By Sonny Clutter

( Reprinted by permission of the Northwest Vintage Radio Society’s monthly publication the CALL LETTER. All Rights reserved.)

Up to 1000 volts may exist in old vacuum tube radio chassis. Make sure that the set is turned off. Always unplug the AC cord, and discharge filter capacitors by shorting across terminals with a test lead or screwdriver blade. (The Editors)

Having a hard time finding resistance line cord? Not enough room inside to install a drop resistor without melting the cabinet? Well, at least for some sets I have a simple solution for you. Let’s take a typical 5 tube set using the following tube line up: 25Z6, 25L6, 6Q7, 6A8, and 6K7. The first number in the tube name is the voltage required necessary to heat the heater (or filament). If you add these voltages together you will find the voltage necessary to operate these tubes heaters in series in this case it equals 68 volts. Since the line voltage is 120 volts we need to drop about 50 volts. This is what the resistance in the line chord does.

In most sets we can eliminate the need for that resistance cord or for a drop resistor by simply changing the tubes to compatible ones with a higher heater voltage. For example substitute a 35Z5 for 25Z6, a 50L6 for 25L6, a 12Q7 for 6Q7, and so on. It is important to make sure that all the substitute tubes have the same heater current rating. The currant rating and other data is easily obtained from the tube manual. When substituting a 25Z6 with a 35Z5 and changing the tube lineup, add a jumper wire from pin 2 to pin 3 of the 25Z6 (35Z5).

This modification requires no circuit changes except using a normal a.c. cord in place of the resistor cord.

Have you ever wondered what causes the white stuff that sometimes appears on old radio knobs? Have you ever tried to clean the stuff off? Well, if you have, you might have tried to scrub it off with cleaners of all types including ammonia and bleach (never mix ammonia and chlorine bleach). You may have used tooth picks and tooth brushes and the stuff still just doesn’t what to come off. Even when you’re successful it comes back later. I’ve found a solution! After much frustration and experimenting I have discovered a very simple way to get rid of the pesky stuff.

The white stuff is a mold that attacks certain plastics. My tip is simple: soak the moldy knob in paint thinner (mineral spirits) for several hours and most of the mold will dissolve. What doesn’t dissolve will easily come off with a tooth brush. Make sure you don’t use lacquer thinner. After you have cleaned the knob well with paint thinner and wiped them dry, give them a thorough coat of ARMOR ALL and polish them. I believe this protects them and may keep the mold from returning.

Unless you saw that old set operate before you bought it, don’t plug it before you take the precautions I discuss in this tip. Chances are that neat old radio you just acquire has not been turned on for 30 or 40 years! If you just plug it in and turn it on , you may cause serious damage. ( I’m referring mainly to radios that are of power transformer design.) The most likely time of failure is the moment power is first applied. How many times have you had a light bulb burn out when you were sitting comfortably in you chair reading? It doesn’t happen that way. The bulb burns out the minute you flip the switch. Well, here’s what can happen when you flip the switch on the beautiful old PHILCO that you just hauled home.

If you are lucky, nothing will happen. If you are extremely lucky it might even play. Chances are if it doesn’t go up in smoke right away, it will develop a loud hum and if you leave it on the hum will get louder and then it will go up in smoke. The point is the old radio hasn’t been just sitting around for that last several decades gathering dust. There is an excellent chance that the electrolytic ( filter) capacitors have deteriorated to the point that they are shorted or leaky. A shorted filter capacitor combined with a good rectifier tube can be deadly to a power transformer, you can forget restoring the set until you locate another transformer (which can be very hard to find).

Here’s how you can make a couple simple checks without destroying your radio’s power transformer. First, make yourself a couple of simple test fixture with a light socket, AC cord and as AC plug receptacle. Mount them on a board and wire them so that the lamp socket is in series with the AC receptacle. If that power consumption of the radio is stated on the model number label use a bulb that matches that consumption (75 watts = 75 watt rating). If not use the following chart:

No. Of tubes Bulb size

1. 50-75 watts

1. 75-100 watts

1. 100-150 watts

1. 150-200 watts

Now for the test: After choosing the bulb size remove the rectifier tube ( usually an 80, 5Y3, 5V4 or some number beginning with a 5). Now plug the set in and turn it on, the bulb should not glow or glow very din. It the bulb glows very bright at this point, you have a shorted power transformer, a short in the filament wiring to the other tubes in the set, or the wrong size bulb in your test socket. Now carefully install the rectifier tube, the bulb should brighten some. It the bulb glows very bright (near full intensity) try another rectifier tube, if it still glows bright then you have a power supply short that may destroy the power transformer.

