"Do you remember my Tanberg radio you repaired for me a while back?"
As it happens, I do... I fitted a new telescopic aerial, and cleaned up the controls a bit.
"It's howling, can you look?"
Yeah, why not...
Well, Trevor made a couple of attempts to get it round to me, each time thwarted by the radio, obviously feeling threatened by a trip to see Doz, putting itself right, and behaving again for days.
Eventually the fault was more or less ever-present.
It's a Tandberg Portable 41, dating from around 1970, and just oozes Nordic quality.
I got the thing on the bench , and sure enough it's sat there ... making a sort of mooing noise, that varied in frequency with the volume control.
Let's get the thing in bits...
First off remove the two screws to the left and right of the handle..
Gently bend the handle out and remove it..
Unhook the aluminium side pieces from the bottom of the case, one each side...
You can now remove the wooden "clamshell" from the chassis..
Remove the knobs..
You can now remove the top. You'll notice I removed the telescopic aerial too, there's a screw in the base, and, once removed, it just slides out of the top. I'm not sure it was necessary to do this.
Remove the two handle retaining plates before they fall off..
(It's worth noting how they sit the the grooves in the top panel to aid reassembly later!)
We can now get at the PCB, but access to the component side is difficult...
So remove the speaker , and place it carefully to one side, you won't find another...
Of course, once it was disassembled, the fault vanished....
Some googling occurred, and a schematic obtained.
This website, had a very detailed description of the set, and suggested the "Stabistor" was at fault...
It's a weird looking thing, and, as suggested in the website, is very corroded.. It's shown on the schematic as an ST1,5, given the designator D501, but it's not some sort of diode. It's drawn as a battery, and that's sort of what it is... it's acting as a voltage stabiliser, feeding a stabilised 1.5V supply to the RF & IF stages.
It doesn't look corroded here, as the green corrosion just fell away. It had sort of made it's way down the wire end, and had even corroded the PCB, which took quite a bit of cleaning up before it could be persuaded to take solder again.
"That's bound to be the fault" ... a big bag of red LED's is opened, and each one tested, until I find one with a sensible forward voltage drop, that I can use in place of the stabistor. It's tacked into the circuit, along with a 220uF capacitor to decouple it, and the radio is powered up .... no mooing! Excellent.
I put the radio back together again , and listen to it for the rest of the evening in the workshop.
The following morning , back in the workshop ... and the cow is back ! Damnit.
If the quiescent current adjustment (R518 on the above schematic) is just barely touched, the fault can be *almost* cleared. The instablility is once again permanent.
(As an aside, you may notice the audio output stage is a mix of silicon and germanium transistors)
The audio input to the output stage is disconnected by lifting C506. The fault still persists, so it's in the output stage somewhere....
There's a few electrolytics in there ... C512 (1000uF) is the output coupling capacitor, C513 (1000uF) is some local supply smoothing. These appear to be in reasonable condition, but changing them clears the fault temporarily. It can be made to become unstable again by adjusting the quiescent current again. Voltages on the output pairs base's are unstable... There are only a few electrolytics in the whole set, so they are all swapped out. The fault vanishes once more. The quiescent current control can be rotated and no instability re-occurs .... The quiescent current is reset by measuring the current flowing into the collector of Q503, set for 5mA.
Here's our stabistor replacement LED glowing away. It provides about 1.45V, slightly less than the 1.5V of the original component, but it shouldn't ever fail, and performance doesn't seem affected.
The guilty parties ... and another saved from landfill !
A while back the boss was having a big clear out of this basement, and, having been a fully signed up, Parker-wearing anorak of old, found a GE Superadio 3.
This was made for the US market only, but benefited from a sensitive FM & AM receiver, with an AM "Wide" mode.
In the US, AM channels range from 525KHz to 1705KHz, in 10KHz channels. This gives the station a maximum audio bandwidth of 5KHz, but I suspect many stations may have used more, hence the Superadio being equipped with "Wide" mode for improved audio quality.
