diode for
    regulation. Stepped-down AC voltage is supplied to all regulator circuits from a
    UL approved "AC adapter."

    AC Adapter

    The AC Adapter is a large version of the type used for calculators and TV game
    products. Inside the plastic case is a single transformer with one primary and two
    secondary windings. The primary circuit is designed for 120 VAC and has an operating
    range of 105 to 135 VAC.

    NOTE. There is a wire fuse in the primary side to meet UL specifications.

    The secondary windings are both center-tapped. One secondary is rated at 14 volts
    AC at 1 amp. This winding is used for the +5 and -5 volt supplies. The other secondary
    winding has diodes connected and it outputs 19.8 VDC at about 350 milliamps. This
    circuit is used for the 12 volt supply. All voltage outputs and centertaps are brought
    into the POWER input (J1).

    + 12 V Power Supply

    Unregulated DC voltage for the +12V supply is inputted at pin 2 of J1 . When power
    switch S1 is closed, C8 filters the voltage and the net result is approximately
    20 volts, which is applied to Q6 and regulator Z2. Figure 13 shows a simplified
    diagram of the internal circuitry in a 723 regulator chip. Figure 13 will help in
    the regulator operation discussion.

    The filtered DC voltage from the Adapter and C8 is applied to pin 12 of Z2 and
    the emitter of series pass transistor Q6. The voltage applied to pin 12 allows a
    constant current source to supply zener current for Za. Pin 6 of Z2 will output a
    zener voltage of about 7.15 volts. Pin 6 is tied to pin 5, the positive input to
    operational amplifier Zb. The negative input to the op-amp is tied to the wiper
    of R10. Initially, pin 4 of Z2 is at ground, forcing the output of op-amp Zb to
    output about 7.15 volts. Transistor Qa turns on, which turns on pass transistor Q6.
    The pass transistor supplies voltage for current monitoring resistor R18 and to the
    resistor network R13, R10 and R12. If R10 is adjusted for 7.15 volts at its wiper,
    the op-amp will be balanced and Q6 will output only enough voltage to keep the loop
    stable. If output voltage dropped below 12 volts, Zb's output would decrease which
    would force the current through Qa to decrease. Qa would cause Q6 to increase the
    current through it, and the output would rise back up to the 12 volt level. If the
    12 volt 1ine increased in voltage, the op-amp would cause Qa's current to increase,
    forcing Q6 to drop down.

    The transistor labeled Qb in Figure 13, is used to protect power transistor Q6
    against over-current damage. If R18 drops sufficient voltage to cause the resistor
    node at Z2 pin 2, to reach 12.6 volts, Qb will take command of Qa. As Qb is turned
    on, Qa turns off which starts turning Q6 off. The voltage at Z2, pin 10, must
    approach 14.7 volts before Qb takes charge of Qa, 14.7 volts at pin 10 means that
    the 12 volt supply is approaching its maximum design current of 480 milliamps.
    If a short develops across the 12V supply, Qb will activate, forcing Qa to shut
    down. With Qa off, Q6's base rises to the input voltage level because of R16. Q6
    snaps off the supply, preventing it from attempting thermal suicide. Once the short
    is removed, Qb will turn off and the system will operate normally.

    Capacitor C13, connected between pins 13 and 4 is a frequency compensation capacitor.
    It prevents the op-amp loop from going into oscillation. C11 and C15 are the supply's
    output filter and noise suppressors. Capacitors C28, C30, C32 and C34 are distributed
    along the 12 volt supply bus for transient suppression.

    +5 Volt Supply

    The 5 volt power supply also uses a 723 regulator. Due to the current and voltage
    requirements, more components were stuck around the regulator for support. But, the
    basic circuit operates the same. Figure 13 will also be used in this circuit.

    For the 5 volt supply, the AC adapter supplies about 17 volts AC at J1, pins 1
    and 3. Full-wave rectifier CR8, rectifies the AC. When S1 is closed, about 7VDC
    is passed through the switch contacts and is filtered by C9.

    The power supply for Z1 and the current source for zener Za is taken from the
    regulated side of R18 in the 12 volt section. Pin 7 is grounded as in Z2, but the
    zener output is handled differently. The 7.15 volt zener voltage is applied to the
    resistor network consisting of R6, R5, and R11. When R5 has been adjusted for a
    5 volt output on the supply bus, pin 5 of Z1 will be at about 5 volts. The negative
    input of the op-amp, Zb, is sourced through a 1.2K resistor, R7, and tied to the
    5 volt bus. The op-amp controls Qa, which controls bias drive for Q3. Q3 is used
    to handle the greater base drive necessary for pass transistor Q4. Q4's collector
    is tied to current sensing resistor R4. R4 monitors the current the 5 volt bus is
    producing just as R18 did for the 12 volt bus.

    Circuit operation is exactly the same for Z1 as it was for Z2. If op-amp Zb detects
    a rising or falling voltage condition at the output bus, it will adjust base current
    to Qa. Since Qa cannot handle the drive requirements for Q4 directly, Q3 is needed
    for current gain. During current limiting condition, Q5 monitors the voltage across
    R4, which is a direct function of bus current. As Q5 begins to turn on, the node at
    R3 and R9 supplies more voltage for base drive of Qb. As Qb takes command of the
    regulator loop, Qa is commanded to start cutting Q3 off. Q3 begins to turn Q4 off
    and the circuit goes into current limiting. The current limiting action of Q5 starts
    to come into play when the voltage across R4 approaches 0.6 volt. Ohm's Law tells
    us the bus current at this voltage level is approaching 1.82 amps.

    C12, connected between pins 13 and 14 of Z1, performs the same compensation function
    as C13. C10 and C14 are the output filter and noise suppressors while the thirty-two
    0.01 microfarad capacitors are distributed all over the Board to suppress transient

    Notice zener diode CR1 on the 5 volt bus. This diode is used as crowbar circuit
    protection in case of catastrophic failure in the RAMs. If something happens in the
    system RAM circuit that causes a short between the 12 and 5 volt buses, CR1 would
    turn on, causing the 5 volt bus to go into current limiting. Since CR1 is a 6.2 volt
    zener, it would protect the TTL devices connected to the 5 volt bus from being damaged
    by a sudden 12 volt supply voltage. Normally, CR1 would be off with no current flowing
    through it.

    NOTE: The 12 volt supply must be working properly before the 5 volt supply will operate
    correctly. Therefore the 12 volt supply must be adjusted before the 5 volt supply.

    -5 Volt Supply

    Source voltage for the -5 volt supply comes from the negative terminal of rectifier CR8.
    When switch S1 is closed, the negative DC is filtered by C1 and about -11 volts is
    applied to resistor R19. R19 is used to limit current for zener regulator CR2, a 5.1
    volt device. The -5 volt circuit is about as simple a power supply as can be designed.
    C4 and C3 is the -5 volt supply filtering and noise suppressing caps, while C16
    through C19 perform the transient suppression function.


  3. #3


    Thanks Larry, much appreciated.
    That should keep me busy at the weekend!


  4. #4


    Hi Larry,

    Thank you for your reply, much appreciated.
    I'll check these out.


  5. #5


    Just got around to having another look at the 5V problem on my model 1.
    Started with cap C9, measured 15VDC. Bridge rectifier CR8 was outputting +10VDC/-10VDC when main switch (J1) was off and +15VDC/-15VDC when on.
    Replaced CR8 for a new one and same result. Could this cause problems?

  6. #6


    Found that CR1, zener diode 1N4735 was dead causing a short. 5VDC back again!
    Now to sort out video corruption problem...


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