eddystone: 940 - Parasitic Oscillations;

ID: 191851
eddystone: 940 - Parasitic Oscillations;  
14.Jun.09 12:39
1733

Bryce Ringwood (ZA)
Articles: 79
Count of Thanks: 7
Bryce Ringwood

After using the set for some time, I noticed spurs at several points on the highest frequency range. Some investigation with a hand-held scanner revealed that the ECC189 was oscillating quite strongly at about 60 MHz. Further checks revealed that the problem was also present on the 2.4 - 5.4 MHz range, but I hadn't noticed because aerial noise drowned out the offending spurious responses. They were audible when the aerial was replaced with a 47-Ohm resistor. The spurs on the highest frequency range  are also very quickly "tuned through" and may not be noticed.

The ECC189 has a grid stopper, and the grid of the second half of the cascode combination is directly grounded via a number of almost inaccesible capacitors. I am not convinced that these have deteriorated, and suspect that the problem may be caused by my using a different make of ECC189 from that originally in the set. Its also possible that many 940s had this problem from new. None of my ECC189s are Brimar or Mullard.

The problem was solved by using 47 Ohm anode stoppers wired to the wavechange switch as in the attached photo. The performance of the set hasn't changed with these additions. Using a single resistor was considered, and rejected because of the difficulty of accesing the valve base.

Comments invited - Bryce

 

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