In the UK (and EU) , we used 2 AM bands for broadcast , one Longwave (LW or GO in France) and Mediumwave (MW or PO in France). Eventually, after things were standardised throughout Europe, our channels were just 9KHz wide between 525 and 1605KHz (with one other allocation at 1611KHz for Vatican City). This allowed only 4.5KHz of audio, and often a narrower bandwidth of 3.5KHz was used to prevent "splatter" on adjacent channels.
The Superadio is connected up to a variac, and supplied with the required 115V. Nothing. Nada. It's taking no current from the supply. I connect my bench supply, set to 9V to the springs where the 6 D-calls normally reside, and there's a click from the speaker, but nothing more.
Measuring the resistance across the American mains plug shows it's open-circuit. Perhaps the 115V transformer was inadvertently connected to our UK 240V mains, and (rapidly) failed, perhaps damaging the radio circuitry itself before expiring.
Removing the tuning knob, and the bass, treble and volume knobs from the radio, and undoing the 6 screws holding on the back, allow the radio to be removed (not without some struggle) from the case.
The speaker wires are unsoldered, and the workshop speaker temporarily connected.
Getting some life back into the radio is simple, a quick squirt of contact cleaner on the switches and pots restores operation, at least from the bench power supply.
Let's investigate the mains. I'm hoping for a blown fuse, on a transformer which I can simply rewire the primary for 240V operation...
Once the screws to the tuning scale-frame/mechanism and plastic PCB frame are removed, desoldering
the AM radio aerial connector from the rear panel, sufficient access is gained to the underside, where the transformer exists...
It must be under that small fibre cover...
... hang on, what are those three wires that look to have been cut ... some sort of test jig used in production? Surely easier to have used a connector?
The small fibre cover is removed, and all becomes obvious. It's just the strain relief for the mains cable, and a small circuit board for the juction between the transformer primary, and the mains lead (and another mystery component, covered in heatshrink... read on). The transformer is absent without leave. The cover just protected the mains junction, and providing some strain relief for the mains lead. Perhaps the boss removed it to prevent damage if connected up here in the UK?
A small 2 x 6V transformer is ordered, and turns up the next day.
The circuit is very simple. It's just a transformer, and a full-wave rectifier. What bothers me is that mystery component, wrapped in heatshrink. It's connected between one side of the mains, and chassis ground of the radio.
A quick look at the schematic show's it's a 2.2M Ohm resistor. What's it purpose? It would leave the exposed metal battery terminal, as well as the ground terminals on the antenna connector referenced to mains, albeit via a large value resistor.
Am I happy about that? I make a post on the VRAT forum for knowledge.
Whilst awaiting a reply, I remove the american mains lead, as it's not double insulated. And fit a new lead. The original lead, obviously had a american two pin plug on it, and it hides away inside a small compartment in the cabinet when the radio is being used on batteries. Now there's no way a UK plug is going to fit inside that compartment, so I'm going to fit a male IEC connector, on a short bit of lead.
The new transformer is a PCB mounted type, as that's what I could get that would fit. It fits neatly in the hole, and I've superglued it in place, however, it needs some sort of bracket to hold it down.
When I started this repair, I had never used my 3D printer before, so this was first time I've designed something, or 3D printed anything.
A model is created in Fusion 3D. Sliced in Cura, and the results fed into the printer...
And 43 minutes later , it's done!
I have to say, I'm very pleased for a first attempt.
So the conjecture on VRAT is that the 2.2M resistor was probably there to assist signal coupling to ground, and is better and safer left out, as I suspected.
The new transformer has it's two secondary windings connected together, to form a centre tap, and connected up. I've tucked the 2.2M resistor out of the way, so if we ever fancy re-instating it, it's there. I take the opportunity to fit a wire-ended 250mA fuse underneath the junction PCB.
I must say I'm very pleased with the results.
An IEC plug is fitted, so the lead still fits in the rear compartment.
The electronics is tested and it's all working well.
The front speaker grille is removed by bending up the tabs...
... ugh. Years of filth! The speaker is removed...
... and the front case and grille head to the kitchen for a bath.
Once the grille is dry, the GE badge is warmed up with a hot air gun....
and removed ready for a coat of paint....
