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Old 9th October 2011, 10:02 PM   #21
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But have you considered the possibility of capacitors in your crossover networks,
Try changing your speakers over and see if the sound changes too...try different speakers. There is little in the Crown that would cause this fault.

Crossover PCBs are subjected to tremendous vibration and solder joints can become very stressed over time. Also Capacitors have a hard time with changing voltages and frequencies.....Good luck
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Old 9th October 2011, 11:37 PM   #22
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Conrad and most here assume the amateur has a lot of equipment. I test transistors with a DVM, forward junction should be ~600 ohms, backward infinity, 2 junctions per transistor. So far I have caught about 20 bad transistors that way. I do have a meter and now a scope, and can now trace signals through circuits while operating. But I fixed a couple of amps just by asking the question "is this set of dc voltages sensible?" Like 0.6 v across b-e, reasonable collector and emitter resistor readings. An audio test probe like a powered computer speaker with a blocking capacitor on the tip and ring and a couple of alligator clips, can also help you find components causing ugly noises. Also, if the amp is over 20 years old, changing electrolytic caps pretty much based on date. As coupling caps they can make nasty crackling noises, as power supply caps they can allow hum and lower power available. You're way ahead of me, you actually have a schematic available. I had to draw up my first amp from the pcb, and large parts were burned away so a lot of component values, even types (PNP, NPN, wattage, specs) were guesses.
While by the hour getting someone to repair your repeated problem may be economic, or even buying a new one, you'll know a lot more when done if you fix it yourself.
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Last edited by indianajo; 9th October 2011 at 11:43 PM.
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Old 9th October 2011, 11:56 PM   #23
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Quote:
Originally Posted by indianajo View Post
An audio test probe like a powered computer speaker with a blocking capacitor on the tip and ring and a couple of alligator clips, can also help you find components causing ugly noises.
Nice idea! I use an old style Xtal earpiece with a coupling cap and alligator clip for ground, insulated point for probing transistor legs......don't need to short those.

Lots of tracing and figuring....steps of elimination and patience is the answer.

I still think you should look at the crossovers!
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Old 9th October 2011, 11:59 PM   #24
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The reason. When capacitors get old or tired they start to pass DC. This can be a noisy crackle like an old volume control, or just an occasional crack as they break down then recover.....eventually it becomes so bad that listening is impossible.....not to mention damage to the output devices if its conducting DC to ground........
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Old 10th October 2011, 05:13 AM   #25
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Quote:
The plastic rod, a certain amount of shaking and banging and some freeze mist might be your best place to start.
Yes, I do assume people have a lot of equipment.

As for the curve tracer, that's the only way to know much once you pass the DVM test, but I offered that more for education than expecting one to be available. Still, my guess is this amp has some low tech problem and easy to find problem, not a full set of defective outputs. Bet that wasn't the problem last time it was serviced either!
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Old 10th October 2011, 11:47 AM   #26
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I appreciate all the inputs from everyone, but am a little confused with the conversation. Please try to not hijack the thread and stay with me here. It may become a sticky or write up. I have done this for my Exotic 82 BMW 633 CSI nb.
Let me list what we know so far.

1. I called Crown the first time the amp acted up a year ago (same symptoms) and had a long conversation with the rep before I did anything and he referred me to the only Authorized dealer near me, the one with the extravagant estimate, charging 80 dollars to blow a little dust off, losing screws and washers in my brand new maybe 3 year old cherish amp just off factory warranty. I will call Crown again and see what they say. Thanks for the suggestion.

2. This authorized dealer fixed the same problem a year ago and the evidence of their work is that they removed and most likely replaced all 16 transistors as can be seen in the pic posted.

3. Based on that, I am assuming that they know what they are doing and are charging for what they need to do. That estimate cost me 80 dollars and I am inclined to want to rely on it as accurate because it is coming from a Crown authorized dealer regardless of what I personally think. I may not agree with paying them to fix it, but they are supposedly the experts.

4. Before it was taken to them, I opened it to look inside; I work and fix laptops computers as a hobby.
When returned, it was obvious that they had removed each and every transistor to test them. The amount of thermal compound is obviously different from the time I gave it to them.

5. Anything I say here is based on recent research that I am doing on this subject before I tackle the problem. There is an elaborate site that tells me, including graphics what I need to buy, what not to buy at the shack, and that (if not almost always) most likely that a transistor(s) must have burnt. If the encapsulated transistor touches the heat sink, it will burn and hence must be insulated (now I see why they plastered each one with thermal compound before returning it to me after testing). If the speaker wires are not properly connected, the transistors will burn. When changing, not only must they all be replaced, must be the same, but they must be of the same manufactured date and to watch out for counterfeits on line and which ones are counterfeit. When buying, in order to receive the same manufactured date,do not buy them in lots of 4 at a time when I need 8 or else they will not react the same and help carry the load evenly. This is from research information Googled after our last conversation.

6. This is a new amp that has left my house only 2 times too much, still smells and looks new.

7. I have read that if the transistors are open, they will send too much current in the output causing the speaker to bounce. Mine seems to react the opposite and unable to push enough juice to push my two JBLs. (no other speakers in my home that can handle 600 watts but the JBLs.)
Having said that, what do you guys think of the diagnosis, assuming that the authorized dealer, let me stress again the authorized dealer that Crown referred me to, that already fixed this same problem, that said that the transistors need to be replaced?

