K100-7 no sound [message #11005] |
Tue, 05 May 2009 20:38 ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
spforster
Messages: 8 Registered: May 2009
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Junior Member |
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Hi, I'm trying to repair a k100-7 for a buddy. Unfortunately, things seem to have gone from bad to worse with my attempts. When I started out with it, I could at least get sound out of the left channel. I traced down a bad transistor in the right channel, replaced it, and when I tried it out, I still had no sound on the right channel, and the left channel was very distorted and at low volume. So then I started messing with the power amp board, trying to test some components, and snapped off the thermal bias diode (duh!). I've replaced that with a 1N4148 small signal diode and now - no sound at all! I get a thump out of the speakers when I turn it on, and that's it.
After checking out just about everything on the power amp board, I can't find a problem. Now, I'll admit that I have much more experience repairing tube amplifiers, but I think that I'm testing the silicon properly (using a DMM with a diode test function).
Any ideas? I suppose if anyone has a schematic for the power amp board with voltage test points on it that might help me get somewhere. Any suggestions on where I should start? TIA for your help!
Best,
Shane
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Re: K100-7 no sound [message #11010 is a reply to message #11005] |
Wed, 06 May 2009 15:32 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
spforster
Messages: 8 Registered: May 2009
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Junior Member |
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Thanks for the reply stevem! The amp is a 2 channel guitar combo amp. I made a few more measurements:
First, there's no DCV present at the speaker jack.
I measured -37.4VDC and +38.1VDC at the big screw terminal filter caps, WRT ground.
The preamp boards are getting +7.7VDC and -7.6VDC, does this seem too low? With the preamps out of circuit I get the same voltage readings.
I don't have a signal generator, but I scoped about 2V P-P waveform on the blue output wire when I play an A on my guitar, volume all the way up on the left channel, and there's no output on the right channel with the same test.
I also unhooked the transformer from the rectifier and measured 59.4VAC on it's output. With the xformer hooked up to the rectifier, but with the output of the rect. unhooked, I get 53.0VDC
I have a schematic for the PC1100 right channel preamp board, but not for the PC1000 left channel board or for the PC900 output board.
It sounds like the voltage to the preamps is too low? The imbalance in the DC supply (-37.4 vs. +38.1VDC) seems problematic too? I'm sorry for my ignorance as to the fundamentals of how solid state amps work, but where do you think I should go from here? Also, if I'm going to inject a signal into the output board, what kind of amplitude are we talking about?
Thanks for your help!
Shane
[Updated on: Wed, 06 May 2009 16:04] Report message to a moderator
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Re: K100-7 no sound [message #11011 is a reply to message #11010] |
Wed, 06 May 2009 18:13 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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Welcome to the board Shane.
spforster wrote on Wed, 06 May 2009 14:32 | First, there's no DCV present at the speaker jack.
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That's good, but I think if there was any to start with you'd have heard a hum.
spforster wrote on Wed, 06 May 2009 14:32 | I measured -37.4VDC and +38.1VDC at the big screw terminal filter caps, WRT ground.
The preamp boards are getting +7.7VDC and -7.6VDC, does this seem too low?
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These voltages all seem close enough to work correctly.
Troubleshooting a Solid State amp is no different than working on a tube amp. You need to isolate the problem to a specific part of the amp and then fix that circuit. I always start with the power supply, then check the power amp and then the pre-amps. Your power supply is working well enough for now, so I'd suggest checking the power amp next.
Try taking your guitar signal and jumper it to the input of the power amp. Or just touch your finger to the input cap, do you hear a hum through the speaker? You just want to see if it is passing a clean signal, so you could use the output from an iPod or similar device to drive the power amp.
Once you've checked the power amp, move on to each of the pre-amps. My first suggestion would be to check all of the transistors. Then check for off value or open coupling caps in the signal paths.
Hope this helps. Bill
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Re: K100-7 no sound [message #11070 is a reply to message #11069] |
Wed, 20 May 2009 18:10 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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A few questions for you.
How are you checking the caps?
What voltage readings are you getting on the output transistors?
On one of the transistors you should have +38 volts on the collector (the case), on the other you should have -39 volts on the emitter (I think the yellow wire).
