Hello, I bought a Luxman LV-103u with a blown K405/J115 transistors on one channel. Long story short, the seller bought new K1529/J200 replacement transistors and told me that the power delivery capacitors probably dried out. I replaced them all with new ones and I'm currently trying to get this thing to work with new transistors on one channel. After swithing it on, the control part works well with LEDs in the front, swiching to different sources works etc. but I can't get any sound. I have replaced all blown fuses with new ones.
One thing I've noticed is a MPC725 resistor between the transistors that's used when measuring voltage for adjusting the bias. It's supposed to be 0,22Ohms but seems to be an open circuit, both when measuring between the two outer pins and between the inner and outer. I therefore haven't been able to measure any voltage between the outer pins and I'm assuming that it needs to be replaced. The same is true for the resistor on the other side altough without the transistors. First of I'd like to ask if I'm right about this and, if that's the case, do I need to replace the resistor with the same one (which I can get on ebay for ~10 EUR) or if there is some cheaper, perhaps more modern alternative.
I'd also like to ask if there is anything else I should think about or test.
I'm aware that I lack the experience or knowledge to figure out by myself what's wrong since the only thing I've done before is replacing caps in some amps. Either way, I decided I could give this a try even if I fail completely.
One thing I've noticed is a MPC725 resistor between the transistors that's used when measuring voltage for adjusting the bias. It's supposed to be 0,22Ohms but seems to be an open circuit, both when measuring between the two outer pins and between the inner and outer. I therefore haven't been able to measure any voltage between the outer pins and I'm assuming that it needs to be replaced. The same is true for the resistor on the other side altough without the transistors. First of I'd like to ask if I'm right about this and, if that's the case, do I need to replace the resistor with the same one (which I can get on ebay for ~10 EUR) or if there is some cheaper, perhaps more modern alternative.
I'd also like to ask if there is anything else I should think about or test.
I'm aware that I lack the experience or knowledge to figure out by myself what's wrong since the only thing I've done before is replacing caps in some amps. Either way, I decided I could give this a try even if I fail completely.
Emitter resistors often fail open circuit when the output transistors burn out. If you have the same open resistor on the other channel, that means that channel has burned out as well. The failure is usually catastrophic, not just output transistors and emitter resistors. You'll likely need to get a lot of components, including new output transistors for both channels. Problem is, these are near impossible to get, but useless fakes are plentiful. Be wary of this.
If there is any fault in the amp, the protection will cut the speakers off and you won't get any sound. Additionally, if there is any fault in the amp, and you power up, a cascade failure will likely occur, and everything will burn out all over again.
If there is any fault in the amp, the protection will cut the speakers off and you won't get any sound. Additionally, if there is any fault in the amp, and you power up, a cascade failure will likely occur, and everything will burn out all over again.
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There are no shorts in the output transistors that I have (I only have one pair for one channel) although one pin from each transistor was shorted through the emitter resistor before I de-soldered it. I'm thinking about buying these:
4x 0,22+0,22 Ohm Metallband Widerstand 5+5 Watt MPC725 | eBay
and see if the channel will work and if it will I'll start looking for output transistors. I'm guessing they are supposed to be exactly the same and that I'll have to buy two new pairs.
http://www.ebay.com/itm/4x-0-22-0-2...137304?hash=item3f5ad96918:g:OvEAAOSwCypWmop-
4x 0,22+0,22 Ohm Metallband Widerstand 5+5 Watt MPC725 | eBay
and see if the channel will work and if it will I'll start looking for output transistors. I'm guessing they are supposed to be exactly the same and that I'll have to buy two new pairs.
http://www.ebay.com/itm/4x-0-22-0-2...137304?hash=item3f5ad96918:g:OvEAAOSwCypWmop-
Today I got the new emitter resistors and was about to solder them to the board. Before doing that, I turned on the amplifier and heard the protection relay click after a few seconds (the amp was not turned on for a few days). So I turned it on and off and this happened about three times until it stopped. I replaced the emitter resistors and still no bias voltage.
