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Power problem on radio circuit

Simon Davis


Power problem on radio circuit | 13 September, 2002

Hi Folks,

This is a totally speculative, if not cheeky post.

I know very little about electronics, but I think you guys do. I have a problem with my car radio, that I'd really like to solve, as I cannot buy a new one where I live (Brazil), and I think it could be a simple problem. I have tried taking it to a car radio shop, but they are not able to help. I'd appreciate some expert advice if you have the time.

Brief description of problem and what I've found so far.

The radio is a Sony, it has radio and MD in the actual unit (hence difficult to replace). It has a multi CD changer in the trunk.

The problem started in the car, but I've set it up in my house using a 12v power supply.

Symtoms - when the power is connected, the unit 'wakes up' as normal - the time etc is displayed. When I change the source to radio or CD, it works OK as well. When I put in an MD, the unit dies.

I've put a circuit tester on it, and found the following;

When the power is disconnected, the voltage from the power source reads 12.02v. When I connect to the radio, it drops slightly to about 11.9v.

When I switch on the radio or cd, the voltage drops slightly, but stabilises at around 11.5v.

When I put in an MD, the voltage drops to about 7v, and the unit dies. The voltage then changes constantly, from between about 4v up to 11v, but the unit is dead.

I have to disconnect the power plug from the radio for about 10 minutes, and keep reconnecting. After a while the unit starts working again.

Other observation - while the CD works ok, the display dims a little, and dims even more as the volume is increased. The display dims a lot just as the unit dies.

So, knowing nothing about it, I have guessed the following:

There is a component that is not working properly. When I put in the MD, this component is overloaded? Perhaps it gets hot? When it has cooled sufficiently it starts working again.

Any ideas what it might be, or if I'm talking rubbish altogether?

Sorry if this is inapropriate for this forum, but I have no other ideas, and would really like to get this solved.

Thanks for any ideas you can give me.


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Power problem on radio circuit | 13 September, 2002

What is happening is your bench supply is unable to supply enough current so the voltage drops. When you turn up the volume knob your asking for more current and the voltage drops more (the led's in the faceplate dim). It also sounds like when you plug in this MD device (what is an MD anyway?) the voltage drops, the radio goes off, voltage comes back up, radio goes back on and as the MD powers up the voltage drops again, etc...

Is your bench supply able to pump out alot of current or is it only rated at 1/2 amp (is the radio really drawing more current then it should)?

When it was in your car and this happended was it blowing a fuse? Were you able to verify this voltage drop while it was in your car?

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Power problem on radio circuit | 13 September, 2002

Thanks a lot for the reply.

The power supply I'm using (12v) is from an HP scanner, so I guess it's always likely that it's not man enough for the job.

There were no fuse problems in the car, it just dies.

As it is the same problem whether in the car or not, I assumed it was the radio itself that was the problem.

MD - minidisk, It's Something Sony invented a few years ago, which has never really taken off. It's a great format though. Roughly equivalent to a cassette player, but digital.

The mystery for me is why when it dies, the voltage goes crazy for a while. The reading just jumps around at random. I'm just sticking the probes from my meter into the + and - cables from the power supply, by the way.

I didn't verify the voltage drop in the car, but it behaves in exactly the same way - dies, then after 10 minutes connect the power and it wakes up. What I haven't done is leave it with the power connected - I can't say if it will spontaneoulsy recover, I've never seen it do it unless I remove the power from it and reconnect.

Having looked through your forum, I realise I'm dropping a really wierd question in here. Hope It's not bothering you guys!

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Power problem on radio circuit | 13 September, 2002

If it's not blowing the fuse in the car then I guess it wasn't drawing a huge amount of current. Check the power wires going into the radio and look for something loose. My first guess is your loosing voltage across a bad connection somewhere and as the demand for more current increases, this voltage potential across this bad connection increases thus depleating what is actually supplied to the radio.

I haven't looked in a radio lately but my next guess is that there is some type of regulation device in it to protect it from spikes and the fact that the voltage source does drift. This may have started to go bad and can't handle the increase in current. Look in the radio and make sure all the divices screwed down to the pc board or the chassis are tight and not coroded.