... duly painted in a fetching shade of satin black..
A break in the handle is repaired..
... and it's primed and painted in the same manner as the grille...
Scratches are polished out of the case with some G3 & elbow grease
... and finally the 3 front panel switches are filled, and the case reassembled...
... and, in doing so I carelessly break the power switch. It's important to make sure the case clears the switch...
Thankfully I didn't lose any of it's parts, and it's quickly crimped back together...
The badge is given a dab of hot glue, and it's refitted to the grille.
The knobs are replaced and the radio tested. It sounds great in 'Wide' AM tuned to Absolute on 1215KHz.
*Further waffle
Then came along "The Pirates" in an attempt to break the BBC's monopoly in the UK. Audio bandwidths were often way wider than the 4.5 KHz permitted, and, certainly channels Like Radio Caroline in the 80's, and the fast-paced Laser 558 produced fabulous (albeit mono) audio. Apparently 9KHz was often the norm, effectively occupying two AM channels. The American Superadio was then a sort-after anorak item for resolving this "extra" bandwidth, despite not being available officially in the UK.
Of course, the last pirate ship fell silent in 1990, but we've got FM (and DAB, I suppose, although quality of DAB can be best described as "variable") now, even if the stations all do sound the same..
Then, after what seemed like an eternity, Radio Caroline managed to get an allocation to permanently transmit on 648KHz. It's got a decent transmitter and aerial (once used by the BBC for the World Service), and can be received further out than it's intended service area of Essex.
Now, as both the boss and myself were interested in it, we set about measuring the bandwidth of Caroline's signal after I commented that it did sound rather "wide". We logged into a few web SDR's to find decent reception, and we measured about 13.8KHz of channel width, corresponding to a very respectable audio bandwidth of 6.9KHz, much outside the required 4.5KHz. Wide indeed. (My boss called people who knew people, who knew people, and ascertained that the audio was indeed 7KHz wide, and the authorities had passed it off, I guess not many people are using AM now, so it's mostly empty, and the problem of adjacent channel splatter has gone away). At the time of writing the BBC is in the process of switching off more medium wave stations up and down the country.
"The kitchen radio's stopped working. It's just making rude noises."
I went to inspect it ...
Sure enough it's just sat there, with it's LCD display flickering and making raspberry noises..
It's a Bush NE-2015 DAB radio with FM and a now long-obsolete iPod dock. It's put in Sterling service for 9 years...
Anyway... let's get it apart. Bush (or whoever makes their stuff these days) have carefully marked the fixings I need to remove with an arrow ... it's almost like they want someone to repair it...
Four screws on the bottom along the front....
... and two on the back, hidden under a plastic cap which just pops out...
There's a knot in the aerial wire, undo it...
Slide the back off, and undo the mains, auxiliary input connectors, and put the back to one side...
It all looks rather nicely made. I'm pleasantly surprised.
Even if they didn't *quite* manage to get the switched mode supply IC in straight!
There's no sign of capacitors having blown their tops, no leakage...
5 Screws sees the main PCB lifted so I can gain a bit of access..
There's some signs of that nasty glue, that turns brown and goes conductive... it's scraped off before it gets worse.
Now a hunch tells me the power supply isn't starting up. The supply is started up in the usual manner, by a voltage fed from the mains rectified voltage, via a resistor chain, and a small capacitor. I check the (two) mains reservoir capacitors are dischanged with a 10K resistor (and they were holding a charge, enough to give the unwary engineer an unpleasant zap!) . The small start-up supply capacitor is removed. It's 47uF at 25V...
The ever-reliable Jingyan MESR-100 ESR meter shows an ESR of 6.67 ohms...
A new one measures 0.34 Ohms, much better..
I also check the two main reservoir capacitors... They're 10uF at 400V each...
ESR is acceptable.
As is the measured capacitance. I'll pronounce those fit, and re-fit them, along with the new start-up capacitor...
A quick re-assemble, and, ace ... it all works!
Another saved from land-fill. I'm always surprised by the audio quality from this little radio, and now I'm impressed by it's build-quality too... a rare thing in this day and age. Well done Bush!