Currently I have no idea what could be wrong. In the service manual, there is a list of expected voltages for the transistors on the board so I checked the output transistors. While the D-pin on both were about right (+/- 51V instead of +/- 49,3V) but the other pins were completely off with about -7V even though the expected was 0V or 2V. But that's probably no surprise considering no bias voltage.
Any ideas on what I could check?
Currently I have no idea what could be wrong. In the service manual, there is a list of expected voltages for the transistors on the board so I checked the output transistors. While the D-pin on both were about right (+/- 51V instead of +/- 49,3V) but the other pins were completely off with about -7V even though the expected was 0V or 2V. But that's probably no surprise considering no bias voltage.
Any ideas on what I could check?
Check the offset of your amplifier. How to Measure DC Offset: 5 Steps (with Pictures) - wikiHow you should have a few millivolt no more.
Also, I'm wondering about the transistor between the output transistors. It is supposed to be a 2SC3423 but was replaced with a NTE374. The original is a NPN transistor while the NTE374 is a PNP, is that correct? The previous owner also sent two NTE373 transistors which are NPN.
Edit: I've also got a pair of the original 2SC3423 transistors.
Edit: I've also got a pair of the original 2SC3423 transistors.
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Looks like it. How does that even work? Well, I guess it doesn't, since you're not getting any bias.
Besides, 160 V / 1.5 A vs. 150 V / 50 mA... with die sizes and current densities being that different, thermal coefficient isn't likely to be particularly close to the original. Which originally was chosen mainly because it conveniently attaches to the heatsink (being in TO-126) and is fast enough not to require a bypass cap.
The least you could do would be putting one of the 373s in if you've got those on hand anyway, doesn't look like they'd be any worse than some BD140s.
EDIT: You do have some 2SC3423s (genuine, I hope)? That's even better, of course.
Well, after I replaced the NTE374 with a 2SC3423, two 100Ohm resistors next to it simply burned up. I'm not sure if I've done anything wrong. It looks like the previous owner might also have burned up those two resistors (and replaced them) since they were of a different color than those on the other channel.
I have not checked all the parts around bus as I've previously written, I'm not getting any bias voltage (or at least I didn't before the resistors from the previous post burned up).
Which are those? Is it the 2SC3423 transistor?
Do you mean with the emitter resistors? I've replaced them and they seem to be fine. My cheap multimeter isn't so good for low resistances like this. It beeps like if it's an open circuit but I can still see the resistance value of ~0,2Ohms or ~0,4Ohms depending on witch pins I measure, after subtracting the resistance of the probes themselves.
I have no idea how to analyse circuits like this and find the cause to the high current that was put through the 100Ohm resistors but I've checked the resistors directly connected to the 2SC3423 (that I just put in) and they seem fine. There is one resistor (R7301) that should be 3k Ohms according to the service manual but is actually a 2,4k (same for both channels). The 380Ohm resistor (R7303) is fine and the potentiometer (VR7301) was around 473Ohms when I checked after de-soldering it.
OK, finally had a look at the schematic of this one. You should do the same (obtained here), as it's not terribly obvious. It's a somewhat unusual concept, there are some drafting errors (Q7105 is drawn as a pnp but actually is npn), the pinout of PCB connectors is not always the same on both ends, and the lousy photocopy quality doesn't exactly help.
So it turns out the FET output driver is preceded by a single pnp emitter follower using a 2SA1381 (Q7109/7110). From its emitter it's going over to the bias circuit using the 2SC3423 (note: minimum bias with 470 ohm trimpot at maximum). It's coming back and going into the emitter resistor, which is the three 9.1k resistors in series, so a good 10 mA given the +/-~97 V regulated supplies. (This to drive fairly big VMOSFETs with 430 pF of Ciss? Seems a bit wimpy. Doing the math, we're talking a good +/- 5 mA at 20 kHz, so it is doable, but linearity would not be all that great at this point. This could be why the distortion spec of this unit isn't good.)
