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
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
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.
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.
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
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
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
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
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
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
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
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
AndrewT said:
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.)
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
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
anatech said:
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.
Hi Kevin,
I use variacs and DC power supplies. You can then do some troubleshooting in all cases. The lamp method doesn't lend itself to much flexibility. The variac has saved me in the same way your lamp has saved you, however, a variac is much more useful in troubleshooting. A light bulb should only be used when there is no access to the proper tools.
So I have to ask you what you really mean. It does appear that you had a problem with my approach to troubleshooting.
Anyway, let's help Brain fix his amp without confusing him too much with this sideshow.
-Chris
I use variacs and DC power supplies. You can then do some troubleshooting in all cases. The lamp method doesn't lend itself to much flexibility. The variac has saved me in the same way your lamp has saved you, however, a variac is much more useful in troubleshooting. A light bulb should only be used when there is no access to the proper tools.
I honestly do not know what you are saying here, but it looks like you do not recommend testing. Either that or you don't give anyone credit for knowing how their meter reacts. I am therefore left with the present scenario since you made no further comments in this direction except to say the readings can be confusing. This furthers that thought.
So I have to ask you what you really mean. It does appear that you had a problem with my approach to troubleshooting.Anyway, let's help Brain fix his amp without confusing him too much with this sideshow.
-Chris
Slightly OT:
Hi Chris,
Actually I don't have any problem with your troubleshooting technique, it is likely not all that different than my own. I do a lot of testing and measurement in both design and debug, not to mention the occasional repair. I first encountered the light bulb as current limiting ballast lamp when I first went to work for Bose, and it proved indispensable in power amplifier design.. The light bulb does have one virtue neither fuses nor variacs have, i.e. current limiting, and for example in the case where you have a partial short on the secondary (load) side of the transformer, neither the variac nor fuse may limit current to a safe value for the power transformer or other components and the bulb can be sized so that it definitely will. I incidentally have 3 different sized variacs, and five dc supplies as well as a very (I hope ) professionally packaged ballast lamp. I have encountered them in quite a few other engineering labs over ensuing years as well.
My remarks in regards to meters relates to bridge rectifiers for example where the VF at the meter's test current is higher than the available drive voltage, making you believe that the diode is bad when it in fact isn't. (This happens with some 1KV bridge I have tested this way.) My Fluke bench meter has just this problem, and my Keithley 2002 (a very expensive bench meter) cannot test diodes at all. (Although with its kelvin connection it can test very tiny resistances accurately.) Electrolytic capacitors take a long time to charge to the point where you know they are not bad, and further the voltages are usually too low to tell you this except in the case of small low voltage types.
Many parts do fail as dead shorts, and these you can always find with the meter, those that don't sometimes are a bit harder to find.
I do a lot of trouble shooting with relatively low voltages applied, and in a particular case with just a variac, due to my careless I lost the (irreplaceable) high voltage transformer in a prized lambda high voltage supply. The fault current was not nearly enough to blow the fuse, but even at the reduced voltage from the variac, the fault current was enough in very short order to fry the high resistance secondary. The culprit was a shorted oil filled 2KV cap operated quite conservatively - it had developed a partial short which did not register at all with the meter I used to check it, but with the 1KV operating voltage applied it broke down and drew maybe 25mA or so, well beyond the secondary rating of that transformer, but nowhere near enough to blow the fuse unfortunately. (Bad design probably, but I am sure they never expected that cap to fail.)
I often use a bulb on the output side of the variac when trouble shooting, and obviously I regard it as an important part of my lab equipment..
I never made any recommendation against testing, what I thought I suggested was the use of a ballast lamp to prevent loss of components during the troubleshooting and repair process.
Fuses aren't really intended to protect equipment, they are intended to protect the venue and user of the equipment in the event of a gross failure representing an imminent safety hazard. My experience with UL/CSA product safety testing really hammered that point home to my consternation. (There's nothing like parts exploding in your face.)
I guess the only thing in my response that might be relevant to Brian is that I made the recommendation in the belief that he had neither a variac nor a regulated, current limited supply. In such a case some current limiting seemed useful.
anatech said:Hi Kevin,
I use variacs and DC power supplies. You can then do some troubleshooting in all cases. The lamp method doesn't lend itself to much flexibility. The variac has saved me in the same way your lamp has saved you, however, a variac is much more useful in troubleshooting. A light bulb should only be used when there is no access to the proper tools.
I honestly do not know what you are saying here, but it looks like you do not recommend testing. Either that or you don't give anyone credit for knowing how their meter reacts. I am therefore left with the present scenario since you made no further comments in this direction except to say the readings can be confusing. This furthers that thought.
