As a result of posting my experiences of the DM63 on this site, I've received a surprising amount of e-mail concerning this unit. This was nice because I felt sure there was an audience for this material, however small.

I've also received offers from a number of people, and as a result I know have another DM63, and a few interesting goodies such as a V3 Differential Plugin.

In the course of overhauling the new arrival, I've discovered a few new faults, so hopefully this section can continue to be of some use to all the Telequipment users out there.

The picture opposite shows the unit as it arrived on the bench. It had been stored in a garage for the last few years and is in need of a good clean and some TLC. But it basically appeared to be in sound condition.

Upon powering up, a trace appeared, but with a lot of hum! I quickly found that I could trigger on the mains hum, so at least IC1 was OK.

This screenshot gives you an idea of the problem. It's pretty bad...

Another problem that happened intermittently - the horizontal sweep would sometimes collapse to just a couple of divisions wide, and display nothing meaningful.

So it was clearly in need of some major power supply work, and the timebase section needed a good looking at. But, even with these faults, the tube appeared to be in good condition - the trace was nice and sharp and the storage modes seemed to work really well.

A quick check showed that the +24V rail had serious amounts of 50Hz ripple. This wasn't quite what I was expecting - dead smoothing capacitors would exhibit 100Hz ripple on the output. Also, the heatsink associated with the two 24V regulators seemed too cool, based on how my existing scope behaved. So, I completely removed the PSU module from the unit and removed the PCB from the metal chassis for a closer look.

As you can see, C405 is a bit unwell! But look closely at D406 - yes, that is a crack! So, this explains why there is 50 rather than 100Hz ripple on the output.

Here's the cleaned up PCB, complete with a new set of diodes - I changed them all to eliminate any doubts about their future reliability. The other picture shows the state of the old capacitors, and compares them to a modern replacement. If you look carefully, you can read "7805" on the cans - this is a date code (week 5 of 1978), so it's perhaps not surprising they're a bit past it!

Once I'd finished visually checking and rebuilding the PSU, I decided to not reinstall it in the 'scope mainframe just yet. Instead I powered it up on the bench so that I could take some voltage measurements - just in case... I was glad I did! The +24V output was more like +35V!

Upon investigation, it seemed the error amplifier wasn't working. TR408 was dead - base-emitter short - so the pass transistor TR405 was saturated and passing the full unregulated output. This explains why the heatsink was too cold before.

While the power supply was out, it was time to attend to the motherboard. The first task is to change all the electrolytic capacitors - this shouldn't need explaining or justifying.

This picture shows the motherboard, free from the brackets and fixings but still attached to the wiring harness. As you can't easily remove it completely from the instrument, it's tricky to work on. You can see the connector for the PSU on the bottom of the PCB, and the pins belonging to the vertical plug-in connectors. This board contains the Y output stages - you can see that the output transistors for channel 1 have been replaced at some point in the past. Unfortunately, the replacements (with their incorrectly fitted heat sinks) aren't up to the job, and the 'scope only manages to reach 7MHz on this channel! I've done a brief survey of a number of these 'scopes and found a variety of different transistors fitted in this role:

Looking through the various versions of the manuals manuals I can find, it seems that only the BD419 and FRB750 devices were the only "official" devices. The BD419 is the best bet because it is best equipped to deal with the heat dissipated, but I haven't managed to source it yet. The problem is the pin-out, as the base terminal is in the middle - this is unusual, as most power devices have the collector in the middle. If you're willing to bend the leads into the required order, you probably can find a large selection of transistors that will work well - devices used in video output stages would be a good bet.

This picture shows the top section of the motherboard, where you can see the connector for the timebase module and the smoothing capacitor for the timebase power supply. It's nice to see that the right devices are still available. I ordered the following parts from Farnell:

The right image above shows the insides of the EHT module. This is quite possibly the worst part of the 'scope to work on - there are no connectors, the unit is hardwired into the main wiring harness. Getting behind the main PCB is very hard - you have to remove the power transistor, and desolder the two high-voltage diodes that connect between the PCB and the transformer. But it's essential to replace the two electrolytic capacitors, as 27KHz pulses caused by the switching action of the unit can leave the unit and contaminate the rest of the 'scope. When this happens, the trace can appear to be a dotted line if you get the right combination of timebase speed and brightness control settings.

Another problem that occurs in here is due to all the high value resistors with high voltages across them - these go high-value or open-circuit with age, causing all sorts of strange faults. I've already mentioned R311 on the last page, but there are lots of other candidates. Basically, if there is any fault that affects brightness, focus or storage operations, then check all these resistors. I saw a really obscure fault caused by an open-circuit R325 (3M9), causing a screen that constantly flooded when the scope was in NON-STORE mode.

Cosmetic restoration:

These old workhorses can get quite dirty with age and use. Personally, I like to regard a full cleanup as an essential part of the overhaul process, and here are some pictures to show what's involved:

This picture shows how grubby these things can get. Dirt and grime gets stuck in the control knobs and on the front panel. The second picture should leave you in no doubt as to the extremes you need to go to! It's time consuming, but ultimately very rewarding.

The rest of the chassis and all the other controls get the same treatment. As you can see, the front sub-panel is slightly bent - this was caused by dropping the instrument at some point in the past. From what I've seen and heard, this is a reasonably common occurrence with these - the tube is clamped at the rear of the instrument and is intended to push against this front panel. It's easy enough to straighten out the panel, but as you can see there is a serious amount of dismantling to be done before you can do the job.

To conclude, this unit has been a useful addition to the workshop. The overhaul detailed above, coupled with lots of time and switch-cleaner, plus a full re-calibration has brought this old-timer back to life. The tube is in much better condition than my original unit, and the storage functions work perfectly. To complete the restoration, I just need to source some better Y-output transistors.