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Old 4th January 2013, 12:49 AM   #41
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JMFahey If Q20 and 21 are shorted, wouldn't that cause the fuse to blow? Now that I can beginning to see how this amp works I am thinking these have been what was causing the intermittent humming and cutting out in the past.

About your comments on this amp. Some people say these amps sound like crap but I have loved the sound of this thing since I got it. It has a very nice clean channel and the overdrive is quite versatile, in my opinion of course. Thanks!
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Old 4th January 2013, 01:48 AM   #42
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Quote:
Originally Posted by nigelwright7557 View Post
If your lamp is coming on then something is talking serious current.
Points to a short somewhere like transformer, smoothing capacitors, output transistors.
So is there a way ti test the transformer and the smoothing caps to verify their integrity? I don't think I will spend the money for the transformer since it is so expensive but the caps are within reason.

BR
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Old 4th January 2013, 02:23 AM   #43
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Default JMFahey Tests

Checking resistance on Q20 18 19 21 are giving me all sorts of crazy readings.
Q20 was 1.1 and going down. 18 was .89 and going down 19 was .67 and going up and 21 was .65 and going down. I am using the heat sink as ground since its ground when the PCB is mounted to the chassis.

Diode test:
Q20 BE .036 BC .036
Q18 BE .623 BC .620
Q19 BE 1.911 BC 1.911
Q21 BE 0 BC 0

Just for Reference I am using a MAC Tools EM710 DMM

Mac Tools Online Store - Digital Multi-Meter with Backlit Display



2) now with amp off, even better unplugged, you will take some resistance readings.
To be more precise, we will use the "Diode test" setting, which applies a couple mA to the probes and shows the voltage reading between them.
We will measure the big suspects, the output and driver transistors.
We *expect* to find around 0.65V (650mV) across BE and BC junctions, one way; open or much higher (over 2V) the other way, and also open CE, both ways.
If any reading in a transistor is close to "0", it's an immediate suspect.
Carefully remove it and remeasure "outside".

Your multimeter will probably show a higher value, say 1/2 or 1 ohm, but that's fine, regular multimeters also add their own internal resistance (wiring/cpnnectors/switches/probes) which never is "0", what we don't expect there is "open" or a *large* value , sy 10 ohms or more.

Post results.

PS: just as an extra test, measure resistance to ground from Q20/21 collectors.
We don't want to find them shorted to ground.[/QUOTE]
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Old 4th January 2013, 12:13 PM   #44
JMFahey is offline JMFahey  Argentina
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Dear badraven, in a nutshell, yes, your output transistors are shorted , replace both oputputs and drivers, meaning Q18/19/20/21 .
Be careful, pamper the PCB, the *one* part that you can't buy anywhere , use a good solder sucker, maybe even a little solder wick to better clean the remaining solder.
You want to be able to pull parts without wiggling, a torn pad is a PITA.
When ordering the transistors, also get some insulating micas for them and a little silicone/thermal grease.
Google and print those transistors datasheets, mainly to know how that case is called , so if it's , say, TO247, you'll need TO247 sized micas, etc.
Also which leg is called E, B or C so we all speak the same.
Calling them "ABC" is confusing
A "rail" is oldspeak to refer to all points in a circuit connected to the same voltage, so "+42V rail", which by the way is written so in theschematic, means all points connected to +42V, meaning Q18/20 collectors, the top side of R88/91/101, C57, D38 cathode, etc.
It really clarifies things, because measuring *anywhere* on a rail must yield the same value.

Note: if you don't know what any word means, google it; I will gladly help you to repair your amp but this is not Electronics 101, this said with the utmost respect and trying to get to the point, which is repairing this amp.
I trust you will
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Old 4th January 2013, 02:47 PM   #45
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Quote:
Originally Posted by badraven View Post
JMFahey If Q20 and 21 are shorted, wouldn't that cause the fuse to blow? Now that I can beginning to see how this amp works I am thinking these have been what was causing the intermittent humming and cutting out in the past.
No, the output transistors wouldn't cause a fault like that - Q20/21 will cause the fuse to blow (and are doing so) - but replacing just those will most likely result in them failing again, you need to replace EVERYTHING that's damaged or faulty in the DC stage of the power amp.

This damage has almost certainly been caused by something you did, as it wasn't like it before you took it apart - any slight short circuit you might cause, even just for milliseconds, can easily blow the amp.
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Old 4th January 2013, 03:03 PM   #46
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If a transformer was damaged, it would probably be visible. Blown outputs also mean more damage downstream. Your driver transistors (little ones also on the heatsink) and the emitter resistors are also most likely toast. In the wise words of Enzo, "throwing parts at a failure is like throwing sponges at a rainstorm."

