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Old 14th September 2005, 06:29 AM   #1
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Default Luxkit A504 repair, long

Hello everyone,

First of all, before I launch into this post, let me apologize for the length of this post and also for the fact that my first post in this community is a plea for help. Thanks in advance for those who are patient enough to read the whole thing and to those who can offer some assistance.

OK, here's the story:
A friend of mine shorted the output leads of his Luxkit A504 amp and blew up one of the output stages. (Actually, as he related it, "Flames shot out of the top." Output stage damage was my diagnosis -- not a hard assessment to make in the light of the cracked and blackened transistors.) Never one to turn away from the opportunity to disassemble something, I agreed to attempt a repair.

Let me digress for a moment to provide a little background. I have a degree in electrical engineering. As part of my job over the past ten years or so, I have designed a number of analog circuits (signal conditioning and data acquisition stuff) and several fairly high-power control circuits for inductive loads (motors and solenoid drivers). I'm not completely unfamiliar with the operation of transistors, diodes, bias circuits, etc, but I must be careful to point out that having never worked on an audio amp, my knowledge of them is academic, rather than practical.

OK, back to the story at hand. Faced with the damaged amp and an irresponsibly optimistic promise to attempt to fix it, I ordered a full set of replacement output transistors (3 each Toshiba 2SA1095 and 2SC2565) and books on amp design from Hood, Slone, and Self. I also attempted to locate, unsuccessfully, documentation for the amplifier. From what little I've been able to find on the web, the amp was a kit version of the Luxman A504 amplifier, and was sold only in Japan. I should add that I have no schematics.

As I mentioned, each output stage consists of 3 paralleled BJT pairs. The output transistors are mounted on a fairly complex "heat-pipe" type thermal sink. Circuit boards are single-layer, holes are not plated-through, all components are leaded, and circuit traces have that old-school hand-drawn look.Most solder joints appear to be functional but ugly -- lots and lots of solder (as is typical of boards without plated-through holes), and no apparent attempt by the builder to remove solder flux. The various boards in the amp are connected with point-to-point wiring.

My first attempt at a repair was simply to replace the output transistors and plug the amp in to the wall. Two things amazed me about that first attempt: 1. the 6" flames that leapt out of the vent holes in the case and 2. just how long the burning smell lingered.

Here is my second repair attempt: I ordered a new set of output transistors. I also began to pursue other damage in the VA stage upstream of the output stage. I found and replaced several 100 Ohm power resistors and a couple of small (TO-220 package) transistors that were blackened. With the power transistors uninstalled, I turned on the amp, and verified that the bias potentiometer adjusted the voltage across the bases of the output transistors. I should point out that the bias voltage swing was about 1 volt higher than the range of voltages that I observed in the amp's good channel. I felt comfortable ignoring this problem (perhaps, in hindsight, not a wise decision) because I expected the base currents to force an IR drop across series resistors in the base circuits. I turned the pot to its lowest voltage, unplugged the amp, installed the new output transistors, and turned on the amp. I suppose I can claim some some measure of success this time because there was no flame. Just lots of smoke, and the amp, she still doesn't work.

Guys and girls, it is clear to me now that this is beyond my capability. I am ready to concede that I will not, without a lot more effort and experience, be able to fix this amplifier. Which leads me, finally, to the topic of this post -- is there someone out there, preferrably in the LA or Orange County, CA area, who feels confident they could fix this thing? I am willing to pay a reasonable fee for the repair, as long as you are willing to discuss the problem and the repair with me. I'd like to get this thing fixed. I'd also like to learn a little more about amps. Maybe I can leverage your experience to help me climb this learning curve?

Thanks for your patience and consideration.

David Sprinkle
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Old 14th September 2005, 07:13 AM   #2
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Hi David

I am not much into SS but have repaired a large number of amps and my experience in releasing fire and smoke from the tiny cans may be of some use to you.

The biggest problem of your approach is that you are assuming the amp is fixed before applying power. If you use a variac to gradually increase PS voltage while monitoring current or at least connect low value/low power resistors in series with the PS rails, you'll be able to greatly reduce the bodycount.

A more time consuming but surefire way of fixing is to test every single semicondutor (and probably quite a few resistors).
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Old 14th September 2005, 08:05 AM   #3
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Hi dsprinkle,

You give up too easily! You must understand that the output stages of a power amplifier only do what they are told to do! The input stage , etc.. do the controlling. If they tell it to blow up it will keep blowing up every time you replace those power (read smoke and flames) controlling devices!

You must seek the culprit earlier on .. in the control stages. If the bias is high you must find the reason. This is your mission... because you want to know?


Cheers,
Greg
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Old 14th September 2005, 12:17 PM   #4
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Probably a stupid question: but after the first attempt of replacing all three output pairs, shouldn't it be more cautious replacing just one pair at the time until it stabilizes?

Isn't the reason for paralleling transistors at the output just taking care of heavier loads?


