Tag Archives: vintage

Soviet Tektronix 7000-series oscilloscope copies

Back in 1970, Tektronix introduced their ‘7000’ series of oscilloscopes. Their clever plug-in architecture offered unmatched performance and flexibility, and they quickly became an industry standard. They were ubiquitous in well-heeled electronics labs in the 1970s, 1980s and into the 1990s, at least in the US and UK. I’ve still got three examples in regular use because they do things that modern scopes struggle with. You want 10 microvolts/division sensitivity? Grab a 7A22 plugin. Want to see the same signal in the time and frequency domains together? No need to go and buy a shiny new ‘Mixed Signal Oscilloscope’. Just plug in a 7L12 spectrum analyzer. Need a 100MHz differential amplifier that can deal with 500V common-mode? No problem: Use the 7A13 plugin. All this stuff is more than 40 years old but still does a state-of-the-art job.

Behind the iron curtain, things developed at their own pace. In 1977, the Lithuanian ‘555’ factory produced the C1-91, and then the C1-115 and C1-122 ‘universal oscilloscopes’. On the right, here’s a picture I found on the web of a C1-115, together with, on the left, a picture of my own Tektronix 7603 taken a few minutes ago.

7603-c1-115Notice any resemblance? Same panel layout, same arrangement of plugins – even most of the knobs are in the same places. Let’s just say the soviet design seems to have been heavily inspired by the Tektronix one.

The soviet scopes don’t appear to have been as common in the east as the Tektronix ones were in the west, but there are some survivors in private hands. There’s a page describing the range on the TekWiki here, which includes links to some owners’ pages. However, there has been very little technical documentation available, and I’m fascinated by the technology: how much of it was copied, and how much was original?

Whilst looking for something else on allegro.pl recently, I came across a set of manuals for the C1-112 mainframe with Я4C-90 Y amplifiers, Я4C-91 timebase and 2K11 calibration plugin. They include circuit diagrams and other technical information. Bidding wasn’t exactly fierce and I got the lot for 5zł. The nice thing about them is that they’re in German, which is easier for a western audience to understand than Russian or even Lithuanian. Sadly they’re not quite the works of art that the Tektronix manuals are, but these even include the calibration certificates!

I’m very curious about what’s inside. In a future blog post, I hope to compare the Tektronix and soviet designs side-by-side and see what they have in common.

Super Breakout to JAMMA, Part 2: Colour

Having got the power supply working for my original 1978-vintage Atari Super Breakout PCB, it was time to get the screen looking right.


At the time Super Breakout was made, video games were mostly black and white. Colour screens were expensive, and so were the relatively complex electronics needed to generate a colour image. But black and white images don’t look too pretty in an arcade, so colour was added by sticking patches of various colours of clear plastic foil on to the screen. Simple, and pretty cheesy, but effective enough to get more coins into the machine. Games like Space Invaders used the same technique.

Fast forward to the 21st century, however, and black and white screens have become rare and expensive while colour screens are standard. My arcade game testing and playing rig uses a colour screen, and I didn’t want to stick coloured plastic on it – it would make other games look very odd! I wondered: how about adding colour to the image electronically?

I had recently been given a MachXO2 pico dev kit from Lattice Semiconductor. It’s a neat little thing, with a 1200-LUT MachXO2 CPLD on it and a built-in USB interface which makes it easy to program. The Lattice Diamond development software is available to download and license at no cost. I wanted to gain experience using this series of chips, since they seem to offer much better price/performance than the older Xilinx CPLDs I’ve used on several projects. Colourising Super Breakout seemed like a neat and vaguely useful example project.

Super Breakout produces a roughly NTSC-standard composite video signal. It’s thoroughly analogue, and there’s no way it could be connected straight to the CPLD. My task was to convert the composite signal into separate sync and video signals, which could then be processed using the CPLD.


