KB3 humming

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yarek
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KB3 humming

Post by yarek » Mon Jul 18, 2016 2:06 pm

Hi guys,

I have the KB3 amp for my keyboards and microphone and it recently got fried by my bandmate. While setting up for our show I asked him for the monitor signal and he gave it to me (I normally plug it into the 'monitor in'). But he gave me a powered output, not line-level. So yeah, I could smell some smoke coming out and then the amp started giving a constant hum (~60Hz).

I took it apart and measured a bunch of elements on the board and as far as I can tell, whatever I tried seem to be fine (resistances and capacitances are all reasonable). Nothing looks burnt or otherwise damaged. I checked the output of the transformer and it seems to be fine (60V on one (i.e. -30 - 0 -+30), and I think 38V (+/-19V) on the other? something like that). The polypropylene capacitor 100nF/275V is in good shape, as are the large capacitors on the board next to the transformer.

Last piece of information: there is no signal coming out to the speaker (other than the hum) when I plug stuff into channels 1, 2, 3 or 4 (monitor). When I plug my headset in, I can get the instrument signal from channels 1 and 2 just fine, but channel 3 breaks up (sound is distorted), and channel 4 doesn't send anything to the headset.

Please help!!!

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Enzo
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Re: KB3 humming

Post by Enzo » Tue Jul 19, 2016 5:31 pm

Have you the skills and equipment to identify and replace parts on the boards?

Loud hum on the speaker? It is usually one of two things in the power amp or a preamp problem: DC on the speaker (which can damage it) or a loss of filtration in the amp. When you turn it on and get hum, does the speaker cone move one direction and stay there? That is DC and turn it off immediately. Better to disconnect the speaker and see if DC is present on the speaker wires. With a loss of filtration, there will still usually be sound, just with lots of hum added. Preamp problems can be a lot of things considering your story.

You story wants me to do this. Plug a cord into each jack around the amp, just one at a time. At the free end of that cord, measure for DC between tip and sleeve. If you get 15v anywhere, there is a problem near that jack. Most likely a shorted protective diode. PV likes to protect inputs and outputs with small clamping diodes to the 15v rails. You may have killed one.

Those diodes are there to protect the op amps, but the trauma you described might mean an op amp was damaged.

yarek
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Re: KB3 humming

Post by yarek » Wed Jul 20, 2016 1:35 am

well, I was never trained in electrical stuff but I went through a lot of schooling so I understand enough physics to make general sense of a simple board like the amp. I just don't know enough to figure it all out by myself.

I have a couple of Fluke multi meters that can test resistors, capacitors and diodes, measure AC (both amplitude and frequency) and DC voltages. And I have a soldering iron. Luckily those elements are not surface mounts, so I can do something about them.

When I first tested the board I thought I found the suspect when I measured ~4Ohm across the 100nF capacitor that seems to be filtering the input or output of the transformer. But when I unsoldered it, it appeared to be spot on, so that't not it.

I will check for DC, and what is present in the jacks. I happen to have a whole baggie of 1A diodes that I bought for making/installing LED turn signals on my motorcycle. They look similar to what I could see on the board. Will check if they are the same, and whether or not anything needs to be replaced.

Thx for the tips and I'll post more questions when I check the things you pointed out here. Really appreciate it!

yarek
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Re: KB3 humming

Post by yarek » Fri Jul 22, 2016 5:20 pm

OK, finally got to taking it out again. There is indeed 41.7V of DC voltage on the output from the board to the speaker. It is also present in the 'External Speaker' output (I know it's in parallel with the internal speaker). No voltage present on any of the inputs or headphone output.

Where do I take it from here?

yarek
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Re: KB3 humming

Post by yarek » Fri Jul 22, 2016 5:46 pm

I checked the impedance of the speaker and it's 7.5 Ohm. Not sure how I can test if it plays OK. I have another amp, but it doesn't give powered outputs (+4dBu is the strongest signal it sends out) so for now I'll assume the speaker is still fine.

So, apart from the 41.7 DC voltage coming out of the board, I found another culprit. There are two sections of diodes and resistors on the board, near the radiator, and then another two sections that have diodes, resistors and capacitors. When I have the speaker unplugged and the board is powered (110VAC and it's ON), three sections stay at about 85F (I used a laser pyrometer to measure the temperature), while one of the sections heats up to 140F. And I think the element R109 actually looks burnt. I didn't spot it earlier because it's so close to R25, a large block resistor. Any hints at whether or not this could be it? and where I can find a diagram that will tell me what these elements are? (i.e. nominal values).

I measured the elements in the two similarly looking sections. Exact numbers below. In general, discounting for elements inherent precision (or imprecision), the only elements that do not correspond to each other (while I think they should) are:

- D21 and D22. D21 seems to be burnt while D22 measures 1.114V across
- D28 and D27. When measuring D28 my multimeter went out of range, while on D27 I measured 1.403V.
- both R109 and R108 measured 292Ohm, but R109 looks a little burnt.

Otherwise things look OK.

section1:
R25: 0.35Ohm
R109: 292. Ohm
D21: 0 V <= measures 0V in either direction
R20: 4.8 Ohm
R41: 22 Ohm
D28: >2.5 V
R2: 67 Ohm
D10: 1.9 V


section2:
D22: 1.11 V
R27: 0.35 Ohm
R108: 292. Ohm
D9: 1.89 V
R1: 67 Ohm
D27: 1.4 V
R40: 22 Ohm
R19: 4.6 Ohm



Very last bit of information: with the board unplugged and the switch in ON position I measured the resistance between posts P8 and P13 (output to the internal speaker). Measured 75kOhm in one direction, and -75kOhm in the other direction :-) And 3.3mF (3300uF) in either direction. With the diode setting it measured 0.5V in one, and 1.6V in the other direction.

