1957 Normende Fidelio

The first step in any tube radio repair is to see what condition the capacitors are in. These radios used wax and paper capacitors as well as high quality mica ones too. Obviously at 67 years old, the paper capacitors have long passed their shelf life and need to be replaced. The old waxies have to go as well. They may still test good but once they get warm, they tend to drip and loose their guts. 

So far so good, but how can you tell which caps are the ones you need to replace? I have found the radio museum to be an invaluable source of information on which caps were fitted to German radios of this era. However, before you grab your soldering iron and start ripping and replacing, it is well worth the while to take a step back and look at the circuit diagram first. Big old smoothing caps can be replaced easily if they test bad but you want to keep away from the little mica, quarz and polystyrene ones around the audio and IF circuitry. Changing those willy nilly, will throw your whole radio out of alignment and replacement is really not necessary. They rarely fail.

A great resource for vintage radio circuit diagrams is the radio museum. Well worth joining if you are really into this. However, limited downloads per day are free and this is usually enough to get you what you want.

The first thing I do is to stitch the circuit diagrams, which are normally spread over several pages, into one big picture. I try to clean this up the best I can using InfranView, which is a great little, free picture editor. I then convert it to a .pdf and open it in the free version of Adobe Acrobat Reader. Adobe offers a very nice free feature whereby you can highlight circuit traces in the color of your choice, which is terrific in trouble shooting and getting the big picture of what is going on in your radio.

In the comments section of Adobe, you can add a line. The round circle shows the color of ink that will be used. I use green for circuit legs I have tested that are ok and red for ones that need attention later. I begin at the power supply, working my way up through the smoothing caps to the anode of the final amplifier. I check resistor values with my ohmmeter and mark good resistors green. At the rectifier output you want to see a good healthy B+ voltage. This should be around 200-300V D.C.

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Disclaimer - if you are following along, you are doing so at your own risk. These radios have potentially lethal voltages that can kill you. If you have not already read my blog on making your workplace safe, head over there first and check it out. 

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Typically the smoothing caps are housed in a large cylindrical, metal container. These are the first caps to suspect as on their way out. They can be tested with an ESR meter to see how leaky they are. There are many articles on the internet as to how best to replace these caps. Some cut the metal container apart at the base and stuff new caps inside. The look and feel of an original radio is best restored this way. Others will leave the metal can as is and clip off the leads of the caps and place new modern caps under the chassis where they can't be readily seen. The choice is yours and depends on the owner's budget.

Next we want to test the output transformer for continuity. If this is toast, the repair can become very costly, or even not possible at all, as output transformers are rare to come across with the correct number of windings and turns.

Again, mark each component and leg of the circuit diagram green as you move through the radio. If the output transformer is good, it is time to take a look at the last tube in the chain which amplifies the sound to a room level for listening to on speakers. Here we want to make sure that our B+ voltage is reaching the anode pin on the tube socket. "How do I identify the anode pin on the tube socket?", I hear you ask. Well that is usually in the service manual. 

Each tube is show with it's pinout as seen from above, i.e. looking into the tube socket. If we are sure we have B+ continuity to the rectifier, then B+ will be here once we turn on the set. The next place we want to turn our attention to is the grid of this final tube. When we remove all tubes later, for an initial power up, we want to be sure there is no DC on this pin. If there is, it will be due to a leaky cap. DC on the grid is bad for the tube. It only expects an AC signal. It will heat up very, very quickly and ruin the tube so we want to pay close attention to that pin as we power up the first time.

Indeed, all grid pins should be checked for zero DC all the way back through the radio, on that initial power on test.

However, for now, we can keep working our way back, from right to left through the tube chain, ensuring we have continuity and resistors are checking out with values that are within tolerance. 

In my case with this radio, two caps were toast and were replaced (see new white caps on the left).

If you haven't already read my post on firing up an old tube radio for the first time, head over there now and read it first.

After having checked the tube train all the way back, looking for shorts and failed caps or resistors, it was time for a first power up. This was done using my isolation transformer, variac and dim bulb tester.

On a 25W bulb, the radio initially started up (all tubes removed) and I could confirm B+ was present where it needed to be. At this point I was sure we had no shorts to ground and so installed the tubes and applied power slowly once more. The dim bulb lit up for around 1 second and died down again and all filaments appeared to be glowing. A quick check of grid DC voltages was taken across all of the tubes. Then the tubes were measured for temperature with an infrared thermometer. No tubes seemed to be excessively hot but the EABC80 was colder than the rest. However, sound could be heard on the TA input (Tonband Abnahme - or external tape recorder input). I could hear my 1 KHz signal generator in great quality in the speakers. Therefore the EABC80 was replaced with a new one and immediately, the receiver sprang back to life on all bands.

The sound of these old radios is absolutely phenomenal. Compared to your Alexa or Marshall, this radio sounds much more realistic.

Therefore, I decided to add a bluetooth interface to the radio on the TA input. These USB devices can be purchased cheaply on eBay and connect directly to the TA input using two resistors and a cap.

The cap isolates the bluetooth input from any DC that may be present. Then the two resistors connect the left and right inputs together with a tap in the middle for the cap. This is a mono amplifier and so the left and right channel have to be joined together to feed the mono input.

Try it out - it will blow your mind just how good these final stages were.

Another radio has been awakened back to life. Here it is on shortwave listening to radio China from Germany.