In the Tip #3 we used the light bulb test to determine if a short exists that could damage the power transformer. Now we are going to take the first steps to check out and repair the power supply.

SPECIAL NOTE: Most early AC radios use a field coil speaker, (speaker cable had 3 or more wires). Never apply power with the speaker disconnected.

Assuming we passed the light bulb test we are now ready to apply full power and take the following steps to check out your radio. ( I always take the first two steps with my hand on the plug just in case I have to remove the power in a hurry.)

1. Observe the rectifier tube; you should see a dull reddish-orange glow from the filament. If you see sparks, an orange or purple glow or the outer plates of the tube grow red, disconnect the power and try again with a new rectifier tube.

2. Listen to the speaker as the set warms up. If you hear a loud hum, popping, cracking or frying noise, disconnect the power.

3. If you have successfully passed the first two tests then allow the set to warm up for a few minutes. Hook a wire up to the antenna terminal and see if it plays. If is plays ok with no hum or distortion, you don’t need to be concerned about the power supply for now. After about fifteen minutes, disconnect the power and feel the filter cans; if the cans are warm or hot, they should be replaced.

4. Most old sets will produce one or more of the symptoms in step one or two. These symptoms general indicate defective filter capacitors. Since they are packaged in a variety of forms. The most common are the cylindrical metal cans mounted on top of the chassis. Other types are usually under the chassis.

5. After removing the chassis and identifying the filters you may notice they have already been replaced, quite possibly several times over the years. The newer ones have very likely just been hung right on the lugs of the old ones! If this is the case the next thing to do is disconnected the original ones leaving only the newer ones connected and return to step one above. Some " technicians" used the poor practice of tacking new filter capacitors right onto the lugs of the old ones when a hum would start to develop. This got rid of the hum until the old filters (still in the circuit) would become more leaky or short out altogether. The original capacitors should always be disconnected.

6. Installing the new filter: Unless you can obtain new old stock capacitors and can reform ( recondition) them, you should use the modern replacement types. These are available from Antique Electronic Supply and other suppliers ( see parts in Part 4). Theses are quite small and can be mounted under the chassis. Attach the new capacitors and the wiring to a couple of small terminal strips you install under the chassis. This makes a much neater and safer job then just hanging them under the chassis. Filter capacitors are marked + and -. Make sure you observe the correct polarity!!

Disconnect the old filters and leave the top mounted "can" types in place so the set will still look original from the top side, The value of the filter capacitors vary and is usually noted on the can or package. ( the package containing the filter also varies.) The capacitors of are rated in microfarad and working voltage. The symbols to identify these values are uf, ufd, mfd, or microfarad for the capacitance. The most common value used in the 30’s radios is 8uf at 400 volts. 10uf at 500 volts is a good replacement.

7. After replacing the capacitors, to go back to the light bulb test and then to steps one and two.

Here we will cover other areas of the power supply and output stage that can cause hum, noise and garbled sound. To make this as simple as possible I will cover just two most common symptom/cause categories:

(1) You have replaced the filters, have installed a good rectifier tube and you still can’t pass the light bulb test ( bulb glows bright, refer to Tech Tip #3)

(2) Same as above except you have passed the light bulb test and operating the set with full power you have hum, noise, and/or poor distorted sound.

Possible causes:

1. Shorted or pinched wiring to filter, output stage or power distribution
2. Shorted by-pass capacitors in the B+ distribution. These can be identified by locating any capacitors wired directly from the B+ distribution to ground (B-).
3. Shorted speaker field or wiring to same.
4. Shorted output tube/s. Some sets use two or more.
5. Burned resistors or other components in power or output stage.
6. Shorted output transformer.

One final test you can make: Remove the output tube’s and see it the light bulb not glows much dimmer (assuming you are still in bright light bulb test mode). If the bulb now glows dim you have isolated the problem to the output stage. The most likely cause would to the tube/s or the coupling capacitors to the control grids of the tube’s. This can easily be determined by simply clipping one end of the capacitor/s loose and trying the test again with the tube/s plugged back in the sockets. If now the bulb glows dim, replace the capacitor/s.

