Being new to the Drake community I've found this list useful in tackling the problem of low sensitivity on a recently acquired R4B.
But, having aligned the receiver as per the handbook, worked on the tips here, swapped the signal path tubes (with admittedly NOS ones), there is no progress. There is both a general lack of sensitivity on all bands - the calibrator peaking at S9 on 3.5, 7 and 14, and a further reduction in sensitivity on 21 and 28, below S5. I've not found much information on correct mixer injection levels, or what gain I should expect from the each stage. Is that information available? Injection to the first mixer is 0.6v to 1.0v p-to-p on all bands except 28 where it is only 0.2v p-to-p. Injection to the second mixer is about 17v p to p (dc-wise grid goes from +6 to -11) on grid 1. But, putting the scope probe (divide 10 to minimise capacity) brings up the S meter reading, with minimal change in cal signal audio pitch. Looking at the cathode signal, what is happening is that without the probe on the grid the injection signal has a large component of harmonics, the sine wave is distorted. Adding a scope probe on grid 1 reduces the harmonics, such that with a pure sinusoid of about 3.5V p to p at the cathode the S meter is up at S9+40, again, with no real change in audio pitch. Before I remove the small vertical PCB by the second mixer to examine - is there anything from your experiences I should be looking for? I have already resoldered the key points on the grid circuitry I can reach to remove the likelihood of a dry joint. Then, a further puzzle - although the S meter with the probe on the grid goes to S9+40, the voltage on TP2 only goes from -5.3 to -6.6, from what I have read I would have expected a much greater change. Apologies for the long post, but this really has me puzzled! thanks for reading. Gwyn Griffiths G3ZIL _______________________________________________ Drakelist mailing list [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
