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Restoration oscilloscope Picoskop EO1/7

A small oscilloscope, a Picoscope EO1/7 is in my possession for a long time. When I bought it maybe 20 years ago for nostalgic reasons for little money, I got an extra chassis as a spare parts donor with it. This chassis was rusted, bent, some parts were missing or broken. Of course, there was no tube left either.

During a cleanup this spring, this chassis was supposed to go into the scrap. Before I had the heart to do that, I made an inventory:

  • all tubes incl. the picture tube are missing
  • selenium rectifier missing
  • no front panel and no case
  • rust and dirt on all parts
  • potentiometer with power switch, mechanics bent, Bakelite switch housing broken
  • potentiometer rear cover missing
  • fuse holder incomplete
  • MP condenser one terminal broken, oil leaking out
  • wiring harness partly removed by brute force
  • one ceramic tube socket broken
  • two resistors destroyed
  • few capacitors missing

But the power transformer, the filter choke and a smaller high voltage transformer were apparently still intact. Furthermore, both rotary switches for X and Y were OK. These are quite good conditions for a rebuild I thought. Unfortunately I didn’t take a picture of the original condition. It really looked like a pile of junk.

The beginning

First, I disassembled the unit as much as possible, cleaned it properly and partially derusted it. After derusting, I sealed the power transformer with an alkyd resin lacquer. Basically you should use a high voltage insulating varnish for this. Since this is not so easy to get privately, it does also e.g. a good colorless boat varnish as in my case.

The next step was to repair the defective components. The leaking MP capacitor got a new terminal lug, luckily a small piece of the old terminal was still sticking out. After that, the thoroughly degreased connection was sealed with epoxy resin.

I also glued the broken tube socket with epoxy resin. In the picture you can see that behind one of the two replaced resistors.

It is important to use the “normal” epoxy resin for bonding. This is resin and hardener, which you usually have to mix in a ratio of 100:60 to 100:40. The curing time is 24 to 48 hours. There are also all kinds of fast-curing epoxy-based adhesives. In my experience, however, these do not adhere as well to ceramics and Bakelite.

Bakelite is the keyword for the next repair, the potentiometer with the power switch. Here I could straighten the mechanics again. Fortunately, all fragments of the switch housing were available. Bakelite can also be glued excellently with low viscosity epoxy resin. Afterwards, the glued area is usually more stable than the rest. After cleaning, lubrication and contact care with Neo-Ballistol, the potentiometer works like new again.

Lastly, I made a new rear cover for the second defective potentiometer. In contrast to the original, I made this from aluminum instead of sheet steel. Thanks to my electromechanical training in the 80s, I can do such work relatively precisely.

This saved the components that could be saved. The function of the two defective pots is completely restored and because of the presumably better lubrication, the expected service life is possibly also longer than with the original.

Spare parts

Some spare parts I had to order. The fuse holder is a standard model from the GDR and still relatively easy to find on the Internet. From the tubes ECF82 I was able to purchase a lot of 8 pieces very well preserved copies from Telam at a good price. It was important to me that all tubes are one make. I think this is a good idea for metrology.

A bit more difficult was to get a cheap picture tube, the B7S1. The offered ones were too expensive for me and that with uncertain function. By chance, I came across an O7S1, compared the base scheme and characteristic values and was of the opinion that the O7S1 is a direct predecessor of the B7S1 from the time before 1945. In the end, I had to find out that this is not so. The base is turned by 180°, the cathode is not led out individually and the three grids have also somewhat different functions. I described the test of this tube in a separate blogpost. So in the end I had to look for a B7S1 and with some luck I found a new and original packed one, even the warranty certificate was still there.

B7S1 OVP, Kassenbon und Garantieurkunde

Reconstruction

First I dedicated myself to the power supply. Also to test first if everything is ok with the hard to repair transformers. Unfortunately the selenium rectifiers were missing and I had to replace these two two-way rectifiers then by four diodes 1N4007 in bridge circuit.

Also, the wiring harness of the power supply was completely missing. I rebuilt it pretty much original. However, instead of the double-wound wire, a modern alternative with ETFE insulation was used. I dedicated a whole blogpost “The wiring harness” to this wiring variant. Of the two electrolytic capacitors in the power supply, one axial 10µF/350V was unsalvageable due to a broken connection directly at the case. In its place, a modern radial variant with 450V dielectric strength is now doing its job.

EO7/1 ausgetauschter Elko

I reformed the 50µF/350V cup capacitor within two weeks. This worked wonderfully, at nominal voltage it had only 80µA leakage current at the end. I have dedicated a whole article to the forming of electrolytic capacitors.

Unfortunately I did the first test of the power supply without load. Because of this and the lower voltage drop of the new rectifier bridge made of silicon diodes, the anode voltage had about twice the nominal value at no load. It went poof briefly and the elaborately formed cup electrolytic capacitor was history, really a pity. At least I learned that you should never test power supplies of tube devices without load and how to restore a cup capacitor or how to hide a modern capacitor in it. There will be a post about that here soon, too. After this repair the power supply was complete and worked perfectly.

The next step was to restore the other wiring in the unit. In addition, a total of two defective resistors had to be replaced.

EO7/1 Kabelbaum oben

I have exchanged all paper capacitors for safety reasons and replaced them with film capacitors. I was lucky and was able to replenish my supplies thanks to Markus from Hamburg, who offers many different values at “ebay Kleinanzeigen”. But I felt too sorry for the basically still working paper capacitors to throw them away just because they have a slightly too high leakage current. I documented my attempt to make them fit again in the article “Paper capacitors restore“. This is certainly better than replacing them, since some of the circuitry is designed to handle these higher leakage currents.

