vrijdag 28 september 2012

Tivoli PAL

Recently, I was able to buy a defective Tivoli PAL radio.

The picture above is from the internet, and how it looks new.

I had heard the Tivoli model One before, and was impressed by the sound quality.

Altough, a Sangean WR1 which is also a retro style radio, has an even remarkable sound, especially the bass region. Being smaller, the PAL would probably not achieve the same result.
I like the design and simplicity of the radio, both the model One and the PAL.
I already have a clone or cheap imitation of the PAL, this one is a Watssound 9300.
Cheap because the tuning has no reduction, the feel is wobbly, and the sound is ... cheap.
On the other hand, a 230V power supply is built in.

The radio I bought was battered and bruised.

The antenna was missing, there are scratches on it and the rubber coating on the corners has gone.
The batterypack was missing too.

After opening the radio, the main PCB was cracked and some tracks interupted. Probaly, one had tried to remove the speakerscrew (which was missing), and damaged the PCB.

Furthermore, all the wires of the ferrite antenna were ripped off, probaly because the radio has fallen.

First problem: how to remove the knobs. Because they are conical, one cannot apply sufficient grip to pull them off as the are firmly on their axes. Even with some mild brute force using a plier and a protective cloth did work. I didn't try it, but a screwdriver between cabinet and knob would certainly create damage.

The soltion was simple: wrap a piece of strong sticky tape (Ductape) on the knob an make a loop so that you can pull on it. Easy and no damage at all!

Repair of the broken traks was done using a piece of wire. 

The wires on the ferrite antenna were colour coded, and on one pin of the mounting bracket you could see where there were 2 connected.

In the original TEA5711 application, just a single coil is used. 
The inductors were measured, and both are identical.
35uH without the ferrite, 205uH on the end as it originally was, and 235uH centered.
As can be seen following the PCB tracks, the AM antenna coil input on the TEA is on a tap, on the middle of the coil. This could improve the selectivity (and sensivity) of the radio.

Finally, power was applied, and... nothing happened.
Strangely, the radio draws more current in the off position than in the on position.
Following the tracks, it seemed that the large transistor Q2 did not switch as it should do, so the pins were bridged, and there was sound!

It was later found out that a MTP2955V is a P-channel 12Amp FET (Ron around 0.2 Ohm).
The TIP2955 is a real transistor, but in a larger housing.

During the repair, the internet was searched for technical info, and nothing serious was found. So it was decided to draw a schematic, and do some tests.

First, to draw a schematic, a picture of the PCB wad used to draw the tracks on it. 
Different colours were used to identify ground, power, audio and other routes.

The integrated circuits in this radio are quite common, so datasheets are usefull.
The IC's used are:

TEA5177T : FM and AM reciever
TDA7056 : 5W (bridged) audio amplifier
TDA1308 : class AB headphone amplifier
LP2950CM : voltage regulator
BQ2002C : NiCd/NiMh fast charge managment IC
3403A (JRC) : quad opamp
LM358 : dual opamp

The radio uses some clever circuitry to quick or trickle charge, to indicate that and to protect the batteries getting too low, depending on the presence of an 12V adapter and the state of the radio (on or off).

Circuit diagram (in progress - click to enlarge)

To be continued.....