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Re: K100-7 no sound [message #11079 is a reply to message #11077] |
Fri, 22 May 2009 11:24 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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What Steve sez is right, isolate the problem and then dive in!
As you may or may not know, with the exception of a few part values almost all Kustom power amps share the same design. Some have more output transistors and a few have circuit tweeks, but the basic circuit is the same from one model to another. The original design dates back to the late fifties and has been used by a lot of guitar amp companies through the years.
Testing the caps with your meter is fine, as long as none of the coupling caps has gone open circuit, you should be okay.
Now that you have a schematic, check voltages as listed there and see how your amp compares. The voltages should be close, but don't stress minor differences. I'd only worry about the left side of the schematic as everything on the right side from Q9 on is the low voltage regulator section.
I never suggest complete shotgun replacement of parts, as it will just as often cause more problems than it cures. Double check anything that you removed and replaced earlier. It's always possible that a solder bridge may have been created or a pc trace could be damaged while doing the earlier work.
Keep us informed as to your progress.
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Re: K100-7 no sound [message #11080 is a reply to message #11005] |
Fri, 22 May 2009 18:20 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
spforster
Messages: 8 Registered: May 2009
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Junior Member |
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Ok, I tried checking out the preamp and power amp sections using the inputs and outputs both chicagobill and stevem suggested and here are my results:
1 Preamps. My testing method was a regular guitar plugged into the input and the output (blue wire) routed to the input of a small bass amp combo. The right channel produced sound, but at an even lower volume than the guitar produced plugged into the bass (testing) amp by itself. The left channel had a very strong signal and was much louder through the bass amp. This pretty much backs up what I was originally reading off my oscilloscope from the beginning and it also tells me that replacing the defective transistor I found on the right channel preamp didn't fix the problem. But, the bright side is that it seems that the left channel preamp is good.
2. Power Amp. I piped both a guitar signal and then the headphone output signal from my ipod into the point where the blue preamp input wires solder into the board. Neither produced an output that I could hear through the speaker. Then, on a whim, I connected my input wire to the other side of the input resistor (R900 or R901 on my schematic, same point) and got a reasonable volume, but heavily distorted output from the speaker.
Not being very knowledgable on the operation of this circuit, but going off of the schematic, I would think that Q900, the first transistor connected to the preamp input, would be suspect at this point? Any other ideas? Thanks guys!
Shane
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Re: K100-7 no sound [message #11081 is a reply to message #11080] |
Sat, 23 May 2009 11:23 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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OK, so now you know that there is a problem with the right pre-amp and with the power amp. I'd work on the power amp first and then get the pre-amp working.
I think the input/mixer resistors should be 47K, what do yours measure?
When you injected the signal into the power amp did you unhook the blue pre-amp wires?
Measure the voltages on the first transistor. There should be somewhere around +6 volts on the collector and +0.7 volts on the base. The emitter is grounded, so there should be no voltage there.
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Re: K100-7 no sound [message #11147 is a reply to message #11005] |
Sat, 06 June 2009 17:52 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
spforster
Messages: 8 Registered: May 2009
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Junior Member |
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Alright, back to work on this one! The input/mixer resistors are 68K and measure out to 69~70K. The tests I did before were with the blue wires disconnected.
This time I've downloaded a 1000Hz test tone into my ipod. I'm injecting this signal at the power amp input (where the blue wires are normally connected - they are disconnected for this test). On the oscope, I get a pretty clean looking sine wave at the speaker output and my DMM reads ~10VAC on the output when I turned it up, with still a little more gain leftover. I do, however, hear some static sounding noise that is always present on the speaker output. Even with everything disconnected from the power amp input (blue wires, test leads), and the amp at idle I still get this static noise.
Also, I tried injecting the 1000hz signal into the left channel preamp (the one that checks out good) and I get a decent looking waveform from the output, as long as I keep the volume down on the ipod. But, when I hook this preamp signal back up to the power amp input, I get almost nothing on the preamp output and the waveform is really badly distorted.
I was able to measure .6VDC on the base of Q900 and 4.3VDC on the collector, which is very close to what my schematic indicates.
