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.

Tube Radio Repair

 

After having fixed many solid state radios, I decided to learn something new (old) and jumped into the world of tube radios. I found my first radio on eBay for €10. The radio lit up, I was told, but had no  sound. Sounded just like what I was after to learn the tricks of tube radio repair and restoration.

But before we jump into that in another blog post, I had some preparation to do. These old radios can have voltages of around 300V D.C. inside and depending on how you put the plug in the wall socket, the chassis can sometimes be hot! Therefore, it is wise to build some protection into the bench. The first step was to source an isolation transformer. This is simply two windings on a transformer that are galvanically separate from one another. There is also no connection to earth.

This means that I can safely touch a hot wire and not get a shock. Obviously, if I were to touch both live and neutral at the same time, I would still get a shock - but hey - I don't plan on doing that. A suitable sized transformer was sourced on eBay.

Now that the power supply was floating, the next issue to solve was how to bring up the voltage in the radio slowly, so that if it did have issues, I could cut the power right away before the magic blue smoke began to escape from everywhere inside the radio. China came to the rescue with a low priced variac.

Now, before I plug anything I get from China into the wall socket, I take it apart and make sure it is electrically safe. In the case of this device, I am glad I did, as the earth connections were made with bolts through the metal case but the case paint had not been removed and thus the connections were not being made correctly. A quick buff with the Dremel and we were good to go. Note that this variac is not an isolation transformer in itself. This unit has a direct connection between neutral on both sides of the transformer. So the neutral wire is still referenced with respect to earth. Therefore the need for a separate isolation transformer. Buyer beware - some ads offer this all in one - and there are devices out there that can do it but you can't be sure until you get it home and test it. 

Finally, a way to reduce or limit the current going into the radio was needed. Often when tube radios are first switched on, they will draw a momentary surge of current and then drop off back to a lower level. This is ok, but if a radio has leaking capacitors, as most do, then they are basically short circuits and they will draw a lot of current, resulting in smoke. A cheap way to implement this is to use a so called dim bulb tester. The hot wire is fed through a light bulb and then onto the socket. So the bulb is in series with the load.

An old tungsten wire bulb is needed - new LED bulbs will not work. The bulb has some neat characteristics which we can make use of.  When it is off, it is almost a dead short with only a few ohms resistance. So the current can flow through it to power the radio. However, once the current draw begins to rise, the wire changes its resistance as it begins to heat up and light. The more it lights up the higher the resistance goes up. The lit lightbulb also warns the tech that high current is flowing and to turn the device off. Similarly, if I were to get a shock the light bulb would like and take most of that current away from me.

Finally, to be able to work on the chassis, it is very helpful to suspend the chassis in a frame. This way the radio can be turned over easily and without damage to the dials, tubes or pulley mechanism. I got this idea from M. Caldeira on his YouTube channel.

I made mine from flat bar aluminum with a profile of 20mm x 3mm. This come in a length of 2450mm which is exactly the right length to make two rectangular frames. One for each side.

Now that everything is prepared, it is time to get working on that first receiver.

Icom IC-7100 Poor Power on Transmit

 If you have an Icom IC-7100 you will know what a disappointment you have felt as you hooked up your transmitter to a power meter and watched the peak output on SSB. Despite your transmitter being set to 100W you will most likely see around 20-30W max on your meter.

After checking all your connections and radio settings, you are probably wondering why your expensive radio is not anywhere near the spec. that it stipulates on the specification sheet. Well, welcome to the real world!

I have no idea what the Icom engineers were thinking when they designed the transmitter feed back loop, designed to avoid overdriving the finals. However, it would seem that they are shutting down the Tx way to early and a simple fix is well shared within the internet community:

The modifications can be done quite quickly if you have a steady hand and a good solder station:

The main board is placed in the solder station with a microscope to enlarge the area to be modified by the addition of a capacitor and a bridge as recommended by DC5WW

The modifications are show in the picture below:

The old man also wanted the radio opened up to transmit and receive to all bands. To achieve this the diode D316 was removed.

After some capstan tape and a blast from the hot air gun, the diode was removed from the PCB. The diode is tiny, take a look for yourself:

An Rx and Tx test showed the transmitter on frequency and with no spurious signals.

Job done!

Kaiser KA-9040 CB Radio Repair

Kaiser is a well known and very popular brand of CB radio sets manufactured in Germany. They were hugely popular in the 70's and 80's and it is great to see these old sets still alive today.

This little set had some very scratchy pots. Just a tiny touch on the volume control would wreak havoc with your ears. The old man also complained of low power output and the 7-Segment display was very intermittent. Parts of digits were not being displayed correctly. All pretty typical problems for this model and year of production.

The customer requested a full workover and alignment so it would work as new out of the box 50 years ago.

The first step was to crack the case open and begin with cleaning those scratchy Pots.

Now many of you may think that a whiff of contact cleaner is all you need. I would beg to disagree. In my experience Pots that are this old and require cleaning, need more that one go. I typically start with the first blast of cleaner, followed by a good mechanical back and forward working to get the cleaner into the spots needed. But then to let them dry out or blast them with compressed air and repeat the process several times.

In the bottom center of the picture above, you can see the rotary switch for the channel selector. This is notorious for bad intermittent contacts, leading to a partial 7-Segment display. The issue is indeed not with the display itself but with the rotary switch. I  required no less than 7 cleaning cycles to restore the display to normal. Yes - with drying out in between - a task than can spread over several days, depending on the amount of time you have available to spend on radios like these.

If you are considering doing this professionally, I can tell you that NO-ONE will pay for your time. It has to be a labor of love.

Once the 7-segment display was working again - best seen on channel 40, it was off to find the source of the low power output.

Now, many of you may be thinking that the finals are shot or a voltage regulator has given up its precious magic smoke, but the very first thing I do when I get a radio, is to try to reproduce the problem that the old man has reported. Quickly, I noticed the issue appears to be with the microphone PTT switch. A slight sideways depression on the mike key showed full power.

I cracked the mic open and cleaned the PTT switch. After a few goes, the radio was back to full power.

These radios were carefully crafted back in the day and a glimpse of the inside confirms the care and attention they gave to the wiring and layout of the PCB.

With a little bit of tender care and loving, this set is back to its original condition and one old man is very happy again.