1.3K burning speakers

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jd907
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1.3K burning speakers

Post by jd907 » Tue Apr 11, 2017 12:43 am

Hello,

This 1.3K literally burned a speaker. The B output is the culprit, the A output hardly puts out any power but it is at least a clean sine wave.
I am fairly new at amp repair - my strong suite is guitar tube and solid state amps, so I feel out on a limb here. What I need help with is a good place to start looking, and some advice from an experienced tech - how much time could this suck up? I would imagine pulling transistors and heat sinks could be really time consuming and I want to give my customer a realistic expectation on the cost of the repairs. Without having gone through an amp like this I am guessing it could be around 2-4 hours, at which point they might be more inclined to buy a newer amp. The other thing I am having trouble with is finding a schematic that has test points on it, do they exist for this model? I have found some, but they do not have any test points.

Thanks in advance,
Jeff

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Enzo
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Re: 1.3K burning speakers

Post by Enzo » Tue Apr 11, 2017 2:39 am

You can get any Peavey schematic by contacting customer service at the factory and asking for it.

There are not many "test points" in the schematics, a technician should have a good idea already of voltages to find.

Always check for DC voltage on the output connectors. That is likely what burnt up a speaker. NEVER connect a speaker or load to a solid state amp that is making DC on its output. Not until the amp is made stable and the DC is gone.

This solid state power amp is like most any other power amp, just bigger. Your 80 watt Bandit amp shares an overall similarity to this.

Power off, measure resistance across each channel output posts. Is eaither side shorted? Often when channels fail, the "crowbar" triac across the output shorts.

I generally do not have to remove parts to determine if they are bad. Esay enough to find shorted ones where they sit. Of course if they are shorted they have to come out. Always check all the resistors associated with any failed transistors, they burn open. Do not overlook the two channel driver boards on the side wall.

Buy a new one? If you are not familiar with high power amps, what is your hourly rate? I charged $60 and hour, but I was pretty good at it, and had 40 years of professional electronics experience. I expect I'd estimate 2-3 hours for a blown up amp of this sort. Plus parts of course.

jd907
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Re: 1.3K burning speakers

Post by jd907 » Tue Apr 11, 2017 1:28 pm

Enzo, thanks for the reply. I've recently encountered a Crown power amp that would have required quite a bit of disassembly in order to check each transistor, which the customer opted to get a new amp considering it's age and relative costs for a new one. I am green - not having much experience with this type of amp, I am trying to estimate how many hours it would take for someone with experience. Your time estimate is close to what I was thinking. I charge $60/hr but take in consideration any new things I am learning along the way. This one may take me more time than I charge, but that is experience I am gaining. I will check back in with some results...

Thanks for the advice.
Jeff

jd907
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Re: 1.3K burning speakers

Post by jd907 » Sat Apr 15, 2017 10:40 pm

Enzo,

I spoke with the customer about the amp and he had it wired for straight through but had the crossover switch on - this is what smoked it.

So I checked across the outputs and they read:

A Output 23k Ohms (held for 30 seconds or so... and kept rising)
B Output 3.5k Ohms (...and rising)

A output will pass a clean sine wave through with little power gain.
B output is putting out enough DC to instantly fry a speaker.

A Side R168 5.6 Ohm reads .2 Ohm
B Side R268 5.6 Ohm reads .2 Ohm

Pin 2 on CR244 has separated from the trace (see picture here https://app.box.com/s/tlg1s5azf31pjshj31v654xb1jpbfa69)

That's what I've discovered so far... going to keep going through the circuit and check the transistors as well.
Any thoughts?

Thanks, Jeff

jd907
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Re: 1.3K burning speakers

Post by jd907 » Sun Apr 16, 2017 6:05 pm

Update:

The power amp board:
I've checked all output transformers between the E&B, all about .46 to .59 VDC. All of the .68 Ohm resistors are good.
R168 5.6 Ohm reads .2 Ohm and R268 5.6 Ohm reads .2 Ohm (these are in parallel with the inductors) I think since they are so out of spec they should be replaced. CR244 (SAC 187) which de-soldered it's pin 3 from the board needs to be replaced.