The industry-standard LM1881 chip separates the sync from the signal. Its output on pin 1 is directly compatible with the CPLD. Getting the video information is a bit more tricky. The video output from the Super Breakout board is about 0.7V peak-to-peak, which isn’t enough to reliably drive any kind of logic gate. I took the easy way out and used an LT1252 video amplifier with a gain of just under 5 to generate a signal large enough to feed into a 74HC132 Schmitt trigger which produces a clean logic output. It was also necessary to add a clamp, the transistor in the top right driven from the blanking output of the LM1881, which forces the black level of the video to a known voltage. Without the clamp, the definitions of ‘white’ and ‘black’ would drift around depending on the picture content, which leads to peculiar black patches and streaks in bright areas of the image.

The resulting waveforms look like this. At the top, the composite video waveform from the Super Breakout PCB. In the centre,  the sync pulse at pin 1 of the LM1881 chip. At the bottom, the digital video ready to feed to the CPLD.

Here’s the circuit built on matrix board, next to the Lattice development board. There’s some more electronics at the bottom left, but more about that in another post.


Generating the colour signal was relatively simple: a counter, clocked at about 6MHz, reset by the horizontal sync signal, indicates the position along each line of video. Some comparisons of that counter with fixed values decide what colour the output should be. A simple AND function of the colour with the video input and – lo and behold – a coloured screen! And no plastic film in sight.





Servicing a Fluke 12 Multimeter

One of my most-used tools on the workbench is my Fluke ’12’ multimeter. I’ve had it almost 20 years, and it’s my favourite meter because it was clearly designed by someone who had to fix things. It’s rugged, especially in its bright yellow holster, and has so many thoughtful features: it has big buttons and a switch, instead of a rotary knob, so it’s easy to use with one hand. It autoranges quickly and reliably. In continuity and resistance modes, it automatically switches to measuring voltage if it detects one, so you don’t need to worry about changing ranges when debugging things. It doesn’t have fiddly extra features. It doesn’t even have a current range, because it would make no sense: measuring current usually involves breaking a circuit, which you can’t easily do when working on a circuit board. It switches itself off when unused for half an hour or so, saving the battery.


It’s been completely reliable apart from needing a new set of test leads last year (for the first time). However, recently the big, chunky buttons had become reluctant to respond, and needed firmer and firmer presses until they didn’t work at all. That meant I was stuck measuring either DC voltage or continuity. Time to pull it apart and see what’s wrong.

Removing the test leads and holster, then taking out the four screws, reveals the view you get when changing the battery. I’m sure Dave at EEVBlog, master of the multimeter teardown, would approve. Lots of chunky components and very solid construction.


The PCB is held in by the plastic clips at the top and sides. Easing them back lets it and the plastic frame underneath it come out:


The black plastic internal frame (under the PCB in this photo) is marked as being made of polycarbonate, so it’s very strong. None of that high-impact polystyrene rubbish. It’s nice that the membrane for the buttons bears against the frame, not straight against the PCB. This is nice industrial design.

The button membrane is connected by a zebra strip to the PCB. The PCB itself looks nice and clean, and the zebra strip is OK, but the contacts on the membrane look tarnished.


I wanted to test the membrane using the continuity check function of my trusty Fluke 12…oh, hang on, it’s in pieces. Break out the equally trusty Avo 8.


The membrane itself works fine. I cleaned up the contacts using DeoxIT D5 on a piece of paper. I also cleaned all the plastic parts, including the holster, in the office sink with washing-up liquid. Here’s the result, showing resistance mode to prove that the buttons work.


Looking good, working as well as it did when new, and ready for the next 20 years.

Sideways RAM for BBC Micro Model B+

There’s an excellent article by Mark at RetroClinic on adding 32K of Sideways RAM to a BBC Micro with just a couple of track cuts and wires. I’ve performed it on a BBC Model B, in order to replace a Solidisk SWR which never worked reliably even when it was new, and the modification works well. However, I wanted to do it on the BBC B+ I use in the office, and the details are slightly different.

The first thing to note is that the B+’s sideways ROM sockets are conveniently already wired to support 32K chips (27256 EPROMs). There is a row of links to the left of the ROM sockets which select whether each socket is configured for a 16K chip or 32K chip. Each link just selects whether pin 27 (A14 on 27256, /PGM on 2764 and 27128) is tied to +5V or connected to the bottom bit of the ROMSEL register IC45. If the link is to the left, pin 27 is tied high, and if it’s to the right, it’s an address bit. To be honest I can’t see any reason not to leave them all to the right all the time, since pulling /PGM low on 2764 and 27128 chips seems to have no effect other than disabling the chip. However, on my B+ they were all set to the left by default.

