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Thread: Powertran Cortex

  1. #141

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    Guys,

    I checked the 5 volt supply across the pins on an IC and got 4.4v and the voltage at the fuse before the 7805 is 10.2v.
    I can probably get hold of an oscilloscope to see if there is any ripple and I think I've got a spare 7805 but what's got me a little confused is I appear to have a different set of component lists / schematic diagrams to those listed on WHTech. I can only assume that they were issued to me in the kit from Powertran. My sheets have additional hand written notes on them and there is a note printed on the power supply overlay saying :
    * = vertically mounted 4R7 resistor (Drawn on overlay adjacent to BASE lead of Q1) and + = vertically mounted 100R resistor (Drawn on overlay between the COLLECTOR lead of Q1 and the output lead of the 7805 IC1) plus the comment "Fit only if the voltage is low (less than 4.8v on the TMS9928 )".
    Looking at my power supply I must have experienced low voltage issues when I first built it because I have the 4R7 resistor in place but I then appear to have by-passed it at a later date probably because it didn't work and I couldn't be bothered to take the PSU out again to remove it.
    It's all very confusing because they have not modified the circuit diagram so there is no way of telling exactly where these two resistors were intended to be connected. (I seem to have tried to fit the 4R7 in series with the 0 volt line of the 7805)
    I realise that this description is probably very confusing, but using your expertise, where do you think a 4R7 resistor and a 100R resistor should be fitted to increase the ouput voltage - I am assuming that as they state the resistors are vertically mounted they are intended to be in series with two of the Q1 / IC1 leads but I don't know which ones.
    The PSU component overlay also states that it is "ISSUE B" PCB.
    Last edited by andyp; January 27th, 2010 at 12:10 PM. Reason: spelling

  2. #142

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    On your schematic differences, there were apparently at least three different revisions of the board, but I have never seen a set of schematics reflecting all of the differences between versions Andy. If yours are different than the ones we have, it might be a good idea to scan them in too as a help to any others who may have that board revision. I'll have a look at mine to see if the resistors are in place as you describe.

  3. #143

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    Interesting & confusing. I understand the circuit in the schematic. So I hope you won't mind me talking through it in order to get a better idea of what's going on.
    the 7805 is using it's output to monitor the 5v rail, if it's a bit low, it supplies a little current. it gets this current from the base of Q1, thereby turning it on a bit, increasing the current flowing through Q1 to the collector, and out onto the 5V rail.

    I'd be separating the psu by now! It's all too easy to put a high voltage onto the main board. Load is crucial to the operation of the circuit so use a dummy load like a car headlight/brake light bulb.

    I'd go back to the original circuit, Isolate the 7805 reg from the transistor output & test the regulator gives 5V. It's possible that the input voltage is not high enough to operate it properly. If it doesn't give 5V perhaps you should replace it with a "low dropout voltage" version.

    (More thoughts in a minute when I've threaded this earth wire & picked my daughter up!)
    "Don't it always seem to go
    That you don't know what you’ve got ‘til it’s gone" (BANG )

  4. #144

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    AAAhhh if you've bypassed the 4.7ohm resistor between +ve & the base of q1, you've turned Q1 off! Which may mean that the computer is running off just (a very stressed) 7805.

    (now covered in mouse poop, & going to pick other daughter up)
    "Don't it always seem to go
    That you don't know what you’ve got ‘til it’s gone" (BANG )

  5. #145

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    Ksarul,

    I've since noticed that the pages I have state that they are:
    "Reprint of article published in Electronics Today International ( With corrections and amendments necessary to be consistent with kit )
    When I get to grips with my 5 volt issue, I'll scan and send them to you.

    Nige,

    Followed your advice and isolated the PSU from the main board and used a car light bulb as a load ( Draws about 1.3 amps at 5 volts )
    I've now removed the whole back panel / PSU / transformer, to make it easier to work on.
    Had some more strange voltage measurements - When I first checked the 5 volt on the output connector I had 4.7v but if I checked it at the floppy disk supply connector I had 5.02v ?? - Even though the 2 supplys come from the same point on the circuit ?? And with power off, when I check for continuity between the floppy 5v and the main board 5v plus 0v to 0v on the different connectors all is OK !
    I then removed the PSU board and decided to tidy it up and remove the by-passed resistor and investigate where these add on resistors were designed to go.
    Judging by the pre-drilled holes in the PCB, the resistors referred to in my amended notes are designed to go as follows:
    100R between the +5 volt output of the 7805 and the CMN terminal of the 7805.
    4R7 between the CMN terminal of the 7805 and the 0 volt rail. ( This was the one I listed as by-passed in my earlier post and it looks like I had also cut the 2nd 100R resistor off the PCB)
    I assume that these 2 additional resistors are intended to bias the 7805 CMN terminal to approx + 0.3 volts to increase the output slightly if required.
    With the 4R7 resistor removed, the CMN lead on the original 7805 was now too short to re-solder to the board, so I have replaced it with one from Maplin (QL31J) - Not sure if this is a "low dropout voltage" version. Everything is now as per the original circuit diagram ( i.e. All additional resistors removed) and the fault is still there.
    What I find most puzzling is the voltage reduces bit by bit as I get closer to the main board ICs.
    These voltages are approximate (from memory) but there is 5.02v at the PSU floppy connector (not used), there is a bit less at the PSU main board connector 4.8v, a bit less where the supply cable is connected to the main board 4.6v and even less across the main board ICs 4.4v.
    I can't see any evidence of dry joints and as the PSU floppy connector seems to have a rock solid output of 5.02v, I was thinking of trying to feed the main board from that point.
    PS. I hope you don't mind me posting all this on the forum.

