The pure *hell* of restoring a PDP-8/S desktop.

Hi. Stabilant 22 was touted highly by Jerry Pournelle in Byte magazine as a contact enhancer, it might help after a good de-gunking. http://www.stabilant.com/ http://www.byte.com/art/9705/sec13/art1.htm "Stabilant 22, the miracle all-purpose contact enhancer. It not only makes for better electrical contacts -- a major use of Stabilant 22 is to get all the electrical noise out of hi-fi systems -- but also lubricates connector insertions. If you don't use Stabilant 22, I bet you wish you did." At 05:23 PM 2/7/00 -0500, you wrote: >Two weeks ago I *had* restored it.... and about every half hour it would >break down with a different problem. I am posting this message in the hope >that if anyone else ever has to restore such a nasty transistor mini then >they can use this as a reference for *everything* that can go wrong with a >minicomputer (excluding the power supply). > >First, the best methods I have found to eliminate the main problems in the >most stubborn transistor minicomputers: > >1) Bad contacts on backplane - Spray it with "Engine Gunk" (seriously, an >automotive engine degreaser/cleaner), then stick a plain extender board with >cloth wrapped around contacts and insert/remove a couple of times. Then use >contact cleaner. > >2) Weak Transistors - Lower the power supply voltage enough that poor >transistors start to fail. > >3) Thermal Transistor Breakdowns - (these are the Fairchild transistors that >*always* fail due to the epoxy inside expanding and breaking contact). I >recommend leaving the minicomputer on for 5 hours and let the poor >transistors completely fail. Nothing worse than an intermittent one. BTW, I >had 18 Flip Chips filled with this problem :-( (thanks for the info >Allison!) > >4) Intermittent Flip Chips - These occur in areas where the mini is running >hot (ie: The MB register) and cause poor solder contacts which *might* be >visible. I recommend pulling these altogether and replacing them. 100s of >ticking timebombs. Get rid of all of them before you go insane. Use a >digital thermometer and check for areas in excess of 115 degrees F (35 C). >In the last 3 transistor minicomputers I have restored over the past 5 >months I have found you will get a lot more intermittents if the mini was >used *a lot* (ie: 12 years) and was stored in an area that varied greatly in >temperature. > >5) Expect your prints not to contain a complete schematic of the mini >computer - I have found many changes to the straight-8, PDP-8/S that were >not listed in FCOs but instead new prints were issued. (1960's) > >COMMON PDP-8/S Problems I have found: > >Since the speed at which data is switched through the PDP-8/S is *much* >faster than the Straight-8 expect problems and replace/resolder the major >registers and *ALL* pulse amplifiers. Most pulse amplifiers are switching at >the maximum speed for such a Flip Chip (R602,603) so the slightest addition >of resistance caused by a poor solder contact will cause endless problems. > > >This PDP-8/S had the following problems (beyond the ones I mentioned before) > >Saul Dinman decided to add a *new* interrupt called "LPC", Low Power >Condition. It will look like a negibus device interrupting the CPU but in >fact is a few gates wired to the power monitor board. Of course, it only >affects *PARTS* :-( of the CPU like... Parity! ,ION, and other nasty *bugs*. >As the original 8/S did not have this installed it was not included in >original prints :-(. I now found a third set with these mods... after >spending a couple of hours pulling my hair out. After polling for interrupts >and a memory parity error it is safe to conclude it is a low poer condition. >Thanks Saul! > > >Of course, everything else *only* failed when the CPU was running a >program.. > > It's OLD instruction set was: >========================================= > >ION when an external device draws *just* enough power when being accessed to >cause an LPC rather than an IO Interrupt :-(. > >Clear Accumulator Sometimes - (external device control) when warm and only >when running a program and oscilloscope is not connected. > >Move AC->MB , 99 out of 100 times asked to. > >Stall CPU Clock 1mS - no longer or it might be seen without scope. > >and my favorite... > >If Zero, Branch Sometimes - BUT branch always in SI mode. > >========================================================================= > >The PDP-8/S has now been running for over 6 hours and every major "bit" >junction has been checked for a poor signal - all Fairchild's removed and >buried. The desktop 8/S is a micro-sauna which means I will probably install >fast boxer fans in place of the two eggbeaters currently installed in the >back. My rackmount 8/s has 4 fans along the sides of the boards + 2 blowers, >one above and one below. > > >I am installing a 20ma->RS232 converter and will get this mini connected to >Telnet with FOCAL loaded as quickly as I can. This mini needs a good month >of continuous running to make sure it is finally... happy. > >PAPER TAPE: If anyone here is using a PC0 with a PC01 controller >(Straight-8, or 8/S) I found internal mods used at DEC to increase the read >speed from 300CPS to 650CPS... error free! 8/S *really* loads FOCAL fast >now! > > >I hope when I get a KA10 it isn't filled with the nasty problems this one >had. > > > >john > > > >PDP-8 and other rare mini computers > >http://www.pdp8.com > > > >