Anyway, let's help Brain fix his amp without confusing him too much with this sideshow.
-Chris
Hi Chris,
Actually I don't have any problem with your troubleshooting technique, it is likely not all that different than my own. I do a lot of testing and measurement in both design and debug, not to mention the occasional repair. I first encountered the light bulb as current limiting ballast lamp when I first went to work for Bose, and it proved indispensable in power amplifier design.. The light bulb does have one virtue neither fuses nor variacs have, i.e. current limiting, and for example in the case where you have a partial short on the secondary (load) side of the transformer, neither the variac nor fuse may limit current to a safe value for the power transformer or other components and the bulb can be sized so that it definitely will. I incidentally have 3 different sized variacs, and five dc supplies as well as a very (I hope
) professionally packaged ballast lamp. I have encountered them in quite a few other engineering labs over ensuing years as well.My remarks in regards to meters relates to bridge rectifiers for example where the VF at the meter's test current is higher than the available drive voltage, making you believe that the diode is bad when it in fact isn't. (This happens with some 1KV bridge I have tested this way.) My Fluke bench meter has just this problem, and my Keithley 2002 (a very expensive bench meter) cannot test diodes at all. (Although with its kelvin connection it can test very tiny resistances accurately.) Electrolytic capacitors take a long time to charge to the point where you know they are not bad, and further the voltages are usually too low to tell you this except in the case of small low voltage types.
Many parts do fail as dead shorts, and these you can always find with the meter, those that don't sometimes are a bit harder to find.
I do a lot of trouble shooting with relatively low voltages applied, and in a particular case with just a variac, due to my careless I lost the (irreplaceable) high voltage transformer in a prized lambda high voltage supply. The fault current was not nearly enough to blow the fuse, but even at the reduced voltage from the variac, the fault current was enough in very short order to fry the high resistance secondary. The culprit was a shorted oil filled 2KV cap operated quite conservatively - it had developed a partial short which did not register at all with the meter I used to check it, but with the 1KV operating voltage applied it broke down and drew maybe 25mA or so, well beyond the secondary rating of that transformer, but nowhere near enough to blow the fuse unfortunately. (Bad design probably, but I am sure they never expected that cap to fail.)
I often use a bulb on the output side of the variac when trouble shooting, and obviously I regard it as an important part of my lab equipment..
I never made any recommendation against testing, what I thought I suggested was the use of a ballast lamp to prevent loss of components during the troubleshooting and repair process.
Fuses aren't really intended to protect equipment, they are intended to protect the venue and user of the equipment in the event of a gross failure representing an imminent safety hazard. My experience with UL/CSA product safety testing really hammered that point home to my consternation. (There's nothing like parts exploding in your face.)
I guess the only thing in my response that might be relevant to Brian is that I made the recommendation in the belief that he had neither a variac nor a regulated, current limited supply. In such a case some current limiting seemed useful.
Hi Kevin,
I have a few variacs as well. I make sure the voltage and current are always displayed with analog meters. This may have saved your transformer.
I am sorry for your loss, no kidding! I have seen a number of high voltage caps fail leaky, rather than short.
One of my favorite tools I use are current sources with 25 ~ 35 VDC compliance. This is valuable for those high drop junctions. Normally I use an HP 34401A. It's diode check will give you a reading of two normal diode drops and slightly higher. What's nice is the 1 mA test current (spec'd).
-Chris
I have a few variacs as well. I make sure the voltage and current are always displayed with analog meters. This may have saved your transformer.
I am sorry for your loss, no kidding! I have seen a number of high voltage caps fail leaky, rather than short.
One of my favorite tools I use are current sources with 25 ~ 35 VDC compliance. This is valuable for those high drop junctions. Normally I use an HP 34401A. It's diode check will give you a reading of two normal diode drops and slightly higher. What's nice is the 1 mA test current (spec'd).
100% agreed. Most people confuse the duty of the AC fuse with protection of the unit. Nope.
That's what it looked like, and why I asked to to clarify what you meant. I was sure it was a misunderstanding and unfortunate wording. The timing of your post with the wording that was used looked far different from what you intended I'm sure. I was thinking that there was no way you meant to say how I interpreted the post.
That's why I tried to point out that we were past the AC power stage so far. Three blown fuses actually!
-Chris
Andrew - yes you are correct on both accounts - so I guess your crystal ball is functioning well today and maybe would be put to good use on sportsbet.com (with all sorts of potential income possibilities....)
Chris, I really appreciate the time you have spent so far.