Finally, I just put in the four ECF82 tubes and the picture tube, and voila, everything worked flawlessly right away. Now the front panel and the case are missing and at the end the picture tube should show the time in analog form. But that’s a new project.

 

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Tie wiring harness

In older devices, for example measurement technology, one often finds cable harnesses tied by hand. This refers to a number of wires that are knotted together with so-called binder twine to form a stable structure. This type of wiring has to be done manually and is therefore very time-consuming. For this reason, it is essentially only used in the aerospace industry today. There are various other ways of producing wire harnesses, such as with cable ties or hoses, etc. But this article is only about the original method with twine. I can imagine that the first wire harnesses were made for telephone exchanges. In any case, the builders of analog switching technology were true masters at crunching wires together.

Basically, cable harnesses have several advantages.

  • The cables are more stable when tied together.
  • Less space is needed for the cabling.
  • Errors during assembly or maintenance are avoided.
  • Overall, the result is very good fail-safety.

Recently, I restored an old oscilloscope and had to add the missing harnesses again. That was the reason why I dealt with this topic in more detail. Here in the picture you can see the first result and I am overall satisfied with it.

First condition to tie a harness yourself is the right twine. In the past this was a waxed thin natural yarn, probably jute, sisal or flax. In very old telephone switching equipment from the late 20’s or 30’s, which I cannibalized as a kid, you can still find such yarn. Nowadays the material is something like dental floss with a lot of wax. There are only a few suppliers left who supply end users with it. SEGOR-electronics GmbH from Berlin carries it in its assortment under the name “Abbindegarn“. As “Abbindeschur” I bought my supply at Bürklin Elektronik. I also strongly recommend to use a binding twine explicitly made for wire harnesses. Other materials do not work as well and may not be stable over time.

I came across an alternative variant that I still like: durable yarn, probably star twine is dipped in hot beeswax.

Now the art of knotting is still missing. For an easy start I recommend to follow the guidelines of the FAA, the “United States Federal Aviation Administration”. This has published in its “Advisory Circular” 43.13-1B, Chapter 11 “Aircraft Electrical Systems” instructions that contain only two variants of tying and are so simple and clear.


Quelle: FAA AC 43.13-1B, Chapter 11

Another tip: You can build up the wiring harness in at least two dimensions on a board with small nails. To do this, hammer in a small nail at each bend and at each branch. The wires can then be bent along there, then tied. If the shape of the tree goes into the third dimension, it becomes more difficult and I would wire and bind it directly in the device.

At mikrocontroller.net you can find a thread which also deals with this topic. From this thread come the following multiple variants for node binding. In the end you have to decide for one variant. I decided like this:

  • Initial knot: “Form A mit Sicherheitsknoten”
  • Intermediate knot: “Form B für glatte Oberflächen”
  • Final knot: “Durchschlungener Schlussknoten”

Grundknoten, Quelle: www.mikrocontroller.net
/attachment/49981/Blatt412_Grundknoten.png

Verlängerungsknoten, Quelle: www.mikrocontroller.net
/attachment/49982/Blatt413_Verlaengerungsknoten.png

Anfangsknoten, Quelle: www.mikrocontroller.net
/attachment/49983/Blatt414_Anfangsknoten.png

Zwischenknoten, Quelle: www.mikrocontroller.net
/attachment/49984/Blatt415_Zwischenknoten.png

Schlussknoten, Quelle: www.mikrocontroller.net
/attachment/49985/Blatt416_Schlussknoten.png

Knotenbund, Quelle: www.mikrocontroller.net
/attachment/49986/Blatt418_Knotenbund.png

The full thread is here: https://www.mikrocontroller.net/topic/64283

 

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Paper capacitors restore

Lately I had to replace almost all capacitors on all GDR tube radios and other equipment from the late 50s to early 70s. The problem is always the paper capacitors of the make “Koweg” (Kondensator Werk Görlitz). Over the years, these capacitors have aged and no longer meet the original specification, especially the leakage current. So far, I have always replaced them with long-life film capacitors. According to my observations, the basic problem of these capacitors is the sealing. At the end faces, this consists of something similar to PUR foam (polyurethane foam). However, this crumbles with time and is no longer present on old and thermally stressed specimens. The second part of the sealant is a varnish that has also become brittle over the years on this series. I suspect it is a nitro varnish. These are the remnants from the last restoration project:

Papierkondensatoren

In the past I simply put these capacitors into the electronic scrap, this time everything should be different. The royal road of any restoration is the preservation of the original substance and from this consideration a plan was born.

My very personal theory is that the paper is hygroscopic. Due to the problem of defective sealing, the paper absorbs moisture over time. Since the paper is insulation and dielectric, a different moisture content will greatly change the properties of the capacitor. So you have to dry the capacitor!

I came up with two viable ways to dry it, in a vacuum or with silica gel in a drying box. Silica gel is a drying agent that is commonly used in closed containers, into which you put the granules and the things you want to dry.

I ordered the granules on the Internet and chose a variant with moisture indicator. In this case, orange means dry, blue means moist. The granules can be regenerated in the oven and reused almost indefinitely.

This granulate now simply goes into an airtight plastic box.

Over it comes a paper towel and then the capacitors on top.

The whole thing now comes over the winter on the heater to further increase the steam pressure. I measured a relative humidity between 5% and 10% in the box.

Next year I will then measure the condensers and see if they have better characteristic values again. In addition, I will then describe here how I do the permanent sealing.

 

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