Issue 3B only sees 16k (and RAMTOP NO GOOD on loading)
A ZX+ with issue 3B board that i got recently, does not see anything more than 32768 (sometimes 37267..) on "PRINT PEEK 23732 + PEEK 23733 * 256 (Enter)".
Following the WoS repair guide, i got 32769 85 - 213 and 170 - 234 poking and peeking.
The guide suggests to replace IC21 at this point, this controls the top ram?
Side note, i did the composite mod on this spectrum, and the signal is a lot better than my toastracks. A guide i found suggests to remove audio by desoldering C126 in the toastrack, it's clear in the schematics.. but i can't find it on the board.. any pointers?
Following the WoS repair guide, i got 32769 85 - 213 and 170 - 234 poking and peeking.
The guide suggests to replace IC21 at this point, this controls the top ram?
Side note, i did the composite mod on this spectrum, and the signal is a lot better than my toastracks. A guide i found suggests to remove audio by desoldering C126 in the toastrack, it's clear in the schematics.. but i can't find it on the board.. any pointers?
Post edited by Asure on
Comments
Your numbers say that D7 and D6 are stuck to the value "1", so IC21 and IC22 are damaged. Those IC's don't "control" top RAM. They "are" part of the top RAM. Specifically, these two chips store bits D6 and D7 (respectively) of every address in top (non contended) RAM.
Do a POKE 32769,0 and PRINT PEEK 32769. You should get 192, which is the decimal for 11000000
Try turning back the PCB. Some issues put that capacitor in the solder side rather than in the component side.
On the other hand, the toastrack TEA2000 PAL encoder doesn't offer the same isolation for the composite signal than previous issues with the LM1889 and associated transistors. I've made some composite mods on toastracks using this little tutorial I wrote some time ago:
http://www.zonadepruebas.com/modules/smartsection/item.php?itemid=1226
In which I use a transistor that normally carries a B/W composite signal to the DIN connector, and change the signal that is present in its base to be a colour signal. That transistor does give me the buffering I need to get a more suitable video signal.
Basically it's as follows (for UK toastracks): remove diode D34 (you actually need to lift the cathode of this diode). With a piece of wire, join the cathode of D30 with the anode of D33 (which happened to be connected to the cathode of D34). That's all: your colour composite video signal is present on pin 1 of your DIN connector.
On spanish toastracks, audio can also be routed to an unused pin at the DIN connector. The pinning in this model is different than the UK model, as you know. So that it's possible to have composite video (colour) and sound in the DIN socket, while maintaining RGB compatibility, and even being able to use the same 3-pole DIN connector that are made for audio/video cables for the Dragon 32/64 computer.
Found a picture of C126 on your site, problem solved :)
Edit: It's not there on my UK toastrack :(
http://www.zonadepruebas.com/uploads/img4c6074b47c9a1.jpg
Regarding the RAM, these things are expensive! What's the best place to shop for them? Can i piggyback without desoldering?
(I have tms4532-20nl3 on the board.)
Use 4164 DRAM's. Cheaper, a bit more reliable (after all, 4532 DRAM's are actually factory faulty marked 4164 DRAMs) and less current demanding. You can even use 41256 DRAM chips.
About piggybacking... I wouldn't recommend that. If in case, you have to cut pins 2 and 14 in the faulty chip, so they don't make contact to the board anymore. Then, connect pins 2 and 14 together in the working chip, then connect these to the board, where the original pins were located. You only need to make one connection per chip, as pins 2 and 14 were joined in the board too. The rest of pins can be soldered to the corresponding pins on the faulty chip.
here you have some supplies of 4164 DRAMs:
http://www.ebay.co.uk/sch/i.html?_nkw=4164+dram&_sacat=0&_odkw=4164&_osacat=0&_trksid=p3286.c0.m270.l1313
- From the pdf these seem pin-compatible drop-in replacements, right?
- Any special things i need to keep in mind, rewiring the bridges at the top etc.?
- Should i get more than say,2x TMM4164AP-15, just to be sure?
- Is it likely more (or all) the upper ram is dead?
Also, what causes this.. old age, bad chips? As they are already partly broken, maybe they were cheaper in the 80's but also likely to break down sooner right?
Use 150ns or faster chips. Don't need to change jumpers, AFAIK.
Absolutely! These can be used to repair damaged low memory as well (4116 DRAM's) with a very little hacking. This is because I recommend to you to buy 150ns chips or faster. For upper memory only repairs, you can go as low as 200ns.
Based upon your tests, it's not likely.
- Upper RAM exposes their data bus on the rear bus, so they can be affected by static electricity coming from your hands while grabbing the computer, if you accidentally touch the tracks at the rear connector.
- Bad power supply, caused by cad capacitors, 7805 malfunction, sudden voltage variations that cannot be regulated fast enough by the regulator. Besides, the 4116 memory chips (those used for the low memory bank) need three voltage supplies, which must be applied and removed in a specific order. Failure to obey this leads to render the chip damaged. The Spectrum internal power supply (not the power brick, but the internal voltage boost circuit) don't apply supplies as expected, so every time you do a power cycle (turn off, wait a couple of seconds, then turn on) you are risking your low memory bank chips. 4164 chips don't have this issue.
- Computer chips exposed to high variations of temperature over years (i.e. stored in a place exposed to sunlight or not isolated from weather conditions) make its internal parts to dilate and shrink, causing small cracks in the metal substrate, eventually damaging the chip. Your synthomps suggest that this may have been happened to your computer.
4532 chips didn't exist as a standard part. They were damaged 4164 chips that had the failure located at one specific 32K bank. If the other bank worked fine, the manufacturer didn't throw the chip, but marked it as "32Kbit DRAM" indicating, with a H or L which bank can be used (High or Low bank). Of course, these chips were cheaper than the 4164, much in the line of Sinclair to lower the machine costs as much as possible.
I can test some of the types you need and send them to you for a small compensation.
Did you find c126 on your toastrack? If not in place on the components side, it's under the TEA2000 on the solder side.
Ben
It would make me very happy if you have the dram ic i need for my 48K+, i'll send you a PM (we're both in the Netherlands).
I got this one boxed of Marktplaats, and it's the third broken one in a row.. lots of sellers there pushing out broken crap :(
@ben: I sent you an email just now, hope i can get this little machine fully working again.
I think i figured out what went wrong, the heat sink slightly blocks the interface port, and any interface you plug in, needs to be carefully inserted with the correct bit of force.. if this is done incorrectly, like @mcleod_ideafix suggested, the rear bus can short easily.