What is the function of a transistor other than to limit the output power as per research results? Just determined to become an expert with your help even if it cost me a couple hundred dollars. Cheers!

Thanks!

Remember I am a nb just relying on the few physics' lab that I had to take in school using an oscilloscope to measure amplitude of sin waves, but am not in possession of one but a credible Crown Authorized Dealer estimate that is just too expensive for some soldering to be done in my view. For 160 dollars, I would have authorized it. They had tried to get me the first time but I haggled but stuck to their guns this time.

Last edited by MaitreG; 10th October 2011 at 12:16 PM.
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Old 10th October 2011, 12:01 PM   #27
Mooly is offline Mooly  United Kingdom
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I don't know what to advise you for the best.

This is not a complex design of amp but it does have the potential for this to all end in tears due to the relatively high voltages and current/power capability of the amp.

Everyone on here will have their own ideas as to how to go about this.

If you are convinced (I am not) that the output transistors are suspect then my line of attack would be to check the DC bias (quiescent) current... and maybe for the purposes of faultfinding make the bias adjustable and reduce it. Use a bulb tester to limit damage if anything untoward happens. Perhaps remove all but one pair of output devices and test from there.

It really also needs a scope and signal generator to get some idea of what is happening.
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Old 10th October 2011, 09:09 PM   #28
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2. Waste of money, didn't fix problem 3. Wrong. 4. Probably what the service writer wrote up, but a waste of time. The estimator has to estimate worst case so you don't get *****ed financially if that is what is wrong. In this case the estimator was wrong, since the problem reoccured and the symptoms didn't match the problem anyway. Probably 90% of newish amps have the shorted O.T. problem due to customer abuse, but hum and crackle is not the symptom that matches that. 6 . If no DC voltage out to speakers when you get faint music with crackle, quit looking at the O.T's 7. This dealer diagnosis was done with the worst case pen, for legal protection, not with a brain. How old is this amp? You and the dealer seem obsessed by transistors, and ignorant of short life capacitors.
Don't read any of my previous suggestions, and don't fix your amp. As I did mine. With a Simpson 260 VOM, an 8 ohm 225 watt load, a car radio speaker for tests, and a pencil and paper for diy schematics.
Oh, and the guy suggesting substitute speakers because of worn crossover also had a valid point. Old speakers are $4-10 at Salvation Army & Goodwill. If that is the problem, repair your good speakers. I don't suggest opening them first, sealing a speaker properly is tricky.
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Last edited by indianajo; 10th October 2011 at 09:26 PM.
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Old 10th October 2011, 09:21 PM   #29
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I agree with Indianajoe. MaitreG, Output transistors are not able to cause the symptoms you describe. They are there to convert signal into power.....what goes in, comes out. That is all.
The noise you describe is from other components or bad joints. Most likely to be capacitors some where in or near the signal path, or plug type connectors not gold plated.
Seriously, If the noise occurs at high power, then the speakers could be the culprit. If at low power, front end of the amp.
Read Indianjoe again and forget about the agents, they probably are using a first year apprentice to fix your amp but they won't tell you that.
You don't need fancy tools, just a little patience. See if its on only one channel, (components, connectors) or two at the same time..(power supply, protection cct). Relax and step through bit by bit.
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Old 11th October 2011, 02:28 PM   #30
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Thanks Guys!

I am very much appreciating all the last entries and still probing. I wrote Crown to see what they would say and awaiting a response.

The speakers are/were new JBL Sound Factors and in no way I will open them and I am sure if I connect them to the Harman Kardon, they will play with no problem but I need more power to run my mini studio.

I found that last time upon inspecting the receipt that they supposedly changed 4 transistors of each (8) and not the 16 and evidence shows that they were all extracted from the board to be tested and installed back with lots of thermal paste.
I have to confess that I checked the speaker wires and did a search and must have had the wires cross changing the banana plugs for the temporary use of a Harman Kardon amplifier while it was at the shop the last time.
I am not doubting your advices but trying to better understand the different kinds of information I am getting on how a transistor would be the culprit and it is even described as a valve and this is a direct quote:
"There are many different ways that an amp can fail but the two most common failures are shorted output transistor and blown power supply transistor"

Let's assume that the transistors were changed and were the culprit, but to save money, because I declined the offer to repair the first time and they had adjusted the price to become more appealing to me, they just changed the 4 input and 4 output that were damaged. Based on what I have read, they all needed to be changed and be of the same manufactured date, still assuming that the transistors were the culprit, wouldn't you guys think that it would eventually fail again and now they have refused to change just the 8 but all, leading to their insisting to hold their price and sticking to their guns?
No tears her Molly, believe me, precaution is key and working around high voltage coil of my BMW for more that two decades and no mishaps is proof that care and major precautions will be applied at all times.
Again Mooly,indianajo, Meshuganah, thanks for the advices, and I have my patience hat on.

Last edited by MaitreG; 11th October 2011 at 02:58 PM.
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