The layout of the parts on the board is:

    +------+ +------+ +------+
S19 |      | |      | |      |
S18 | IC62 | | IC68 | | IC71 |
S15 | ROM  | | ROM  | | OS & |
S12 | banks| | banks| | bank |
S11 | 8/9  | | A/B  | | 1/F  |
S9  |      | |      | | BASIC|
    | (S15)| | (S18)| | (S19)|
    +------+ +------+ +------+
    +------+ +------+ +------+
    |      | |      | |      |
    | IC35 | | IC44 | | IC57 |
    | ROM  | | ROM  | | ROM  |
    | banks| | banks| | banks|
    | 2/3  | | 4/5  | | 6/7  |
    |      | |      | |      |
    | (S9) | | (S11)| | (S12)|
    +------+ +------+ +------+

Note that S19 (for IC71) behaves a bit differently. IC71 is always the OS ROM, which is not a sideways ROM, but can be a sideways ROM (in bank 1 or bank F) as well. If S19 is placed to the right, IC71 should have the OS in the top 16K and a sideways ROM image, conventionally BASIC, in the bottom 16K. Which bank this ROM image ends up in depends on the setting of S13. If S13 is set ‘north’, the lower half of IC71 appears in bank 1. If it’s ‘south’, it appears in bank F. Normally bank F is convenient for BASIC.

The key ingredient of the sideways RAM upgrade is to provide a write enable signal to the RAM chip. This isn’t normally present on ROMs because it makes no sense to write to a ROM. The RetroClinic upgrade takes a write signal from IC77 pin 8. This is the write strobe for the uPD7002 ADC and the 8271 floppy disc controller. The corresponding signal still exists in the B+, and is used by the ADC and the 1770 floppy disc controller. It comes from IC27 pin 6, however, which is conveniently close to the ROM sockets.

However, there’s an annoying difference between a 27256 32K EPROM and a 62256 32K RAM. The EPROM has A14 on pin 27, but the RAM has A14 on pin 1 and /WE, the write enable signal, on pin 27.

On the Model B, it’s not hard to cut the tracks to pins 1 and 27 and solder wires on the bottom of the board. However on the B+, the tracks to those pins are on the top of the board, mostly concealed by the socket, and are a royal pain to get to. I could have desoldered the socket, done the modifications and put it back, but that was relatively risky and a lot of effort. I decided to just take pins 1 and 27 of my RAM chip outside the socket and run a wire from pin 27 to IC27 pin 6, and pin 1 to IC45 pin 14 (the source of the address line), on the top of the board. Here’s what it looked like.


My final discovery was that the Sideways RAM utilities are provided by the 1770 DFS ROM. My B+, however, only had DFS version 2.10, which doesn’t include them. I programmed a ROM with DFS version 2.26, and took the opportunity to shuffle the existing ROMs into 32K chips to save sockets.

This done, the SRAM commands were now available, but the Sideways RAM wouldn’t work. I’d put my RAM chip in socket IC57, which is banks 6 and 7. It seems that the B+ and DFS SRAM utilities only understand about Sideways RAM in the banks which the B+128 would have it: banks 0, 1, C and D, exactly the banks which are not covered by the ROM sockets. Doh!

I needed to move my RAM chip to appear in banks 0 and 1. This was easily accomplished by moving pin 20 outside the socket and soldering a wire from it to IC46 pin 15. IC46 is the address decoder for the ROM sockets, and pin 15 is its /0 output.

Now, when switched on, the machine shows ‘Acorn OS 96K’ (cute!) instead of its previous ‘Acorn OS 64K’, and the *SRLOAD command works to load images into banks 0 and 1.


