Alba C22; 2 x 4.5v to PP9 conversion.

[Pip]

Hi, I'm Pip, never done this before but I was hoping for some help with an old, probably late 50s early 60s mw/lw Alba radio, can't find a model name or number. It works on a pp9 battery, when I first connect the battery, I'm getting good sound and volume control, all works well for a minute or so then the sound begins to fade and goes completely. Every time I take the battery connections off, then replace them again, the same thing happens. I don't know much about radio repair, I just love old radios and enjoy restoring the cases, but I'd appreciate some suggestions and if the solution is pretty simple, I'll give it a go!
Many thanks, Pip

[tivo]

It would be helpful to narrow the model down a little as there are a quite a few Alba transistor models of this era.

Any chance you can attach a photo to aid identification and thus dig out the relevant schematic?

I have had several PP9 driven radios that exhibit your symtoms simply because the (new) battery was either of dubious quality or ending its useful life.

[paulsherwin]

This sounds a bit like thermal runaway, when the output transistors draw more current as they get warmer, which means they draw more current, which means they get warmer... Feel the output transistors as the radio fades away (they will be a pair of metal clad transistors, probably on small metal heatsinks). Do they feel hot? If so, that confirms thermal runaway - ask for more help if that's the case.

Worth checking the health of the PP9 battery though, as tivo suggests.

Paul

[Pip]

Here's the radio, it's model No. C22.

The battery is brand new.

http://img412.imageshack.us/img412/4...ure5973yp2.jpg

[paulsherwin]

Originally Posted by Pip

The battery is brand new.

Ah, you can't assume anything from that, since PP9s tend to sit around in shops for months on end and some of them aren't manufactured well.

If you have a test meter, measure the battery voltage both off load and as the radio fades. You don't need anything fancy to do this - my local open market sells suitable meters for £2.50.

If you don't have any sort of meter you can eliminate the PP9 by using another battery. Any 9V battery will do, even the PP3 in your smoke alarm. Just make up a couple of bits of wire to connect the PP9 connectors to the battery connectors. Be very careful to connect the battery the right way round (positive to positive, negative to negative). If your symptoms are the same with a different battery then your PP9 is probably OK.

Nice old radio by the way.

Paul

[Pip]

Thanks for that, I have a pp7, that I know is working well, I'll give it a go in the morning, fingers crossed, Pip.

[tivo]

The Alba C22 was first released in March 1960.

(Although this model was designed to run from 2 4.5v batteries rather than a 9V item)

Any chance of a photo of the circuit board with the back off?

[paulsherwin]

Originally Posted by tivo

(Although this model was designed to run from 2 4.5v batteries rather than a 9V item)

Aargh! It's possible to run this sort of design from a single 9V battery, but you need to make some circuit modifications (basically, the speaker connection would have gone to the halfway point between the two batteries, so you need to add a big electrolytic capacitor in the speaker circuit and connect it to +ve).

If the radio has been converted in a less than expert manner this may explain the problems.

As Tivo says, a good quality pic of the circuit board of the set would be very helpful.

Do the PP9 connectors look original? Do they look nearly 50 years old?

Paul

[Paul_RK]

Originally Posted by tivo

(Although this model was designed to run from 2 4.5v batteries rather than a 9V item)...

The chassis and later variants were used by several manufacturers (Defiant being another common one in this form), and these early sets always used a PP11 battery with its two 4.5 volt sections. The "centre tap" of the battery went via the loudspeaker to the common connection of the output transistor pair. Some modification must have been performed for 9 volt working or one end of the loudspeaker would be left floating, so the question is whether the modification is to blame for the likely thermal runaway or whether a fault has developed since...

Paul

[JoshWard]

I have a C22 in the same colour and the chassis was used by several manufacturers. I have a Defiant model and it suffered from the same problem. Are the output transistors getting warm? They are located at the top left hand of the chassis. If they are I would change the electrolytics which should get it going again. Josh.

Looks like Paul Sherwin suggested the tes with the o/p transistors first!

[Pip]

Hi again,
I wish I'd had the time to read your replies earlier, as I've just taken the whole radio apart! I think I can put it back together though. Tried a different battery this afternoon but had the same problem. The tip about feeling for heat as the sound faded worked and I found out where the problem was, I think they are called transistors, silver coloured metal things, marked OC 81, with three wires , red, black and yellow attached to the circuit board, there are two of them next to each other on the PCB one marked x5 and one x6, it's the x6 that's getting hot. I think Tivo was right about the battery, I thought there was something not right about the connectors. I took a photo of the inside before I took it apart, you can see where someone has joined the wiring together.

http://img359.imageshack.us/img359/4...ure5974dd6.jpg

[Paul_RK]

Looking at that "conversion", it rather appears that 9 volts is being applied between red and green wires (where there should be 4.5 volts), while the black wire (which should be 4.5 volts below the green) is left floating. Time to congratulate X6 for surviving, torture it no more, and procure a high value electrolytic as Paul Sherwin says: unless you'd rather make up a 2x4.5 volt supply, that is...

Paul

[Pip]

For a total novice like myself, would it be easier to change the capacitor or rewire to take two 4.5v batteries? I have only a vague idea of what a electolytic capacitor does and wouldn't know where to get one or fit it, or maybe I'd be better off binning the whole thing and taking up an easier hobby??

[Paul_RK]

It's a nice radio, launched within three years of the very first UK transistor sets, and looks in unusually good condition - much too good to bin! An electrolytic to do the job would cost very little, and it's a simple enough component: there are a few in the set already, the three tubular items with one red end, one black end and yellow tape round the middle, and the larger orange coloured one with translucent sheath.

Electrically, though, if I'm right in my estimate of what's been done to the set so far, reverting to 2x4.5 volt working would be the easier option. The only problem then is finding a good way to fit the batteries: six 'C' cells, or even six alkaline 'AA' cells would give a pretty good working life. Maplins or any such supplier could provide a holder for them - connect the set's red wire to positive, the black to negative and the green to the middle of the chain of six cells, and all should be well.

Paul

[Pip]

Now that sounds like something even I could manage! I didn't really want to bin it but I'm more confident about restoring the external bits than meddling about with the workings. This is the third radio I've bought recently, the others were a KB rhapsody and a lovely little hacker, but not one of them worked when I brought them home! I'll try the battery thing though, and thanks for the help, Pip

[Paul_RK]

You're very welcome. I hope I'm right in what I take to have been done to the set - the apparent state of the battery wiring and the overheating X6 certainly point in one and the same direction. Don't give up on the Hacker or the Rhapsody either, always assuming you haven't binned them

Paul

[Brian R Pateman]

Originally Posted by Pip

Now that sounds like something even I could manage! I didn't really want to bin it but I'm more confident about restoring the external bits than meddling about with the workings. This is the third radio I've bought recently, the others were a KB rhapsody and a lovely little hacker, but not one of them worked when I brought them home! I'll try the battery thing though, and thanks for the help, Pip

The Rhapsody (in one of its many versions) uses exactly this supply arrangement.

These are capable of ecxellent results and well worth the effort. Keep up the good work.

Regards,