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    • #8823

      In Part I I reported my experience with the refurbishment for old and heavily used blocks and groundplates.

      I forgot to mention the little magnets behind the contact plates – these are essential for a good contact, too. Each contact + magnet must be able to hold a small block 27×27 mm at one of his contacts:

      Otherwise one of the magnets has to be turned (north / south pole) and a new test will give a – hopefully – good result. I had several times this effect – therefore please check before using… and not always is the slash-marked side the outside…

      Another critical point can be the not completely spot-welded connection of the contactplate togues in the connection blocks: T / L / – / GND / + and so on. A few, perhaps 2 promille of all of my blocks had not on all “planned” connections with each other… So this had to be verified and – in the case of a missing connection(s), re-soldered (here the test with an Ohmmeter):

      Several resistors of the old versions had too big tolerances. I expect max. +/- 10 % – but if I have a block with embossed 47 Ω on the cover and my Ohmmeter says 69 Ω then it’s time to change the resistor!

      The disc-shaped ceramic capacitors in the blocks (47 nF, 0.1 µF) are designed for 12…16 V. This construction is nearly 100 years old and until nowadays without any problems, so there is absolutely no need for a change – but a control I would always prefer! The tolerances may be up to +/- 20 % – but I had a couple of weeks before a 47 nF version where my C-Meter said: 68 nF (!). Of cause I changed the capacitor – I think that this capacitor was wrong sorted after production before marking, it is not a chemical problem.

      The electrolyte capacitors may have a tolerance of +/- 20 % – some versions with 30 % deviation may occur are are quite good, but a nearly zero capacity (=dry) or nearly the double (=holes the metal foils) … that’s a chemical problem and causes errors in any circuits! That means: exchange, but with a minimum voltage of about 16 V.

      The coils, ferrite antenna etc. can be verified with the L-Meter – there was already a forum part (tuning block, ferrite antenna etc.)

      Diodes, transistors and other semiconductors should be tested with special circuits – we had several forum parts with this theme. Additional checks can be done with a Universal Tester, e.g. M328 / LCD 12864 / All-in-1 LCR for about US$ 12, to be found in the wellknown internet auction platform…

      Care has to be taken with the 2-stage transistor amplifier #8054 – there sometimes happened rather strange things…

      The relay block with an additional diode on the cover print must not have the diode inside…

      Light bulbs / lamps found in old and used Lectron training sets from schools:

      We may guess which ones can be used with Lectron experiments… the answer is: NONE of these! NEVER EVER!

      The electric data are in the range from 2.5 V to 12 V – and the current comsumption between 200 mA and 500 mA!

      That are (Volt x Ampére = Watt) 0.5 W … 3.5 W! Even a silicon power transistor 2N3055 mounted on a huge heatsink will have problems to “manage” this energy on his base/emitter path! How will a Germanium small signat amplifier transistor react? He will die immediately – perhaps a little smoke cloud, and that’s it…

      Really too bad, that not at the start of each experiment using the light bulb the warning

      “PLEASE USE THE LECTRON BULB: 6 V 50 mA (=0.3 W) TOGETHER WITH A RESISTOR 47…120Ω!”   was printed in red letters.

      As an alternative – and if not a suitable light bulb is available – we may use a LED instead. In my spare parts I found a very old yellow jumbo LED. This fits quite good in the original lamp block, with a resistor of 390 Ω the current is 15 mA @ 9 V.

      Two hours ago I completed the assembly:

      The red point is the (+) side. By the way: the above LED/Resistor cover came from an early production (197x ?), where a lot of blocks had a wrong assembled LED – the battery (+) had to be connected to the right side (…?!). Meanwhile I changed it so that the cover symbols are correct.

      Good luck,



      • This topic was modified 2 weeks, 2 days ago by Guenni75.
      • This topic was modified 2 weeks, 2 days ago by Guenni75.
      • This topic was modified 1 week, 6 days ago by Guenni75.
    • #8834

      Hi Günther,

      This is such helpful information to address status of legacy parts.  Your points about the old style light bulbs is especially important.  Following your recommendations will ensure protecting the ever fragile PNP germanium transistors from passing catastrophic current!

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