Good luck

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Power problem on radio circuit | 13 September, 2002

You right Simon, this forum is not for that type of question, but since you have been polite and desperate, i'll shoot some help. Your right, sounds like it is the unit, there is probably a component failed that is shutting down the unit when it gets warm (drawing lots of current). I am suprised that it has not blown fuses, unless you have stuck pennies in them. A car battery can supply hundreds to thousands of amps if required. Unless you have some electronics background, I would take the unit to a sony authorized dealer and have them repair it. If it is worth the couple hundred bucks and yeah md players are rare now. Good luck...

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Power problem on radio circuit | 13 September, 2002

I'm not clear, but sounds as if the problem symptoms are the same in the car as on the bench. => If that's the case, it's most likely the radio is bad.

Again, I'm not clear, but it sounds like using the MD is the trigger to shut the player down: * Stop using the MD. * Fix the MD or some of the associated power supply regulation circuitry.

For this unit to suck the current down like it is doing, it has to be generating a boatload of heat. Touching components may be a useful troubleshooting technique.

In real life, as someone else stated, you're not goung to get too far without replacement parts and a circuit diagram. Maybe just go over to Best Buys, if you can't live without the MD.

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Power problem on radio circuit | 13 September, 2002

Thanks to all of you. This is very kind.

I'll have a good poke around the circuit board for loose / hot components - good tips.

I think it might be a throw away job, but I thought I'd give it a last try. You guys have actually given me much more advice than any of the car radio specialists I've spoken to. Take a bow!

Hope your SMT's all work well (whatever they are).



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Power problem on radio circuit | 16 September, 2002

simon- davef is on the right track. 1st year electronics courses teaches us that you should always check the power supply first. In this case, the power supply in question is not your power source but the regulated power within the unit. I suspect one of the voltage (power) regulators is compromising the remainder function of the unit. It is probably shorting out once it warms up. The internal metal of the regulator expands once power is applied and heat is generated. since your audio unit is a multi-purpose device, there will be several voltage regulators to individually power each of the units functions. The small voltage drops and "lights dimming" that you have observed are probably not a regular occurence, with respect to the radio function. Where things are getting bad is when you activate the MD. The regulator for the MD is either bad or originally marginal to spec. Voltage regulators are specialty transistors. Typically, voltage regulators have 3 leads. Input voltage, output voltage and ground. You can verify these leads with your volt meter. You will measure 12 volts coming in and most likely 5 volts coming out. The voltage regulators are identified as having a large heatsink (metal plate) underneath them and are usually held down with a machine screw. One of the best meters we possess are the finger tips. Yes, be careful, when something is HOT, you can blister the end of your finger quickly. Try hovering rather than direct touch, it also helps to wet the end of your finger before touching/hovering. Voltage regulators will have some latent heat to them, usually a lot of current runs through them. Just look for the one that is much hotter than the others. Isolate that regulator (disconnect from the circuit). Verify it is the one for the MD unit by seeing that the MD no longer operates. If successfully isollating the MD, verify the characteristics previous described for the radio. do you still get "dimming", does the volume control work without " dimming", etc.

Replace the suspect regulator with a new one, one that matches the electrical specs of the original regulator. See if the MD now operates as intended.

Oh, by the way, just thought of something. It could be that a 2nd or 3rd operations tech misassembled your unit at the factory. You could actually have a screw or bare metal contact to the power or ground circuit(s) on the printed circuit board, especially the power circuit for the MD device. Carefully dissassemble the unit, looking for that possibility. Look for uncoated copper traces. A scratched surface that is in close proximity to a voltage or ground potential. Do you see evidence of burnt spots on the printed circuit board?? That can indicate a failed component as well. good luck.

By the way. SMT or SMD is the intials for Surface Mount Technology or Surface Mount Device. These are the materials we in this industry are using to create, assemble and frustrate our selves with in our daily worklife pursuits.

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