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How to troubleshoot efficiently?

A K2 needs a block diagram

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#1 AC4AQ

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Posted 23 April 2014 - 05:30 PM

Hello to our fellow techs.

I'm wondering if there is a audio technician out there who has developed a method for rapidly locating the faulty circuit area, but not necessarily the failed components in Crown audio power amplifiers.

I've been trying to do this for two years now and am still having to create a technique for locating the trouble area when I encounter a particular Crown model for the first time. I am a technician who has had 20+ years experience in component-level work in RF power amplifiers,  servo-controlled devices and off-line converters. In the case of the servos, a failure anywhere causes symptoms everywhere, so it is necessary to 'open the loop' to troubleshoot. Inasmuch as as Crown doesn't really sell "amplifiers" such much as "speaker servos", the similarity is obvious.

However, I am now facing a K-2, with symptoms everywhere. It draws excessive line current unless both output modules are disconnected. No failed FETs, only fault found on either module was U51 (low side 12 VDC reg). Both high and low sides toggle if I feed them 12VDC, Vcc and drive the LEDs. Looking at the drive board , we have a nice sawtooth but IC 101 & 103, 201 & 203 are all locked. Going farther back, there are many pathways that can induce this failure mode.

I am not looking for the answer to this problem, but a better way to find it. The schematic tells all, too much in fact, it takes a long time to find where a bus like -LVA might go to. And there are multiple closed loops, so you can't open the loop, at least not all of them.

What  would be ideal is either a block diagram, with the test points shown so that it not necessary to waste hours looking where the fault is not. Or maybe a Truth Table for the protection circuit, to identify quickly which failure is present. Or, a flow chart.

Right now, it is the K-2. But there is an IT-6000 waiting for me next, and a smoked IT-6000 will make the K-2 look like child's play.

Thanks for any thoughts on this.

Jeff. 73 de AC4AQ

#2 joust

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Posted 24 April 2014 - 04:05 PM

Hello Jeff, (Note : You haven't responded to my PM...)

I won't go into the amplifier troubleshooting guide now, we can do that in PM. Now, as for your K2 :
First thing : The symptom described above, was this the original symptom? Can you confirm you have the manual? There are a lot of flow charts inside!

OK. Now you'll have to let me know your different voltages : You should have a +15V, a -13V (!), and +/- 105V (around) for the HV rails.
Since you removed the output modules, might as well refurbish the beasts! Verify the filter caps, and if possible, measure them. (Note : There are high voltages present! It would be important to discharge the output rails before attempting to remove or repair the K2. The owner's manual explains how to use TP1. section 5-2 in the manual).

I have found numerous times that C23 and C24 (-13V circuit, 100uF 25V) may have changed value; I replace them as matter of fact (especially if the units date code is older than 2002).

So now you have issues with the output boards : Are both defective?? How are the Silpads? Hopefully, you have been very gentle with them! They isolate the positive HV from the chassis. If holes are discovered, the Silpad should be replaced; You could use small bits of Kapton tape to cover these holes.
Verify the buffer ICs (34151) and the HS optos (2601, 2611), since the 5V regulator died.

You mention that you replaced U51 : This is not a 12V regulator but a 5V regulator.

As for the IT6000, let's do it when you'll be evaluating it.

Peace.

Alain

#3 AC4AQ

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Posted 25 April 2014 - 10:59 AM

Thanks for your help,  Alain.

It looks like the data I have is incomplete. It consists of eight files when unzipped:

100522M    100442C   100436D
100195C    100946F    100848B
100012B     100550A.

Each of these is a schematic scanned from the original paper into a .pdf file.

In the case of this K2, it trips the overcurrent on my bench Mains source, which prevents further damage. Both output modules appear to be good as if they are supplied with Vcc, AC, -Vcc, -Vcc+12 and a suitable drive signal to the optos they will commutate. In the case of U101/103, U201/203 one is switching, the other is over to the rail.

Until a few moments ago I suspected that -Vcc+12 was absent, resulting in shoot-through in both output modules however this seems unlikely now, perhaps Crowbar is actually causing the overcurrent.

I will look for the PM, as I somehow missed it. Also please note that none of what I wrote previously was meant as criticism of Crown, as my problem of accessing the data seems not specific to any manufacturer but perhaps to the system where I work.

Thanks again and I will post any developments.

Jeff / AC4AQ

#4 AC4AQ

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Posted 25 April 2014 - 11:13 AM

Duh, There is extensive data after all. I forgot we're dealing with 20th Century technology here and the bulk of the service information is on paper and didn't make it to the server.

