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VINTAGE RADIO By JOHN HILL Troubleshooting vintage radio receivers - the basic techniques There are few tasks more daunting to a new and inexperienced radio collector than the repair of a receiver that does not work. If one is unfamiliar with valve radio repairs, finding a fault can be quite difficult. Here are a few tips to get you started on vintage radio repairs. Many vintage radio collectors are totally divorced from the electronics trade and often have little or no knowledge of the workings of a radio, whether it be valve or otherwise. Many such enthusiasts simply collect their treasures, clean them up so that they look nice and display them. If a set happens to be in working order, it is a special prize. However, even if it does go, there is a good chance that it will not work for very long before something breaks down and the set no longer functions. This month's Vintage Radio column presents a simple troubleshooting chart (Table 1) and discusses some of the problems associated with fault finding. The chart contains a list of problems (or symptoms) and the pos- sible reasons for them. The chart does not contain every conceivable fault or remedy but most of the common ones are covered fairly well. Multiple faults A neglected 50-year old radio can be a mess of problems and there is often more than a single fault to find and repair. For example, a partial short circuit in a high voltage capacitor may not stop a receiver from working but the leakage problem can overload other components. Items such as valves, resistors, loudspeaker field coils and high tension chokes are vulnerable to overloading of this nature. It is, therefore, pointless to replace the overloaded component without also replacing the component that caused the problem in the first place. One of the most potentially destructive faults for the vintage radio enthusiast is an open circuit speaker transformer (see Table 1). It should be carefully checked before the set is turned on. Failure to apply HT to the plate of the output valve will cause the screen to carry excess current and to run red hot - a valuable warning sign but one which may come too late. The red hot screen can release (occluded) gas, making the valve gassy and useless. Most valves can withstand this abuse for brief periods but a few suffer instant destruction. The type 59 valve was notorious for this weakness and there may be others. This was tragic enough in the days when these valves were popular but at least they could easily be replaced. Another very common fault listed in Table 1 is weak and (often) distorted audio. This was almost always due to the first audio stage screen resistor - usually 1.5MQ- going high in value, typically to several megohms. The high value resistors of those days (ie, anything above lMQ) were notoriously unreliable. Paper capacitors _ ___,_ _....,__ _ _ ___,_+3V 22k .001 --■---411--PROBE ,___ _ _ _ _ _ _ _ _ _ _ ovEARTH CLIP B EOc VIEWED FROM BELOW 6 SILICON CHIP Fig. 1: the signal injector circuit uses two transistors wired as an astahle multivibrator. It produces a 2kHz tone that is rich in harmonics up to 20MHz. At the risk of repeating myself, I will say again what I have said quite a few times in the past. Faulty paper capacitors are the cause of a great deal of trouble in old valve radios. These ancient capacitors are unreliable and are the direct cause of many problems. In fact, they can be so troublesome that I feel justified in discarding all of them. That's · right! Total replacement of the paper capacitors is part of my routine restoration procedure. By do- RESURRECTION RADIO Vintage Wireless Specialists A simple signal injector can be built around two common NPN transistors. This unit operates from a 3V supply which is provided by two penlight batteries. Repairs - Restoration - Sales Our skilled technicians offer QUALITY repairs and restoration. We also have a large stock of bakelite and timber radios fully restored and for SALE. Parts are available for the enthusiast, including over 900 valve types, high voltage capacitors, transformers, dial glasses, knobs, grille cloth etc. Circuit diagrams for most Australian makes and models. Send SAE for our catalog. WANTED: Valves, Radios, etc. Purchased for CASH Call in to our showroom at: 51 Chapel Street (PO Box 1116), Windsor, Vic 3181. Telephone: (03) 529 5639 capacitance is better than one of the more basic types. The signal injector When using the signal injector, the earth clip is connected to the chassis and the probe is connected to the control grid of the valve being tested. A signal injector can quickly locate a faulty stage in a radio receiver. ing this, one can eliminate about half of the common problems that would otherwise be encountered when restoring a valve receiver. Total capacitor replacement (using modern polyester types) also minimises the likelihood of future capacitor breakdowns