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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment. |
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2nd Mar 2021, 12:13 pm | #41 |
Pentode
Join Date: Nov 2015
Location: Maidstone, Kent, UK.
Posts: 131
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Re: 2708 EPROM Power Sequencing
OK, only one other observation again regarding the relay, the mounting orientation is important as reversal would mean the inbuilt catching diode would short the -5v rail. Typical problem with symmetrical pinouts. the spec says pin numbers are not printed on relay case, the only indication being a molded "pip" - very easy to miss!
Orakle42 |
3rd Mar 2021, 10:25 pm | #42 | |
Octode
Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
Posts: 1,440
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Re: 2708 EPROM Power Sequencing
Quote:
Yes, the first time I encountered some Python code I was rather disappointed to see it relied on indenting, having been used to Pascal & C style languages with begin / end (or { / }) indicators on a code block, with indenting only a style issue - Which caused enough debate over position of the { } brackets and indentation of these indentation / the code block. And for a single line of code of code, after a conditional test etc, you didn't need the { } as these were only necessary to group together more than one statement etc. but many still preferred to use these for clarity (but sometime putting all on the same line. Yes, I had previously suggested putting some moderate-power zener-clamps across all the rails, to guard against surges, lifting from parasitic-powering effects and also some reverse-protection. And may help guard against mis-connection of the PSU / faults in it. |
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3rd Mar 2021, 11:29 pm | #43 | |
Octode
Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
Posts: 1,440
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Re: 2708 EPROM Power Sequencing
Quote:
I've just seen this and had a quickly look over this / done a mini design review, having a few observations: LDE10-20B05 PSU AC 5V PSU This has quite a low max 264Vac operating range compared to even a cheaper mini open frame 3W one in RS. And they do show an MOV across the input on the datasheet circuits, so it may be a good idea to include this. They also recommend a 2A fuse, rather than 3A one you have - which I think should actually be the more 'standard' 3.15A (that they do have when using an extra EMC input filter. And it looks like you may also need input filtering for EMC compliance). They also specify a low-ESR / High freq. Electrolytic on the o/p, whereas you haven't mentioned a special type required. And they also have a TVS diode across the o/p, to guard against converter failure - so it looks like they have got too much confidence in it not producing an excess o/p voltage, and including this may be advisable. Plus they have 1uF rather than 100nF high-freq. ceramic etc. non-polarised capacitor across the output. I know it doesn't really apply that much to a DIY project, that not supplied commercially, but I think I'd be tempted to just run the whole board from an external 5V 2A etc. 'wall-wart' PSU, that should have all the EMC / EFT surge & safety compliance certification approval - as I've lots spare, and they are very cheap (But obviously avoiding too cheap fake Apple USB chargers etc.) Should C1 main reservoir value be higher, to cope with surges - or is it that as this PSU has a high-switching =frequency relative to 50/100Hz, then a much lower one is OK. I think R3 should actually be 470R, not 4k7, to give 25mA min load. (I presume the other load resistor values take account of 7824 & relay coil currents, to make-up the total to the min. load required) I'd probably be tempted to include LED's in series with the load resistors, to give a visual-indication of that rail being on - Although 25mA is a bit high to put through many LED's so might want to still have a resistor shunt in parallel to keep LED's currents a bit more equal / not too bright (Even the designed for 45mA panel LED's are still quite bright at 2mA and just as bright as many '2mA' ones are). I'd probably also put some status LED's on all other rails, to assist in showing it's all working OK (although won't be too visible if it's all inside an enclosure without remote mounting in case wall etc). But I know you were also quite a fan of having status 'lights'.... It's also unclear from their datasheets if the +/- output types need the min. load from each output to ground, or whether this is between the + and - outputs, and connection to 0V centre is not required. I presume there aren't capacitors on the Softy, that combined with the 47uF's on the output of the DC-DC's, won't take these over their max. output capacitance spec. I'm not sure if the 47uF's are really required on the outputs of the DC-DC converters, as there doesn't seem a min. o/p capacitance spec. for these. But I presume C8 is much higher than typical 7824 required output capacitor to cope with surge Vpp currents. And these regulators aren't too fussy about a bit extra capacitance causing instability. Also, I've never seen a 47V rated ones - should they be 35V / 50V ? The DC-DC converters also show input-side conducted-EMC filtering / surge-protection, but I presume this should be required in this case, if fed from a stable supply, and tracks to it will be fairly short and unlikely to radiate. It may also be better to thicken-up the power tracking on the PCB, as quite a bit of space around these to do some copper-fill to lower resistance / maybe add some thermal cooling if they connect to pins of regulators / converters. |
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13th May 2021, 10:28 am | #44 |
Pentode
Join Date: Jan 2021
Location: Northampton, Northamptonshire, UK
Posts: 131
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Re: 2708 EPROM Power Sequencing
I powered up my SOFTY 1 a couple of days ago to program some 2708s and it had died, after a bit of probing and finding nothing obvious I swapped out the 2708 (OS) and then it worked ? With the suspect 2708 in the SOFTY programming socket it killed the SOFTY so looks like its fatal ?
I'm still powering it from 4 bench supplies and have been careful to switch the -5V first and last ? Anyway thats prompted me to finish the power supply design, its now at JLCPCB. Chris |
13th May 2021, 3:22 pm | #45 | |
Octode
Join Date: May 2018
Location: Northampton, Northamptonshire, UK.
Posts: 1,440
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Re: 2708 EPROM Power Sequencing
Quote:
In which case, I wonder if you can measure any shorts on the now-died 2708? It could be overloading PSU rails (Although might spot that if you have current displays / limit set on your bench-PSU's) Or maybe it's not responding to the chip-selects to deselect it, so is causing a clash on the common data bus etc. Knowing how this 2708 has failed may be useful to see what might have caused it. So could also try powering just the 2708 up from three PSU's with it de-selected etc. Not sure if these old ones mind too much if address etc. inputs are floating, or whether you need to tie High/Low etc. |
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13th May 2021, 6:30 pm | #46 |
Pentode
Join Date: Jan 2021
Location: Northampton, Northamptonshire, UK
Posts: 131
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Re: 2708 EPROM Power Sequencing
I will investigate what has gone wrong with the 2708 but I'll wait till I have the new PSU board working just to be sure !
Chris |