Hi all,

I found a 1980's General Electric 7-4630A clock radio, which I'm restoring after spending many years in an abandoned shed. The cockroaches have had a grand time inside it, consuming any loose labels, and the speaker cone! I'm trying to find a suitable replacement for the damaged speaker, but have a somewhat limited stock of spare speakers. The speaker used by this set is rated at 45Ω 0.8W.

The closest replacement I have is a 16Ω 1.6W speaker. Would I be able to use this speaker with a 29Ω resistor (or appropriate combination of resistors)? Does the wattage of the speaker matter?

Thanks,

Chris

45 ohms sounds very unusual, I'd be thinking it might be 4.5 ohms. Have you measured it?

Chris
if the power rating (watts) of the replacement speaker is the same or greater than the original that will be fine.

I would be tempted to try the replacement without a resistor.
Speaker impedance is more than a simple DC ohm measurement.

Using a lower impedance speaker may make it louder.

Are you sure the original isnt 4.5 ohms?
ben

I'm pretty sure that the speaker is 45Ω. There is no decimal point, and no space between the two numbers to accommodate one.

Chris

Missing some info: What is actually driving the speaker?

Valve? Transistor?

Marc

It's all solid state. This is a 1980's solid state clock radio.

Here's a Youtube video of the innards of the beast:
http://www.youtube.com/watch?v=kqiJg_w0d5A.

Chris

I'm pretty sure that the speaker is 45Ω.

Take the speaker out of circuit and measure its DC resistance with an ohmmeter. A rough guide is to divide the expected impedance by 1.2 and see if the DC ohms come close to that. So, if the speaker is indeed 45 ohms, I'd expect to see around 37 to 38 ohms DC resistance. If it's in fact 4.5 ohms, should see 3.5 to 3.75 ohms on the meter.

Obviously this will only work if the coil hasn't gone open circuit. If there's a short somewhere in the coil you'll get some reading that doesn't fit the above ranges.

Unfortunately, I no longer have the speaker. My mum decided to throw it out without my knowledge, and it's garbage day today. I'll try and find someone else who has one, and see if I can get some information.

Chris

Thats rather what I expected. The speaker is likely in series with the collector and 45 Ohm is realistic.

You may have to consider an output transformer, or a resistor and feed the speaker with a large capacity capacitor.

More Q's: What is the diameter. Someone may have?

Voltage is not the issue with transistor OP of that age, it is the wattage & polarity when in series with the transistor. Check the transistor PNP's tend to go short when they fail & that may have actually cooked the Voice coil?

Marc

My mum decided to throw it out without my knowledge

Mothers do that sort of thing. Back in my schooldays I solved the problem by getting a secondhand Brownbuilt steel cabinet with lock on it. Kept all of my tools and parts and other treasure in there. She was happy because there was nothing to "tidy up". I was happy because nothing that I wanted to keep got tossed out as rubbish.

I'm in need of some help with the mains cable for this radio. It is two-core black cable, though one wire has white writing running down it. Both leads are marked as 'PWR CORD' on the PSU, so I have no clue which is which. There are some small blue components though, which have a red dot on one end and a black on the other.

http://imageshack.us/photo/my-images/6/oys8.jpg/.
http://imageshack.us/photo/my-images/832/8sav.jpg/.

Thanks,

Chris

Not being familiar with clock radios from this era I am only guessing but the two blue components could be capacitors or inductors to act as voltage drops in lieu of a transformer.

What is it you wish to do with the power cord? If you want to replace it then this shouldn't be a problem though if the cord is single insulated 'figure 8' flex then I would recommend substituting this with double-insulated two-core flex which Jaycar should have in stock.

If you wish to check polarity this should be easy to trace with a multimeter if the plugtop is still on the flex. The active pin is on the left with the pins facing away from you.

It appears that all circuitry on that board operates at mains potential too.

Brad, sorry, I should've been more specific in my post. The plug has been cut off, and I want to replace it. I've seen those blue components in other consumer electronic devices (televisions and monitors especially). I have no idea what they do, but have observed several dot colour variations.

Chris

The blue components may be varistors for over voltage protection.

Because of the shadow, I can't quite make out their component numbers printed on the PCB. Do they say R-something or C-something? Or something else altogether?

GTC, if memory serves me right, they had '201' and '202' printed under them.

Chris