Hopefully on topic

[Martin Crockett]

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, <cube1 at 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
>