Carlos
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Old 14th September 2005, 01:27 PM   #5
jcarr is offline jcarr  United States
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David:

The A504 was never sold as a finished product, AFAIK. It was _only_ sold as a kit, so naturally some were built well and worked well, while others were not.

Also, the design was quite flexible, which in the right hands made for an interesting amp, but in the wrong hands, could be the source of niggling construction errors, or outright mayhem. At the very least the A504 could be configured as class A or class AB, single amp or BTL, with global NFB or without. There may have been other options, for example battery operation, but I don't remember whether this applied to the A5xx series or only the later A9xx series.

Also, these amps were originally sold in the early 1980s, and especially given the possibility of class A operation, they weren't known for running cool. The electrolytics on the pcbs should probably be all replaced for safety, and prior to inserting new electrolytics, the pcbs should also be inspected for copper trace damage due to leaking electrolyte.

Given the range of possible configurations and construction quality, I think you should try to get a schematic at the very least, and preferably the building manual. OTOH, the documentation may be only in Japanese, which could be a problem for the building manual, but won't affect the schematic.

If you contact Luxman, they should be able to provide at least photocopies of the documentation.

http://www.luxman.co.jp/

Some of the Luxman kit amps were sold in North America under the "Fuji" brand (I think), so perhaps this may be another lead to pursue.

Quote:
The various boards in the amp are connected with point-to-point wiring.
Now that's an expanded definition of P2P wiring that I haven't encountered too frequently. Sure you haven't also worked in marketing?

hth, jonathan carr
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Old 14th September 2005, 01:30 PM   #6
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Quote:
Originally posted by carlmart
Probably a stupid question: but after the first attempt of replacing all three output pairs, shouldn't it be more cautious replacing just one pair at the time until it stabilizes?

Isn't the reason for paralleling transistors at the output just taking care of heavier loads?


Carlos
Thank you, Carlos, for pointing this out. Yes, I considered just replacing only one output pair, but remember that I had convinced myself that proper bias required the base current to provide an IR drop across the base resistors. As I mentioned in my original post, this was in hindsight not a wise decision.
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Old 14th September 2005, 02:08 PM   #7
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Quote:
Originally posted by dsprinkle


Thank you, Carlos, for pointing this out. Yes, I considered just replacing only one output pair, but remember that I had convinced myself that proper bias required the base current to provide an IR drop across the base resistors. As I mentioned in my original post, this was in hindsight not a wise decision.
Yes, bias should be moved to the least current position. Or at least stay in class B or AB grounds before going further on.

JCarr's suggestion to get a schematic and/or building plans, as well as looking for 'lytic leaks are very important.

Did you check if the "fire-cracking" effects did not cause any pcb problems or if that might be the reason for the fire?

In any case you should certainly put 5 or 10 watt resistors in series with both supplies to check for problems before total power connection.


Carlos
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Old 15th September 2005, 04:48 AM   #8
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Jonathan, thanks for the pointer about the manufacturer. I had a native Japanese coworker call Luxman today. We were able to order the original build docs for a small fee. Should arrive in a couple of weeks.

Carlos, thanks for the suggestion about installing a power resistor in series with the supply rails. Shoulda thought of that myself.

Finally, any suggestions for variac specs? I have a chance to pick up a 7.5A Variac model 116. Am I correct that this seems a bit too small?

Thanks again.

-dave
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Old 15th September 2005, 10:27 PM   #9
anatech is offline anatech  Canada
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Hi Dave,
Definitely not! It's kinda large really unless you are working on large class A amps. I mostly use a 2A model, I have 10A units I can drag out when needed (CJ Premier 1 for example).

If it's a good price, pick it up. I find an ammeter and voltmeter permanantely connected is very helpful. You can easily watch for problems.

-Chris
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Old 16th September 2005, 12:56 AM   #10
Eva is offline Eva  Spain
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dsprinkle:

I'm sorry to say that but I think you are suffering a tremendous lack of inventiveness and creativity, no matter if you have an EE degree (and blowing output devices in groups of six is definitely not good for anybody's budget).

There are thousands of tricks to avoid such catastrophic failures, being the simplest of them to wire a 100W or 200W light bulb in series with the mains cord of the amplifier the first time you plug it after repairing (my favourite). Among other methods, you can insert power resistors or 500mA fuses in the supply rails, or test the amplifier without output devices (and without load!!) in order to check input stage and VAS, etc...

Also, your understanding of audio amplifiers would improve a lot shall you take a pencil and a piece of paper and face the job of drawing a complete schematic of your broken amplifier, so that you can analyze it thoughtfully while resting on your favorite armchair or bed (I learnt this way). Such an analysis may reveal interesting facts, for example: when output devices fail, drivers, pre-drivers and the bias circuit will be subject to overcurrent, overvoltage or reverse bias, and will also tend to fail (so by replacing only the output devices and pluging the amplifier directly to mains you are going to get smoke quite often).

Finally, remember to test all semiconductors against shorted junctions before powering a repaired amplifier, this takes little time and doesn't require to unsolder componentes but will reveal most blown devices since the most common failure mode is a short.
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