#5 joust

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Posted 25 April 2014 - 11:33 AM

Can you describe the original symptom, with indicator information, if available, and the initial hypothesis you developped. There are some inconsistencies that I want to unravel.
Here are some indicator information for troubleshooting :
All indicators dark, power indicator dark : you have no power to the amp, possibly the crowbar was activated;
All indicators dark, power indicator on : initially, it will be a bright green, meaning the amp is enabled; After a few seconds (between 6 secs and 1 minute) the power indicator should become a lighter green : the amp should be disabled, it is now in Sleep mode. Inject a signal into one of the two inputs and lift some level control and the power indicator should become bright immediately, meaning the amp has 'wakened' from Sleep.
One side has all indicators on : The side that has the all the indicators on has a problematic output module; This also happens when an output module is not power up or has its harness removed (ie. for inspection).
Both sides have all indicators on : Now both output modules are problematic.
If the amp is not powering up, remove the power harnesses from the output modules and power up : The power indicator should now light up and the front indicators should all be lit! Then verify LV power supply voltages : you should have the +15V and -13V rails. If you don't have -13V, replace C23 and C24 (100uF 25V).
Note : When you point out a component, such as U101, please designate from which module it comes from! :)

Hope this helps!

Alain

#6 AC4AQ

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Posted 25 April 2014 - 01:30 PM

I believe the work is now progressing rapidly.

Our symptoms are: Power up, relay clicks, IOC, Clip and Enable LEDs briefly flicker follower by second click and high line current. Symptoms unchanged with either output module disconnected. With either Output module connected but P2 on Input board disconnected: Power up, click, three seconds later second click with overcurrent, no LEDs light. With both disconnected, IOC, Clip and Enable LEDs illuminate, +15 VDC and -13 VDC bus good. Triangle waveform of 7.7 Vp-p, 4 uSec, on pin 3 of Main module U101, U103, U201 and U203. Triangle waveform of 3.6 Vp-p on pin 4 of same ICs, DC is nil.  However only U101 and U201 have square-wave output, 50% duty cycle output, U103 and U203 on Main board have both outputs high, suggesting these two ICs are damaged. I am now proceeding to extract them, replacements are on hand.

I suspect this came to pass due to technician ignorance, the rear panel was not in place, leaving J8 ungrounded and this likely damaged the ICs. Previously, I found U51 bad on one of the output modules, likely this was the source of the original problem but now I have to correct the self-inflicted wounds.

Thanks again for your attention.

#7 joust

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Posted 25 April 2014 - 03:54 PM

Did you get my most recent PM?

Usually if the regulator shorted on the output module, the 34151 has probable shorted outputs; Crown suggests, that in the event of one side failure on the output module, to replace the opto and the buffer/driver at the same time.
Have you tied the ULOL (underlap/overlap) adjustment? If in the event that you cannot square the signal, it is possible that wrong output devices may have been installed.
If you measure AC current while powering up, it is possible that a large current flow (3 to 5A) may pass for an instant; After powered up, there is little current flowing (0,75A) once unit is aSleep. When unit is awake, there may be sensibly more current than when aSleep.

Cheers!

#8 joust

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Posted 25 April 2014 - 03:55 PM

Correction : Have you 'tried' the ULOL ... and not tied!

#9 AC4AQ

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Posted 07 May 2014 - 10:04 AM

The answer to my question of how to troubleshoot a K-2 efficiently, at least in this case, is seek out the necessary information. It turns out there was adequate data available had I searched beyond the LAN. In the end, this K-2 needed C57, U2, U51, U52 on the output module replaced and U102 on the main module. Also both opto-isolators (U2 and U52) on the other module were damaged.

As the amplifier arrived, it is likely that the only failed components were C57 (leaky, pulling down the Vss-12 source) and possibly U52. All the other components were damaged by attempting to power-up the amplifier without the rear panel in place, which ungrounds part of the main module and isolates the speaker filter network.

So the answer is: In amplifier repair and in other things it is a goodly lesson to looke ere ye leap.

Now on to the IT-6000, which went Pop! and has a leaky FET in the output stage. This one is encouraging as it uses the familiar International Rectifier Class D audio power amplifier scheme. Time to research and not rush in where angels fear to tread...

Jeff/AC4AQ

#10 joust

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Posted 07 May 2014 - 11:33 AM

Jeff, I believe I've posted enough troubleshooting information that will help you. Check previous posts on Itech repairs for more detailed information.
How old is the amp? When has it refurbished for the last time?
Now for a few pointers : Verify the BCA coils : If cracks appear at the solder joints : Remove the old solder and replace with non-leaded solder. Also, make sure the leads extend enough to properly solder the connections. BCA coils can become very hot : these components have the task of nullifying the carrier. A lot of current is processed and the heat generated can soften the solder connection and create intermittent noise and operation.
Whenever an o/p device is replaced, I suggest to refurbish the rest of the devices : remove thermal compound off the heatsinks and devices and reapply a slight coat to each surface.
When installing a new or multiple o/p devices, use (install temporarily) the heatsink and clamp to align the device properly with the main board before applying solder.
With older clamps, it may be necessary to reform the forks (slightly!) so that they create enough pressure on the output devices that when you try to pry the device under the clamp, it shouldn't budge. Note : I use a Dremel sander to lightly buff (roughen) the curved surface of the clamp where it makes contact with the output device. The rough surface created holds the device better.
I would replace the t-sens (thermal sensor) foam, installed at the HS forked clamp. Clean the sensor, apply a small amount of compound in the hole of the device's tab, direct the sensor into the tab hole and stick the foam on top of the sensor. Then, install the clamp.
If an output device fails, you have to investigate the gate drive card for gate current buffer failures (J1Y or K1Y) and associated resistors.
Note : A failure to correct any subsequent output fault (verify the zener diodes!, gate drive card repair, etc) can cause the output to fail again! See my other posts for solutions.

