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Old 9th July 2007, 06:09 AM   #1
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Default Optimus MPA 125 Blowing Fuses

Hi There,

I purchased from a friend (yes I know where you're going to go with this) a used Optimus MPA 125 that supposedly (operative word) worked fine. So, notwithstanding all of the "don't buy from friends,etc" lessons, here I stand with an inoperative amp.

Ok, so here is my thought/s and please tell me if I'm way out of line or out of my mind (which could be quite possibly true!) Unlike Jeff Spicoli of Fast Times fame, my father is not a TV repairman. But, I have however repaired quite a few electronic devices in my day (not to the level of all of you guys though!). I would like to fix this amp, and I think with some help from a few (or more) of you electrical gurus, I think I can (or so the little engine said).

As far as skills/equipment go: soldering/soldering equipment - check, voltmeter - check, mechanical abilities - check, oscilloscope - no, diagnostic abilities on SS boards - none really.

Ok, so anybody want to lead a blind man through a mine field over a telephone? (So I'm not blind, there is no mine field, and most of us aren't utilizing a telephone, but the analogy does kind of fit huh?!)

What should we do first?

Cheers,
Brian
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Old 9th July 2007, 12:36 PM   #2
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Check for shorted power rectifiers and shorted output transistors. You may be able to disconnect different parts of the circuit to narrow it down a little before you start pulling parts off of the board. The problem will almost certainly be in the power supply or the output stage.
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Old 9th July 2007, 01:31 PM   #3
AndrewT is offline AndrewT  Scotland
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Hi 60,
do you have a spare mains light bulb and multimeter with a fair set of AC & DC ranges?

Build up a plug top feeding a light bulb socket then feeding a socket outlet.
The light bulb socket must be in series with the live feed only.
The neutral and the earth go from plug top to socket outlet.

Attach it all to a box so that you can't come to any harm from loose or exposed wires. An insulating box may be safer than an earthed metal box.

Now plug in the light bulb and then plug this light bulb tester into the mains and switch on. The bulb should remain Unlit and should not flash nor glow dim.

Now plug a working unit into the tester socket outlet. When you switch on the bulb should flash briefly and either go out or glow dimmly. If it glows bright try a 60W bulb instead of a 40W bulb.

Unplug and plug in your faulty unit.
The bulb should again flash but this time may glow bright or dim.
The fuse should not have blown.

You now have some voltage in your faulty unit to allow you to start taking some test readings.

Get this far and come back.
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Old 9th July 2007, 03:32 PM   #4
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Tubewade - thanks for the reply....

Hi Andrew,

As far as decent multimeter - yes I have a couple. Re: spare mains light bulb - I don't have one (nor do I know how to make one - but if you can give me a quick tutorial - i did a search and can't find any info - I'll make one straight away). I'm sorry if my skillset is too basic at this point - but I do learn really fast

I'm a little confused as to what to do next (I may need to walk a little slower right now)

Thanks again for all of your help,
Brian
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Old 9th July 2007, 03:40 PM   #5
anatech is offline anatech  Canada
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Hi Brian,
Let's start fresh. What exactly is wrong with it?

If it doesn't power up, look at the fuses and let us know. Do not replace a fuse if there is a lot of black or silver on the glass.

Also, look for a schematic. There may be one in the rear of the owner's manual. Sounds like a Radio Shack brand.

There is nothing wrong with buying from friends. It's when they start fixing things for you when they are not properly trained where the troubles creep in.

-Chris
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Old 9th July 2007, 04:08 PM   #6
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Hi Chris,

Thanks for the reply. Ok, so here is the situation: I first attempted to power on the unit - nothing happened. So, I checked the fuse, and yes it was blown. I went down the the local store and purchased some fuses, put one in and as soon as I turned the power on the fuse blew. I put another fuse in (just checking if it was a bad one) and it blew as well. I pulled the cover off to see if there was anything "obvious" like burned wires etc. but nothing visual...

I did just check all of the blown fuses - and yes they are definitely black etc.

I looked online (yes apparently this was a radio shack brand) and found an operating manual (but no schematic)...

Back at 'ya
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Old 9th July 2007, 04:22 PM   #7
anatech is offline anatech  Canada
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Hi Brian,
Well, understand that you may have extended the damage, but let's sally forth on it.

