Trevor's Tandberg TP41 and the mystery of the "Stabistor"

 Trevor called..

"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...

https://www.electronics-notes.com/articles/history/radio-receivers/tandberg-tp41-circuit-block-diagram-maintenance.php

The what? Stabistor?

So, the schematic is consulted.... 

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 !