This radio belonged to a friend's dad. Before he passed away, told my friend to give me this radio. I saw (listened) this radio fully functional, back in the days, but I received it rusty, dusty with broken wires, and the FM wasn't working.
I´m on electronics since my youth and worked on an electronics service while I was in college, studying for an automation engineer in the late 90's-mid 00's. In that period, I repaired tube TVs and radios so I have some experience.
In case you aren't repairing or restoring this same radio, take this tutorial as a general guide if you have electronics skills but you aren't familiar with tube radios. I was able to repair mine, looking on forums where people discussed the same failure on my radio, but on other models. Or you can simply take a look at how "old tech" looked like.
I must say that this is more electronics-oriented than to aesthetic restoration. I don't want a vintage piece of furniture but a fully functional device.
I performed this restoration with simple tools (I didn't use my oscilloscope or anything else)
-Multimeter (with capacitor meter)
-Solder, soldering iron
-Brushes, compressed air to clean.
-Electronic spare parts (resistors, capacitors, tubes, etc)
-Anti-rust liquid, contact cleaner, alcohol
-Primer, paint, lacquer etc
-Sandpaper, drill accessories for grinding
-Arduino capacitor meter (my multimeter isn't accurate on picofarads)
Step 1: Tips & Warnings
Safety first! "The high voltages stored in large capacitors can kill! If a radio has been turned on in recent weeks some of the capacitors (especially the electrolytic capacitors) may be holding deadly voltage charge. Before working with these capacitors they should be completely discharged. This can become by (bridging) connecting the two ends the capacitor in question with a high wattage 1000 ohm resistor via insulated clips and leads".
No PCB. Most of these devices are wired point to point. Be careful to not bend components. If some component's leg touches another, that is not supposed to be joined, you may cause a malfunction or a short circuit!. The same recommendation if you want to measure voltages, be careful with the multimeter's leads.
Tubes are tough. I'm not saying that they can't fail, but you better look first for bad resistors and capacitors.
Variable components. You will find variable capacitors, variable inductors, etc. Most of this elements are covered with wax or resin. Don´t move anything unless you know what you are doing.
Tuner zone. The most delicate part of these radios. Special care with the tuner main capacitor (the moving aluminum plates), the large ferrite core (AM), and the wires(string-like) from the coils in general.
Chemicals/Solvents. Be careful with this. You may delete component's values/marks or some chemicals can "eat" the wire's isolation from coils and transformers.
Take pictures from different angles, just in case you forget something or broke something by mistake.
Read. Yes, read everything you can about your device. Look for schematics, vintage radio sites, and forums. Here you have some suggestions:
I do recommend to read these tutorials. They helped me to refresh my memory about the tube radios:
Step 2: First Impressions
Well, the cabinet didn't look bad from the outside, some little scratches but ok for the age. The inside was very dusty and some wires were broken on the AM ferrite core. So I took out the entire chassis to begin my restoration. Only the speaker stayed on the cabinet.
I used a brush and compressed air to clean the chassis. I found a few "toasted" components. With the schematic, I replaced those components and rewired the AM Ferrite coils. I was hoping that those components were the cause of the FM failure, but I was far from that.......but the AM signal became stronger.
The chassis was very rusty, my first thought was to clean with an anti-rust chemical but I didn't want to damage components and would be difficult to paint again, so in order to preserve this radio, I decided to remove everything, to grind the chassis.
-Tuner string wore out
-Dirty switch's contacts
-Noisy volume and tone control
-Burned dial lamp
-Missing knob cap
Step 3: Restoring the Chassis
With the schematics and taking some notes, I removed the components in groups: the components attached to the tube sockets, the cans, transformers, switches, tuner, etc. I used my drill with a grind accessory and sandpaper to clean the chassis. Then I repainted the chassis.
If you want to re-paint your chassis, just remember that the chassis acts as a common ground for components and metal parts like transformers, switches, the main capacitor can, IF/RF cans, tuner, etc. Cover this zones or peel the painting before installation. Check everything with the continuity function on your multimeter.
It'sstill winter here in my country so I didn't want to put some lacquer on the cabinet. I painted the chassis and used a box with a lamp inside to dry it, but I didn't want to do the same with the cabinet.
I used a common cloth cleaner to clean the speaker cover (fabric) and soft cleaner for the keys, dial panel, etc. I used lemon's juice to remove the rust from the fabric(very effective).
I immersed the rusty screws, bolts, nuts, etc in an anti-rust chemical.
To replace the knob cap, I bought 2 copper tube caps (1 1/4") and then reduced to the size. Then I painted the caps with golden paint.
Step 4: Electronics
With all the components out of the chassis, it was easy to check the elements. As you can read on the links that I gave you in step 2, I looked for off specs components:
-Paper capacitors: I found some of them. The paper capacitors can be replaced with film ones. For this task, I used the Arduino capacitor meter to test the capacitors in picofarads and my multimeter for nanofarads and microfarads.
-Resistors: I just changed a couple.
-Electrolytic capacitors: I replaced all of them, except for the main one. These capacitors usually have more than one inside, 3 in my case (50 uf + 50 uf + 4uf), and share a common ground with the metal can.
-Functions switch: disassembled and cleaned. It's a complex mechanical part, so be careful. I took note and pictures of the components attached to it and the initial positions. The schematic also shows the switch positions.
-Tuner section: I cleaned the AM-FM switch, and checked every component inside. Everything was fine (I thought). I didn't want to mess with the tuner, so I just cleaned the metal and painted the surface with a little brush to protect.
-Transformers: I cleaned the main and audio one, and put some lacquer to protect. Also, I replace some output wires because the originals were too cracked.
-Tube sockets: I looked for poor contacts, but everything was fine.
-Volume and tone potentiometers cleaned with contact cleaner.
When you are checking components, you must desolder one leg (if needed) to prevent wrong measures.
Arduino capacitor meter code I2C (no extra components). LCD address 0x3f by default
LCD to Arduino:
Vcc to 5v
GND to GND
SDA to analog 4
SCL to analog 5
Leads/Probes= A0 and A2
Step 5: Circuit Assembly
It's time to put everything back together. Guided by the schematic and my notes, I reconnected everything again. I was hoping that the FM should work with all the fresh components.....but didn't. At that point, I blamed the tubes, so I bought the ECC 85 and EBF 89 because the problem should be in the RF/IF section. I didn't buy the ECL 86 because the amp was working fine. I swapped the tubes, but nothing happened.
I was really confused because I had tested every component on the radio. It was time for Google. I googled "Grundig 96 no FM", "Grundig 96 FM failure" and such, but I didn't find anything about my model. "Grundig FM failure" gave me some results and some advice pointed to a capacitor (4.7-5 nf) in the tuner section for the Grundig 97. Looking at the schematic, I found that the circuits were very similar, but couldn't find the capacitor on my device. Well, this capacitor was located between two plates in the tuner section so I wasn't able to see it. This radio was assembled in my country, Chile, so I don't know if this capacitor is in the same location for other models.
This capacitor is on the same track than my "toasted" 1K resistor. Some people reported this capacitor in short circuit, but in my case was open. So I installed a 4.7 nf film capacitor and the FM is back again!!!
Step 6: Final Assembly
Now the radio is fully functional. I fixed the tuner string following the service manual and put the dial panel and knobs in place. I didn't find the dial's lamp (7v), so I installed LEDs instead. I put the chassis back into place and cleaned the cabinet with furniture cleaner to achieve a nice finish.
Watch the video!