test-drb January 2012

test-drb@ccmp.vtda.org

201 participants
274 discussions

Debugging CRT monitors (Philips CM8833 Mk2 / Acorn AKF17)

by cctech＠beyondthepale.ie

> >With anything over about ten years you start by replacing electrolytic >capacitors. If you want to test instead of shotgun then pick up an ESR >meter, but it usually shows more than a few are failing. If the monitor >is over twenty years old, then just replace all the electrolytics and >then troubleshoot - it there is still a problem that is. > I don't think I've got anything less than ten years old. I can't remember having cause to replace more than an occasional electrolytic here and there except in the case of a television from the 1970's which has required about five or six over years of heavy use. I'm sure if I checked everything with an ESR meter it would show up a bunch that are below par. However, many applications for electrolytic capacitors can tolerate very large variations and they may well have years of usable life left in them even though the meter condemns them. There may be a case for checking critical capacitors in power supplies etc which may be at risk of overheating or leaking, however there is no reason to check every electrolytic. I think replacing large numbers of electrolytics is likely to be be more trouble than it is worth and may even introduce new problems that weren't there before, particularly on old pcbs where the print was never well stuck to the board, even when new. Far better to look for a schematic if available and wade in with the scope looking for the problem with deductive reasoning. > >Do take care to match the caps to the job they do - low ESR caps for the >horizontal, and caps that handle AC ripple for the linear power supplies. > I will agree there. Regards, Peter Coghlan.

12 years, 2 months

Commordore 64 character color cycling (Ethan Dicks)

by tpresence＠hotmail.com

> > The Color SRAM is a 2114. Very ordinary and, in my experience, prone > to failure. > > If you have another C-64, you could try swapping the PLA. You will > need an 18-pin socket for the SRAM. The chips are available from a > number of surplus places like Unicorn Electronics, BG Micro, and more. > Speed ranges for the 2114 are, IIRC, 250ms-450ms. I don't remember > what speed the C-64 requires, but you can typically sub a faster chip > than the circuit requires. > > -ethan I have already purchased a replacement sram for the 2114 (acutally 2 of the units, one P2114 and one MM2114N-2) and intend to replace the IC sometime today. I do have another C64, but no chips are socketed, and I don't want to ruin it, thus I'll just try replacing it in the bad unit. My understanding is that I need a chip that has a speed faster than half a machine cycle for the system to work properly. The spec sheet on the P2114 states that it has a 450ns access time. For the MM2114N-2 it looks like its 200ns. I understand that the access speed requirement is .5*(cpu freq/1)/machine-cycle. I don't know the machine cycle for this operation, but assuming the worst with an IC having an access time of 450ns, the machine-cycle couldn't be over 2.25 for things to be ok. If I use the 200ns IC, I should be good with a machine-cycle of just over 5. I could probably solve this by opening up the unit and just pulling the IC scribe marks and looking up the data sheet, but the location with the ICs is 50 miles from the house, and I happened to be nearby yesterday...so I just winged it. Can anyone checksum my thoughts here? Is my algorithm and its implementation ok? I also bought a replacement for the PLA, however, I had misunderstood the requirement. I thought it was just a plain ole N82S100, but now I understand that it is a specially programmed N82S100. Although I have found a ROM file I could perhaps burn on the chip, I simply don't have the gear. I'm hoping this isn't the problem, as I have to mail order the part, but I understand that this is the most common IC to fail on the C64 board. Since I'm getting only character color palette cycling, and have a border and don't have a problem with a blank screen, I am encouraged that this IC is probably ok. Just to be safe, I bought a replacement for U29, as the description in the c64 chip and common problems guide has content that really looks similar to my problem, moreso than even the description for problems with U6 (the 2114). Thanks for your commentary though. I'm likely to buy sockets for these chips as well, as they aren't that expensive, and it may save me issues should the selection I made be suboptimal. Kevin

12 years, 2 months

C compilers and non-ASCII systems

by jonas＠otter.se

At 11:21 AM -0500 1/31/12, Sean Conner wrote: > > So now I'm wondering---besides Baudot, 6-bit BCD and EBCDIC, is there any > >other encoding scheme used? And of Baudot, 6-bit BCD and EBCDIC, are there > >any systems using those encoding schemes*AND* have a C compiler available? > > Not sure what you mean by 6-bit BCD? For current systems which use a 6-bit character code, the Unisys 1100/2200 class systems (ClearPath nowadays) use Fieldata as well as 9-bit ASCII. They have a C compiler. These machines are 36-bit, so to fit ASCII characters in, they extended the characters to 9 bits, fitting 4 "ASCII" characters in a word instead of 6 Fieldata characters. It is possible to use all 9 bits which creates interesting problems when you need to migrate the code to a different architecture. I have come across (ancient but "modernised") applications which mix Fieldata and 9-bit ASCII... Also IIRC there is no reasonable way to handle Fieldata in Unisys C, so you have to treat Fieldata characters as hex or octal values. /Jonas

