Wednesday 27 January 2016

Telequipment D32 trace rotation fix

Another ancient piece of equipment I have is this Telequipment D32 mains / battery operated portable oscilloscope.

Telequipment is an old brand name used by Tektronix in Europe. I think it was made in about 1972 for servicing TVs. I bought it in a broken state from a friend of a friend in the mid 1980s. The six NiCad D cells had gone short circuit and the mains transformer was burnt out. I replaced both of those and as far as I can remember that was all I had to do to get it working.

I did get a lot of use from it as for many years it was the only working scope I had. I haven't used it much in recent years though because I now have several much better scopes. It is the only battery powered one I have, although I do have two USB digital scopes which are battery powered if I run them from a laptop.

I powered it up fully expecting the NiCads to have died again, but amazingly they still work to some extent. The only thing wrong with it was the trace was slanted and the trace rotation control didn't have enough range to correct it fully. It seemed to max out in one direction and then have a large dead band.

I did a bit of research and managed to find the schematic. I must admit that somehow the fact that a beam rotation circuit is needed because of the earth's magnetic field had completely passed me by, or else I once knew and have since forgotten. This is despite building my own scope with valves (vacuum tubes) when I was 13 and using CRT scopes for many years after.

The rotation circuit is below. After looking at it and making some measurements I was surprised to find it to be a design fault rather than a faulty component.

The base of TR301 can be varied between ±7.5V with R303, so one would expect the emitter follower to drive the coil over that range with a VBE offset. The problem is the coil has a resistance of about 1kΩ, so the pull down resistor R314 can't drag it below -3.5V. This means the control has no effect for a significant part of its travel and the range of the correction is asymmetrical.

To fix the problem I removed R314 and replaced it with a PNP emitter follower. That gives push-pull symmetrical drive between about -6.8V and 6.8V. The only dead band is a small one around 0 due to the VBE drops (classic crossover distortion). It also has the advantage of not wasting the current through R314, all the current now flows through the coil.

This is a view of the board before the mod. It is ancient technology, all discrete transistors and hand routed PCBs, including vias made with what looks like soldered in brass rivets.

I don't know why all the transistors are socketed. The only other time I have seen that was in a Russian transistor radio, but they were germanium PNP transistors, so may have been unreliable. These are silicon planar epitaxial so should be reliable and never need replacing. Perhaps they did it because they thought soldering would damage them. Or perhaps they were just old school designers used to valves.

Anyway the scope is a joy to work on because all the PCBs hinge out to allow access to both sides with the scope still operational. See for a look inside a D32.

Here is a view of my modification: -

I replaced R314 with a 2N4403 (any PNP transistor should work) and linked the base to a handy via. Don't worry about the proximity to the ceramic capacitor's lead, it is the same net.

So a simple mod but I don't understand why I need it now. Perhaps the earth's magnetic field has changed since I last used it. It was about a decade ago!


The scope has always had a delay between switching on and starting up. The power LED comes on about a second after it is switched on.  This got longer and longer and then it stopped starting reliably at all. I had always assumed the delay was caused by a capacitor charging in a bias circuit for the inverter but on examination of the schematic I fount that wasn't possible because all the capacitors have a low impedance supply.

It turned out the be the on off switch. I measured about 3V across it in the on state. The contact resistance measured 0.3Ω but that should only drop 0.3V at 1A, so it seems to be a strange non-linear resistance and it must get lower over time as it heats up.

The switch is part of the brightness potentiometer and it is operated by pulling the knob out and pushing it in. Normally potentiometers with switches operate it at the start of the rotation but this allows the brightness setting to be retained. It is also a small form factor so I didn't think I would be able to find a replacement. As it is riveted together I didn't think I would be able to repair it or even get any switch cleaner into it.

It is actually a two pole switch, the second pole is used to change the charge current of the battery to include the scope current when on. I figured 0.3Ω would not affect that circuit as it has 150Ω in series, so I swapped the connections over. It now starts instantly although it does take time for the tube to warm up of course.


  1. When I was training, people used to warn us of avoiding damage to the muMetal screen. A hypothesis is that the MuMetal screen has been banged, so the beam is now more affected?

    P.S. I never hit one to find out if it was true! I would have lost my job, they were ohh so expensive.

    1. After hinging down the PSU board that covers the tube to fix another fault (the on off switch gone high resistance) I discovered the tube is loose and can rotate easily, so perhaps it just rotated.

      I don't think the MuMetal screen has been damaged as it is totally surrounded by aluminium chassis on three sides and the PSU PCB underneath.

  2. Yeah, it used to happen, usually after a trip in the boot of a car. Servicing them was when I was warned about not damaging the screens. At the time I posted the comment I could not recall why I knew about muMetal. Today at work, I remembered why we took the screens out. To twist the coils..... Hence my popping by. Glad you got it sorted.