and makes the set more reliable than it may have been when new. If your wallet dictates that you restore with restraint, then at least replace those paper capacitors that have some measurable voltage potential across them - particularly where high voltage potentials are involved. However, vintage radio repairs often involve more than just replacing a few suspect capacitors. There are many other things that can go wrong and stop a receiver from working. To trace these faults requires more than a troubleshooting chart. Some basic tools and instruments are also needed. Two very useful instruments are a multimeter and a signal injector. Little needs to be said about multimeters other than that you only get what you pay for. A meter that has a good ohms range and can measure A signal injector is another very useful piece of test equipment and can be made up quite cheaply for only a few dollars. Fig.1 shows the circuit details of such a device, while one of the photos shows the constructed item. An injector is used as follows: first, attach the earth clip to the receiver chassis, then switch on the injector and place the probe on the grid pin of the output valve. If the output section is OK, the injector signal will be heard through the loudspeaker. If the signal is not heard, then there is a fault somewhere in the output stage. In such a case, the problem could b_e a defective valve, a faulty coupling capacitor, a corroded valve socket connection, an open output transformer, no high tension at the valve socket, a dry solder joint, a speaker plug not making contact or faulty wiring inside the speaker plug. On the other hand, if the output stage tests OK, then one must go back AUGUST 1992 7 Protect your valuable issues Silicon Chip Binders These beautifully-made binders will protect your copies of SILICON CHIP. They feature heavy-board covers & are made from a distinctive 2-tone green vinyl. They hold upto 14issues& will look great on your bookshelf. * High quality * Hold up to 14 issues * 80mm internal width * SILICON CHIP logo printed in gold-coloured lettering on spine & cover Price : $A11.95 plus $3 p&p each (NZ $6 p&p). Send your order to: Silicon Chip Publications PO Box 139 Collaroy Beach 2097 Or fax (02) 979 6503; or ring (02) 979 5644 & quote your credit card number. Use this handy form l _ _ _ _ _ _ Illa _ _ _ _ Enclosed is my cheque/money order for $_ _ _ or please debit my O Bankcard O Visa O Mastercard Card No: Card Expiry Date __/__ Signature _ _ _ _ _ _ __ _ _ Name _ _ _ _ _ _ _ __ _ _ Address_ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ P/code_ __ ·-----------· 8 SILICON CHIP The author's signal injector was built into a convenient cabinet that once housed a low-cost burglar alarm. to the preceding valve and again place output a little but is a small price to the probe (ie, inject the signal) on the pay for protecting the transistors. control grid. If that stage is working, The advantage of a signal injector is sound will again be heard through the that it will quickly locate the defecloudspeaker (it should be louder than tive section of a malfunctioning rebefore) and you then move back an- . ceiver. Once a particular stage beother valve until the faulty stage is comes suspect, it then narrows down isolated. the search by a considerable margin. A signal injector will not indicate If you don't have a signal injector, the exact cause or location of a fault removing a valve while the set is opbut it will indicate where you should erating usually produces a loud look. If a signal injector is designed "click" in the speaker as the high tenproperly, it will deliver a signal that sion is disconnected from the valve. produces an audible sound in the A click accompanying the removal of loudspeaker no matter what part of an output valve would be a reasonthe set the signal is fed into. This able indication that the valve and the signal should be heard regardless as loudspeaker are operating and that to whether it is injected into an audio the fault lies elsewhere. Shorting out or a radio frequency (RF) stage of the the grid with a clip lead may also receiver. It should even be heard from produce similar results. the aerial terminal, if the set is funcCompiling a troubleshooting chart tioning normally. is a difficult exercise, as it is impossible to cover every conceivable situaPrecautions tion. Valves can have internal faults, Care should be taken when using transformers can have open windings, the injector probe. It is unwise to in- and dry solder joints can be difficult discriminately place the probe any- to find. Dry contacts such as those in where, unless the injector output is adequately protected. Touching any high tension connection could damWARNING! age the transistors. Protection is normally provided by Dangerous voltages are present means of a small capacitor in the probe in valve radio sets so always exlead. The original injector circuit ercise