Electronics Repair Liabilities

[dr*audio]

I agree that DC voltage was probably not the cause of the blown tweeter. One possible cause could be a high frequency oscillation. If the amplifier was oscillating at a supersonic frequency you would not be able to hear it, but, the tweeter could be damaged.

Have you measured the DC offset of the amplifier? Have you tried measuring for AC voltage at the output of the amplifier?

Oscillation is a good possibility. That would kill the tweeter, and then the output transistors would fail and the amp would go into protect mode.
The oscillation could have caused the original failure or it could have been caused by the replacement transistors.

 

[Okke 70s]

I think this is the way.

Happy to help! I hope thing turn out alright eventually.

 

[Jimbo40]

Yeah, that's also what I'm thinking.

That was my techs explanation for what may have happened with the first instance of the protection circuit turning on. What other possibilities might there be?

Clipping! How hard are you running it.
Get a dummy 8 ohm load, run a test tone up to or near rated power, and measure dc on the line.
Could have a bad crossover or just be driving it to hard with a weak cap in the xo.

 

[petes'picks]

Holding a tech responsible for damage done by a Nakamichi amp that was built 30 years ago? I doubt you would be able to get Nakamichi to buy you a new tweeter if amp was purchased new last week. Your amp was blowing tweeters BEFORE your tech worked on it, I bet he will take another look at it for free but I wouldnt expect anything more than that.

 

[Jimbo40]

Holding a tech responsible for damage done by a Nakamichi amp that was built 30 years ago? I doubt you would be able to get Nakamichi to buy you a new tweeter if amp was purchased new last week. Your amp was blowing tweeters BEFORE your tech worked on it, I bet he will take another look at it for free but I wouldnt expect anything more than that.

Yeah good point, also if the tech ran it hard for 3 days without issue, what would you suggest he did wrong?

 

[petes'picks]

Dont know if he did anything 'wrong', I think you have component that goes flakey every so often causing DC to go to speakers or causes clipping. I guess you could pay a tech to watch the signal on an oscilloscope for 8 hours but that could get pricey and isnt a normal routine of any tech ever.

That isnt a simple amp. Have him look at 1kz square wave, watch distortion meter and higher frequencies on oscilloscope but he may well have done those things already.

 

[titanstats]

Yeh, it sounds like the kind of problem that can appear to have been fixt, but actually is still hiding somewhere. I agree the guy has to get back at it to finish the job, but can't really be charged with the cost of the fallout. It's a tough break for you, but I recon you guy might also have to take a deep breath.

On a side note: especially with this sort of problems I never feel secure without extensive testing. Several occasions several hours in all the relevant modes.

Yeah, I play a piece for days after I repair it, and use all functions. Gremlins really only show after hours of use, I won't release a machine that hasn't played clear and clean for several days; once you've done that though, what else can you reasonably do? Old electronics are old, stuff happens.

 

[62vauxhall]

In the mid or late 1970’s I got an Amcron aka Crown DC300A. My speakers were double Advents.

Had it for week and half way through an LP, suddenly there was the loudest noise imaginable. Myself and those in room were actually immobilized. Then smoke drifted through the grille cloth.

After turning off all the power switches , I began taking drivers out of the speaker cabinets. The smoke was from all four tweeter voice coils burning up.

The tinsel wires on all four woofers was vaporized and the insulation inside the boxes had black scorch marks and was melted in spots.

The next business day, the amp went back to the shop I bought it from.

It was sent to the distributor and was quickly fixed. But they did not disclose what happened or what they fixed. I refused taking the amp back and got something else.

I made a point of stating that the distributor should cover the cost of replacing all four woofers and tweeters. The request was emphatically denied.

 

[Okke 70s]

Well that is intriguing.. Did they just not mention what they fixed, or did they refuse to tell? A (structural) safety hazard maybe, they feared to be sued for?

It is very different from the OP's dilemma I feel, although it both ands with blown tweeters.

 

[UncleAng]

Nakamichi PA-7A II for the curious

No damage done to speakers or anything else but my PA-7A II has required quite a bit of work in the past 3 months. PM me if you want a list of what has been replaced due to aging and being out of spec.

 

[UncleAng]

He's pretty certain that he set the bias to 100% factory standard

That would be 40 mA. At idle the amp's cooling fins should be warm and both channels should feel at the same temperature. After playing at volume for a few hours the fins would be warmer but not so warm that you can't keep your hands on them. If the channels are at different temps either at idle or after prolonged play something ain't right. These amps do run in class A mode for the first 10-15 watts; their Threshold counterparts are biased at 100 mA and are in class A for up to 20% of their rated power. They are a lot warmer at idle, etc. and require more servicing plus are prone to more failures because of these extreme settings.

 

[KentTeffeteller]

Some speaker makers install a lamp on one side of the tweeters to shunt excess voltage.

Thus protecting tweeters.

Fuses work too...

Yes, I recommend this highly. Some amplifier designs which are finicky, need this protection. Beats DC in a loudspeaker driver, if things go awry on the amplifier end. Note: Crown always recommended this in their manuals, from the beginning of the DC series. Common sense.

 

[UncleAng]

Some speaker makers install a lamp on one side of the tweeters to shunt excess voltage

I believe my Walsh 4's have three of those mounted by the tweeter

 

[egellings]

Is it possible that the speaker's load characteristics cause the amplifier to go into HF parasitic oscillation, which would make short work of a tweeter?