Hi all, I apologise if I posted this request in the wrong forum, but I'm new to this; not electronics, I've got over 30 years bench experience, but asking for help :-)
The answer you have given me is perfect. It confirms my suspicions that it is C14. Now I have a value, I'll check it out in the morning. I appreciate your assistance. Many thanks and Cheers, Martin Sent from my Nokia 6110 On Tue, 5 May 2020, 22:45 Jay Jaeger, <cu...@charter.net> wrote: > On 5/5/2020 2:08 AM, Brent Hilpert via cctalk wrote: > > On 2020-May-04, at 8:10 PM, Martin Crockett via cctalk wrote: > >> .. > >> I noticed there is a capacitor that has vaporized, but I cant determine > >> what value it is. > >> > >> I have the DEC VT-100 maintenance guide but it is very blurry in the > >> relevant area. > >> > >> I cant even read the board designation. > >> > >> This is the area of the circuit, I can trace the 2 Zener diodes on the > -23V > >> rail, to one end of the cap, the other end seems to go to ground. The > >> obvious culprit is C6, but that doesn't match the mud map of the board, > as > >> in C1x. > >> https://imgur.com/a/tm8mn8b > >> > >> This is the capacitor in question. > >> > >> It looks like C1x where x is undetermined. > >> > >> I think it was a ceramic monolithic capacitor, it seems to be different > to > >> any other caps on the board, i.e. slightly larger and a different colour > >> blue. When I look at this hires photos on Google images, it is the blue > >> capacitor circled below > >> > >> Does anyone have a VT-100 and mind checking what the value of this > >> capacitor is please? > >> Ideally value and voltage or even just the nomenclature written on it, I > >> can work out the value and voltage from that. > > > > > > > > You sure that isn't C14, connected to pin 2 of E24, Vgg PS pin on the > non-volatile RAM? > > (bitsavers schematic pdf pg 15 / "VT100 BASIC VIDEO Sheet 2 of 6" / > upper middle of page). > > See also the board pics on bitsavers. > > > > 0.22uF, 50V CER > > > > This same question with the same subject appeared on the S100 computers > mailing list. I replied there with basically the same conclusion, based > on my own schematic (and also with advice that this is a better place to > ask.). Maybe he has a spam filter issue, and so didn't get the reply? > > "Yeah, that looks like it is the printset from bitsavers. Even smudged > it looked like C14 to me, and I confirmed it *IS* C14 on my very clear > but very slightly older printset: VT100-0-1 Rev. C July, 1978. (Could > probably figure it out by elimination, too. C6 is on the edge of the > board opposite the RS232 connector in both printsets. One could look > through the rest of the parts for the other C1x capacitors to eliminate > those as possibilities as well. > > My printset (Rev W schematic for this board) does NOT show the C59 that > is on the bitsavers pritnset (later, rev. AA), next to C14, and the > schematic in that area varies as well. But its proximity to C59 pretty > much seals the deal (see below regarding the bitsavers schematic.) > > C14 is 0.22uf, 50 V (+80%-20%), on both my printset and the bitsavers > printset. > > To verify it is C14 on your board: > > On *my* schematic (Rev W), one side goes to +12V, the other to E24 > (NVRAM) pin 2, R67 (22K), one side of W7 (which goes to -23V) and the > collector of Q6 (2N3904) > > On the *bitsavers* schematic (later, Rev AA), one side goes to ground > (instead of +12V), but the rest it the same as above. > > Your board should have an indication of which circuit schematic it is. > > So, its a bypass cap for the NVRAM. On older boards it bypasses to > +12V, and on newer boards it bypasses to ground, and C59 bypasses +12V > to ground as well, which seems like a sensible change. ;') > > For future reference, when asking about DEC stuff, the classic computer > mailing list might get you an answer quicker. ;) " > > JRJ >