In summary, to add 32K of Sideways RAM to a BBC B+ 64K:

  • Move S12 into its right-hand position
  • Bend pins 1, 20 and 27 of a 62256 RAM chip so they fall outside the socket
  • Put the chip with bent pins into socket IC57
  • Solder a wire from pin 1 to IC45 pin 14
  • Solder a wire from pin 20 to IC46 pin 15
  • Solder a wire from pin 27 to IC27 pin 6
  • Make sure your DFS is version 2.26

Now you have Sideways RAM in banks 0 and 1. Job done.


That syncing feeling: classic arcade games that won’t stay still

I’ve got a collection of classic arcade games from the ‘golden era’ of the early 1980s. They’re not the whole wooden cabinets with flickering lights and cigarette burns, but just the circuit boards from inside. They are easy to store and easy to plug in to a joystick and monitor to play.

However, some of them have always been a bit tricky to see. The monitor I use, a spiffy Microvitec bought surplus from Display Electronics in 1990, has fantastic picture quality but is a bit fussy about its input signal. Specifically, it seems to expect that the sync pulses – the bits of the signal which indicate where lines and pictures start – must conform more-or-less closely to broadcast standards. Unfortunately, the people who designed the old video games weren’t too worried about complying with standards. The result is that, on my monitor, some games tend to flicker and roll, or require very finnicky adjustment of the controls.

There’s loads of information about what video sync pulses are supposed to look like on the web.  This link has plenty of detail. However, the important things here turned out to be that the horizontal sync pulses should be fairly close to 4.7 microseconds long, and the vertical sync pulses should be pretty much three lines, or 192 microseconds, long.

I compared the outputs of various games – one which had never given problems (Mr Do’s Castle) with three which were troublesome (Phoenix, Pleiads and Q*Bert). The results were interesting. Here’s the vertical sync period from Mr Do’s Castle:


In all the scope pictures, the red trace is the sync output from the game, and the blue line is the vertical sync from my electronics which I’m just using to trigger the scope at the right time.

In this case, the narrow red pulses are the horizontal sync pulses, and the broad area between the dotted lines is the vertical sync pulse. It’s six lines, or about 386 microseconds, wide, which seems to be good enough to keep the monitor happy.

Examining Pleiads and Phoenix, which have the same video electronics, here’s the horizontal sync pulse:


Oh dear, It’s only 3 microseconds wide when it ought to be 4.7. And the vertical sync pulse?


It’s a full eight lines wide, or more than 500 microseconds. Those numbers are way off what the monitor is expecting. The result is that the monitor refuses to give a stable picture, which makes playing the game very tricky:


Looking at Q*Bert, its horizontal sync pulses are nearly three times as wide as they should be, at 12.6 microseconds:


and Q*Bert’s vertical sync pulse looks like this:


It lasts more than a millisecond! That’s miles off. The effect on the picture looks like this:


That’s as stable as I could get it. Notice how the left hand side (which is actually the top – the monitor is rotated 90 degrees)  is curved and wobbly. It was fairly hard to adjust the monitor to get the picture stable enough to take a photo.

There are solutions to these problems which involve modifying the game boards themselves, but I didn’t want to do that. I think they’re interesting historical artefacts (even the bootleg ones) and I try to keep them as original as possible. I wanted to fix the sync problems outside the board.

After a bit of experimenting, I came up with a little circuit which regenerates the sync pulses to be a bit more standard.


Untangling the rat’s-nest of wires, the schematic diagram looks like this:


The circuit is simple and cheap, using two standard TTL logic ICs. It ought to work with 74HC series chips as well, but some of the resistor values might need changing. The diode is any common-or-garden signal diode: a 1N4148 or 1N914 is fine. It works like this.

  • IC1B is a monostable which triggers on every sync pulse, generating a pulse 4.7 microseconds long. These become the new horizontal sync pulses.
  • IC2B combined with D1, R3 and C3 form a sync separator which triggers the monostable in IC1C only on sync pulses which are longer than about 40 microseconds.
  • IC1C is a monostable which generates vertical sync pulses about 200 microseconds long.
  • IC2E combines the new horizontal and vertical sync pulses into a new sync signal.

The output isn’t what you might call broadcast standard, but it’s close enough to make the monitor happy. I’ve tried it on a few games and it works even on games which the monitor was happy with before. Here’s the results from Pleiads:


Nice horizontal sync pulses, with a vertical pulse 193 microseconds long. There are a few extra pulses around but the monitor doesn’t seem to mind.


