The subject of this page is the inability to trigger on any signals, due to the failure of IC1 in the timebase circuit. I've put the remaining faults on a separate page...

While this section might seem rather obscure (even by this sites standards!), I've found that the information here may be useful to other people out there who have the same 'scope. These are well-built pieces of equipment, and I guess that there are many of them still in use...

Apparently IC1 had failed a couple of times previously - the analogue 'front-end' of this custom-made IC seemed to be vulnerable... Having said that, I haven't heard of another instance of IC1 failing, so I suspect I might have had another fault with my scope. Looking at the diagram it's easy to see how a problem in the proceeding stage (TR2, etc) could damage the IC.

Back at uni, we had an identical scope which worked perfectly - none of the lab-techs there could recall that particular scope ever going wrong. Also, despite having had lots of correspondence regarding this model over the last few years, I've not come across another instance of this.

Anyway, by 1990 the IC was no longer available, so the instrument was 'thrown out' in my direction. It had 2 dual vertical plug-ins giving up to 4 traces and a storage tube, so it was too good not to repair.

The only answer was to design a replacement circuit for IC1. It has to do a reasonable amount - in addition to accepting the analogue trigger signal and controlling the sweep generator, IC2, all timebase functions, such as auto-bright-basline, holdoff, single-sweep and X-Y lockout are controlled by IC1. I began by running a ribbon cable from the IC socket to a prototype board. This evolved into a Veroboard prototype (which worked well for 7 years) Recently, as part of a total overhaul, I decided to fix the final bugs and produce a PCB version.

Vero Prototype:

This bodge worked well enough for a few years, but was not perfect. The performance of the comparator could have been better (due to poor layout, of course). Also, at fast sweep-rates, you'd get double-imaging and instability - I found later that this was caused by insufficient hold-off time (hard to measure without another 'scope!)

Once these and other minor details had been sorted, I was able to think about producing a PCB version. I spent ages looking at the analogue trigger circuitry on the scope - the various signals are switched by diodes and inverted by a single-transistor preamp stage (TR2), before level-shifting diodes apply the signal to the IC...

After taking lots of measurements, I decided that this transistor stage had to go. One reason for this is bandwidth - as the original IC was only able to trigger up to 15MHz, this stage was equally slow. It was the only bandwidth bottleneck in the system - the plug-ins are 50MHz (although the Y-amps of this scope are only 18-20Mhz). LS/HC TTL is capable of at least 40MHz, and I was planning to use a high-speed comparator, so it seemed silly to throw away performance.

Examining a similar scope (from the same range, and using the same vertical plug-ins) revealed that the trigger amps in the plug-ins seemed to expect a 75 ohm load. I removed TR2 from the timebase board, and connected a 75 ohm resistor in its place (which probably wasn't too big a change, because TR2 is a CE stage with the emitter connected directly to ground). I then developed a better preamplifier, which used 5 transistors, had a gain of around 10, and was pretty flat up to at least 30MHz (the limit of my function generator).

Next to think about was the comparator, and a little bit of research led me to a device made by Maxim. The MAX909 has built-in trigger-points set at ±2mV - this means no external resistors to worry about. This got me thinking - the low threshold points will result in good sensitivity, better than I would have risked if I was using external components in the conventional manner. Just how necessary was my pre-amp?

A breadboard test revealed this device gave excellent results - trigger sensitivity was around 0.2-0.3 divisions at l.f. (better than any scope that I was able to test at work), and it performed well up to at least 30MHz. It also didn't seem to suffer from spurious triggering (the danger with a sensitive comparator). So this IC enabled me to drop a whole load of analogue circuitry, providing an easy solution to a potentially tricky problem...

This enabled me to begin laying out a PCB. Once I had fixed the position of the major components, I removed the timebase module, and stripped it down for easy access. A total of 26 components were removed from the PCB:

Some minor modifications to the metalwork were required. Two holes in the top panel hold the PCB via small right-angle brackets. While I was at it, I drilled some holes above the X-output stage to help improve ventilation.

The copper-clad board offers screening from the fast-edged TTL signals that float around the board. A sheet of insulating plastic is essential, of course...

The cut-out in the copper-clad board was already there, but it fits perfectly with the ribbon cable that connects to IC1's socket. This cable is kept short to avoid problems. The analogue trigger signal enters the board via a short length of 75 ohm cable and an SMB plug. This is terminated with a surface-mount resistor directly under the socket on the PCB. This works well, despite the SMB connector being 50 ohms. I guess 30MHz is still DC as far as the SMB connector is concerned...

This picture show how the new PCB fits into the timebase chassis. The two Molex connectors are for the front-panel LED and ±24V power supply. The existing 5 volt power supply (a BC109 and zener diode) has been removed, replaced by a 7805 on the new board. Also, the 'single-sweep ready' LED and connections have been removed in favour of a dual-colour device. In addition to the normal red LED, there's now a green trigger-signal indication...

This shows the (almost) completed board. The hole between the two tantalum capacitors (bottom left) is to enable R2 adjustment, without having to remove the board.

The layout might not look all that compact, but I really did get the board as small as possible without going to much tighter design-rules. The main priority was to ensure the board was easy to etch and assemble.

I added a small heatsink to the 7805, as you can see in the next pictures. It only dissipates a watt or so, but my el-cheapo in/out thermometers claim the ambient temperature near this board is in the 60-65°C region with the covers on. The only other components that get warm are the three resistors near to the top-left Molex connector. TTL LS runs reasonably cool in applications like this...

This shows the final timebase assembly, just before installation back into the 'scope. I think it's worked out really well - there's just enough slack on the cables to operate the unit with the additional PCB hanging to one side for access, and it doesn't look too much like the total bodge that it is!

Finally, this picture shows how the hole (mentioned above) enables calibration of R2 without disassembly. That worked out perfectly, but I have to admit that luck was on my side as it was quite hard to make any accurate measurements...

I haven't got around to producing a decent copy of the circuit, but if anyone is interested, I'll make a copy of it available here. If you've got this model, and think it may be suffering the same fault, then I'd be happy to discuss it with you, and help you with your own PCB.

The only remaining problem with mine is the tube has aged, and the storage functions don't work as well as they should. Part of the calibration routine is to adjust the storage cathode current (there's separate heaters and guns for the storage part of the tube), and I couldn't get close to the required value... But it's quite usable for most tasks.

On to the remaining faults...