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Old 10th March 2008, 12:16 AM   #1
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Default power amp repair

Hi all.
I'm a newbie to this forum.
I'm working on a very old Crown power amp and am looking for a good reference book that has waveform analysis. ie:if I feed a 1k sine wave into it and get something else on the scope, the book can lead me in the right direction as to what the new waveform means. I have a dual trace scope.
I'm in the Toronto gta.

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Old 10th March 2008, 03:05 AM   #2
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Get the schematic.

All power amps tend to follow a similar receipe:

gain stage in the front end/phase inverter, something to drive the outputs, something to spread the voltage apart from the drivers for the upper and lower rails, and then usually a current gain output stage in the form of a follower.

Since these amps have big feedback, you'll get garbage out or nothing much out if any of the devices are toast.

What one usually wants to do is to look for obvious failures like burnt resistors, and shorted transistors.

You may want to look for the magic ~0.6vdc base to emitter drop on the transistors - if it's not there, then there is a problem.

Excess current is also a clue...

Often one actually has to remove the transistors and test them.

The early Crowns used an IC in the front end, that can die too.

There is no book that I know of that explains amplifier troubleshooting very well, or if there was one, one that you could just read and follow along without some real knowledge of how circuits work.

Sorry for the bad news...

Otoh, folks here can answer questions about how the circuit works...


PS. if you need a very good book on electronics, The Art Of Electronics by Horowitz and Hill is excellent and unusually well written... it's actually readable!
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- every once in a while I say something that makes sense... ]
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Old 10th March 2008, 04:00 PM   #3
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Thanks for the quick reply.
It's a D150.
By the way, the OD leds for both channels go on when I turn it on and whether I have a signal going in or not.
The gain works as well when I turn it up or down.
Also, if I can manage to get a jpeg of the output from the scope,
would you be able to get some idea of what's going on?

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Old 10th March 2008, 10:41 PM   #4
jaycee is offline jaycee  United Kingdom
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Crown have service manuals on their website with full test procedures. This would be a good place to start.
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Old 11th March 2008, 03:09 AM   #5
d3imlay is offline d3imlay  United States
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Check all of the transistors/diodes for shorts. Check the power supply caps for equal DC voltage and ripple.
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Old 11th March 2008, 04:05 AM   #6
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I know about the manual. I've downloaded it.
As I mentioned before, I need a reference to help me analyze the output waveform when a 1k sine wave is fed into the amp. ie:
what does oscillation look like on a scope?
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Old 11th March 2008, 05:08 AM   #7
gootee is offline gootee  United States
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If there is a known problem or goal, it would sure help if you stated what it is. You have not given much information. And it's much more difficult and time-consuming for anyone to give you advice in generalities. So please try to state the known problems, or your goals, and present the results of whatever tests or measurements you've performed. Then be prepared to perform suggested testing and report back. There are many people here who would like to try to help you. But you have to help them help you, as much as possible, by providing all information that might be relevant.


Rather than starting by looking at the amplifier output, I suggest you start with the power supply rail voltages. If those are not within the published specs, it doesn't make sense to look at much else. (And if one of them is low, for example, you'd first have to try to determine if a power supply fault is responsible for not keeping it up, or if, instead, something downstream is dragging it down.)

If the unit is very old, the first likely suspects are probably all electrolytic capacitors. If you don't have an ESR meter, maybe you can use this: http://www.fullnet.com/~tomg/esrscope.htm . (If you don't have a suitable square wave or pulse generator, you should be able to slap one together with a couple of op amps, or an NE555 IC, for a couple of dollars or so. Schematics are on the web.)

Other good possibilities for causes of problems in old equipment include dirty or worn-out potentiometer elements and switch contacts, as well as connectors and sockets. Get some Caig De-Oxit (see Caig.com), probably in both the spray-can form and the concentrate's needle-applicator form.

Before even using any test equipment, do a thorough visual inspection, looking for any discolorations of components, wires, PCBs, case interior, etc, and for leaking or bulging electrolytics, or evidence of past electrolyte leakage, and anything else that looks slightly odd, plus evidence of past repair attempts, damage, spills, mice, etc. A magnifying glass and a bright lamp should be helpful.

If you have the service manual, as you said, and it has a recommended troubleshooting procedure, or a calibration/alignment procedure, follow that first. If there are test-points' voltages or waveforms, check them against your unit, using the specified controls settings and operating conditions.

Regarding your question about what oscillation looks like on an oscilloscope: It depends. If the oscillation is at a very-much higher frequency than your scope's bandwidth, which you failed to mention, and has a relatively low amplitude, then you might not see it at all. If the oscillation is 'big-enough', but is beyond your scope's bandwidth or its trigger bandwidth, it might just look like a 'fuzzy trace'; anything from small to very large, depending on the oscillation and your scope, settings, etc etc. But if your scope IS capabable of a fast-enough sweep speed, and can trigger on it, a high-frequency oscillation will probably just look like some periodic waveform. If something is coming out that is at a much higher frequency than you're putting in, it's probably 'not a good thing', and could also damage or destroy components in the amp.

Later on, after the amp is working pretty well, you can also bang some square waves into the amp's input, and see what sort of ringing you get, on the leading edges of the output. That might tell you something about the frequencies at which it would 'like' to oscillate, and might even point you toward some compensation loop that needs trimming, or something.
The electrolytic capacitors ARE the signal path: http://www.fullnet.com/~tomg/zoom3a_33kuF.jpg
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Old 11th March 2008, 08:23 AM   #8
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Point taken.

My scope is a Tektronix 10mhz storage oscilloscope.
I have a sign wave gen. I don't have a square wave gen. but will get one or make one soon. One problem that is describable is that the OD leds go on when the amp is turned on(i/p signal or not). It probably is something in the power supply. I've tested the power caps. They are fine. I've cleaned the pots with a quality spray. I've also swapped the op amp. The amp is very old and the paper caps probably need replacing anyway.
Thanks for your extensive reply.
I won't post anything further till I undertake your suggestions.
I've attached a jpeg of the scope reading.

cracklin audio
Attached Images
File Type: jpg scope.jpg (94.5 KB, 514 views)
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Old 11th March 2008, 09:27 AM   #9
d3imlay is offline d3imlay  United States
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On the two waveforms displayed, where is the scope connected?
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Old 11th March 2008, 12:20 PM   #10
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The top waveform is from the gen. 1k tone.
The bottom is taken from the output of the amp( spkr lugs).
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