The results from Q*Bert are also good, though the vertical sync looks even more odd:


The vertical sync pulse is a sensible length, but there’s a long pause after it before the horizontal pulses start again. That doesn’t bother the monitor, though, and all the wobbles have gone away. Here’s the test setup in use.

Img_7777Sorry about the mess, but at least the display on the monitor is tidy. Mission accomplished.

Repairing a floppy disc drive

Why on earth would anyone want to repair a floppy disc drive? It’s quite a while since most of us bade them good riddance and started using USB sticks and Flash memory cards. However, I still use floppies from time to time, mostly with my trusty BBC Micro which still sits in the corner of the workshop.

Recently I was asked to recover some documents from some old 5.25″ BBC Micro floppy discs. The documents themselves were in an unusual format, about which more another time, but the first step was to simply get the data off the discs. The discs were 80-track ones, and I have a pair of Chinon FZ-506 80-track drives for the Beeb.


Out of all the various Beeb drives I’ve had over the years, I’ve kept these two because they’re housed in a compact casing with a mains power supply, and they have handy 40/80 track switches on the front, not hidden round the back. One drive has always been a bit reluctant to start spinning, but for occasional workshop use that wasn’t a problem. I’d got in to the habit of just opening and shutting the door a little which would kick the motor into action. However, when it came to intensive use backing up these old discs, which needed both drives, the failure to start became a real pain. I didn’t have a spare drive, and finding another one (especially in Poland) isn’t easy these days.

My curiosity got the better of me and I decided to open up the drive and find out what was wrong with it. It wasn’t hard to take apart and I soon had the motor revealed. Here’s a photo of it sliced into its component parts.


It’s a ‘pancake’ motor, so called because it’s (nearly) flat. The shiny silver bit on the left is the turntable which drives the disc, and it sits in a bearing. The next layer is the circuit board containing the windings and controller circuitry, and below that is the rotor which is a multi-pole magnet on a steel disc.

The motor is controlled by a Mitsubishi M51785P motor controller chip. The chip’s data sheet revealed that the motor has three phases, each of which has a coil to drive the rotor round and a hall effect sensor for feedback. This particular one is arranged with two coils per phase, but occupying 6/7 of a revolution, so the motor goes more slowly than the chip is driving it. At least, I think that’s what’s going on. Here’s a closeup of the circuit board. You can see the six coils, and the coloured wires I soldered on to measure things while the motor was running. Because of the way it’s built, it’s impossible to access most of the circuit board while the motor is assembled.


The controller chip seemed to be doing all the right things: its oscillator was running, and the outputs to the coils were doing sensible things. The coils themselves were all undamaged and measured the same resistance as each other. But I noticed something odd about the hall effect sensors. There are three of them, HG1, HG2 and HG3. I noticed accidentally that if I shorted together the two wires taking the output of HG1 to the controller, the motor still ran but sounded very rough. Not surprising. The same happened if I shorted the output from HG3. But shorting the output from HG2 had no effect at all. Aha! Only HG1 and HG3 seemed to be having any effect on the motor. I swapped HG1 and HG2 just to see what would happen, and the fault moved to HG1. That proved to me that I had a faulty sensor, not a faulty chip.

Where to get a replacement sensor, though? This drive was made some time in the late 1980s, and I couldn’t find hall effect sensors in today’s electronics catalogues which would fit mechanically and electrically. I had a rummage around the workshop and found a scrap 3.5″ floppy drive. A squint at the circuit board revealed a suspiciously hall-effect-looking device of the right shape and size nestled next to the spindle rotor, used for index sensing. Well, it had to be worth a try. I extracted it and fitted it to the 5.25″ drive in place of the faulty one.

Success! The motor now ran more smoothly, and shorting each of the hall sensors in turn had roughly equal effects, so they were now all working. Best of all, the motor started reliably every time. Interestingly it wasn’t as quiet as the other drive, but I suspect the scavenged hall sensor is optimised for magnetic fields from the side rather than the front, given how it was mounted, so it’s probably not perfect.

