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Thread: Tarbell Floppy controller modification

  1. #1
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    Default Tarbell Floppy controller modification

    What is the purpose of this giant ceramic resistor on the back of my Tarbell floppy controller? Obviously it's not stock.

    IMG_4160.jpg IMG_4161.jpg

  2. #2
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    It's a shunt around the series regulator (+5) to improve the current handling capacity at the expense of regulation accuracy. If you don't like it, you could replace the TO-3 regulator and resistor with a switching regulator, which would run much cooler. You saw this sort of thing on early MITS boards with 7805 regulators. You can see from the discoloration on the PCB that the regulator is running very hot.

  3. #3

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    Under estimating the thermal load seems to be quite common on these early S100 boards. The +8V line is often around +10V adding significant thermal load on the regulator. The heat sink on this board is even small for a TO-220 regulator. For a TO-3 running anywhere near its rated current it is a miss. Most of these engineers know Ohm's law. It is not rocket science to get the thermal resistance of these heatsinks.
    The temperature that this board was running at was close to melting solder.
    Dwight

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    Quote Originally Posted by Chuck(G) View Post
    replace the TO-3 regulator and resistor with a switching regulator
    I only see one source for those, and they are about 8X the price of the <1$ Chinese things not in a TO-3 footprint.
    https://www.ezsbc.com/index.php/revi...ct/list/id/19/

    Is there anyone else making these?

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    Is there any particular reason to keep the TO-3 appearance? LM2596 "simple switcher" based regulator modules are pretty cheap and small. 3A output.

    Figure it this way, if your supply is +10 and you're drawing 2A at 5V, that regulator will dissipate 10W, which is a lot even for a PCB-mounted TO3 with a too-small heatsink. A buck regulator won't break a sweat at that power.
    Last edited by Chuck(G); August 23rd, 2019 at 05:10 PM.

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    Quote Originally Posted by Chuck(G) View Post
    Is there any particular reason to keep the TO-3 appearance? LM2596 "simple switcher" based regulator modules are pretty cheap and small. 3A output.
    Other than having the pinouts to the board the same, no.
    I ended up just buying some cheap modules and will make a board with the TO-3 pinouts and solder them onto it.

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    Well, if there were a demand, it wouldn't be difficult to make a TO-3 shaped PCB. Maybe glitch wants to do some--not my thing.

  8. #8

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    Quote Originally Posted by Dwight Elvey View Post
    Under estimating the thermal load seems to be quite common on these early S100 boards. The +8V line is often around +10V
    Dwight
    It is because of this I run my SOL-20 from a Variac and set it so it is around 8V, if it gets too close to 7.5V it reaches the 2.5V in/out difference required by the standard 7805 and ripple appears in the supply output. Over 8V to 8.5V its not good as the heat dissipation rises significantly. I installed a small volt meter in there, but I found that at the point that the input voltage is too low, close to 7.5V, ripple appears in the video out that is easy to see on the VDU, so I just increase the line voltage until it just goes away and a little more and I don't have to have the top off the computer to see the voltmeter.

    Some of the early S-100 memory cards were very power hungry too with multiple regulators to share the load.
    Last edited by Hugo Holden; August 23rd, 2019 at 08:03 PM.

  9. #9

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    One thing to realize is that a regulator in a TO-3 can will get to the same temperature ( all other things equal ) if it is rated at 1.5 amps or 3 amps, with less than 1.5 amps. So, putting an over rated regulator is just costing more money that could be invested in a better heat sink. In fact for a S100 card, you are more likely better off putting two regulators on the board with the 1.5 amp rating than one 3 amp one. Another point is that it is better to put them on different ends of the board because the slight drop across the board will help them share the current load better( some RAM board designers never got this ).
    Also, check the heat sink's designer recommendations. Changes in orientation can make a difference. The one used on this board was likely the worst from what I recall but it is only a small difference.
    Dwight

  10. #10
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    You have to wonder what's drawing the current. I have the original Tarbell 1771 floppy board and it uses regular 78xx TO-220 regulators that don't get overly hot. I don't remember that 8257 DMAC getting really hot on my other boards.

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