I hope this helps!

Alain

#11 AC4AQ

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Posted 07 May 2014 - 03:49 PM

IT-6000 Update: This particular amp was manufactured 12/28/2004 and is serial number 8000492671. It doesn't appear to have had previous service and does not have the BCA drive board with red capacitors for C526 and C527.

After I replaced a bad FET (Q1007) I just attempted to start the amplifier with the Front End modules removed, as in the manual. However, Q1004 exploded after a few seconds and the blast wave was such that my ears were ringing for several minutes afterward. I think it should be recommended that hearing protection be worn when an amplifier is serviced where there is a fair chance that semiconductors may detonate.

I think it is reasonable to substitute 200 ohm 25 watt resistors for the toroids while troubleshooting, as this will prevent excessive current and allow non-destructive testing.

Tnx...Jeff

#12 joust

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Posted 09 May 2014 - 10:06 AM

Hello Jeff, ITech BCA troubleshooting and repair is quite tricky and can be explosive! The most probable cause of this failure is that the fet was not properly clamped down on the heatsink and thermal runaway destroyed the device. (You can verify for yourself : look under the failed device : if the compound has not flattened out then the device was not properly set on the heatsink. Make sure that when the fets are properly clamped down, that the devices don't move sideways : Use a pick and try to slide the device under the clamp. If the device moves then it is not clamped down enough. You may have to reform the clamp to have more pressure on the devices. Beware not to screw down the clamps too hard : you're going to strip the heatsink's screw hole. I also had success if the clamp's forks contact with the devices is 'roughened' a bit. I used a Dremel's sander to roughen the curved contact portion of the forks. I believe an ordinary file will also do the trick.
And, in regards to BCA troubleshooting, I remove the BCA coils to test and 'validate' the BCA repair. Note : If the BCA circuit is not properly troubleshooted and repaired, when power is applied devices can potentailly fail again and you'll have to redo the repair!

The BCA module contains two amplifier output circuits, and each amplifier circuit is composed of a HS (high side) and a LS (low side).
The HS section has the +VCC (also located as the RED cable on the power harness) on the heatsink while the LS has the output rail on the heatsink. The -VCC is located on the LS side, at one of the fet leads or the BLUE cable at the power harness.
If the BCA coils have not been removed, disconnect and remove the coils. Power up the amp : The unit should power up normally (CLIP indication may come on), and power is applied to the output section but not modulating. If the power up fails, then you have another problem to solve first.

When you investigate and find the location of the fault, keep in mind which channel and side has the issue : You will have to repair the gate drive section associated to the fault location.
** DO NOT POWER UP THE ITECH WITHOUT THE GATE DRIVE CARDS INSTALLED! ** The fet gates need a path to ground and cannot be high Z or else the device may fail, or may be stressed and eventually fail on a gig.

Check the heatsink voltages : LS = -VCC and HS = +VCC (referenced to ground); If LS = 0V (when referenced to ground) then you still have an issue!
Locate and measure the gate lead on the LS devices; It should be at -VCC+12V and the fet should be conducting. If you verify the HS gate lead, this one should be at cuttoff: 0V. As you can conclude, you should notice this : The output section has clocked once : The LS section is conducting while the HS is not. You cannot have both sides not conducting at the same time!

Also you can try this : Remove the output module; Remove and identify the gate drive cards and BCA coils; Identify the front-end cards;
Verify all fets and diodes : Remove heatsinks and clamps; Note which device(s) are faulty and replace them; Reinstall heatsinks and clamps; Note where the fault was identified and repair (if necessary) the associated gate drive card section or you could simply replace the gate drive card. Reinsert the gate drive/front-end cards in their respective location, previously identified, but do not re-intall the BCA coils yet.
Re-insert and re-connect the output module (still without the BCA coils);

Power up : If the repair was successful, you should have the values mentionned above on the heatsinks : LS = -VCC, HS = +VCC;
If LS = 0V (not conducting) : ** Make sure that you have 12V on the zener diodes! **
If diodes are conducting normally, start by swapping gate drive cards; If the symptom switches side then the gate drive card is defective;
If no change, swap front-end cards : If symptom switches side, then the front-end is also faulty.
If no change, remove gate drive cards and verify output devices minutiously!
** DO NOT REINSTALL BCA COILS IF A FAULT STILL EXISTS WITH THE OUTPUT SECTION! **

If all is well, power down, reinstall the BCA coils, after the coils have been refurbished) and power up, load amplifier and pass some signal; Once the fans have started to oscillate it is the right time to verify and adjust ULOL settings.

Good luck!

A.