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VINTAGE RADIO By JOHN HILL A fault with a difference One intriguing aspect of vintage radio repairs is the number of obscure faults that one has to deal with from time to time. Few repairs are straight­forward and there is often a hidden and challenging problem to solve. It’s not really surprising that vintage radio receivers can produce obscure faults. No doubt the age and poor con­ dition of some of these old receivers has some bearing on the matter. A classic example of this was a fault in an old 1932 model Precedent, an early 5-valve superhet with 2.5V valves and an 8-inch (200mm) elec­ trodynamic loudspeaker. The restoration had been without incident and the set worked quite well for an old timer. However, there was an annoying problem that resulted in noticeably distorted sound when the set was hot. Turn the set off and then on again a few hours later and it would be OK. It took quite some time to establish a connection between the fault and the length of time the receiver was in operation. The problem never be­ came apparent until the set had been operat­ing for several hours. From then on, the distortion would gradu­ ally creep in. After several unsuccessful attempts at locating the fault, the problem was finally traced to the loudspeaker. To cut a long story short, gradual heat build up in the field coil cre­ ated physical distortion between the speaker frame and the field coil housing. Twisting the housing with one hand would increase or decrease the audible distortion, depending on which way it was twisted. This heat induced warp eventually caused the voice coil to foul the centre pole of the electromagnet. Once that happened, the sound deteriorated until the unit was switched off and the speaker cooled. It was noticed that although the voice coil did not rub on the magnet when the speaker was cold, it was very close on one side. As there was no central “spider” on the cone, I de­ cided to reposition the pole piece to see if that would solve the problem. One favourable aspect of early elec­ trodynamic loudspeakers is that they are held together with good old-fash­ ioned nuts and bolts. This allows them to be taken apart for repairs. In later years, speakers were riveted or spot welded together, which made dismantling extremely difficult, if not impossible. Several nasties The large bolt head on this Precedent loudspeaker at top holds the central pole piece in place, making it easy to remove for repairs. 74  Silicon Chip The Precedent speaker was of the nuts and bolts variety and it required little effort to remove the field coil and the cen­tral pole piece of the electro­ magnet. This revealed several nasties. First, despite being plated, the pole piece was now very rusty. This had the effect of enlarging its diameter, thus reduc­ing the clearance between it and the voice coil. Second, the field coil was wound on a cardboard spool which was a loose floating fit on the pole piece. Vintage Radio This pole piece shows obvious signs of damage. Note the scuff marks at the end where the voice coil has been rubbing against it. Repairs Sales Valves Books Spare Parts See the specialists * Stock constantly changing. * Top prices paid for good quality vintage wireless and audio amps. * Friendly, reliable expert service. Call in or send SSAE for our current catalogue RESURRECTION RADIO 242 Chapel Street (PO Box 2029) PRAHRAN, VIC 3181 Tel (03) 9510 4486 Fax (03) 9529 5639 Analogue... VMX410 $1,899.00 - Very high quality video mixer for S-Video and Composite. Full-frame TBC, 60 wipes, borders, colours, mix, fade, cut, picture-in-picture, chromakey, freeze, etc. Fix colour bleed - H and V!, four memories, GPI trigger, etc. Stereo mixer with four inputs + mic input, headphone output and metering. The speaker cone on this unit is still attached to the frame. Note the “spider” in the centre of this model, which is used to posi­tion the cone. Shrinkage over the years had caused this problem. The cardboard spool is perhaps the cause of most faults that develop in electrodynamic loudspeakers. The spool absorbs moisture from the at­ mosphere when the speaker is not in use. When the field is operative, the heat generated within the coil drives the moisture out of the cardboard and it condenses on the cold pole piece. Even though the iron was originally electroplated, 60 plus years of heating and cooling can eventually result in severe rusting and a fouled voice coil. Moisture in the field coil spool can also cause electroly­ sis which was possibly the main cause of field winding failure. If moulded bakelite had been used instead of cardboard, ...or Digital? We've got it! The latest digital editing cards... DPS Spark & Edit Bay, Miro DV300, FAST DV and AV Master, etc. SCSI cards and drives. Whole systems too. Want prices, pictures and info? Visit our website at www.questronix.com.au/~questav - it's updated frequently - or come in. Kits, components, accessories - our list is growing all the time! 2/1 Leonard Street, HORNSBY, NSW, 2077 P O Box 548, WAHROONGA, NSW, 2076 Fax (02) 9477 3681 Ph. (02) 9477 3596 March 1998  75 This back view shows the speaker frame and voice coil. The coil is wound on a thin