A new, old radio...
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Every now and then, we recieve questions from the less experienced
"I bought that and that radio. How should I start restoring?" There are a lot of people who just "plug it in", and see what happens. It doesn't take a scientist to figure out that's not the most recommendable method. That is wy I wrote this.
Every restorer has his own ways of dealing with his patients. My way is primarily aimed at avoiding the risk of vital parts burning out. Maybe others will put their ideas on this subject on paper? In this article, I will restrict myself to TRF radio's on AC power, so radios with a mains transformer.
If any, remove the back and bottom covers. Make a small drawing on where the
tubes are, and write the numbers down as soon as you take them out. Clean the
tubes, especially the pins, and inspect them visually. Broken tubes, or ones
with loose parts inside must be replaced.
If the glue with wich the tubes are attached to their bases has come loose, follow this procedure: Solder loose the connection wires. usually they are attached to the tips of the pins. Pay very good attention on "which wire comes from where", this saves you a lot of searching. Extend the existing connection wires by soldering a piece of thin copper wire to it. This eases re-fitting them. It also cures broken (too short) wires. Now you can take some abrasive paper (large grain) to roughen up the glue residual. Lead the wires through the pins. Use superglue to attach the base to the glass again. Finally, solder the wires back to the pins, and cut off the ends. If you use the superglue carefully, the result will be very good. If you have a tube tester, you can check the proper functioning of the tubes, and put them in a safe place.
The next step will be the removal of the chassis. Don't forget to mark the wires you need to take loose. Dust off and clean the chassis thoroughly. The (waveband)switches can be sprayed with contact spray. If nessecary, drill a small hole in the housing at a safe place.
When we are finished with that, we can proceed to an inspection of the wiring
and components. Are there any loose parts? Are there any burnt or broken parts,
no capacitors that have been leaking? No touching blank wires. Measure suspected
components, and replace defect ones. Capacitors in the output section usually
need to be exchanged. If there are any loose wires or parts, try to find out
where they once belonged. Sometimes, exchanged components are soldered back in
very illogical places. For the connections on the tube sockets, we need to have
the tube data. Having the schematic of the radio gives a big advantage. After
this, we are going to check if the mains voltage is set correctly. If we can't
find it out, we need to call assistance from the variac. Look up the filament
voltage of one of the tubes. Connect your universal measuring unit to the
filament connections. Set the voltage selector for the highest value (highest DC
resistance on mains connection), and slowly power up the variac. If, with the
variac at mains voltage, the filament voltage does not reach 110 to 115% (The
transformer has no load yet !), then set the voltage selector one step down and
repeat the procedure until you find the correct setting. Finally, check if there
is a high voltage across the anode connection(s) of the rectifier. If that is
not the case, check if one side, respectively the center tap of the H.T. winding
is connected to the chassis through the NGB (negative grid bias) resistor. Check
the resistor for interruption. If it is still OK, Measure directly on the HT
winding. If the transformer is equiped with connection strips, find out if the
taps of the transformer are firmly connected to them. If nothing can be
measured, that's bogus for us, because then we are in need of substitute
If all is OK so far, we can put in the rectifier tube. If the radio uses a speaker without a permanent magnet, but has a "field coil", we need to connect the speaker too. This is because the coil is used as HT choke, and all the anode current is flowing through it. Place a voltmeter across the connections of the first capacitor, and put a sensitive ammeter in series with the cathode connection of the rectifier (on a directly heated rectifier, you might also take the anode connection) If you have trouble getting a decent voltage, and a large current (100 mA or more) starts to flow, or if you see any strange light effects inside the rectifier, that means at least one capacitor is as leaky as a hose. This one will have to leave the scene, and be replaced by a good one. If all goes right (about 50V combined with a very small current, we let the settings rest for a while. Meanwhile, we measure the anode-and screen grid voltages of the other tubes. They will have to measure 30 to 50 V. Is this voltage missing somewhere, check the concerned resistor, and above all the decoupling capacitor. Solder the latter loose; as this brings back the voltage, replace the capacitor by a new one. The indicator tube usually is cut off if grammphone is selected. If all voltages check out right, wait about 10 minutes, and then slowly turn the variac up, until the rectifier delivers about 100 V. If electrolytic smoothing capacitors are used, it is better to turn up the voltage slowly, and with intervals, so that the electrolytics get the chance to "re-form" themselves. Now and then, check if they become warm. If that is the case, they are defect (when checking this, keep one hand in your pocket for safety) You can also see this on the ammeter; it will not go back to "almost zero" A leakage current of about 2 mA at working voltage is permissible. In the meanwhile, use your eyes and ears to detect smoke clouds, sparks, and smells. If such a undesired symptom occurs, don't panic! pull it out and determine what's wrong by measuring in the surroundings of the symptom.