12 years, 2 months

Visiting Bristol UK

by huw.davies＠mail.vsm.com.au

I'm yet again visiting Bristol UK for most of January 2012. I guess during the week I'll have to attend the training course that work is sending me to, but I wondered if anyone had any classic computer recommendations for the weekend? Assuming the weather is OK, I'm happy to drive a reasonable distance but local stuff is easier. I did Bletchley Park last time I was in the UK (November 2010) and I've already promised myself that I'll visit the SS Great Britain and the Newport cable bridge, but wondered if there were other CC related things in the area. If anyone in the area wants to catch up for a beer, I'd happily shout a pint of the local bitter. Huw Davies | e-mail: Huw.Davies at kerberos.davies.net.au Melbourne | "If soccer was meant to be played in the Australia | air, the sky would be painted green"

12 years, 2 months

C compilers and non-ASCII systems

by tshoppa＠wmata.com

IBM has a very handy page on C compatibility with EBCDIC system services: http://www-03.ibm.com/systems/z/os/zos/features/unix/bpxa1p03.html

12 years, 2 months

C compilers and non-ASCII systems

by cym224＠gmail.com

On 31 January 2012 12:21, Zane H. Healy <healyzh at aracnet.com> wrote (in part): >> And of Baudot, 6-bit BCD and EBCDIC, are there >> any systems using those encoding schemes *AND* have a C compiler >> available? z/OS is not only POSIX, it is UNIX (see http://www.opengroup.org/openbrand/register/brand3470.htm). (Sorry for not replying to the OP -- I just saw this now.) N

12 years, 2 months

Looking for Andrew Back

by dbetz＠xlisper.com

Does anyone here know how to contact Andrew Back? I have some equipment to send him and I need a mailing address. Thanks, David Betz

12 years, 2 months

Repairing a TI-1270 calculator

by jgessling＠yahoo.com

Thanks Tony.? As you suggested there are 10 wires connecting the main board to the keypad.? As in 4 x 6.? I fiddled around a little, gently.? I've ruined another similar unit by cutting those wires in hopes of reattaching them so I was avoiding that.? I guess I made something that was loose become a circuit at least for now.? I works great.? Really a treat to see it work.? Maybe not an HP, but they rarely break anyway. Jim

12 years, 2 months

Commordore 64 character color cycling

by tpresence＠hotmail.com

I have a malfunctioning C64 that I would like to repair that is exhibiting the most odd of behaviors. The system operates fine, except displayed fonts cycle through the color palette. If you remember the old atari 2600 game "Adventure" its behaves like the chalice does, every second or so changing colors. I haven't started probing the system at all, and although I can use a soldering iron, my troubleshooting skills are pretty rusty. I was considering this a good project for a refresher. Sound doable, or is it likely I'll be looking to replace lots of components on the board? Kevin

12 years, 2 months

C compilers and non-ASCII systems

by cctech＠beyondthepale.ie

At 11:21 AM -0500 1/31/12, Sean Conner wrote: > A friend recently raised an issue with some code I wrote (a hex dump >routine) saying it depended upon ASCII and thus, would break on non-ASCII >based systems (and proposed a solution, but that's beside the issue here). >I wrote back, saying the code in question was non-portable to begin with >(since it depended upon read() and write()---it was targetted at Posix based >systems) and besides, I've never encountered a non-ASCII system in the >nearly 30 years I've been using computers. > > So now I'm wondering---besides Baudot, 6-bit BCD and EBCDIC, is there any >other encoding scheme used? And of Baudot, 6-bit BCD and EBCDIC, are there >any systems using those encoding schemes *AND* have a C compiler available? > > -spc (Or can I safely assume ASCII and derivatives these days?) A unix called UTS (Universal Time Share?) by Amdahl ran (amongst other things under VM) on an Amdahl 5870 I worked with in the early 1990's. This was an EBCDIC machine. I never did much with UTS but I'm pretty sure it had a C compiler. There was also C/370 for VM/CMS and MUSIC. Regards, Peter Coghlan.

12 years, 2 months

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test-drb January 2012