last modified: Saturday 30 January 2016 (UTC)
author: Hales
markup: markdown

Repair: Jena 5020 vintage radio

In its natural habitat:

This radio is about 50 years old and sports the latest in silicon dioxide technology. Notably it has a phosphor bar-graph tube right under its badge to indicate signal strength:

It's one my father's favorite radios. Only the FM mode has ever worked and it's mono.

About a week ago we turned it on and found it had several problems:

Opening it up

Taking off the bent backing board:

We have seven vacuum tubes, six of which are mounted to the main chassis. The leftmost is partly shielded and sits atop the FM tuning circuitry. The rightmost is a rectifier for the main DC supplies. Hiding horizontal to the right of the speaker is the bar-display tube, held on by a spring.

This is supposed to be a German built unit, however my TV education says that Germans are precise and neat people. My suspicion is that this particular model was built by pastafarians:

The PCB only covers a proportion of the design. Overall it feels like a mess and the construction method caused me many issues. They could have used to the PCB to do a lot more of the work, but it would have required jumper links and a lot of physical redesign.

I'm suspicious of how long the assembly would take for each of these radios. Soldering and assembly is something that appears on the outside to be easy, but reveals its true nature when you awake from the flux coma sometime after midnight. I have suspicions that this radio would take hours to build from scratch.

Fixing the hum

A mixture of cleaning the PCB (as best I could) and jiggling the valves around in their sockets fixed the hum problem. The whole board is still dirty, but nothing short of desoldering most of the parts or dunking the whole unit into solvent would fix this.

Originally I suspected that the 50Hz hum was caused by the main (electrolytic) filter capacitor:

These tend to dry up and lose value over time. I don't have equipment capable of measuring 50uF, however the tube-jiggling magic fixed the problem so presumably this is still ok.

The polarity of the cap confused me. I asked for some help on the EEVblog forums and had the answer within minutes: this can contains two seperate capacitors with the case being used as a shared negative. I presume each cap smooths a seperate DC rail, but I never measured to confirm.

Getting at the switches

I could not see any way to get at the switch mechanisms, so I took the front knobs and glass off. This didn't go too well.

The knobs are held on using worm screws. One of the knobs was missing half of it's screw head:

Getting it off involved lots of gentle plier work and prayer. I ended up breaking the potentiometer for the tone control (bass/treble), leaving the radio stuck in the 'no bass' position.

Repairing the pot

The potentiometers for volume and tone are double-stacked. Here I have already dismantled the culprit. Pretend you can't see the plier scratch-marks on the shaft:

You may also notice the mesh of parts soldered directly to these pots. These were a pain. One test cycle I forgot to put the tagstrip back into the correct location and (I presume) some leads were touching. Something went bang, but I never found out what it was or if it caused any issues.

The pot had two faults. Spinning around the central shaft is a black piece of plastic with a metal ring attached. A small piece of the plastic (to the left in the picture below) is supposed limit rotation but had broken off. The metal ring is supposed to contact the carbon-coated disc below, however it did not when I tested it.

I superglued the plastic back together and soldered in a small amount of brass wire as a brush. I had to bend it over a bit more than is shown here for it to fit, but it works perfectly now:

The pot stack was held together by small steel tabs that you bend over. To repair and test the pot I had assemble and disassemble it a few times and the tags broke off due to fatigue.

To stop the pots falling apart I clipped and hot-glued them together:

I hope that readers are not upset by the final appearance of this pot. It's now returned to full working order and the glued clamp is removable for future repairs. I regret the damage, not the repair.

Things always go wrong during electronics repair, despite the best preparations. What can go wrong will go wrong, and what can't go wrong will still go wrong. Replacement double-stacked audio-log 1 meg-ohm pots are not common and nothing that exists today would suit the exact same dimensions. I believe it's a blessing when the part that breaks is one that you can repair.

Collateral damage

One of the wires from the pots snapped its connection from the PCB. I suffered this problem a few times when working on this radio.

The makers used wire that's only just long enough to get the job done, not long enough for parts to be easily moved or worked on. The wire is also solid core so it takes very little flexing before it breaks.

Identifying where to reconnect the wire wasn't easy. Before I started work I took what I thought were lots of pictures of the unit to use in situations such as this. As it turns out they were not enough, and those that I did take were blurry due to poor lighting.

For example: the yellow wire at the top of this photo is the culprit. Can you tell where it terminates?

Somewhere away from that DIN connector it solders onto a tab sticking out of the PCB. I identified it by the tiny copper cross-section left on the tab from the break.

Morale of the story: take pictures of all junction areas under good lighting before starting. Ignore family concerns about the albums of solder joints you collect. It's a sign of sanity.

Triage of the switches

Some investigation revealed that the main switches could not easily be disassembled. Everything was constructed onto and around them, so almost all of the radio would have to be desoldered before I could get to their contacts. In the end I kept cycling the switches until they cleaned themselves and this seems to have solved the problem.

An interesting discovery was that the leftmost switch on the front panel is a hard power switch:

'Aus' means 'exit' in German. For years we had been partially pressing another key down to release the FM (UKW) switch. This left the bar-graph tube on, making the unit look faulty, until we killed power at the power-point. We presumed that 'Aus' was yet another frequency range that didn't work.

Bad part hunting

Next step of the repairs: hunt for out of spec parts.

Some parts very politely label their values:

Others use coloured dot codes:

Then there are the mystical parts that go "I'm measured in units of Gf!":

I've never come across these units and could not find any information on them. I have a suspicion that these are inductors.

Decoding resistor dot notation

By writing down the coloured dot combinations and comparing them against measured values I found that they approximately followed modern colour codes.

