Crate KX-80 Keyboard Amp fuse pops immediately

[ajsemtb]

Hello, I have a Crate keyboard amp that I use as a monitor that has been dying for a while, I think, and finally bit the dust. What happened this time is I turned the power switch on and immediately I saw a bright white/blue flash inside the case around the side of the cover, and all the LED's blinked and shut off immediately.

Prior to this, when the power switch has been turned off, it clicks or pops, then a high pitched squeal that goes up in pitch for a second or so then it shuts off.
But that has been going on for a long time.

So, i opened up the case, replaced the 2Amp 250V slow blow in line fuse, tried it again, immediately it blew.

I have pictures of both sides of the board, I did notice an area that looks like it is burned or was heated. Can anyone identify what the components are in this area? I know what capacitors look like, but its the black thingies that I don't know what are.

Anything that would immediately come to mind would be appreciated also.

thanks in advance, AJ


Also, I don't have a capacitance meter, but I'm going to go buy one. I have a nice multi-meter, but it doesn't have the capacitance function. Any recommendations on a meter would be appreciated. Probably going to fry's unless you can recommend something on Amazon much higher than I could get at fry's. thanks

[ajsemtb]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

photos...

[retiredcaps]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

1) Circled in red are four diodes that probably act as a bridge rectifier. If your fuse is blowing as soon as you apply power, one or more of these diodes could be shorted.

2) Use your multimeter's diode test function to see if they are bad. Post your measurements. You should have 8 readings (2 per diode).

3) What is your budget for a multimeter with capacitance measurement capabilities?

4) Post a picture of your current multimeter for one retiredcaps' bonus point.

5) If you are going to do more troubleshooting, an ESR meter is more useful than a capacitance meter.

[999999999]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

The darkened area appears to have a couple diodes, but at least on one I see "1N 53..." which is usually a zener diode. Can you get the complete part # off both of those?

Is this a single output channel amp, meaning it only drives the one or pair of integrated speakers? My first guess would be it blew one of the output transistors on the heatsink.

[kc8adu]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

those caps near the resistors are cooked.they should be yellow like the others.
shorted diodes,shorted transistors on the heatsink are the top suspects.

[ajsemtb]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

Well, I haven't taken any readings yet, but I'm going for the RetiredCaps' bonus point!!!! My meter is a Commercial Electric HDM4400 from Home Depot, which I got for $20 because it was misshelved and I argued false advertising... LOL (It was supposed to be $50, but there were a LOT of them on the hangers with the other price. The associate had no problem giving it to me for that price...)

My price range is at or less than $100 for a meter. I would ultimately like to be at around $40, but as much as I do this, it's time for me to get the right tools for the job. I would like an ESR meter, recommendations welcome. Does an ESR meter also check capacitance? (shows what I know) Also, I was looking at some, and can you use some meters to check components while still attached to the board? That would save some serious time there!!!

I will try to get the part #s off the diodes tomorrow. I will pull off the transistors from the heat sink and get the #s from those as well.

And you said there should be 2 #s you get from the diode test? is that running the contacts one way then the other?

Thanks, as always, you guys rock!

AJ

[ajsemtb]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

OK, the diodes that 999999999 was asking about: There's an "M" in a circle, then a "K"
next line, 1N
next line, 53
next line, 53B
next line, (maybe) 229

And KC8AdU, I can confirm that those are supposed to be the same yellow, the markings on them are the same. 100uf 35V Q192L

And yes, its 1 channel, to a 15" speaker and a piezo tweeter or something like that.

there are 2 transistors that I see on the heat sink, maybe a third tiny something or other w/ 3 leads, but I don't know what that is.

Ok, I think I answered all questions. Except for readings. I'll get those tomorrow.

[retiredcaps]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

My meter is a Commercial Electric HDM4400 from Home Depot, which I got for $20 because it was misshelved and I argued false advertising... LOL (It was supposed to be $50, but there were a LOT of them on the hangers with the other price. The associate had no problem giving it to me for that price...)

One retiredcaps' bonus point awarded.

I'm not sure who the ODM iss, but Sperry's DM-4400 is identical. See

http://www.ebay.com/itm/200687691351

I almost got this multimeter off ebay for 99 cents (plus 5 other assorted brands) from one pawn shop seller, but the seller pulled out of all the auctions with 12 hours left to go claiming the items were no longer available. All my bids were canceled.

Sure enough, one week later, he had them all for auction again but this time with reserve prices. I should have reported him.

My price range is at or less than $100 for a meter. I would ultimately like to be at around $40, but as much as I do this, it's time for me to get the right tools for the job.

An ESR meter will cost around $65+. That only leaves $35 for another multimeter. If you don't mind buying used, I suggest getting an used Fluke off ebay. Let me think about which one you can realistically get in the $40 price range that will measure capacitance.

I would like an ESR meter, recommendations welcome. Does an ESR meter also check capacitance? (shows what I know)

Some will. I don't own one, but the Test equipment thread has had many discussions regarding this very topic.

https://www.badcaps.net/forum/forumdisplay.php?f=33

Also, I was looking at some, and can you use some meters to check components while still attached to the board? That would save some serious time there!!!