If no disturbing signals have been recieved at a DC voltage of about 250 volts, things are beginning to clear up. Don't turn up the variac too high, the tension over the filter capacitors may run too high, because the rectifier is still unloaded. Now, we connect the ammeter between the anode of the output tube, and the output transformer, and we temporarily disconnect the coupling capacitor to its first grid (if the radio in question uses transformer coupling, of course there is no capacitor). Now we put in the other tubes, and slowly turn the variac up again. Watch the voltage on the smoothing cap, and also the current taken up by the output tube. If the latter is the same as (or lower: worn out tube) the prescribed value, we reconnect the coupling cap and look if the anode current remains the same. If yes, the cap is OK. If the current rises, the capacitor has to be renewed. If the radio gives a hum, the smoothing capacitors become suspected again. Temporarily exchange them with good ones. If the hum leaves, the capacitor will have to too. Now we connect the antenna, switch the radio to Medium Wave (BC band), and turn the volume control halfway. If all is right, the radio will receive several stations as we turn the tuning capacitor.
Pay attention to the fact that the anode lead of the output may never be interupted if the tube is in use. The screen grid takes up too much current, and fails soon. If the tube is transparent, check if it doesn't start to glow. If the radio doesn't give any sound, we are going to check the AF part first. Touch the center connection of the volume control with a screwdriver; a loud hum indicates the AF section is working. Is the AF in order, we call in the aid of the signal detector. Connect it to the grid of the RF tube. Tuning the radio, a few strong stations should be heard. If not, inspect the coils, waveband switch, and the tuning capacitor.
If there is a short between the plates (oxidation, dust, etc.), we can play
an old trick: Disconnect the wires, charge a 50 uF electrolytical with some 250 V
and connect it to the suspected section. tune the radio up and down. If we are
lucky, the short burns away. If needed, repeat the procedure. if some of the
plates have been bent, we can carefully try to flatten them again. At the final
stage of this job of patience, we take a short-resistant low voltage power
supply. Set it to approx. 10V/1A and connect it to the capacitor. Turning the
capacitor reveals the remained shorts.
Of course this method doesn't give us a perfect parallel operation on a multi-section tuning cap, but better have that than a useless capacitor. You could always look out for an intact replacement.
Continue with the signal detector, in the direction of the detector tube. Is there a signal on the grid of a tube? Can the amplified version of this signal be found on the anode? Does it also reach the grid of the next tube? By systematically going through the entire radio this way, eventually will get the radio to do its duty again. Is the radio playing, but also producing whistling noises, try replacing the decoupling caps of the RF tubes. Probably, their capacity has decreased. Is there a constant rattling audible, insert a 001uF/1500V between the anode(s) of the rectifier, and the chassis. If there already are these sort of capacitors, you can almost be shure their capacity has diminisched by a large percentage.
Furthermore, I would like to point at the fact that a radio that has been inactive for a long time, may have terrible cracking distortions in the beginning. When it is playing for a while, these distortions disappear automatically. Suspectedly, the Cathodes of one or more of the tubes have a very irregular emission which improves as they are used for a while.
Finally, if whe get the radio to play well again, don't forget to write down the major voltages and currents. Might a defect occur later on, these data could come in very handy. If you have the service documentation, you can see the voltages you measure are higher than indicated. This is because voltages were measured with universal units having a much lower internal resistance than the digital ones we use nowadays.
If you set the voltage selector "on the feeling", as described earlier, check the filament voltages once more, to be sure it is correct under load. Low filament voltages usually cause insensitivity, but may also lead to damage done to the cathode surface.
Finally, i give you the schematic of a simple signal detector, as seen above, which can be connected to a normal AF amplifier. Leaves me with wishing you a lot of succes, and much radio fun!!