Spectrum+ Refurbishment & Repair - Part 1

I don't normally have a lot of luck when it comes to picking things up on the sale groups in Facebook. Thanks to the FB algorithm, I normally see posts about bargain stuff four or five hours after it's first posted meaning that the item has already gone.

This time, though, I saw something almost the second it was posted. A Spectrum+ for a very, very reasonable price. I bought it immediately. It was sold as needing a new keyboard membrane (no surprise there) but condition otherwise unknown - to be honest I didn't ask at that price!!

After a brief wait, it finally arrived. A quick inspection showed that it needed a bit of a clean but otherwise looks in good condition. It was the unit only but, as luck would have it, my rubber keyed buddy's power supply will provide the power I need.

Speccy Plus! Yeah!

And then I made a mistake.

There is an excellent video by Joules per Coulomb on YouTube (see HERE), which runs through the basic tests to perform prior to plugging in a ZX Spectrum, the idea being that any major faults can be intercepted and corrected, thereby avoiding unnecessary additional faults being caused. For example, the 4116 lower RAM chips require three voltages to operate (5v, -5v and 12v). If any are missing then the lower RAM can easily be fried.

So, of course, I followed standard procedures and carefully ran through all the required tests, didn't I?

Nope. I was a dumbass. I plugged it straight in. This was the result:

Oh s**t!!

In addition to the red border and garbage on screen, there was a random screech from the Spectrum's speaker, just to add to the excitement.

At this point, I opened him up and decided to run through the tests that I should've done at the start. (I noticed the keyboard membrane was borked but that's for later.)

Issue 4A. Nice.

The board is a 4A with a 6C001E-7 ULA, made in August 84. Out came the multimeter.

Test one: Voltage regulator left terminal resistance to ground. PASS
Test two: Lower RAM ground rail resistance. PASS
Test three: Lower RAM 5v rail resistance. PASS
Test four: Lower RAM -5v rail resistance. PASS
Test five: Lower RAM 12v rail resistance. PASS
Test six: Inductor coils end to end resistance. PASS
Test seven: Inductor coils cross coupled resistance. FAILED

Fsck. So reading resistance through each coil was fine i.e. the resistance was very low. But testing across the windings results in low resistance too. JPC suggests that this is down to TR4 failing with a short across its collector and emitter.

Test eight: Diode test base to collector on TR4. FAILED
This test should show 0.6 to 0.9v but instead the meter read 0.00v.

Test nine: Diode test base to emitter on TR4. FAILED
This test should show 0.6 to 0.9v but instead shows O/L i.e. no connection.

Test ten: Diode test base to colelctor on TR5. PASS
Test eleven: Diode test base to emitter on TR5. PASS
Test twelve: Diode test (reverse polarity) base to collector on TR5. PASS
Test thirteen: Diode test (reverse polarity) base to emitter on TR5. PASS

After all that, TR4 is toast. I removed it from the board and re-ran the tests and got the same results i.e. base to collector read 0.00v and base to emitter read as open circuit. So, I need a new TR4. These were promptly ordered from RS Components for the princely sum of about 35p each or so in a pack of five. They were not the original ZTX650 but actually ZTX651 which are an appropriate drop in replacement.

A day later, they arrived and I set about soldering a new one. This was straightforward as the ZTX651 has the same pinout as the ZTX650. Some other replacements that could be used may not have the Base, Collector and Emitter in the same order.

ZTX651s and some extra caps too.

With a new TR4 installed, I went through the initial tests again and, this time, they all passed. Back to the desk for switch on number 2.

Bugger.

With the Smart Card V2 plugged in, this was all I could get. Vertical black and white bars. The only thing that changed was the colour of the background which would sometimes be blue and sometimes black, very occasionally it would be white. Now, at this point, I'd like to say that I started to channel my inner Sherlock Holmes and deduced that, if the display is working but nothing will start then it is most likely that the CPU is toast. I'd like to say that but I can't as I had to ask for assistance from Twitter and the ZX Spectrum Facebook groups. Many suggestions were made but the one that kept coming up was that the CPU was toast so I decided to look at it more closely.

Rather annoyingly, the Z80 is this Spectrum was soldered to the board. In my Issue 2 the Z80 was in a nice, convenient socket. So I would have to remove this one from the board if I was to have any chance of proving the problem. I did not need to order a new Z80 as my CPC has (had) a socketed Z80 which I could 'borrow' for a while. What I was missing was any good way of de-soldering a 40pin DIP.

I looked through RS Components website and found decent quality solder wick but I have never really had much luck with it. The good stuff can also be expensive and is, obviously, single use.. Then, on Amazon, I spotted a de-soldering soldering iron for a fiver. What could possibly go wrong? :)

Cheap - but will it make me cheerful?

It's essentially a standard mechanical solder sucker but bolted into a soldering iron. And although there are plenty of reviews and YouTube videos suggesting these are a bit, well, crap, my experience so far, is that they are massively easier than solder wick and a standard solder sucker. I practiced on a scrap board and the components were just falling out of the board into my lap. Which was nice.

To cut a long story short, I successfully de-soldered the Z80 and carefully removed it from the board. I was lucky to have a 40 pin, turned pin socket that would drop straight into the empty space, closely followed by my borrowed Z80.

Socketed Z80

Back to the desk for switch on number 3.

Almost there....

The excellent Smart Card V2 started up this time so the Z80 was clearly the original problem. But just as my excitement grew, I heard the deep beeps that told me I had a RAM problem. IC17 to be precise.

Out came the de-soldering iron again and the offending chip was quickly dispatched and socket installed. Next problem, I wasn't sure I had any appropriate RAM chips to fill the gap which needed to be type 4532 or 4164. I didn't have any of either but I did have a few 4264s. Google was my friend and it confirmed that a 4164 can replace a 4532 but a 4264 could replace a 4164. By my logic, this meant that a 4264 should work.

4264 RAM Chip Installed

And, by jove, it did. :D

Lower RAM Done - What about the upper??

GET IN!

The main board is repaired but still needs to be re-capped. But for now, let's get the top on and try out that keyboard. The unit was sold as having a keyboard fault, presumably the membrane so I had already ordered and received a new one. Sure enough, the original membrane looked a bit crappy and a portion of keys in the centre refused to work.

And then the real problems started....(to be continued)