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Thread: Model 1 reading 20v on Z2 pin 11. It doesn't seem happy about it.

  1. #21
    Join Date
    Sep 2011
    Melbourne, Australia


    In my experience most M1s sent to me with power problems have Z1 or Z2 or both which have failed, socket and replace them then maybe 10% of the time there is something else on the power also on the fritz.
    *There is never a charge for Tech Support even if you don't purchase from me - We are Enthusiasts Helping Other Enthusiast and that is just the way it is

  2. #22


    Quote Originally Posted by TRS-Ian View Post
    In my experience most M1s sent to me with power problems have Z1 or Z2 or both which have failed, socket and replace them then maybe 10% of the time there is something else on the power also on the fritz.
    That does jibe with what SAM's guide suggests. I don't know from where they are coming, the regulators that I ordered from Amazon are said to arrive in mid September. The minimum order was 10, btw, so if anyone needs a couple let me know.

    Meanwhile, any thoughts on avoiding this over-complicated PSU entirely by simply removing the entire power chain and injecting five volts only, after replacing the RAM with pin-modded 4164's?
    Last edited by Towmater; August 27th, 2018 at 10:44 PM.

  3. #23
    Join Date
    Mar 2013
    Chaffee, MO


    I seemed to remember helping a fellow with the Z1 or Z2 IC. It took a bit to locate but Posting #13 here is a good reference
    for troublehooting the Pin 6 Vref problem.


  4. #24


    All the voltage measurements indicate that Z1 is not working.
    pin3 is more positive than pin 2 indicating that it isn't in current shutdown.
    Also the voltage at z1 pin 5 is higher than pin 4 indicating that Z1 should be outputting current.
    pin10 is essentially at the same voltage as Q3 emitter. This can be either there is no output from Z1 pin 10 or Q3 has an emitter base short. It would be wise to check Q3 emitter to base first. If it is shorted, it could damage Z1.
    Check that Q3 doesn't have an emitter base short if so, replace both Q3 and Z1. If not shorted, just replace Z1.
    In reference to the last post, Vref was not an issue. It was measured.
    I should note that we are replacing Z1, not because it was bad for someone else but because we have done enough measurements ( actual trouble shooting ) to have a high assurance that it has failed. We are also making sure that Q3 didn't damage Z1's output as it could have done that. We are not easter egging this!
    Last edited by Dwight Elvey; August 28th, 2018 at 04:39 AM.

  5. #25


    Common part numbers for 723 are:

    be careful to check the package type with the manufactures data sheet. These also come in a can with wire leads, as well as a dip. There were also variants of reference voltage but these are rare. If so, they'd look like LM723-5.0 for one with a 5V reference.

  6. #26


    Looks like Q3 is not shorting.

    For the replacement, i purchased this@amazon)/gp/product/B01IL91EB0/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

  7. #27


    New 723 regulators came in today, but the issue is still the same as before, no +5V on pin 10 of Z1. I guess my first question should be "are the through holes plated?" There's a slim chance the solder didn't go through to the component side if they are not plated vias? I am at a loss here. Perhaps I should "inject" 5V after the regulator?

  8. #28


    Scratch that "plated through hole" question, I realized that this could be easily tested with Z1 out of the socket. I gotta stop thinking out loud. As long as Z1 is out of its socket, are there any revealing tests that I can do with the socket empty?

  9. #29
    Join Date
    Mar 2013
    Chaffee, MO


    By replacing Z1, that tells me there is something on the Motherboard around Z1 that you have overlooked.

    Check for 10.6 VDC across 10,000 MFD Cap C9. This voltage is generated by Bridge Rectifier CR8.
    You could have a shorted or open Diode in CR8.

    Remove Z1 from Socket. Turn R5 Full CCW. Measure from Pin 5 to 6. You should have
    1200 OHMS. Turn Pot FULL CW while watching your meter. It should go to 2200 OHMS
    with no drop outs.

    If that works check again from Pin 5 of Z1 to GND at FULL CCW (=3300 OHMS) and
    FULL CW (=4300 OHMS) This will have checked all three components in the pot circuit.

    Check pins 2 thru 7, 10, 11, and 13 of IC Z1, with Power ON.
    Pin 6 (Vref) should be 7.15 VDC.

    If the 5.0V is incorrect (HIGH or LOW), check the adjustment of the 5V Adjust Control (R5)
    to see if the Wiper changes voltage at Pin 6 or Box 22.


  10. #30


    C9: measures 17 volts
    R5 turned measuring Z1 pin 5 to pin 6: 1150Ω to 2200Ω
    R5 turned measuring Z1 pin 5 to gnd: 3350Ω t0 4250Ω

    Z1 Socket (with / without reg in place)
    Pin 2: 1.1V / 150mV
    Pin 3: 0.5V / 150mV
    Pin 4: 0.5V / 150mV
    Pin 5: 4.1V / 0V
    Pin 6: 7.3V / 0V
    Pin 7: 0.3,V / 0V
    Pin 10: 0.5V / 210mV
    Pin 11: 12.15V / 12.1V
    Pin 13: 0.6V / 20mV

    Z1 Pin 2 is .66V more positive when measured against pin 3. Earlier posts stated that this indicates shut-down conditions? But this is a new Z1, so maybe it hasn't broken yet?

    C9 at 17V seems anomalous vs. the 10.6V suggested?

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