During the recent overhaul, I fixed many minor faults. Here are the ones I that can remember:

Hum on trace:

This caused much headscratching, and I can't claim that it's entirely fixed. It got really bad about a year ago, which is what prompted the overhaul.

I started by doing a blanket replacement of all of the electrolytic capacitors in the instrument. This might seem a little extreme, but bear in mind that they were all dated 1974, and obviously well past their prime. Also, there wasn't actually that many, so it wasn't too expensive or time-consuming...

After that, things were better but not perfect. Looking back at the notes I took, I had 350mV, 100mV and 150mV of ripple on the 105V, 24V and -24V rails respectively. The C-change had brought this down to 130mV, 25mV and 15mV. The 105V rail still seemed too bad, so I changed TR416 (an MJE340 that runs very hot) which brought it down to 50mV. Further changes made little difference, and I still had visible ripple on the trace.

After much thought, I realised what was going on. As with all the best faults, it's obvious with hindsight! The waveform of the ripple had been confusing me because it wasn't the normal 100Hz sawtooth that you might expect. Rather, it was pulses, positive and negative going - the classic full-wave rectification current waveform. This picture will explain all:

The power supply connects to the mother board (PC175) via edge-connector SK802. The only earth connection is via pin 5 - nothing too worrying about that in this particular application... until you notice the bridge rectifier hidden away on the mother board!

With age, the contact resistance had risen. The capacitor charging current pulses were causing a voltage to appear across it, thus causing the ripple. The answer was to remove the connections from that winding of the mains transformer, and install a tightly-twisted pair directly from the transformer to a Molex connector. The other half of this connector is soldered directly to the diodes on the mother board. This has more-or-less cured the ripple - I've run out of energy to chase any residual for now!

No X-Y mode, no alt-triggering on RH plugin:

The same fault caused 2 symptoms, although you can be forgiven for not immediately seeing the connection. It was complicated in my case by my Veroboard bodge - I couldn't remember when the X-Y mode stopped working, or even if it ever worked since my initial 'repair' - shows how much I use that mode!

After lots of measurements, I decided that my Veroboard version of IC1 was doing the correct thing in X-Y mode. So, time to try and figure out all that diode switching...

Returning to alt-triggering: The 'scope presents the timebase module with 2 trigger sources - LH and RH. However, the mother board can obtain each of these trigger signals from two places, depending on the 'Alt-control' signal from the appropriate vertical plug-in (the voltage on pin 6 of each vertical plug-in edge connectors). There's a range of different modes available, depending on the vertical plug-in:

• V1 - standard single plug-in: 'Alt-control' signal is high, the mother board takes the trigger signal from the main Y signal (via TR805/806 or TR807/808).
  
  
• V5 - single delayed plug-in: The trigger signal needs to be taken from before the delay-line contained in the plug-in. In this instance the 'Alt-control' signal is low, and this causes the (balanced) signal to be taken from pins 7 and 10 of the Y plug-in.
  
  
• V4 - dual plug-in: The 'Alt-control' signal depends on which trigger source you select. You have a choice of 3 modes - channel 1 or 2, or Alt triggering. The latter lets you trigger on two completely different signals at once when you are viewing them in 'Alt' mode, and achieves that by triggering from the main Y-signal (pin 6 high). However, if you are triggering on channel 1 or 2, the scope needs to accept a trigger signal on pins 7 and 10 of the Y plug-in (pin 6 low).

To complicate matters even further, when the scope is switched to X-Y mode, the X-amp is fed with the RH trigger signal. To ensure consistent operation, the switching is arranged to force the RH trigger source to be via TR807/808 instead of pins 7/10 of the RH plug-in. This overrides the 'Alt-control' voltage from the actual plug-in.

The way all of this is achieved is slightly strange (to me, at least), and understanding it is not helped by the diagrams. But once you've worked it out from the rather terse circuit description, you begin to see how the two faults are related. TR807 (an MPS6518) was the culprit. So, as always, a simple cure to a complicated problem!

Reduction of trigger sensitivity on one channel:

This fault relatively appeared recently, and it was a simple matter to prove that the fault was in the plug-in. I started trying to examine the trigger circuitry in the plug-in, but based on my experience of the above fault, I decided to change the obvious things before wasting time trying to work out the details. I was a good plan, as I found the fault almost instantly - another MPS6518 (TR758).

BTW, these transistors aren't easy to find, but I found that a BC327-25 works perfectly. According to Towers, the only spec that is worse with the BC327 is Ft, but that doesn't seem to cause a problem - despite checking, I've not observed a loss of h.f. triggering sensitivity...

Reduction of deflection, beam present at minimum intensity:

This fault occurred some years ages ago, and it was easy to realise that the EHT had gone high. The supply is generated and regulated by a 3-transistor circuit in the metal EHT can at the back of the unit. TR302 conducting more has the effect of reducing the EHT. R311 (120K) looked suspicious, having around 100V across it, and it was a good hunch - it had gone O/C.

Variations in EHT voltage:

This drove me mad while trying to calibrate the unit! Initially, I noticed that if you used the storage modes, the trace was a bit soft when you selected normal (non-store) mode. Also, it seemed to go slightly off-calibration. It took be a while to notice that the two symptoms were connected, but once you've made the connection, it was obvious that the EHT was changing. It seemed that changes in beam current, caused by the storage modes would trigger it. Power-cycling the unit would fix it, until the next time...

The EHT generator is the worst part of the scope to work on. So, before spending time thinking about the circuit, I took the easy route - replacing the 2 socketed BC109's fixed it...

DC BAL Mod:

You'll probably notice how hard it is to set up the front-panel DC-Bal control on these units. The control has far too much range, and as soon as you get it right, it drifts off again. This in turn affects the remaining DC-bals inside the plug-in

Looking at the diagram, you'd think it would be a simple mater to change R603/R604 (220K) to reduce the range of the control. Not so! Due to the way these things are put together, it's impossible to get to the resistor without unsoldering lots of other wires...

The easiest solution is to solder an additional resistor directly to the potentiometer - lift the centre (wiper) contact, and solder a 1M resistor in series. On 1 of the four channels (shown here), I couldn't quite get enough adjustment, so I had to add another 1M resistor, again directly on the pot, between the new resistor and the end of the track...

Having done this, I subsequently discovered that later models have addressed this issue by adding a 680 ohm resistor in parallel with R605/R606, reducing the value of these from 1K2 to around 430 ohms. This is better than increasing R603/R604, as my mod does, because it's always good practice to minimise impedances wherever possible. This picture shows how Telequipment applied this mod:

Onto the next section - 'Scope 2 - with even more fault reports.