Hopefully on topic

Jay Jaeger cube1 at charter.net
Tue May 5 08:15:14 CDT 2020

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

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.  ;)  "


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