Re: Malfunctioning VT240 - help please

On 6/21/20 10:41 AM, Jon Elson wrote: On 06/20/2020 09:41 PM, Charles wrote: On 6/20/20 8:31 PM, Jon Elson wrote: I confirmed the bad one by removing the piggyback and the failure returned. Now I need to desolder the bad one without ruining the board. I may just cut the leads off close to t

Re: Malfunctioning VT240 - help please

I confirmed the bad one by removing the piggyback and the failure returned. Now I need to desolder the bad one without ruining the board. I may just cut the leads off close to the bad chip, and solder the replacement to the stumps. (Normally I remove the legs and install a machine-pin DIP s

Re: Malfunctioning VT240 - help please

On 06/11/2020 02:29 AM, Mattis Lind via cctalk wrote: >/If that would be the case I think the system would fail />/quite soon rather than on test 5. A guess is that this is />/a memory problem. / That was a good guess, everyone ;) I got some new 4116's and piggybacked (dry, no solder) two of the

Re: Malfunctioning VT240 - help please

Make that "fuse-link PROMs". My mistake. -Charles On 6/15/20 10:41 AM, Tony Duell wrote: My fear is that one of the PALs has altered itself from tin-whisker migration (fuse regrowth) :( I've finally looked at the printset for the VT240 and I must be missing something The only PALs I can f

Re: Malfunctioning VT240 - help please

> My fear is that one of the PALs has altered itself from tin-whisker > migration (fuse regrowth) :( I've finally looked at the printset for the VT240 and I must be missing something The only PALs I can find are the 'logic units' in the video memory updating circuitry. That circuitry is pret

Re: Malfunctioning VT240 - help please

I remember testing memory chips with a thumb. The one that took yoir fingerprint ofd was always faulty. Hopefully it's not whisker regrowth. That would be really frustrating. On Fri, 12 Jun. 2020, 12:15 am Charles via cctalk, wrote: > > On 6/11/20 2:29 AM, Mattis Lind wrote: > > > > > > torsd

Re: Malfunctioning VT240 - help please

One trick I found with the -5V if it is driven by  a charge pump: check the voltage. If it is being pulled down since the charge pump cannot supply the current, just disconnect the charge pump and put a lab supply in its place.  The increased current will clean out whatever is shorting it to gr

Re: Malfunctioning VT240 - help please

On 6/11/20 3:15 PM, Mattis Lind wrote: The T11 is not halted - it's looping endlessly in the first ROM. There is a brief burst of DRAM select activity on the scope just before it hangs in the loop.  That burst of DRAM activity might indicate a DRAM problem. One thing I have tried in the pas

Re: Malfunctioning VT240 - help please

On Thu, Jun 11, 2020 at 2:37 PM Adrian Graham via cctalk wrote: > > The 4116's are soldered to the board, too > > You’ve just mentioned the magic 4116 word, I’d bet some of your dollars that > it’s either one of those that’s gone south or the -5V required to run them Definitely check the -5V

Re: Malfunctioning VT240 - help please

> The T11 is not halted - it's looping endlessly in the first ROM. There is > a brief burst of DRAM select activity on the scope just before it hangs in > the loop. > That burst of DRAM activity might indicate a DRAM problem. One thing I have tried in the past is to put a known good DRAM on top of

Re: Malfunctioning VT240 - help please

On 6/11/20 10:31 AM, Mattis Lind wrote: Now that you know the T11 is good I think it a good idea to attach a logic analyzer on the bus. I would then disassemble the ROM code and match that with the logic analyzer execution trace. Then it should be possible to find out w

Re: Malfunctioning VT240 - help please

> On 11 Jun 2020, at 15:14, Charles via cctalk wrote: > > = > > Thanks for the tip. I didn't see in the manuals that the keyboard light > pattern was actually a binary code, but that makes sense! I would have > expected an error message on the screen, but as I previously noted, the

Re: Malfunctioning VT240 - help please

On 06/11/2020 02:29 AM, Mattis Lind via cctalk wrote: If that would be the case I think the system would fail quite soon rather than on test 5. A guess is that this is a memory problem. Sure, once you call a subroutine and then try to return, corrupted memory would throw the processor into neve

Re: Malfunctioning VT240 - help please

> > > > Now that you know the T11 is good I think it a good idea to attach a logic > analyzer on the bus. > > I would then disassemble the ROM code and match that with the logic > analyzer execution trace. Then it should be possible to find out what is > going on. If one can rely on the fault code

Re: Malfunctioning VT240 - help please

On 6/11/20 2:29 AM, Mattis Lind wrote: torsdag 11 juni 2020 skrev Charles via cctalk >: On 6/10/20 4:31 PM, Jon Elson wrote: On 06/10/2020 12:48 PM, Charles via cctalk wrote: That leaves the unlikely possibility that one of the octal

Re: Malfunctioning VT240 - help please

torsdag 11 juni 2020 skrev Charles via cctalk : > > On 6/10/20 4:31 PM, Jon Elson wrote: > >> On 06/10/2020 12:48 PM, Charles via cctalk wrote: >> >>> >>> That leaves the unlikely possibility that one of the octal TTL devices, >>> or ROMs. has developed a weird internal pathway that only interfere

Re: Malfunctioning VT240 - help please

On 6/10/20 4:31 PM, Jon Elson wrote: On 06/10/2020 12:48 PM, Charles via cctalk wrote: That leaves the unlikely possibility that one of the octal TTL devices, or ROMs. has developed a weird internal pathway that only interferes with DAL3 & 1 on some bit patterns, but not all the time. Seem

Re: Malfunctioning VT240 - help please

On 6/10/20 4:31 PM, Jon Elson wrote: On 06/10/2020 12:48 PM, Charles via cctalk wrote: That leaves the unlikely possibility that one of the octal TTL devices, or ROMs. has developed a weird internal pathway that only interferes with DAL3 & 1 on some bit patterns, but not all the time. Seems

Re: Malfunctioning VT240 - help please

On 06/10/2020 12:48 PM, Charles via cctalk wrote: That leaves the unlikely possibility that one of the octal TTL devices, or ROMs. has developed a weird internal pathway that only interferes with DAL3 & 1 on some bit patterns, but not all the time. Seems like a zebra rather than a horse. The

Re: Malfunctioning VT240 - help please

OK. the keyboard is working properly as far as I can tell, data is going in and out, and I even swapped it for the keyboard on my VT220 and the same symptoms persisted. I just verified all four ROMs on the T11, and the ROM for the 8085, against the images I found on the MAME site. So far so go

Re: Malfunctioning VT240 - help please

On 07/06/2020 18:27, Charles via cctalk wrote: > Until a few minutes ago, my VT240 was operating normally, but now it's > unresponsive (fails during power-on self test). > > Normal behavior was: display a checkerboard, then two different > intensity all-white bands growing slowly up from the bottom

Malfunctioning VT240 - help please

Until a few minutes ago, my VT240 was operating normally, but now it's unresponsive (fails during power-on self test). Normal behavior was: display a checkerboard, then two different intensity all-white bands growing slowly up from the bottom of the screen, then a beep and the expected "VT240