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Enzo
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Re: KB3 humming

Post by Enzo » Fri Jul 22, 2016 7:20 pm

The speaker. You can test to see if the speaker functions by briefly touching a 9v battery to its terminals. A working speaker will pop or thump with each touch. That won't tell you how good the speaker sounds, but it will tell you if it works.

Your amp makes DC, so Q6 and Q10 are immediately suspects for shorts. And that means Q8 and Q11 are also suspects. Note D21 and D22 are in parallel with output transistors, so it is WAY more likely that Q6 is shorted rather than D21.

yarek
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Re: KB3 humming

Post by yarek » Fri Jul 22, 2016 8:55 pm

Enzo, thank you for your reply.

Transistors are something I'm not very familiar with. I mean I know the physics behind, but I don't know what should I measure, and where? And what the outcome should be.

Will test the speaker. For sure!

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Enzo
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Re: KB3 humming

Post by Enzo » Fri Jul 22, 2016 10:38 pm

Can you read the schematic? Transistors have three legs, no two should be shorted together. I mention the schematic because in some places ther is a low value resistor between two legs, so you will get a low reading, but in particular, the emitter to collector should never measure low resistance.

For example Q6 and Q10, the main outputs, each have only about 22 ohms between base and emitter.

If you have a modern meter, it should have a "diode test" function. You can use that to do rudimentary tests of transistors. Just google "diode test for transistors" and you will find a number of tutorials on that process.

yarek
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Re: KB3 humming

Post by yarek » Wed Jul 27, 2016 1:32 am

I finally found another bit of time to look into it.
Connecting the speaker to a 9V battery causes some sound to come out, to my taste it should be fine.

Regarding the board, the Q6 and Q10 transistors don't give me the same measurements. But if you're saying that what I measure between the nodes is affected by other elements (e.g. resistors, capacitors and diodes that may be connected in parallel to those nodes), then that doesn't tell me what's wrong: the transistor, vs. the other junk. I guess the only way to find out is to unsolder those elements and measure them free-standing. I only have a simple soldering iron, but I guess I can do this. Have done that with the 100nF capacitor on the power line, I can do it with the other things. It just won't look as neat when I'm done with it ;-)

I won't have the time to get to it, at least until Thursday evening, so any hints on what to try first, will be appreciated.

PS: I found this on the web: http://www.vetco.net/blog/?p=184

I'm not sure what are the types of those transistors in KB3, so it's not very obvious to me whether or not what I'm measuring makes much sense...

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Enzo
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Re: KB3 humming

Post by Enzo » Wed Jul 27, 2016 9:36 pm

I am at first concerned that transistors are shorted - as in almost zero ohms between two or more legs. Yes, external junk can make the transistor APPEAR shorted when it is not. However there is no parallel junk in this world that will make a shorted transistor appear not to be. So you measure between two legs of a transistor right where it sits. If it appears shorted, then it has to come out. We then retest it to find it it was the transistor itself or the parallel junk. If it does not check shorted in the circuit, we can leave it there, because it is not shorted. It may have other problems, but at least shorting isn;t one of them.

Q6 and Q10 are opposite polarity types, PNP versus NPN, so they will read different for that. But if we start with emitter to collector, neither one should show as shorted. Base to emitter of each ought to show about 22 ohms, and base to collector ought not show as shorted.

The schematic calls out the Q6 as 2SA1302, and the Q10 as 2SC3281. That is a fairly common pair of high power transistors in audio.

yarek
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Re: KB3 humming

Post by yarek » Thu Jul 28, 2016 9:54 am

gotcha.

I'll give it another shot during my lunch break (working from home is sweet sometimes).

I think I do have silver paste to re-mount the transistors on the radiator if need be. Haven't looked that well on how to approach this. Will give it a shot at noon.

Thx Enzo!

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Enzo
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Re: KB3 humming

Post by Enzo » Fri Jul 29, 2016 1:42 am

WHOA...

Check your silver paste with an ohm meter to see if it is electrically conductive. The white silicone grease does not conduct, and we do not want the collectors of these transistors conducting to ground (the heatsink is grounded). Some silver greases are conductive.

yarek
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Re: KB3 humming

Post by yarek » Sat Jul 30, 2016 12:38 am

well, ran out of time, will give it a shot tomorrow. I think I will first try to unsolder some of the elements on the board and test those. They are simpler to take out, and solder in again. If they check out fine, then I'll know for sure it's the transistors.

and yeah, didn't think about the electronic conductivity hurting it. I have a bunch of stuff, I bet I can find something that will allow good thermal contact, without conducting the electricity.

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Re: KB3 humming

Post by yarek » Sat Jul 30, 2016 3:30 pm

I unsoldered D21 and now both transistors give reasonable readings, so I'm pretty sure it's D21. Where can I find the specs for that element?

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Enzo
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Re: KB3 humming

Post by Enzo » Sat Jul 30, 2016 10:08 pm

That would be a very unusual failure, but any part can fail. How does that part test on its own?

The specs? Right on the schematic or right on the part. It is a 1N5402. Also could use MR502, either way a 200v 3 amp plain old rectifier.

Peavey part number is 70400502, you could buy it from them, or order from any electronics parts supplier, like Mouser.

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