The pre-amp boards:
A channel side passes visual inspection
B channel (our problem channel) - BURNT TOAST! https://app.box.com/s/7bj72xt6f7pu3bd3rkok519hqz5sxoeb
I'm going to look into replacing components or seeing if, by chance a new board is available from Peavey.

Jeff

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Enzo
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Re: 1.3K burning speakers

Post by Enzo » Mon Apr 17, 2017 7:01 am

Stop and think. Two 5 ohm resistors measure 2/10 ohm, and have parallel inductors. Inductors are just coils of wire, so of course teh resistor will measure loow, it has a wire across it. Resistors really have no way to go low in rsistance, they generally fail high or open. Wirewounds like those are pretty much open or OK.

Nurnt driver card. A Qtip and alcohol will clean off most of the soot. Yes, you will need to rebuild that circuit.

jd907
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Re: 1.3K burning speakers

Post by jd907 » Mon Apr 17, 2017 3:37 pm

:D Stopping and thinking. Duh. Ok, I figured I would have to rebuild - I think this one was made in 1996 and would be surprised if they had replacement boards still. The customer wants to repair it. I'll post up when I have completed the repair. Thanks for the help!

Jeff

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Re: 1.3K burning speakers

Post by jd907 » Wed Apr 19, 2017 11:10 pm

Need some help on cross references...

70466531 - MPS6531 NPN Transistor (which crosses to a 2N3642 and is also obsolete) I still haven't found a clear cross on the web for this, help!

70340054 - This is C104 100pf 10% 50VDC, it looks like a resistor. I can't find any direct replacements on digikey or mouser. Is this a tantalum cap? I'm wondering if I can just use a ceramic instead...

70400099 - SAC187 Triac (TO220) I found this on Digikey http://www.digikey.com/scripts/DkSearch ... 2737981980 I think I am OK on this one, the current model on digikey is rated for 700V @ 16A versus 400V @16A.

Another note...
There is an "extra" resistor on these pre-driver boards which isn't listed on the schematics I have. Does anyone know anything about this? I don't think it will change anything I do with my repair as the values I have on the board match to the values I find on the schematics for the items I am replacing. If you look at the big wire wound resistor on each side of the pre-driver board you'll see an 82k 5% 1/4W resistor right next to it. You can see it in my original picture https://app.box.com/s/7bj72xt6f7pu3bd3rkok519hqz5sxoeb

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Enzo
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Re: 1.3K burning speakers

Post by Enzo » Thu Apr 20, 2017 7:31 am

C104 is just a plain old ceramic cap, yes they look like resistors, except they are usually a different color. Pretty darn rare for them to fail. Anything is possible, but way more likely the transistor it is across is shorted than the cap might be.

Your Digikey link is just a search page, please specify the part number you found. The triac can be higher voltage, of course. You can also contact Peavey customer service and ask what they are using in place of teh SAC187 these days.

MPS6531. Mouser has thousands of them in stock right now for 78 cents or less apiece, so why cross them at all? To cross they are just generic audio signal type transistors.

Your extra resistor? I have no idea, it is not on my prints, so it is a later revision change. Unless it is burnt up, I'd ignore it. Just like if you drive down a familiar highway, then one day a new side street appears and some new houses. It doesn't change the rest of your commute.

jd907
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Re: 1.3K burning speakers

Post by jd907 » Thu Apr 20, 2017 11:35 am

Enzo,

Here is the correct link to the triac... I'm not familiar with alternistors, but deduced that variant of triac is not required in this case. They make this also in a "Snubberless" alternistor, but this is the standard triac.

https://www.digikey.com/product-detail/ ... -ND/725395

Another mentor has indicated that he doubted the PF cap would be bad as well. I was considering it's proximity to the burned resistor and if it would be a safe bet to replace. I have some ceramic caps in stock, so I'll leave this one in there and if I have any problem later on I can replace it. I have an ESR meter and a Fluke 117 but neither of them will read these PF range caps - I'll be looking at getting a piece of equipment to measure these. Though just for practice I guess I could build an LC circuit with known L and find the Q.

Mouser! Ok... I don't know how I missed that. Getting locked into Digikey. I'll move my shopping cart over to Mouser. Thanks.

Yep, just ignoring the extra resistor! Nice analogy.

Jeff

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