Mystery 8085-related IC identification needed please

[Brent Hilpert]

On 2016-Dec-16, at 5:43 AM, Adrian Graham wrote:
> viewdata side of things is powered by a Plessey MR9735-002 teletext
> processor supported by a pair of 2112 RAM chips and an SAA5070 "LUCY".
> 
> http://txlib.mb21.co.uk/main.php?g2_view=core.DownloadItem&g2_itemId=2034

On 2016-Dec-17, at 10:34 AM, Adrian Graham wrote:
> One problem I have is that I've already found a few chips with dead outputs
> so I've no idea if these will be any different. The pinouts I have match the
> LS92 since pins 2/3/4/13 are NC. All testing so far has been done with a DMM
> and cheap logic analyser. Since one of the possibly-LS92s is out of circuit
> I'll build a little test circuit to see if it does actually count given a
> clock source...

The counters may be the beginning of a video timing divider chain or a clock divider for the LUCY chip.
(e.g. a diagram in the document you mentioned shows a div-6 between a 6MHz crystal osc and the LUCY chip.
The division factor may vary of course in the executel depending on the crystal.)

Also to keep in mind as you trace it out, 749x counters were often used with gates between the outputs and reset inputs to change the count modulus / division factor.


On 2016-Dec-17, at 10:34 AM, Adrian Graham wrote:
> It's a 5-band red-red-black-black-violet so either 220R or 70k? Based on
> what Pete said about the Z80 I'm going for 220R without pulling it out of
> circuit.


Looking at the photo in your earlier message
	http://www.binarydinosaurs.co.uk/stcexecuteltimingcircuits.jpg
the wide red band on the resistors would be a 2% tolerance indicator.

As with Dwight's suggestion about colour confusion, if the resistors in question are those to the left of the two unmarked chips (or similar), they look like brown-black-black-red-wideRed, which would be standard 10K / 2%.

You can try measuring resistors in circuits like this with a DMM.
Switch the DMM leads to get both polarities through the resistor.
It's not guaranteed as it depends on the connected circuitry and the R value, but one of the readings will often be your R value, or may confirm or give direction as to the value.