cardboard former which often goes out of shape over the years, especially if the cone has sagged. A typical field coil housing. Unlike the Precedent loudspeaker, this unit has the pole piece riveted in place which makes repairs more difficult. electro­ dyna­ mic speaker fields may have been more reliable. Proposed repair The proposed repair for the Pre­ cedent loudspeaker was to clean up the pole piece and reposition it centrally in the voice coil. However, the job would not really be complete unless something was also done about the moisture absorbent cardboard. So, after leaving the spool out in the Sun to dry for a few hours, it was given 76  Silicon Chip several coats of clear lacquer to seal it. A couple of thin cardboard wash­ ers were also made, so that the field coil would be held firmly in place and these washers were sealed from moisture too. All that remained was to reassemble the speaker and the distortion prob­ lem would be solved. Unfortunately, one cannot really expect to solve dif­ ficult problems as easy as that. It would appear that electrody­ namic loudspeakers were originally assembled from rear to front. In other words, the cone was glued into posi­ tion last. Putting a speaker together with the cone already in position is not so easy. No matter how the bolts that hold the frame and field housing together were jiggled and twiddled, the voice coil always rubbed on the pole piece. After going through this routine often enough to realise that it wasn’t going to work, the situation called for drastic measures. The pole piece was set up in a lathe and a quarter of a millimetre was turned off its diam­ eter at the front where it fits into the voice coil. After that, no further problems were experienced with the speaker’s assem­ bly. The cone moved freely without a hint of interference. Now one would expect that increas­ ing the clearance between the pole piece and the voice coil would reduce the speaker’s sensitivity. If that’s the case, it was by no means noticeable. What’s more, if the same problem arises again with other speak­ers, I will have no hesitation in taking a skim off the pole piece in order to give it adequate clearance. Because the voice coil is wound on a thin cardboard former, it is asking a lot to expect it to be perfectly round 50 or 60 years after it was made. Reducing the pole piece diameter is one way of compensating for an outof-shape voice coil. A cone that sags and loses its form is one reason that voice coils go out of shape. While on the subject of voice coils, remember that they are only glued to the speaker cone and a touch of lacquer to rein­force the area of at­ tachment is highly recommended. If you have ever encountered a speaker with a loose voice coil or voice coil winding, you will appreciate the need to pay attention to that part of the speaker while it is accessible. Other methods There are other ways of clearing fouled voice coils without having to resort to the drastic methods previ­ ously described. One way is to move the cone in and out while applying side­ wards pressure to the cone. In other words: try to loosen the dust, grit and barna­ cles by forcing the inside of the voice coil to rub gently on the pole piece. After that treatment, lay the working was used. Unfortunately, this speaker is of slightly different construction, which just goes to show that some may be easier to repair than others (note the “spider” shown in the photograph of this model). One pr o b lem t h a t would arise if doing a similar repair on this second speaker is the fact that the pole piece is riveted in place instead of being held by a bolt, as was the case with the Precedent speaker. No money In conclusion, I see little point in spending large sums of money on old radios, especially The hole in this backing plate is a neat fit around the voice coil. In some instances, this hole when the price of some may require enlarging so as to clear a coil that has gone out of shape. repairs exceeds the re­ ceiver’s value. This par­ speaker face down on the workbench Once again, give the speaker a run ticularly applies to re­placement field for a while so that any rubbish can in the face down position so that the wind­ings and speaker cones where work its way out of the gap. gritty granules can find their way out. one can easily spend $100 on an old If that fails to do the trick, slide a In my case, the repaired Precedent speaker restoration. thin piece of shim brass between the speaker was reinstalled in its cabinet Why pay to have these things done voice coil and the pole piece, work­ before I realised it had the potential when they can often be restored by ing it all the way around if possible. for a Vintage Radio story. So for the either a straightforward repair or by This procedure is likely to be more purpose of supplying photographs, combining various good components SC effective than the previous method. another electrodynamic loudspeaker into one working unit? This 2.5kΩ field coil is wound on a cardboard former. Sealing the card­board with lacquer will help to moisture-proof it. Most electrodynamic loudspeakers employed a hum-bucking coil which was used to neutralise hum induced by the field coil. March 1998  77