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by tcat
Sat Dec 09, 2017 1:40 pm
Forum: Hardware
Topic: The Mandatory Modifications
Replies: 6
Views: 493

Re: The Mandatory Modifications

Hi Cristian,

Just observations, need to be proved by experience of others.

I reckon you have Iss#5 QL board to service, I wish to learn from your steps, if you shared here in the forum, I may do the same in parallel.

Tomas
by tcat
Fri Dec 08, 2017 10:26 am
Forum: Hardware
Topic: ZX Interface I - under the hood
Replies: 5
Views: 172

Re: ZX Interface I - under the hood

Hi Charles, Your post was quicker than mine. I see Q11 transistor. My Speccy is Iss#3 so perhaps a later model. I was confused when looking at ULA pin1 (with a dot) and its opposite pin 40 ~ with +5V VCC, this is where one transistor leg is fitted through a wire? Compared to the diagram, annotated b...
by tcat
Fri Dec 08, 2017 9:27 am
Forum: Hardware
Topic: ZX Interface I - under the hood
Replies: 5
Views: 172

Re: ZX Interface I - under the hood

Hi Prof,

Thank you. I have assembled it again. But it seems middle transistor leg (possibly its base), is attached to ULA pin 10? Perhaps service manual has some info, that I do not get. I have only a circuit diagram inside the manual of poorer quality.

Tomas
by tcat
Fri Dec 08, 2017 8:55 am
Forum: Hardware
Topic: ZX Interface I - under the hood
Replies: 5
Views: 172

ZX Interface I - under the hood

Hi, I would have normally posted to `World Of Spectrum' forum, but I got SSL handshake failed, so I hope may get some attention here as well. I have recently recapped my Speccy also I1, now looking inside it, I would like to know the function of a transistor flying on top of ULA. Please see picture....
by tcat
Thu Dec 07, 2017 12:55 pm
Forum: Hardware
Topic: The Mandatory Modifications
Replies: 6
Views: 493

Re: The Mandatory Modifications

Hi Cristian, We went through `Microdrive design' topic together :-). Being no expert, please find my observations. This is a summary of mandatory modifications. 1. Add resistor R104 (82 Ω) in TR1 collector circuit. It should be noted that this resistor is fitted in a non-standard way in the collecto...
by tcat
Tue Dec 05, 2017 5:14 pm
Forum: Hardware
Topic: White Screen, no Microdrives Activity
Replies: 7
Views: 491

Re: White Screen, no Microdrives Activity

To diagnose which chip(s) are affected sort out the memory chips (TMS4164) VSS and Q pins. Pine 16 and 14. On good memory chips this should produce another black line on the screen. When you short out a memory chip without the screen image changing that is one of the affected chips. The memory are ...
by tcat
Mon Dec 04, 2017 8:36 pm
Forum: Hardware
Topic: Microdrive Unit Design
Replies: 118
Views: 7534

Re: Microdrive Unit Design

c) The presence or absence of the foil screen appears to affect reliability in either slot #1 or #2 (i.e. reliability improved with a foil-screen in both positions.) NB - in my QLs, I've switched-out the linear 5V regulators for various designs of switched-regulator - these may well generate more n...
by tcat
Mon Dec 04, 2017 1:19 pm
Forum: Hardware
Topic: ULA ZX8301 - TV Picture Capabilities
Replies: 62
Views: 5386

Re: ULA ZX8301 - TV Picture Capabilities

3) 16 color mode. This can be done externally with some logic that has to extract a 10MHz clock from the available 15MHz, and use it to sample and process RGB. An extra control bit could also be implemented using external logic. Hi, Dealing with some wave shaping at the Microdrive design topic, als...
by tcat
Mon Dec 04, 2017 11:26 am
Forum: Hardware
Topic: Microdrive Unit Design
Replies: 118
Views: 7534

Re: Microdrive Unit Design

Hi Martyn, Thank you for a very nice summary, I could not have done it better. a cheaply built hardware design, still working within a wide ranges of tolerances after over 30 years. I like the above argument, MD technology is so simple and we can still read very old tapes, to be compared with data C...
by tcat
Mon Dec 04, 2017 8:31 am
Forum: Hardware
Topic: Microdrive Unit Design
Replies: 118
Views: 7534

Re: Microdrive Unit Design

Hi Charles, Martyn, The picture with encoding is perfect. Now I learnt about phase locking of the clock, the series of "0" zeros purpose. I was also surprised to see 4-bit overlap on DATA tracks, I only thought of byte interleave given some info in Service Manual. There still remains a mys...

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