Last edited by ouimetnick; 4th January 2013 at 03:14 PM.
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Old 4th January 2013, 10:41 PM   #47
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Smile JMFahey

JMFahey - No offense taken. You are correct, this is not necessarily a training forum for electrical terms. Even though in a way it actually is. Leaning what terms mean and how things work i.e. conventions etc. is also a matter of learning where to find the info I need. i.e. definitions etc.. I'm learning to do all these things. I appreciate your patience as my learning curve accelerates in this area.

Thanks for clarifying what a rail is. I have been seeing that and now I understand what it mean.s

With that said, I understand about being gentle when desoldering. I have soldering wick and I have a premium sucker if I need it.

No one has answered my questions about where to get these transistors. I am thinking of checking with Radio Shack here in town in hopes they have these items in stock. But they are normally kinda high on their prices and I have never thought of them as carrying quality items. So maybe that is a bad choice.

As for the pin conventions of the transistors, you said B C E (L to R) to alleviate confusion so that is how I am listing. If you look at my earlier post you will see that I acknowledged your request and agreed to follow it.

Again Thank you!

BR



Quote:
Originally Posted by JMFahey View Post
Dear badraven, in a nutshell, yes, your output transistors are shorted , replace both oputputs and drivers, meaning Q18/19/20/21 .
Be careful, pamper the PCB, the *one* part that you can't buy anywhere , use a good solder sucker, maybe even a little solder wick to better clean the remaining solder.
You want to be able to pull parts without wiggling, a torn pad is a PITA.
When ordering the transistors, also get some insulating micas for them and a little silicone/thermal grease.
Google and print those transistors datasheets, mainly to know how that case is called , so if it's , say, TO247, you'll need TO247 sized micas, etc.
Also which leg is called E, B or C so we all speak the same.
Calling them "ABC" is confusing
A "rail" is oldspeak to refer to all points in a circuit connected to the same voltage, so "+42V rail", which by the way is written so in theschematic, means all points connected to +42V, meaning Q18/20 collectors, the top side of R88/91/101, C57, D38 cathode, etc.
It really clarifies things, because measuring *anywhere* on a rail must yield the same value.

Note: if you don't know what any word means, google it; I will gladly help you to repair your amp but this is not Electronics 101, this said with the utmost respect and trying to get to the point, which is repairing this amp.
I trust you will
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Old 4th January 2013, 10:52 PM   #48
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Smile Nigel

Nigel I'm not trying to say I didn't cause a problem. I'm just saying I can't find a single joint that is shorted. The couple of joints I suspected and desoldered had a track between them and were resoldered. Every time I sit down to do something with this amp I am looking over all the joints to see if I am missing something obvious. But nothing so far. I am using a lighted 5x magnifying glass and 2x reader glasses. The truth is I only reflowed the joint on the board from the end of the PCB with the power supply connections to the right side (in the picture I uploaded) of Q21. In the pic follow the right side edge of the chip across the board (down in the pic) I got interrupted and didn't reflow any others.

I am still learning how to read the schematic so maybe you could tell me the items down wind of Q18, 19, 20 and 21 that need to be replaced. I might as well get it all done.
BR

Quote:
Originally Posted by Nigel Goodwin View Post
No, the output transistors wouldn't cause a fault like that - Q20/21 will cause the fuse to blow (and are doing so) - but replacing just those will most likely result in them failing again, you need to replace EVERYTHING that's damaged or faulty in the DC stage of the power amp.

This damage has almost certainly been caused by something you did, as it wasn't like it before you took it apart - any slight short circuit you might cause, even just for milliseconds, can easily blow the amp.
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Old 4th January 2013, 11:03 PM   #49
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As Nigel says replacing output transistors doesn't always fix the original fault.
If the amp output was shorted then sometimes jut replacing output transistors will fix it.
If some part of the driver or bias circuit has gone wrong then you will keep blowing output transistors.

Not for a novice but what I do is remove all output transistors ( or disconnect them) and route the driver output back into the LTP and then I can check bias and DC offset without blowing fuses.

Otherwise you need to check all resistors/transistors in the driver circuit too.
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Old 4th January 2013, 11:34 PM   #50
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Default nigelwright7557

I'll do what I need to do. The amp is broke so it just experience for me at this point. Can you identify the "Driver Circuit" for me or are you saying to pull and check every piece on the board? Thanks!

BR


Quote:
Originally Posted by nigelwright7557 View Post
As Nigel says replacing output transistors doesn't always fix the original fault.
If the amp output was shorted then sometimes jut replacing output transistors will fix it.
If some part of the driver or bias circuit has gone wrong then you will keep blowing output transistors.

Not for a novice but what I do is remove all output transistors ( or disconnect them) and route the driver output back into the LTP and then I can check bias and DC offset without blowing fuses.

Otherwise you need to check all resistors/transistors in the driver circuit too.
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