RE: DECstation 220. Another Impasse

> -Original Message- > From: Brent Hilpert [mailto:bhilp...@shaw.ca] > Sent: 27 August 2017 09:00 > To: r...@jarratt.me.uk; General Discussion: On-Topic and Off-Topic Posts > <cctalk@classiccmp.org> > Subject: Re: DECstation 220. Another Impasse > > On

RE: DECstation 220. Another Impasse

> You have some of your reasoning reversed regarding comparator behaviour. > Comparators are typically: +in > -in --> output transistor off --> output TRUE / > HIGH with pull-up R. > I have double-checked the datasheet (http://www.ti.com/lit/ds/symlink/lm339.pdf). It says "The output NPN sinks

Re: DECstation 220. Another Impasse

On 2017-Aug-26, at 11:09 PM, Rob Jarratt wrote: >> You have some of your reasoning reversed regarding comparator behaviour. >> Comparators are typically: +in > -in --> output transistor off --> output > TRUE / >> HIGH with pull-up R. > > > I have double-checked the datasheet >

Re: DECstation 220. Another Impasse

On 2017-Aug-26, at 9:57 AM, Rob Jarratt via cctalk wrote: > I have replaced several parts in the area of the monitor sense circuit, > including the comparator and the buffer it is driving, but the comparator > does not seem to be doing what you would expect. I have reverse engineered > the circuit

RE: DECstation 220. Another Impasse

> Just checked and found the missing resistor. I have updated the schematic. Its > value is 340R and that corresponds exactly to the 0.36V reference I measured. > So now the schematic makes sense, but the behaviour does not. > I drew it incorrectly, now fixed with the resistor for the

RE: DECstation 220. Another Impasse

> Yes it makes sense that the voltage shouldn't be 5V. I will check later this > evening for other components and compare with the other board too. > > Just checked and found the missing resistor. I have updated the schematic. Its value is 340R and that corresponds exactly to the 0.36V

RE: DECstation 220. Another Impasse

> > FWIW I would expect a voltage rather less than 5V there, otherwise the > comparators will never switch. But your schematic wouldn't produce such a > voltage, so I suspect an error. > Yes it makes sense that the voltage shouldn't be 5V. I will check later this evening for other components

RE: DECstation 220. Another Impasse

> -Original Message- > From: Tony Duell [mailto:ard.p850...@gmail.com] > Sent: 26 August 2017 18:39 > To: r...@jarratt.me.uk; Rob Jarratt <robert.jarr...@ntlworld.com>; General > Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org> > Subjec

Re: DECstation 220. Another Impasse

om>; General >> Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org> >> Subject: Re: DECstation 220. Another Impasse >> >> On Sat, Aug 26, 2017 at 5:57 PM, Rob Jarratt via cctalk >> <cctalk@classiccmp.org> wrote: >> > >> > I

Re: DECstation 220. Another Impasse

On Sat, Aug 26, 2017 at 5:57 PM, Rob Jarratt via cctalk wrote: > > I have replaced several parts in the area of the monitor sense circuit, > including the comparator and the buffer it is driving, but the comparator > does not seem to be doing what you would expect. I have

RE: DECstation 220. Another Impasse

I have replaced several parts in the area of the monitor sense circuit, including the comparator and the buffer it is driving, but the comparator does not seem to be doing what you would expect. I have reverse engineered the circuit and I simply don't understand how the circuit can behave in the

RE: DECstation 220. Another Impasse

> -Original Message- > From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Noel > Chiappa via cctalk > Sent: 19 August 2017 23:26 > To: cctalk@classiccmp.org > Cc: j...@mercury.lcs.mit.edu > Subject: Re: DECstation 220. Another Impasse > &

Re: DECstation 220. Another Impasse

> From: Rob Jarratt > I have just written a new blog post > the comparators are not producing valid output, the input signals are > varying abave and below the reference voltage, but the outputs never change. Don't forget that when the output of chip A is wrong, that might be

RE: DECstation 220. Another Impasse

> > How many connections does it have? If just 2 then it could well be a fuse. Not > really to protect the machine in the event of flashover in a monitor putting > high > voltages on the lines, but rather to meet the legal requirements in some > countries that any device linked to a modem

Re: DECstation 220. Another Impasse

On Sat, Aug 19, 2017 at 6:14 PM, Rob Jarratt via cctalk wrote: > For the time being I have decided to take a look at the other spare board, > which works less well in that it does not produce any kind of video output > at all, but on the basis that the 82C206 seems to be

RE: DECstation 220. Another Impasse

> > I have made an interesting, but sad, discovery. I traced both the two apparently > shorted pins on the gate array to two pins on the Chips & Technologies 82C206 > Integrated Peripherals Controller. Checking the 82C206, I found lots of pins on > that chip appear shorted to ground. I fear the >

Re: DECstation 220. Another Impasse

Hi Rob, I've seen lots of PLCC packages soldered directly to the board. Many BIOS chips were socketed, presumably because they were the last to get inserted unto a motherboard as BIOS firmware updates were issued during manufacturing. Removal won't be easy but you could either: - Slip a

Re: DECstation 220. Another Impasse

On Wed, Aug 16, 2017 at 7:10 PM, Rob Jarratt via cctalk wrote: > I suppose that, happily, it looks possible to procure replacements for this > chip. I am slightly puzzled by the packaging of this chip though. It is > surface mounted on my CPU board, but the packages

RE: DECstation 220. Another Impasse

> I was out most of this evening, but I just had a quick look again and think I have > found something significant. Comparing the gate array pins on the two boards I > have, I see that on the board where the video is corrupted, there are two pins > that appear connected to 0V that are not

RE: DECstation 220. Another Impasse

.@classiccmp.org] On Behalf Of shad > via cctalk > Sent: 15 August 2017 21:53 > To: cct...@classiccmp.org > Subject: RE: DECstation 220. Another Impasse > > Hello Rob, > please try to check the status of the OE input, comparing it to the other > motherboard. > > Andrea

RE: DECstation 220. Another Impasse

Hello Rob, please try to check the status of the OE input, comparing it to the other motherboard. Andrea

RE: DECstation 220. Another Impasse

> > Some great ideas Peter, thanks! I will double check the scope, I wonder if mine > doesn't have the bandwidth to show a glitch that is occurring? I will consider > cutting the track, which is also a great idea. I really do have to hope that it isn't > the gate array, it is my last resort,

RE: DECstation 220. Another Impasse

> -Original Message- > From: cctalk [mailto:cctalk-boun...@classiccmp.org] On Behalf Of Peter > Coghlan via cctalk > Sent: 15 August 2017 09:48 > To: General Discussion: On-Topic and Off-Topic Posts <cctalk@classiccmp.org> > Subject: RE: DECstation 220. Another I

RE: DECstation 220. Another Impasse

> > I have looked at this problem a little more. I have two motherboards, > neither of which work, but one at least produces a corrupted video pattern. > The one that works best appears not to be writing to the video memory. When > I look at the EMEM pin on the Paradise PVGA1A chip I can see a

RE: DECstation 220. Another Impasse

> -Original Message- > From: Rob Jarratt [mailto:robert.jarr...@ntlworld.com] > Sent: 13 August 2017 20:31 > To: General Discussion: On-Topic and Off-Topic Posts (cctalk@classiccmp.org) > > Subject: DECstation 220. Another Impasse > > I have reached another