  6. #146

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    Posting the data here is a very good thing, Andy--especially if it helps someone else avoid a similar problem once the final details of the isolation process are completely documented here. I'll have to go into mine soon to add the floppy controller and the memory mapper sections of my board, so all the data you've posted here may come in very useful, as it will tell me both normal and abnormal behavior for the motherboard. If your schematics say they are the corrected ones, it would be very good to put them up on WHTech.

  7. #147

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    A quick update on my dead / dying Cortex and a plea for advice on what to check next.
    As per my earlier post, after tidying up the PSU board I could only achieve 4.4 volts on the main board.
    With the bias resistors re-fitted, as per the construction guide and a new 7805, I achieved 5.25v at the PSU output and 4.9 Volts on the main board.
    All the ICs were then removed, cleaned with a fibreglass brush and re-seated
    I then fitted my new BASIC eproms and applied power. The Cortex gave a continual alarm tone, the "RUN" light flickered and the "MEM" light was lit.
    I then fitted my known good FORTH eproms and the fault was still there.
    Assuming the PSU could still be suspect, I took a known good PSU from a working desktop PC, checked the output and cabled it to the Cortex.
    From memory I was then seeing +12v, -11.65v and +5.25v at the PSU output and +4.9v across the ICs on the main board.
    With power supplied from the new PSU the "RUN" and "MEM" lights were lit as before but there wasn't the same continuous tone from the speaker. However I did hear a faint "crackling" sound from the speaker after pressing the reset button, this sound would disappear when the LEDs had settled down.
    Being a total novice when it comes to this sort of thing, can anyone suggest what I should be checking next.
    I am working on the following assumptions
    1) If I continue using the desktop PSU we can eliminate any power issues.
    2) At this stage I don't need the video output.
    3) At this stage I don't need the cassette interface.
    4) At this stage I don't need the E-bus.
    5) At this stage I don't need the keyboard.

    Could any of the above interfaces be dragging the system down? I can unplug the keyboard but should I be removing any ICs to the isolate the others or is this a waste of time.
    I have access to a DVM, a logic probe and possibly a scope - Can anyone suggest a way foward?
    Last edited by andyp; February 2nd, 2010 at 12:37 PM. Reason: spelling

  8. #148
    Join Date
    Jul 2009
    Location
    Southampton, UK
    Posts
    169

    Default Dead Cortex

    Andy,

    Could someone perhaps supply you with a single EPROM containing a small program that does nothing other than cycle one of the LEDs on and off? This would confirm that a bare minimum of circuitry was working. Whether this works or not, it would give you a known base to start testing from.

    I'd also be inclined to replace IC44 and IC97 - a pair of 74LS245s that you can get off Ebay for a few quid. The '245s are quite prone to blowing, as far as I understand.

    Stuart.

  9. #149

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    Stuart, - Thanks for the info, I'll follow your advice and get some spare 74LS245s.

    I notice from the ETI description that one of the 74LS245s is a data buffer connected to the DRAM chips and the other is used by the EBUS - Could I not just swap these around or try them one at a time in position #44 to see if it makes a difference. Or do they both have to be fitted at the same time?

    Does anyone know if ALL the DRAM chips have to be fitted for correct operation and if one of these memory chips was to be faulty could this be preventing my initial start-up sequence
    It occurred to me that if the contents of the eproms are transferred into the DRAM at start up, then in theory could I get away with fitting 3 x TMS4164 at a time.
    I could then try different combinations of my 8 DRAM chips in batches of 3 to eliminate them as faulty. Would this be a usefull exercise?

  10. #150
    Join Date
    Jul 2009
    Location
    Southampton, UK
    Posts
    169

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    The 74LS245 at position IC44 is the critical one as it buffers the DRAMs. You could try swapping them over. If IC97 is not fitted, that shouldn't be a problem as far as I can see.

    Each DRAM is only 1 bit wide - so you always need 8 of them to form an 8-bit byte. If any one of them is bad, it is possible that could be causing the problem (or of course it could be any of the other chips on the board).

    The processor itself has 256 bytes of RAM, so it should be possible to develop a routine to test the DRAM, with the routine in EPROM and using the processor RAM for workspace, and giving feedback through one of the LEDs. If no one else chips in with an offer of help then I'll see if I can get a TMS2564 and cobble it to work on my programming board, which supports only up to a 2532. What would be useful is if a UK owner was able to test any such routine on a known-working Cortex just to make sure it works first.

    I'm up in Southampton. You got a new Maplins in Barnstable last year was it?

    Stuart.

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