In Trouble

Radio Record, Volume II, Issue 13, 12 October 1928, Page 31

In Trouble

‘Y a A Batch of Questions CORRESPONDENT writing from Cromwell submits a batch of questions which are dealt with in these columns as the matters raised are of general interest. [A query regarding the "Southern Cross" generator was dealt with in our article on that subject. ] "T)IOGENES" (Cromwell) writes :- I have discovered a circumstance which seems decidedly queer. I can disconnnect the battery end of the B positive lead from my detector valve (on Browning Drake 4), and the set. will produce 2YA on the speaker though not so loud. If I take the H.T. wire off the valve socket it stops. With three volts on the plate of the detector I. can get all the volume I want on 2YA. Into the bargain my aerial is bad and. the earth rotten-in fact, I can get as much volume without the earth-though the set is less stable. . I notice some of your correspondents from Christchurch have been feeling proud of themselves over picking up 3YA’s broadcast of 5SW on the signing of the Peace Pact. It came in quite well down here, and was intelligible in the initial stages, though 3YA seemed to be having some difficulty in keeping station tuned in later on. ‘I, would be pleased if you could identify a Morse station for me. It-is operating on a somewhat shorter wavewas sending a succession of V’s. interspersed with occasional variations which we could not decipher. It has a high, squeaky note, and goes fairly fast, though not so fast as Awarua. than Awarua (600 metres), and

I have a Philips latest B and C eliminator, and have been storing up a list of questions in connection therewith. If you could enlighten me on the following points I would be greatly obliged: 1. Does it harm the eliminator to be left connected to the power when the A battery is turned off, and how long can it safely be left? 2. What voltage will the plugs put out on a Browning Drake 4 containing a Philips 609A, Alton 201A, Radiotron 201A, and Philips 605B? 8. Can any device be put in the detector lead to make variations in the voltage at will? 4, Can I drive the detector off dry batteries and the rest off the eliminator, using the same negative lead with two terminals? 5. I get a slight AC hum no matter how I arrange matters, and it is the same with or without bi-pass. Can this be avoided? 6. Often but not always when the set is on the verge of oscillation it breaks into an intermittent popping-not motor boating, but what motor boating would be like if the pops were spaced about a fifth of a second apart. The popping is not regular and the set does not do it on dry batteries. 7 By turning the reaction off and turning up the rheostats I can produce a heavy drumming sound like a powerful motor. What is this? 8. Does the eliminator need a good earth or will any old thing do? Does it matter if eliminator and set earth are run together away from the set? Mine are entirely separate and both bad. 9. Will a Philips safety fuse work on an eliminator lead? 10. Does it harm the eliminator to run the set off dry batteries and the grid bias off the eliminator? —

Y thank you in anticipation of your reply to this list of inquiries.Diogenes (Cromwell). In Answer. QTATION 2YA having such great output can be brought in with a minimum of voltage on the valves. In this particular instance when the positive B was removed from the battery a certain amount of audio frequency current could filter back to the plate of the detector. When the plate terminal of the socket was disconnected this was impossible, so that the path of the impulses was broken. The Morse station was in all probability a ship and cannot be identified without the call letters. The queries on the Philips 3003 B. and ©. eliminator, ‘on being referred to that house, are answered as follows :- (1) Practically no harm occurs to the 3003 B. and ©. unit through the eliminator remaining connected to the power supply with the A battery switch disconnected. The rectifier filament burns, but this does not reduce its life, owing to emission ceasing. True, an increased voltage strain is placed on the filter condensers, but these are very robust and very rarely give trouble. Hliminators have been left for days. It -is, however, recommended that the eliminator be switched off at the same time as the set. (2) The voltage delivered at the -plugs depends on the total milliampere draw and the draw on each plug. A printed sheet goes with the Philips 3003 Bliminator explaining this fully. Querist should use tap 1 for detector, tap 2 for the R.F. valve, tap 4 for the first a.f. valve, and tap 6 for the power valve. Under these conditions the first a.f. valve would

have approximately 90 volts on the plate and the bias should be 4. The 605 would have 180 on the plate and the bias should be 20 to 25. (3) A high variable resistance such as the 50,000 ohm Centralab can be placed in series with the detector tap. The Centralab should be bridged with a condenser of at least .006 mfds. capacity. (4) Yes; you can use dry batteries on the detector valve using a common negative lead. This gives perfect results on short wave regenerative sets. (5) The hum you get is almost certainly due to a poor eliminator earth. No hum should be audible, only the usual soft "breathing" of the set. A poor set earth can cause a hum. (6) The ‘popping’ experienced at tkmes is due to rather high voltage on the detector ; sec reply to No. 3. (7) The drumming sound you mention is apparently due to some peculiarity in your set, and not to the eliminator. (8) A good earth is desirable for the eliminator. The only legal earth is one attached to the wiring conduit through the third pin of the 3-pin plug which regulations lay down must be used. See also answer to No. 5. Separate earths should be used. (9) The Philips safety fuse can be used with an eliminator. It may be noted that this eliminator has a very valuable safety feature limiting the total current that can be supplied on short circuit. It would be impossible to burn out your present set of valves by ernnecting the full eliminator voltage directly across the filament battery terminals. (10) It is impracticable to use only the bias voltage from the eliminator and no sound reason can be seen why the querist should desire to attempt this.