The dot nearest the end is the first digit, the middle dot is the second digit and the last dot represents how many zeros you should add after the first two numbers. White means 6 (rather than 9) and silver means 3. I hope this helps someone else out at some point.

Whilst doing research on old resistor codes I came across a video on identifying old resistors. It looks my resistors are called 'dog bones'.

Replacing resistors

I only replaced a couple of resistors. Some parts were a little outside their tolerances but I left them alone.

I mostly only have a collection of bog-standard axial 1/4W resistors, so I was lucky that the resistors I had to replace were all high in value. Even at a few hundred volts they don't need to dissipate much power. None the less, many resistors of this size are not rated above a couple of hundred volts so I used multiple for good measure.

I installed them on the opposite side of the board to the other components. This allows access to them without having to dismantle the whole radio:

Yes there is heatshrink on these, it's just transparent. They're not going to short against the other tracks :)

A finger test whilst running the unit confirmed that only a miniscule amount of power was being dissipated through them.

Getting the Schematics

Ha. Ha ha ha.

After a brief search I found this download page that gave me a scan of the schematic and list of parts. I kept looking for a PCB layout diagram, as matching a schematic to a real circuit can be difficult.

Lo an behold a full manual with pcb layouts exists at radiomuseum.org. Only some low quality and cropped previews are available on the page, however it tells you to right click the images to download a full copy. I couldn't get this to work.

As it turns out you have to be a member of the site first. I'm showing screenshots here for reference, in case the site changes:

Okay, so what does that entail? I'm open to joining new communities -- sometimes they limit your ability to download things simply to reduce bandwidth costs. According to the Application for membership page:

As I read this warning bells started going off in my head. They want to share my private phone number? Paying money to a Swiss bank account? Uh huh.

Instead of paying your way in you can earn your admission through community activity. I would probably enjoy contributing to the site's forums, but not in this way. As a hobby I enjoy electronics because so much information is freely shared. I don't feel comfortable being part of a community where you can pay for the privilege.

The overall feeling given to me by this page is that they want to control the online vintage radio community. They might run it really well, I don't know, but years of data loss has told me that putting any single authority in charge of information is a bad idea. What will become of this only scan of this radio's schematic in another ten years? Will this site still be around?

Alternate routes

Reverse image searching some of the previews yielded nothing useful. No one else seems to have these scans of the PCB layout.

Radiomuseum.org didn't do something daft like hide a download button on the page by default. The download links are either not sent to my web-browser unless I am logged in (the correct way of doing things) or are hidden in deep layers of javascript (which I'd probably prefer to pay $25 than read :P).

Reverse engineering

When the world won't give you information on your terms, make the information yourself.

I started with photos of both the bottom (copper) side of the PCB and the top (populated) side. Using the perspective tool in the GIMP I was able to align the two layers:

I've done this before to help understanding other boards. Tips if you ever want to do this:

Lining up layers using the GIMP's perspective tool is a pain, because when you adjust the area near one corner the other parts of the layer distort too. I went in circles, re-aligning all four corners over and over again, each time getting the pictures a little bit closer to perfect.

Ideally there would be a tool that lets you define exact points/corners you want to distort the layer from. The cage tool in the gimp does exactly this and is perfect in all but one way: it only distorts the pixels within the cage, not the entire layer. I like to align my layers based on visual cues such as vias/holes that go through to both sides, not the absolute corners of the PCB, so this method causes too much of the image to be lost.

Anyway, once the pictures were aligned I loaded them up into Inkscape to draw out the parts. First was the copper layer:

Then working off what I saw I filled in some information on the components:

I didn't complete the job but I took it far enough to help me fix the display tube problems. Much of the electronics are either off-board or floating above the switches -- drawing these in would be a messy nightmare.

Fixing the display tube

Using my reverse engineered diagram I was able to locate the two resistors I had replaced on the schematic:

These are both used to control the EM-84 bar-graph tube and were probably what was causing the problems with it. I also replaced an out-of-spec cap on the same line, shown right next to the tube in the diagram.

The cap's role would likely be to smooth the control signal to the tube. Without it the tube would (try to) flicker at the carrier or audio frequency, depending on where they are sourcing the signal form.

I probably could have gotten away with not replacing the cap, but I was suspicious as to why all these parts had failed together. The replacement had a higher capacitance than specified so now the bar graph is a little sluggish.

Old capacitor, after scraping the wax off to read the label:

Many of these wax capacitors have lost their ends, aka 'boogers'. Much snot was found rattling around in the case during my repairs. I suspect the remaining wax caps are going to succumb to moisture sometime soon.

Closing notes

I've been putting together an order of new replacement parts for the radio. Getting a variety of small (3 to 100pF) 500V rated leaded film capacitors isn't crazily cheap or easy these days and I don't want to resort to SMD ceramics with leads soldered on.

Overall everything took about a week. I didn't attempt to fix the other bands on the radio and I could have replaced many more of the parts, but I'll leave those repairs to another day. For now the radio sounds and works great.


2016-01-30: I've been in contact with the previous owner:

It's so fantastic that my radio found such a great home. It was in a flat in East Berlin that I lived in. There where these old valve radios in lots of the flats in the building this Jena probably the smallest. One was the size of a big Juke box with incredible deep rich sound.

I used to listen to the John Peel sessions on the British armed forces radio on this Jena

PS the short wave (LW) and medium wave (MW) did work . the FW I never used as UKW the main FM band was better Thanks for post

I'm not sure if I offer the safest home to this radio, but it does continue to see regular use. I think that's the best thing.

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