Further troubleshooting of the power supply and other areas will require an understanding of basic electronics and the use of test equipment. ( A multimeter for checking for shorts and proper voltages is very handy--ed.)

Several members have recently asked how I deal with the block capacitors Philco used in the 30’s radios such as the models "70" and "90". First, it is important to understand that some of these block units have more than one component in them and some include resistors. I believe that simple clipping the old lugs off and hanging components can also lead to other problems such as shorts, etc. Here’s what I recommend for a good restoration:

1. Remove all the bakelite capacitor blocks and draw yourself a good layout of all the wiring to the individual blocks. ( These blocks are identified by a part number.) Remove them by unsoldering all the leads; don’t just clip them off. A dental pick is a very useful when unsoldering

2. Place the old blocks in a pan of water ( hopefully not one you plan of cooking with later). Bring them to a boil completely submerged in water. I neglected to mention you will need a stove or and extremely hot power transformer ( just kidding!) for heat.

3. After boiling for about 5 minutes, remove from heat (and also the kitchen as what comes next will be messy). Using a pair of long nosed pliers (to hold the hot blocks) and a dental pick or some other narrow, sharp tool, dig out all that’s inside.

4. After scraping out as much as possible, place the blocks in a bath of paint thinner or other solvent you might prefer that will dissolve the remaining tar residue. This makes it much nicer to reload them later without getting all that black stuff all over your hands ( wear Protecting Gloves—Eds.) and everything else. After initial bath and cleaning I use a fresh batch of solvent to complete the cleaning, You can us Q-Tips or small pieces or rags to wash the empty blocks out clean. I save and reuse the old solvent for the first bath the next time I do this. I pour it through a paint strainer back into the container.

5. When the blocks are clean and dry, remove the remaining tiny wires that are still attached to the lugs. You are now ready to reload the clean blocks with new modern components.

6. Use the Philco service bulletin #289 published in 1937 and included in some Riders Manuals. This publication gives all you need to know (but were afraid to ask) about Philco capacitor blocks. If you can’t come up with this publication, call me and I can arrange to get you a copy.

7. I do not refill the old blocks with anything other than new components, as a filler is unnecessary.

Aside from rebuilding the block capacitor (Tech Tip #6) there are numerous other challenges that one might encounter, such as: not getting the stations to track properly even though you’ve spent hours aligning it! Well believe it or not, the 260 KC IF frequency shown in the Riders does not apply to all versions of the Philco 90. If you’ve aligned one of these babies and it just isn’t right, try it again at 175KC ( or KHZ for you new timers). I submit you’ll have no trouble getting it to track afterwards. The 90 was made for about 3 years (1931-34) with at least 3 different versions. Some of the variations are obvious, but this difference in frequency is not, and either version will peak at either frequency.

Another common symptom with the Philco 70, 90, and a lot of other models is low or weak volume when everything else seems OK. If you touch you finger to the grid cap of the RF amplifier tube you will get lots of volume, but touching the antenna transformer produces little or no effect. The culprit is the antenna transformer which you may find open. It is usually an easy repair.

First, take a continuity check to make sure it is at fault. It so, remove it and examine the fine coil winding associated with the lugs that failed the continuity test. You will usually see the telltale greenish signs of corrosion on an area of the coil windings. Probe carefully around the affected windings using a small pointed pick (I use a dental pick). You may find one or more of the windings not intact. They may even fall off with careful prying. You can remove two or three turns and get away with it but more will affect performance. Rewinding a small section of the coil may also be in order. With patience and effort it can be done. Be sure to use the same size enamel-coated wire.

The volume control on the Philco 70 is a tough one, if it is bad. It is nearly impossible to find an original replacement. To replace this special dual control: use a suitable 10 to 15 kohm, 2 to 4 watt, wire-would control and wire the center lug (wiper) to ground, and the left lug to the cathode of the RF amplifier. Wire the antenna terminal and the coil winding together to the right lug. Next install a 1kohm, 2 watt resistor in series with the wire marked F (using schematic in Riders) the cathode of the #24 tubes (same place we hooked the left lug to). The other wire marked G goes to the cathode of the oscillator tube #27. Install a .05 microfarad, 600volt, capacitor from point C to ground. This should work quite nicely.

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