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MG MGA - Frying electronics

I have blown three power transformers in my old Blaupunkt radio, which are not cheap components. As a test I intalled a new (modern/cheap) car radio and it blew in a couple of weeks (it still lights up but audio output transformer is fried). All the rest of the car's electrical system seems fine -- no blown fuses, good lights, signals working properly etc. Ammeter indicates normal charge/discharge status. I don't have a voltmeter, that is my next test. What would cause the radio(s) to fry without blowing the (5A inline) fuse first?
Jim Paul

Jim,
Does the impedance of your speakers match what the radio's requirements are?
Henri

Connect a voltmeter across the battery and run the engine at around 3,000 rpm. If voltage exceeds 14.6 volts your regulator is overcharging the system. Check the connections from the radio to the speaker, make sure that neither wire is connected to the body or frame.
John H

Chech generator output at 3500 rpm with all components off,over voltage can damage components. fuse only protects against amps. 10 amps @ 12 Volts is less energy than 10 amps at 16 volts. Voltage regulator is probably at fault. do headlights get brighter when you rev engine?
Randy
R J Brown

Hi Jim. Is your old Blaupunkt radio 6 volt or 12 volt? Some radios could use either oif these voltages and the dual voltages were selected by a switch. If you have a 6 volt radio, and are running it on 12 volts, that certainly could be the reason that it is failing! If you are running the radio on 12 volts, and are running speakers of correct impedence, then your radio might have some type of internal flaw. Also as a thought, is your radio positive or negative ground? Originally, MGA's were all wired positive ground, although many have been converted to negative ground. I suspect that if your radio was wired with the wrong grounding for the radio, that it would not work, and might do damage to your radio's circuitry, especially if the radio was a transistorized model. Just some ideas. Glenn
Glenn

Jim,
If I could add what you already have heard, it's your voltage regulator. Also insure that your earth connection is good.
mike parker

Thanks for all responses. Yes it is a 12v negative ground Blaupunkt Frankfurt. Properly grounded with matching 4 ohm vintage speaker. My headlights do get a little brighter when revving the engine but not abnormally (in comparison to my other cars). I suspected voltage regulator initially but had no other problems with electrical system so disregarded it. I will measure the voltage at 3500 rpm.
Jim Paul

Jim,
You have negative ground radio? I know you have the radio properly grounded, what have you done to the antenna? I take this from the following website:
http://www.oconnorclassics.com/techtalk_neg.php

"Isolate the radio from the rest of the car.
This requires careful mounting of the radio and antenna so that they do not touch any metal part of the car. The radio is then wired so it's metal case is "hot." Any contact between the case and the car's body, dashboard, etc., will produce a blown fuse, burned wires or worse. Anyone working on the car who doesn't know that the radio is wired "backward" may trigger an expensive mistake."
Hope it helps.
mike parker

If a negative ground radio is being used in the car, the car's polarity should be changed to negative ground. To try isolating the radio from ground is an inventation for a disaster. One slip of the screwdriver can wind up frying a harness in an instant. Additionally, one side of the antenna installation goes to the car's ground, which will put the isolated radio chassis to ground through the antenna cable. If the antenna is also isolated from ground, don't count on very good reception and possibly a lot of engine and generator noise in the radio. Without a ground, the antenna is nothing more than a piece of wire dangling outside the car, it might work, but nor very well. The speakers, if mounted on any metal need to be checked to insure that one side of them is not grounded to the speaker frame, and thus to the car's ground. Good luck - Dave
David DuBois

These are the closest gauge I've found.
I have one installed in my MGA

http://cgi.ebay.co.uk/Smiths-2-inch-battery-condition-gauge_W0QQitemZ7994907425QQcategoryZ72215QQssPageNameZWDVWQQrdZ1QQcmdZViewItem

I don't think there are any Jaeger equivalent one's.

Oh and I also have a battery starter pack that has an inbuilt volts gauge, which I used before I got the above gauge to sort out my electrical system.

Hope this helps.

Cheers <MARK>
Mark Hester

Modern car audio equipemnt uses 4 ohm speakers, but some older cars used speakers with impedence up to 32 ohms...especially true with vacuum tube units. Standardiziation was not an issue, as there wasn't a lot of aftermarket stuff. Never assume the speaker is correct for the radio just because it's 4 ohms. Too low of an impedance acts like a short circuit and can overload the circuitry.
R. L Carleen

Thanks for all the ongoing responses. I was a bit of a maniac right after I bought my MGA last year. It was almost entirely original and when I saw so many vintage parts for sale on eBay, I avoided Moss and other contemporary vendors and got the spare parts online. Plus I attended a few car shows and saw there was a noticeable difference between repro parts and vintage parts. I HATE seeing a modern radio in an old dashboard, it just "looks wrong" however practical it might be. Plus we all spend so much $$$ restoring our cars, why cheap out with an $80 radio from Best Buy? Anyway I documented my experience on my website. The car was converted to negative ground (thanks Barney Gaylord) and all components are correct (speakers, etc.)

http://www.jimpaul.tv/mga/mods.htm

Seems the one mistake I made was not installing a voltmeter sooner...at least temporarily.
Jim Paul

Jim - Ok, now we have that all sorted out, you have a negative ground radio in a negative ground vehicle and the speakers are not ground, disregard all after "dot net" in my previous post. Cheers - Dave
David DuBois

This thread was discussed between 24/08/2005 and 26/08/2005

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