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Old 9th July 2007, 07:30 PM   #11
anatech is offline anatech  Canada
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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.

Quote:
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.
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
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Old 9th July 2007, 07:41 PM   #12
AndrewT is offline AndrewT  Scotland
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Hi,
I reckon I could make a bit of money betting 60F hasn't a variac.
And probably doesn't know what we are talking about.
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Old 9th July 2007, 07:56 PM   #13
anatech is offline anatech  Canada
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Hi Andrew,
Quote:
I reckon I could make a bit of money betting 60F hasn't a variac.
That's entirely possible, and acceptable.

Hey Brian,
How are things going?

-Chris
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Old 9th July 2007, 09:07 PM   #14
kevinkr is offline kevinkr  United States
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Slightly OT:

Quote:
Originally posted by anatech
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. 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

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.
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Old 9th July 2007, 10:01 PM   #15
anatech is offline anatech  Canada
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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).
Quote:
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.)
100% agreed. Most people confuse the duty of the AC fuse with protection of the unit. Nope.
Quote:
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.
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.
Quote:
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.
That's why I tried to point out that we were past the AC power stage so far. Three blown fuses actually!

-Chris
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Old 10th July 2007, 04:09 AM   #16
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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
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Old 10th July 2007, 07:31 AM   #17
AndrewT is offline AndrewT  Scotland
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Hi 60F,
this is the Forum for you.
Stay with us and build up that bulb tester (ballast as Kevin calls it).
You can use it for testing ALL projects in the future and you don't have to stand back as you switch on.
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Old 10th July 2007, 01:32 PM   #18
kevinkr is offline kevinkr  United States
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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.
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Old 10th July 2007, 02:13 PM   #19
anatech is offline anatech  Canada
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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
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Old 10th July 2007, 05:25 PM   #20
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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/...87a1d96a_b.jpg

http://farm2.static.flickr.com/1122/...5ccd0b00_b.jpg

http://farm2.static.flickr.com/1383/...0130438f_b.jpg

http://farm2.static.flickr.com/1109/...c0a2e1d3_b.jpg

- Brian
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