The last job was to realign the head slightly, because this drive was a bit fussy about reading some discs. I found a disc that it struggled with but that the other drive would read every time, and tweaked the position of the head stepper motor each way a little until this drive read that disc reliably. You can see in this photo that the stepper motor has elongated mounting holes, so it’s possible to loosen its screws (there’s another one just out of shot to the right) and turn the motor a few degrees to adjust the position of the head.


After all this work, the drive read all the discs I asked of it without any problems. I hope it’ll be OK for the next decade or two.

The BBC Micro linear power supply

The BBC Micro is the machine that really got me into computing. It was designed by Acorn in a tearing hurry for the BBC Computer Literacy Project in 1981 – see the film Micro Men for a great dramatization of the story. Because it was done in such a rush, some things weren’t quite finished for the early machines. One of those things was the power supply.

Why is the power supply interesting? Well, later BBC Micros had a perfectly normal, reliable switch-mode power supply, not dissimilar to the one in a modern PC. But early ones had a linear power supply, which gained a fearsome reputation for overheating, exploding, and being incompatible with nearly everything. I recently dragged an early, Issue 2, BBC Micro out of the loft and realised it contained this elusive beast, the linear power supply. I thought I’d see what all the fuss was about.


First things first: does it work? Well, yes. Despite not having been switched on for probably 20 years, it powered up absolutely fine. Boop-beep. No smoke or flames. Good. Time to see what’s inside.


This particular machine started life as a model A, with just 16K of RAM and very little else. However, it got upgraded at some point into a model B, with the full complement of RAM, and has an unusual Opus double-density disc interface in it as well as a couple of extra ROMs including Wordwise, the word processor. That’s the sort of thing that’s supposed to be impossible with a linear power supply. You can see the black box on the left – later power supplies are a gold colour. Here are a couple of closeups of its labels:

DSCN8939  DSCN8931

I took it out and examined it. The first thing I noticed was that it doesn’t really fit very well: there are some odd washers sandwiched between the power supply and the case, and they’ve used nylon screws to fit it, for some reason.

DSCN8942 DSCN8943

You can probably see from the side view that it’s riveted together. The rivets were quickly dealt with, revealing the insides:


Pretty straightforward stuff: a toroidal transformer, rectifier and smoothing, and a row of regulators. Here are closeups of the circuit board and regulators. Note the little orange tantalum capacitors. I suspect that’s where the reputation for explosions has come from: when used like this, they do tend to fail short-circuit and go off like little fireworks from time to time. These ones, however, are rated at 35V, so with only 5V or 12V across them they should be fairly reliable.

DSCN8950 DSCN8949

It didn’t take long to trace out the circuit.


What is interesting is the way the 2.25A output at 5V is achieved. Rather than use one big regulator, they’ve chosen to use three 7805s, rated at 1A each. Three sets of wires leave the power supply and are delivered to three places on the Beeb’s motherboard. There is no connection between the three 5V rails on the motherboard according to my meter. They’re entirely independent. This is good, because it means that the three 7805s won’t end up fighting each other if their output voltages are slightly different.

I took a few measurements while the machine was running. The voltage on the 4700uF smoothing capacitors was 10.5V. The currents delivered by each output were:

VCC1: 0.49A
VCC2: 0.75A
VCC3: 0.84A
-5V: 15mA

I tried removing the disc interface to see what difference that made to the power consumption. It reduced VCC1 to 0.35A and had no other effect.

The total current flowing at 5V is just over 2A, which is sailing fairly close to the wind given that the power supply is only rated at 2.25A. However, nothing in the power supply is under great stress, and I see no reason why it should fail unexpectedly. Unlike a switch-mode supply, it would also be easy to repair. If it was going to be used for a long time, I’d be very tempted to replace the 1uF tantalum capacitors with modern ceramic or electrolytic ones, just because the tantalums do tend to commit suicide randomly with old age.

These machines are now becoming collectable, and early ones like this are worth preserving in their own right. If you’ve got an early Beeb, there’s no reason to fear the linear power supply and replace it. It’s part of the story, and it’s possible to keep it going more or less indefinitely.