euratele: Euratele 412 - repair report

I recently repaired two of the above signal generators. Both of the kits seem to have been delivered around 1973-74 based on the date codes on the electrolytic capacitors. One of them has the original selenium rectifier in the power supply, the other seems to never have had the selenium rectifier fitted, instead there is a BY298 silicon diode. One of them bears the Euratele brand, the other is branded by the swedish education institute NTI and has swedish text on the front panel.

In one of them, the output attenuator potentiometer was open circuit (probably burnt from someone accidentally connecting DC to the RF output). Additionally, it had a tendence of squegging in the LW band. The builder had tried to fix it by connecting some capacitors in apparently random positions around the waveband switch. The reason for the problem was wrong value of the grid capacitor in the RF oscillator, C19. It was 4,7 nF instead of 500 pF.

In the other one, the AF oscillator for internal modulation didnt stop reliably when the unit was switched to external modulation. It was still running and giving maybe 10-15 % AM modulation in the MW and SW bands but stopped in LW and UKW. This problem persisted after checking all wiring and replacing relevant resistors that had drifted too much, but it was solved by changing the ECF80 valve. Trying a few valves, all ok according to the valve tester, did show that the original Telefunken valve and one used Philips, gave the problem with AF oscillator not stopping. The Siemens valve from the other signal generator and one used Cossor (probably in fact manufactured by Mullard / Philips), worked well. So the conclusion is that not every ECF 80 will work well here, there is a need to select a useable specimen. In the external modulation mode, the modulation switch will connect R112 (47 kohm) in series with two resistors of 82 kohm and 22 kohm placed at the external modulation input, in parallel to the grid leak of the pentode part of ECF80. Apparently, if the pentode has a too high slope, this isnt enough to stop the AF oscillator.

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