My ignorance re: this subject was plainly communicated by me in my initial post. My true hope was that someone would be willing to share some of their knowledge and time, and help me learn (with the added bonus of fixing my amplifier). I have been in the "mentor" role for most of my adult life (giving freely of both time and knowledge) and I thought this might be the time when I could delve into a subject that has always interested me with some help. It doesn't feel like thats the direction we're going here. SO, can anyone recommend a forum/site that would be better suited to my skillset/situation?
- Brian
Chris, I really appreciate the time you have spent so far.
My ignorance re: this subject was plainly communicated by me in my initial post. My true hope was that someone would be willing to share some of their knowledge and time, and help me learn (with the added bonus of fixing my amplifier). I have been in the "mentor" role for most of my adult life (giving freely of both time and knowledge) and I thought this might be the time when I could delve into a subject that has always interested me with some help. It doesn't feel like thats the direction we're going here. SO, can anyone recommend a forum/site that would be better suited to my skillset/situation?
- Brian
Hi 60fairlane,
We're all delighted to have you here, sorry if our prior conversation misled you, as Andrew T indicated it would be a good idea to get your hands on the so called ballast lamp.
Chris has many good points on technique, and lots of experience as do I and others.
Do you have a schematic, and as important can you read it?
It would help greatly if you can scan and post it here or perhaps you'll be really lucky and find a copy on line.
One really important thing is to verify that your version of the schematic which should have a range of valid serial numbers matches with your unit's serial number.
Pictures of the unit are always helpful as we can identify parts you may be unfamiliar with. In this case even a cell phone camera can be welcome.
More thoughts later, you have definitely come to the right place.
We're all delighted to have you here, sorry if our prior conversation misled you, as Andrew T indicated it would be a good idea to get your hands on the so called ballast lamp.
Chris has many good points on technique, and lots of experience as do I and others.
Do you have a schematic, and as important can you read it?
It would help greatly if you can scan and post it here or perhaps you'll be really lucky and find a copy on line.
One really important thing is to verify that your version of the schematic which should have a range of valid serial numbers matches with your unit's serial number.
Pictures of the unit are always helpful as we can identify parts you may be unfamiliar with. In this case even a cell phone camera can be welcome.
More thoughts later, you have definitely come to the right place.
Hi Brian,
Yes, as Kevin and Andrew mentioned, you are in the right place and we are very happy to help you out. We all enjoy assisting others or we wouldn't be here.
You should build the current limiter and buy a bunch of bulbs before they are gone. Incandescent lamps may be outlawed in the near future. Green you know!
Depending on how much fun you have, you may find yourself in the mood to learn more and perhaps even build something. At that point you should consider picking up a variac as well off Ebay or a local garage sale. Those things are very handy.
Radio Shack also gave each piece a number code, please post that. I'll have a look in my manuals to see if I have that model, a very slight chance understand.
-Chris
Yes, as Kevin and Andrew mentioned, you are in the right place and we are very happy to help you out. We all enjoy assisting others or we wouldn't be here.
You should build the current limiter and buy a bunch of bulbs before they are gone. Incandescent lamps may be outlawed in the near future. Green you know!
Depending on how much fun you have, you may find yourself in the mood to learn more and perhaps even build something. At that point you should consider picking up a variac as well off Ebay or a local garage sale. Those things are very handy.
Radio Shack also gave each piece a number code, please post that. I'll have a look in my manuals to see if I have that model, a very slight chance understand.
-Chris
Hi All,
Thanks for the replies, as for the specific questions:
Kevin, Chris
The only "printed" reference material I could find:
http://support.radioshack.com/support_audio/49019.htm
They only list a parts list and an owners manual.
I think I could muster my way through a schematic if I had one.
pictures of amp:
http://farm2.static.flickr.com/1076/770328284_ba87a1d96a_b.jpg
http://farm2.static.flickr.com/1122/770328268_0a5ccd0b00_b.jpg
http://farm2.static.flickr.com/1383/770328240_a60130438f_b.jpg
http://farm2.static.flickr.com/1109/770328336_50c0a2e1d3_b.jpg
- Brian
Thanks for the replies, as for the specific questions:
Kevin, Chris
The only "printed" reference material I could find:
http://support.radioshack.com/support_audio/49019.htm
They only list a parts list and an owners manual.
I think I could muster my way through a schematic if I had one.
pictures of amp:
http://farm2.static.flickr.com/1076/770328284_ba87a1d96a_b.jpg
http://farm2.static.flickr.com/1122/770328268_0a5ccd0b00_b.jpg
http://farm2.static.flickr.com/1383/770328240_a60130438f_b.jpg
http://farm2.static.flickr.com/1109/770328336_50c0a2e1d3_b.jpg
- Brian
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