So now I'm a bit confused. The left channel preamp works ok by itself, the power amp seems to work ok by itself, but together they hardly work at all? It seems to me that there's still got to be a problem with the power amp, since I'm still getting the noise and there seems to be some attenuation of the input signal (I'm sure my ipod's output is much higher than the guitar amp's preamp output and that's why I can get the higher output from the power amp when I'm driving it with the ipod).
Should I just start checking all of the voltages listed on my schematic for the power amp?
Thanks!
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Re: K100-7 no sound [message #11219 is a reply to message #11005] |
Mon, 15 June 2009 21:53 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
spforster
Messages: 8 Registered: May 2009
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Junior Member |
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Ok, so I'm back on track. With the 1kHz test tone, I tried to replicate your test, stevem. I was able to produce ~.150-.153VAC from my ipod (I assume you meant .153VAC, or 153mVAC), the volume control is a bit touchy, go figure its an iTouch! I really need to get a signal generator. Anyway, I measured 1.33V on the preamp output, with the bass and treble controls half way and volume all the way up. I turned bass, treble, and volume all the way up and measured 5.48V. This test was with the blue wires disconnected. When I soldered the blue wire back in place, manipulating the preamp's volume control I measured up to ~24V on the speaker output (8 ohm, 50W speaker - I don't have a load plate). I wasn't willing to go any higher though there was plenty of headroom.
Plugging a guitar into the amp now I get a reasonably loud/clean tone. The only problem I still have, besides the other non-functional preamp is that there is quite a lot of noise. There is your normal, though relatively loud hiss, which I'm not overly concerned about at this time. But there is also a static noise, that sounds as best I can describe like a light rain falling on a piece of cardboard. That noise is always present, independent of any volume controls, and with the blue wires connected or disconnected, so I assume its a fault of the power amp. Could this maybe be a carbon comp resistor being noisy somewhere in the circuit?
Thanks!
Shane
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Re: K100-7 no sound [message #11230 is a reply to message #11219] |
Fri, 19 June 2009 23:43 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
terminal
Messages: 20 Registered: June 2009 Location: GEORGIA
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Junior Member |
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I have a similar problem. I have a K100-1. it has two preamps. when I got it, the right side (reference from facing the amp from the front). did not work. no volume at all. the left side sounded great, and I played it several times without trouble.
the left side has failed now as well. I get some volume out of the output speaker, but it is very quiet. when I roll the volume all the way up, when it gets to about 8/10 the volume tapers off to silence.
I have a signal generator and an o-scope.
left channel: on the volume pot, I get signal on the "input" outside "lug", but never on the center lug.?? this was the channel that was working. a failed pot? perhaps...
right channel: no signal at the "input" lug at all.,, I don't have a schematic, but I guess its a ceramic cap or a transistor before between the input jack and the volume pot.
in addition, here are voltages I'm seeing at the power transistors:
left transistor (once again, from the front facing the back of the chassis):
red wire: 0vdc
blue wire: -38vdc
yellow wire: -38vdc
right transistor:
red wire: +39vdc
blue wire: 0vdc
yellow wire: 0vdc
how do these voltages look?
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Re: K100-7 no sound [message #15409 is a reply to message #11005] |
Mon, 14 March 2011 17:56 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) ![Go to next message Go to next message](/FUDforum/theme/vk30/images/down.png) |
chicagobill
Messages: 2005 Registered: April 2003
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Senior Member |
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Welcome back!
Failing resistors or caps or transistors can all make noise. If you have a scope, try monitoring the power supply lines and see if there are any visible spikes when you hear the noises. If there are, trace them along the supply busses til you get to the source.
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Re: K100-7 no sound [message #15416 is a reply to message #11005] |
Wed, 16 March 2011 17:54 ![Go to previous message Go to previous message](/FUDforum/theme/vk30/images/up.png) |
stevem
Messages: 4759 Registered: June 2004 Location: NY
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Senior Member |
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At this point Shane I would start by replacing all the transistors in the driver/output stage other than the 2 metal cased drivers and the two outputs and see what noise reduction results you get, since this is no more than 3 bucks in semiconductors its worth just shot gunning it to hopefully be done with it!
If those fail to due the trick then replace both drivers.
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