The two smoking 100 ohm resistors are the gate stoppers for the output FETs, and unless one of the protection diodes had a problem or there were oscillation, the FETs are likely to be toast (a FET by definition has just about zero DC gate current). The excess current probably went through the 2SC3423 again. Clearly you inherited whatever the original problem was that caused it to fail in the first place.
I suggest you unsolder the output FETs on the faulty channel for now (good opportunity to check for any shorts) and run, I dunno, a 1 kOhm resistor between the gate and drain connections of one of them - just so we get any kind of connection to the output node at all and the feedback keeps working. (Do it for both if you want the unit to come out of protection.) Then you can measure the bias voltage and adjust it without anything blowing up in your face.
If you find an oscillation problem with a scope, I'd put my bets on either the output Zobel network (the 33 ohm and 0.022µ) or one of the 4.7µ/160V electrolytic bypass caps, as these exist separately for both channels. Finding problems like this can be quite nontrivial though.
So it turns out the FET output driver is preceded by a single pnp emitter follower using a 2SA1381 (Q7109/7110). From its emitter it's going over to the bias circuit using the 2SC3423 (note: minimum bias with 470 ohm trimpot at maximum). It's coming back and going into the emitter resistor, which is the three 9.1k resistors in series, so a good 10 mA given the +/-~97 V regulated supplies. (This to drive fairly big VMOSFETs with 430 pF of Ciss? Seems a bit wimpy. Doing the math, we're talking a good +/- 5 mA at 20 kHz, so it is doable, but linearity would not be all that great at this point. This could be why the distortion spec of this unit isn't good.)
The two smoking 100 ohm resistors are the gate stoppers for the output FETs, and unless one of the protection diodes had a problem or there were oscillation, the FETs are likely to be toast (a FET by definition has just about zero DC gate current). The excess current probably went through the 2SC3423 again. Clearly you inherited whatever the original problem was that caused it to fail in the first place.
I suggest you unsolder the output FETs on the faulty channel for now (good opportunity to check for any shorts) and run, I dunno, a 1 kOhm resistor between the gate and drain connections of one of them - just so we get any kind of connection to the output node at all and the feedback keeps working. (Do it for both if you want the unit to come out of protection.) Then you can measure the bias voltage and adjust it without anything blowing up in your face.
If you find an oscillation problem with a scope, I'd put my bets on either the output Zobel network (the 33 ohm and 0.022µ) or one of the 4.7µ/160V electrolytic bypass caps, as these exist separately for both channels. Finding problems like this can be quite nontrivial though.
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I think I've realised that I'm not capable of repairing this amplifier. I'm thinking about taking it to a repair shop but I'm not sure if it's worth it or if I'll be better off selling it to someone who may want to repair it. Do you guys think it's worth paying for professional repair of this amp?
I've de-soldered the output transistors and one of them is shorted. I feels like a waste of money to buy new ones considering their price and given how easy it is to destroy them like I've done. If I understood what was going on and could prevent damage it would have been another thing. At least I think I've learned a lot.
I've de-soldered the output transistors and one of them is shorted. I feels like a waste of money to buy new ones considering their price and given how easy it is to destroy them like I've done. If I understood what was going on and could prevent damage it would have been another thing. At least I think I've learned a lot.
One thing to learn is that for an amplifier to work properly, ALL parts in it have to be within spec. If even one is not, you have a non-functioning amp, and more likely one which will produce more damage to itself. I.e. if you have a 10 transistor amp with 9 working transistors, you do not get a 90% working amp, but rather 90% possibility of further (catastrophic) damage. Power amps work some of their components almost to the limits, so if something goes wrong, it is very easy for things to get over limits, resulting in very expensive smoke.
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