Dansette Companion output transformer

[Andy_Clarke]

Got a little Dansette Companion with no sound. Gently rocking the output transformer restores the sound. On removing the transformer it was obvious there was a broken wire on the input side, where it was soldered to the board, no problem to extend and resolder it. However there are two other wires on the input side that are joined where they are soldered to the board, one of these has broken where it disappears into the transformer windings.

Is there any way to rescue this or does it mean trying to source a replacement transformer?

[AC/HL]

There are two versions of the Companion, but they both have the same amplifier.
From your description the problem is the centre tap. Unless you can find a way to get at the broken wire it does look like a replacement.

[newlite4]

I have just done a couple of Dansette Gems, one of which had this same problem. It seems that if dropped, the transformer frame doesn't have much structural rigidity and can move on the pcb which leads to fractures of the connecting wires.
Neil

[crackle]

Have a search on Ebay for Eagle LT700 transformer, it might do the job.

Mike

[Andy_Clarke]

Originally Posted by AC/HL

From your description the problem is the centre tap. Unless you can find a way to get at the broken wire it does look like a replacement.

The transformer connections in that schematic don't look the same as mine, not that I'm too hot at reading schematics. Two of the wires are definitely joined, one does look like it's the centre tap.

Originally Posted by crackle

Have a search on Ebay for Eagle LT700 transformer, it might do the job.

Thanks, I'll get one.

[Andy_Clarke]

Fitting an LT700 transformer has resolved the lack of sound problem, however the sound is very distorted at low volumes, not too bad with the volume control on max.
Any ideas what to check, the old Plessey electrolytics have all been replaced.

[Herald1360]

Check the quiescent current in the ouput stage- measure voltage across the 4.7R resistor. Should be around 10-15mV or so. Ideally about the same current should be in each output transistor, but this is trickier to measure without removing component legs.

If it's lower, try shunting the upper bias resistor (4k7) with about 47k and see how much it increases. Higher shunt R, less increase and vice versa.

Worth a check that each output device's VBE is pretty much the same.

If nothing improves, you may need to try another, reasonably matched, output pair.

[Andy_Clarke]

Originally Posted by Herald1360

Check the quiescent current in the ouput stage- measure voltage across the 4.7R resistor. Should be around 10-15mV or so.

12mV rising to 80 at full volume.

[Herald1360]

That sounds reasonable for quiescent current and something's obviously doing its class B stuff.

Maybe the output devices (or one at least) has degraded.

Try measuring voltage across each half of the output TX primary at zero volume. It won't be much but may give an idea about balance between the two transistors.

I don't have the front end circuit- could be something iffy there, though I'd expect that to cause trouble at high level.

Have you got a pair of general purpose PNP silicon transistors? Might be worth subbing them. Increase the 100R bias resistor until the voltage across the 4R7 is same as with present devices and see if it's any better. That would point to output device trouble.

[Techman]

Classic speaker voice coil rubbing with distortion at low volume but clears when volume wound up.

[Andy_Clarke]

Originally Posted by Herald1360

Try measuring voltage across each half of the output TX primary at zero volume. It won't be much but may give an idea about balance between the two transistors.

Well, the problem is, as I mentioned above, that the output transformer was wired differently to the circuit diagram posted above, in that two of the primary connections were joined where they were soldered to the board, there also appears to be an additional capacitor under the board. It all looked original. Have to say I'm not to hot at comparing a schematic to the actual physical board but I think I might have a go at rewiring this board to match the schematic and see what happens.

Don't suppose anyone's got one of these Dansette Companion radios and could take a look at the board?

[Herald1360]

The two joined primary connections simply make the centre tap. Circuit diagrams are rarely laid out in the same way as the actual hardware, they are ( or should be!) just identical connectivity wise.

Do investigate Techman's suggestion first before diving in! Easiest way is to substitute a known good speaker.

[Andy_Clarke]

Sorted I'd just made a silly mistake when fitting the replacement transformer
It sounds good - just sitting here listening to the Archers...

Just need to find a replacement for the missing chrome bezel for the volume control.

[Herald1360]

[crackle]

Thats good

Mike