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I Really Blew It!!!

KingBubba

KingBubba

"Too Much Stuff"
I have over time worked up to recapping my SA-9500II because of a leaking filter caps. I have taken my time researching my cap choices and all that went into the project. I was successful at this project. The caps were replaced and after careful checking I got to the point of adding speakers and having a listen. I hooked up my Insignias and fired up one of my favorite testing songs. Incredible! the sound was unbelievable, crisp, clear, with more bass than I have ever heard coming out of the Insignias before. I am proud but not finished. I have to set the bias. I have the manual which I read and understand. I follow procedures carefully and get the bias set at 30mv on a cold amp. I then let the amp run for the requisite 30 min and then test and readjust the first set of TPs to 30mv I then test the second set of TPs for a value of 30mv+- 10mv and it comes up high at 45mv. I readjust carefully and got to the point of removing the test probes from the second test points to return to the first test points again. I am using the small test probes with the hook to be safe and not run the risk of touching other TPs. As I remove the test probe there was a spark and the left side fuse blows. To my knowledge I did not cross any other TPs. I have no fuses so I decide to check the right side. Following the same procedure, I test the first TPs and then the second TPs. They are within spec so I figure I am okay on the right side. Again, as I remove the probe, I see a spark, The right fuse blows and I am dead in the water. Today I get some fuses and install them and as soon as I apply the on switch both of the fuses blow immediately.

I am now out of my range of knowledge and wondered if anyone from AK could lend me some assistance in sorting this out. I honestly believe I did everything right. The only thing that I can think of is that I removed the positive side test probe first. Was that a mistake? Any assistance would be appreciated. Thanks.

Gary
 
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Gary
Hard to say what exactly caused the problem.Are the fuses protecting the +/- rail supply voltages?Off hand I would say that either drivers and/or output transistors are shorted -collector to emitter.Can you send me a scanned schematic.

Dave
 
I'm sorry to hear what happened. You sound like you were pretty thourough with what you did. It does'nt take much to short something out. Could you have pulled a jumper out by accident near where you used the probes?
I usually use aligator clips when setting bias etc, and I usually cut power before moving probes.
I've done what you have done previously.
Someone with more knowledge will chime in I'm sure.
 
Detailman, I was using test probes that have a small hook, entirely enclosed to make shorting almost a non issue, which is what my intention was in using them. I was very careful, as this amp is my baby. I am extremely distressed but determined that I can learn enough to fix this.

Avionic, the schematic can be found on the AK Pioneer Manual Database. Go here:

http://audiokarma.org/forums/showthread.php?t=91705

To access the database you must open the PDF file in the first post to get the password and the username to enter the database. This keeps this database for AK members who are signed in. It is a great site and many thanks to the folks that made it possible. There is two service manuals for the SA-9500II amplifier. One is a high res version that is very good indeed. Page 21 is the adjustment instructions, which I followed to the letter. Thanks for your your concern and any assistance you could lend.
 
Gary
With the unit disconnected from power.Measure the resistance from the output transistors -collectors to chassis ground on both the left and right channels.Should not show a short ie. zero resistance.


Dave
 
Echowars, yes, the meter was set on the lowest DC , 2000mv, and it will autorange. It is a fairly good quality DVM though I cannot offhand remember the maker. It is not a Fluke, but is of good quality. Avionic, excuse my ignorance, but I am not stupid enough not to declare my ignorance in this situation. There are four output transistors on the heatsinks and they are of the square/flat variety. They have three contacts entering the sockets. I do not know which contacts are the collectors. These contact pins are, however easily accessible without removing them from the heatsink. Are the outside contacts considered the collectors? Please excuse my lack of knowledge but I am a superior learner if given half a chance. If it would make sense to do so, I will take some pictures tomorrow night to describe my situation and display what I am working with. Thanks again to all for their concern.
 
Gary
Measure the resistance from( pins 2 to pin 17) and (pin 11 to pin 17).On both the left and right channels.

Note: pins 2,5,8 & 11 are the collectors and pin 17 is tied to pin 18 chassis ground when the speakers are hooked up.



Dave
 
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Sorry to hear of diffuculties KB :tears:

Not sure we will ever be able to actually recreate what happended but the fact that both sparks were heard/see as you were removing your test clips each time, one would have to think that as you were releasing the clips they touched something as they were extended out?

I guess a safe procedure step for you or anyone new to this kind of procedure would be to power off the unit then placing test clips on posts, then power up and measure/adjust. When done, first power off then remove test clips. This kind of thing has happened to others in the past and I'm sure will also in the future, so simple steps like powering on and off before installing and removing test clips could save lots of grief :thmbsp:


Lefty
 
Believe me when I say, the power switch will be off every time I move any contact from now on. My ass is killing me from all the kicks I have given myself.
 
Meters don't usually have a 2000 mv scale. Are you sure it wasn't 2000 ma scale? if set to this range it will short across the probes thus explaining the problems you were having.
 
Here is a photo of the meter. As I explained to avionic in an email, I bought this used and have never had a manual for it. The scaling on this is very odd, especially the ohm settings. I've never been able to figure out the math on this thing. I generally use this at work for plain voltage and continuity test so have never had to really understand it. When I first started this thread I thought I was using another autoranging meter that I have. This meter is old but works great, I just don't quite understand its method of scaling.

Meter.jpg
 
It is a little odd but with the probes plugged in as shown the 8:30 position would be the correct setting. The label is 2000M.
 
I was in the Millivolt setting, so that is not an issue. I think it is a given that I shorted somehow but I don't know how. I was being soooooo careful. A friend at work explained the scale to me so it is clear to me now. My red probe blocked out the mark showing K ohms so the scale on 20 tops out at 20,000 ohms on up to 2,000,000 ohms. How simple can you get.

Avionic is helping me sort out the TPs and what to look for. I am getting a dead short on the right side collectors. The left side is acting like a charging cap with the values increasing up to a point where they slow down considerably. He has not gotten back with me yet but from what he said, anything less than 20 ohms is a problem. I'm just not sure which problem. He has been very patient with me and deserves kudos for his willingness to help. I am learning. That is what I came to AK to do. I am appreciative.
 
Here is a pix to identify lead orientation. :D
Center lead is collector ........ B C E ......

Dave
 
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Avionic, you are a font of knowledge. Thanks. I have them all out and will test them soon. They look exactly like the picture.
 
Bubba
I have a bunch of old transistor substitution books.The 2SD588 subs to a ECG380 just happened to have a old ECG sub manual from 1989.

Dave
 
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