due caution when troubleshown used a .0lµF capacitor in this shooting. In particular, take care role, which was too large for high with the power supply and high voltage (valve radio) use. Consetension (HT) circuitry in the set. quently, any high tension contact Note also that some vintage repromptly wrecked the transistors. Receivers were AC/DC models with placing this capacitor with a value of no power transformer and these .00lµF solved the problem and made should be left strictly alone unless the signal injector more suitable for you know precisely what you are valve work. doing. The smaller capacitor restricts the _TABLE 1 Possible Cause Symptom · PCB and SCHEMATIC CAD No power; nothing lights up Faulty mains plug connection; faulty on/off switch; open circuit power transformer primary; disconnected mains wiring inside set. ..... . Receiver lights up but does not work Faulty HT winding on power transformer; faulty valve; open circuit winding in aerial coil, oscillator coil and/or IF transformer; open circuit winding in field coil or HT choke; burnt out resistor; shorted high-voltage capacitor; faulty valve socket or speaker plug connection; missing parts; disconnected wiring; short circuit caused by perished insulation. . Receiver works but is noisy Troublesome dry solder joint; dirty socket or switch connections; faulty valve (tapping may pinpoint); faulty components (resistors, capacitors, etc). Severe oscillation (receiver squeals) Faulty decoupling or bypass capacitor; ineffective valve shield; faulty valve (usually in RF stages); wrong valve. Motorboating Open grid or cathode circuit (usually in audio section); faulty valve; faulty second filter capacitor. Distortion Faulty coupling capacitor to output valve; incorrect grid bias; gassy output valve. Hum Inadequate filtering of high tension; stray coupling from AC source to grid lead; short between valve cathode & heater; unshielded audio leads; gassy or faulty rectifier valve; filament resistor in early output valves; faulty electrolytic capacitor. Pronounced hum Shorting electrolytic capacitor; internal fault in rectifier valve; shorted field coil or HT choke. Intermittent loud/soft volume Faulty coupling capacitor on output valve; valve with loose internal elements; faulty connection (eg, valve socket or solder joint); faulty screen grid bypass capacitor Static like noises when tuning Tuner plates scraping; faulty earth connection on moveable plates. Set crackles or makes frying sounds Faulty connection or component on HT line; faulty valve (tapping may pinpoint) ; badly worn volume control. Very soft volume Worn out valves; low HT voltage; incorrect valve types; leaky high-voltage capacitors; alignment problems; open circuit audio-coupling capacitor; high value screen resistor on first audio valve. Low high tension Weak rectifier valve; damaged or faulty power transformer; wrong primary tapping selected on power transformer; incorrect loudspeakar (ie, wrong field coil impedance); faulty valve. . -· ···· ·-- ··· · -"::t ·= m.li!.m ·- TV IF AMPLIFIER - . . : )r. • • ........ ....... -::. ~ i ---- ------------ .______ ___________ __ J C•nitifl4♦ EASY-PC • Runs on PC/XT/ AT/286/386 with Hercules, CGA, EGA or VGA. • Design Single sided, Double sided and Multilayer boards • Provides Surface Mount support • Standard output includes Dot Matrix/Laser/Inkjet printers, Pen Plotters, Photo-plotters and NC Drill Double spotting Incorrect alignment. • Award winning EASY-PC is in use in over 12,000 installations in 70 Countries World-Wide Overheated choke or field coil Excessive HT current caused by faulty HT component, probably an electrolytic capacitor. • Not Copy Protected Rectifier anodes glow red hot Direct short on HT line (eg, electrolytics in HT filter short circuit). Output valve screen grid glows red hot Open circuit anode on output valve (probably open circuit output transformer). Note: this can damage the output valve. No shortwave reception Dirty wave-change switch contacts; leaky paper capacitors; resistors gone high in RF stages; low HT voltage; weak frequency changer valve. • Superbly Easy to use Options: • 1000 piece Schematic symbol library • Surface Mount symbol library • Gerber Import facility For full info 'phone, fax or write: valve sockets, wave change switches and some tone controls are just waiting to give trouble and often do. The list could go on and on. Hence the accompanying chart lists mainly common problems and has been compiled for the benefit of the novice. Remember, a good vintage radio res- toration should result in the set looking and working as it did when it was new. Restoring a vintage receiver so that it looks as new can be a difficult and time-consuming task in some instances, but there's no reason why it cannot function as new. Hopefully, my troubleshooting chart will help you in this regard. SC BTC PO BOX432 GARBUTT 4814 QLD. PH (077) 21 5299 FAX (077) 21 5930 AUGUST 1992 9