Fuses blowing straight away is normally a shorted rectifier or output(s).

With your meter, check the rectifier diodes and output transistors for shorts. Also, not common but can happen, Shorted filter capacitors or even the ceramic ones across the AC line or primary. Same for secondary. Keep an open mind.

Also, make sure there are mica insulators under the power transistors. I've even seen some people use cardboard under transistors. Yes, they blew. A missing bushing is harder to spot, and I've seen that as well.

-Chris
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Old 9th July 2007, 04:34 PM   #8
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by AndrewT
Hi 60,
do you have a spare mains light bulb and multimeter with a fair set of AC & DC ranges?

Build up a plug top feeding a light bulb socket then feeding a socket outlet.
The light bulb socket must be in series with the live feed only.
The neutral and the earth go from plug top to socket outlet.

Attach it all to a box so that you can't come to any harm from loose or exposed wires. An insulating box may be safer than an earthed metal box.

Now plug in the light bulb and then plug this light bulb tester into the mains and switch on. The bulb should remain Unlit and should not flash nor glow dim.

Now plug a working unit into the tester socket outlet. When you switch on the bulb should flash briefly and either go out or glow dimmly. If it glows bright try a 60W bulb instead of a 40W bulb.

Unplug and plug in your faulty unit.
The bulb should again flash but this time may glow bright or dim.
The fuse should not have blown.

You now have some voltage in your faulty unit to allow you to start taking some test readings.

Get this far and come back.
Andrew T is giving great advice, and the same I have repeated countless times in the past.

This is just a conventional 120V 40W to 100W (or more) light bulb you can get at a hardware store or home center along with a socket, box, wire and other items. I have been a strong advocate of this approach for years as it prevents serious damage to the device being repaired by limiting the maximum current it can draw to that of the light bulb used. I usually refer to this as a ballast lamp.

Mains are just another name for the juice that comes out of your wall socket.

DVM's are great, but interestingly depending on the available voltage and current on resistance and diode ranges can give almost useless results when measuring large power rectifiers with forward drops of more than 800mV. This may be further confused by large electrolytic caps which can look like shorts to some cheaper meters, (and some expensive ones too.) due to charging current flowing when you are trying to make the measurement. (It can take many seconds to get a valid reading.)
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Old 9th July 2007, 04:50 PM   #9
anatech is offline anatech  Canada
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Hi Kevin,
We already know it's blowing fuses. Therefore the light bulb will glow at close to full brilliance.

Let's assume for a second that Brian knows how to use his meter. He will come back with useful information.

Let's assume now that Brian does not know how to use his meter. He will come back and ask about his results.

Either way, we already know that a meter must be used. Not unless you'd advocate that he connect it and light the bulb, then let's start cutting things until the light bulb goes out. Then possibly something else may smoke.

So, what are you saying?

-Chris
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Old 9th July 2007, 07:07 PM   #10
kevinkr is offline kevinkr  United States
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Quote:
Originally posted by anatech
Hi Kevin,
We already know it's blowing fuses. Therefore the light bulb will glow at close to full brilliance.

Let's assume for a second that Brian knows how to use his meter. He will come back with useful information.

Let's assume now that Brian does not know how to use his meter. He will come back and ask about his results.

Either way, we already know that a meter must be used. Not unless you'd advocate that he connect it and light the bulb, then let's start cutting things until the light bulb goes out. Then possibly something else may smoke.

So, what are you saying?

-Chris
I'm not advocating the cut things out approach at all, nor was the previous poster that I quoted, however the ballast lamp is a very useful troubleshooting tool in conjunction with a meter, voltage measurements and enough knowledge to interpret the results. In addition it greatly reduces the risk of damage once you reach the point of needing to power the unit to determine whether or not you have found all of the problems.

Fuses blow for a variety of reasons including partial shorts, component failures, and many circuit issues that may be trouble shot in some cases more easily with some (nicely current limited) voltage present in the circuit.

The one thing I can say is it has saved me lots of money and aggravation when I missed one bad part in a power amplifier during troubleshooting and repair.

Meter readings can and do tell you a lot, resistance measurements in direct coupled solid state amplifiers can also be very ambiguous and hard to interpret. Depending on the meter measurement current the results can be downright misleading.

Some combination of both approaches have worked well for me for countless ages.
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