Vintage radio: the recap
Sep. 4th, 2025 12:17 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
arisoturaLately, (among other projects) I have been replacing the vintage radio's capacitors. Most notably, the paper and electrolytic ones, which are known to be failure-prone at this age.
It can get tricky to work on this kind of circuitry, where there's no PCB and everything is wired point to point. It's a mess. Sometimes you have to get creative.
I also couldn't do all of it in one go, because the schematic is a bit odd. For example, take the way capacitor values are expressed:
* 300 = 300 pF
* 1k = 1000 pF (= 1 nF)
* 2k2 = 2200 pF
* M47 = 0.47 µF
* 5µ = 5 µF
And then there was one capacitor which was labelled "2,2k", which seems to be a print error? The actual capacitor was 22 nF.
There was also a couple capacitors which didn't match the schematic values (ie. 39 nF instead of 33).
So I had to adjust to that...
But eventually, I got all the capacitors replaced. Then I quickly gave the radio a test.
I connected it to my laptop and played music. It was a night and day difference! Suddenly the sound was nice and loud, and there was no distortion. The volume controls were also much less weird.
However, I was still unable to receive any radio station. We even tried running a SDR to see if the radio would pick it up, but nope.
Then I looked back on the tubes that weren't right.
As I said before, one of them is an EF86, instead of an EF89. It turns out their pinouts are different, making them incompatible. The other tube that should be an EF89 is, well, I don't know. There are no markings on it. It doesn't seem to be an EF89, judging by how pins 1 and 6 don't have continuity, so chances are it isn't right either.
Then we have the modern GT12AY7, which is an ECC81 replacement. But we need an ECC85 there. The two seem to be almost compatible. Pin 9 is the filament center tap on the ECC81, while it goes to the shielding on the ECC85. Other than that, the rest matches. So, not great.
This situation would explain why the radio receiver part isn't working. Meanwhile, the audio amplifier involves a different tube (EL84), so there's no problem there...
I ordered replacement tubes for the three problematic tubes. The other ones are the correct types, so for now we'll leave them alone.
Hopefully fixing this should let the radio receiver come to life. I'm excited!
It can get tricky to work on this kind of circuitry, where there's no PCB and everything is wired point to point. It's a mess. Sometimes you have to get creative.
I also couldn't do all of it in one go, because the schematic is a bit odd. For example, take the way capacitor values are expressed:
* 300 = 300 pF
* 1k = 1000 pF (= 1 nF)
* 2k2 = 2200 pF
* M47 = 0.47 µF
* 5µ = 5 µF
And then there was one capacitor which was labelled "2,2k", which seems to be a print error? The actual capacitor was 22 nF.
There was also a couple capacitors which didn't match the schematic values (ie. 39 nF instead of 33).
So I had to adjust to that...
But eventually, I got all the capacitors replaced. Then I quickly gave the radio a test.
I connected it to my laptop and played music. It was a night and day difference! Suddenly the sound was nice and loud, and there was no distortion. The volume controls were also much less weird.
However, I was still unable to receive any radio station. We even tried running a SDR to see if the radio would pick it up, but nope.
Then I looked back on the tubes that weren't right.
As I said before, one of them is an EF86, instead of an EF89. It turns out their pinouts are different, making them incompatible. The other tube that should be an EF89 is, well, I don't know. There are no markings on it. It doesn't seem to be an EF89, judging by how pins 1 and 6 don't have continuity, so chances are it isn't right either.
Then we have the modern GT12AY7, which is an ECC81 replacement. But we need an ECC85 there. The two seem to be almost compatible. Pin 9 is the filament center tap on the ECC81, while it goes to the shielding on the ECC85. Other than that, the rest matches. So, not great.
This situation would explain why the radio receiver part isn't working. Meanwhile, the audio amplifier involves a different tube (EL84), so there's no problem there...
I ordered replacement tubes for the three problematic tubes. The other ones are the correct types, so for now we'll leave them alone.
Hopefully fixing this should let the radio receiver come to life. I'm excited!