The very same threads will tell you that it is with technical reasons why.

[ajsemtb]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

http://www.ebay.com/itm/Peak-Atlas-E...ht_2194wt_1270

Anybody use this or have any feed back on it?

[mariushm]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

It's ok but expensive. I think this Atlas meter was designed (or based at least on work) by Bob Parker, the same guy who's currently selling the BlueESR meter here:

http://www.anatekcorp.com/blueesr.htm

My suggestion is to get in touch with RusMike here on this forum, he's making the ESR micro which also measures capacitance:

http://www.radiodevices.info/esr/esr4.htm

I have one and it's a good device.

So yeah... something overheated there, maybe the large resistor, maybe the diodes... check for shorted diodes. If the resistor is overheated, then it may be caused by a transistor on the large heatsink.. follow the traces...

[999999999]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

Keep in mind that while there is a darkened area, it may not be the fault that blew the fuse, might just have been a bit hot running the whole time, though certainly the caps that look like toast should be replaced.

Diodes are dirt cheap, you can just clip those off and put in new ones for a few cents cost. The transistors may or may not be easy to find and if not then you'll want to replace both with modern equivalents. Pretty sure the resistor isn't the heat source, the traces where its leads are soldered would be the darkest area instead of off to the side. One of the top down pics shows this fairly well,

https://www.badcaps.net/forum/attach...2&d=1338160327

[ajsemtb]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

What are the big long white things?

[mariushm]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

They're wirewound resistors... basically resistors that can handle 3, 5 watts or even more through them. The normal small ones can handle 0.125w - 0.25w

example:

http://www.digikey.com/product-detai...2R0-ND/1740836
http://www.digikey.com/product-detai...0W-10-ND/18768

[999999999]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

^ wire-wound power resistors. They should have markings on the face what values they are, but since they are upside down on the PCB we can't read the markings.

[budm]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

The two power transistors mounted on the heat sink with screws are the power output transistors, more likely to be complimentary pair (NPN/PNP) direct coupled output set up so it needs bi-polar power supply set up (Class A/B amplifier). The little transistor is part of the output transistor biasing circuit, it is also mounted on the heat sink so it can sense the temperature of the heat sink, as the heat sink gets hotter (the output transistors get hot), it will reduce the bias current of the output transistors to protect them from thermal runaway (the black small black trim pot close by is for Bias current adjust, it should be set so the output transistors will draw about 20~30 mA at idling). The small transistor with a small heat sink is the Phase splitter driver for the output transistors. The two large resistors by the heat sink are for the Emitter of the output transistors, the other by the other corner of the board are for voltage drop resistors for dropping the main supply voltages (around +/-35Vdc) down to abiut +/-12~15Vdc for the OP-Amps, there should be two Zener Diodes in that section.
I need better pictures of the bottom side of the board where the two output transistors are mounted so I can tell you which pins to check for shorts on the transistors (if they are shorted).
And just to be sure that we do not have shorted out power transformer, you should disconnect the two red quick connectors (they are the secondary winding of the transformer, there is another wire for the center tap of the transformer, it looks like the green wire) from the board and see if it still blow the fuse.
If it does not blow the fuse, then also measure the AC voltage on those two red connectors so we will know what DC voltage this thing is running at.

[budm]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

Here are the test points for you to check.

[ajsemtb]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

Ok, all this testing was done w/ the diodes in circuit and transistors in circuit. I didn't desolder anything at this time. If that is bad, or would give wrong readings, shame on me and I'll redo my tests...

Bottom line, 2 of the 4 diodes that are in the bridge rectifier that RetiredCaps' described are 0'd out. So, i'm assuming those are toast. The other 2 read .755V and .458V (one way then the other, they both read this same amount).

The 2 by the burned section of the board, that 99999999 described as zener diodes read .785v/.523V and .487V/.522V respectively.

For BudM the transistors, again in circuit, read 2.4K ohms between E and C that you described on your photo. The other one read 8.35K ohm on the same points if the E C and B are in the same place.

Please let me know if I did this right, and if I need to remove them from circuit to get a correct(er) reading.

My course of action for sure so far is to replace the 2 burned up capacitors, and probably the matching pair just for fun, and the 4 diodes that make up the bridge rectifier.

I completely removed all wires and such and have the board completely isolated from the case, so at this time I can't check the voltage to the transformer, sorry BudM.

[budm]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

OK, I would just lift one leg of the Diodes that are tested bad to verify (when you say 0'd, you mean ZERO Ohm, right?) and take the reading again. The output transistors are OK.
The Zener should read like Diode (conduct one way only), but it does not mean it is good since the Zener Diode are use in reverse bias mode. At this point I would concentrate on the Bad Diodes, if one or two are bad, I would replace all 4 at the same time.
By the way, what is the part number of the Zener?

[ajsemtb]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

on Ohm setting, they are 0.2 ohms or 0.1. On the diode setting they are 0v. the others read voltage across them. ?

[ajsemtb]

Re: Crate KX-80 Keyboard Amp fuse pops immediately

I don't know if I'm looking at the correct thing, but There's an "M" in a circle, then a "K"
next line, 1N
next line, 53
next line, 53B
next line, (maybe) 229