Luckiestmanalive Posted April 14 Posted April 14 (edited) Hi everyone - I've been running a Luxman tube power amplifier for a couple of years now so I've been looking for a Luxman preamplifier to go with. I placed a cheeky bid on a Japanese auction (100V to match my power amp) and got a Luxman C-03 preamplifier and it arrived last week. Despite the pictures appearing to be clean and unmolested, I was warned the tone controls were hit and miss and I've seen posts that suggest these old beauties will need new caps and extensive glue removal (it turns corrosive). I tried it out and found the front lights weren't going at all and there was only one channel on phono input. On the plus side, the sound from CD source was every bit as good as I could hope for (note to my future self and anyone else buying from an auction, you are rolling the dice, especially if there aren't any good pics of the innards). Here is what greeted me when I took off the top cover: Someone has obviously been here before me and done an extensive recap but there is still some residual glue to clean up. Note the large caps on the left next to the transformer are somewhat fancy but there are cap spaces to the right that are unfilled. Interesting... Also, what the hell is going on in the top right?! That right hand side of the board (on either side of the blue switch lead) is the phono input circuit and there are a six(!) missing capacitors at the beginning of the circuit (2x 6800pf, 2x 47pf, 2x 100pf). I haven't looked underneath the bandages but it seems the pair of inductors on each channel have also been replaced with a single loop of wire?! The previous owner might have been to the school of removing components one at a time until what you have left is only what is necessary?!? Anyway, after a good poke around (mostly with the aid of a can of contact cleaner) I found I could get the lights at the front and the missing channel on the phono input going for a little while before they dropped out again. So I had to bite the bullet, take the front off, and take out the board at the front (see pic above) - the one with the rats nest of grey wire connectors (label each of them as you remove them!). Then I found I could turn the board over and spray it thoroughly with iso-propyl (ie a few times) to remove the stuff they spray their boards with (once you spray it the first time it becomes sticky so you have to remove it completely). Then I let it dry before using my flux marker on all the pins on the board and re-soldering them all. Then I cleaned the board of the flux residue with iso-propyl and reinstalled all the connectors and attached the board back to the front panel (only two screws). Now the lights work reliably and the phono input has two channels! The sound from all the inputs is really lovely so, lucky for me, it turned out to be well worth the effort. Unfortunately, the power button lamp doesn't work now but I will be replacing the missing caps and possibly some others so I will resolder the connection to the mainboard then and remove the last of the glue residue. This is it put back together and playing music: Any tips on restoring this beauty would be much appreciated! Edited April 15 by Luckiestmanalive 6 1
wadiman Posted April 16 Posted April 16 I had one of these a few years back - gave a pretty good account of itself! 1
andyr Posted April 16 Posted April 16 On 14/04/2024 at 6:13 PM, Luckiestmanalive said: Any tips on restoring this beauty would be much appreciated! You're a better man than I am, Gunga Din! 1
mbz Posted April 21 Posted April 21 On 14/04/2024 at 6:13 PM, Luckiestmanalive said: I will be replacing the missing caps Not uncommon for parts to be "missing" for whatever reason. Check the solder pads to determine if parts were ever soldered in. Will add to my shopping list along with a sansui alpha and... 1
Luckiestmanalive Posted April 24 Author Posted April 24 Bad news! I've not posted because I've been sad and busy with troubleshooting. The unit was working perfectly the night I 'fixed' it. Actually, it was not so perfect because a couple of times one channel made a loud but brief static noise, go quiet very briefly, then return to normal. I didn't think much of it given it was only a couple of times in a session over an hour long. The next day, I turned it on and although everything seemed to be the same, there was no sound out of either channel. The loud, brief static issue suggested to me it is a faulty, intermittent connection issue. However, I took some DC voltage measurements and found voltages over a diode around what looks like a protection switch that don't seem right. The same switch is in the phono stage and it clicks in after turn on with less than 1V on one side of its diode and 12V on the other side. On the other switch, both sides read 12V (the schematic confirms it should be 1V and 12V). So now I need to find the source of the problem. I unsoldered the switch and gave it a clean with a piece of paper soaked in contact cleaner. That didn't change anything. The only good news is that I fixed the power light so everything looks like it works!
Luckiestmanalive Posted April 25 Author Posted April 25 Maybe that noise was from a faulty and now stuffed transistor?
mbz Posted April 27 Posted April 27 It would be helpful if you used designations from the schematic rather than referring to some diode and a switch. I'm guessing your talking about D7001 and relay RL7001, RL7001 controls the audio output. The unit uses IC5001(TA7317P) for protection functions, ie, power on mute, excessive Vdc, over current... The fault scenarios are, - failure in either L or R channel, eg, excessive Vdc - failure in the protection circuit, eg, IC5001, RL7001,,,, - failure in power supply to protect circuit. A good place to start is to test for excessive Vdc at R7502 20/5W(R), measure either side to GND, expect less than 50mV. Repeat for other channel R7501(L) 12V is expected on both sides of D7001 when the order to operate RL7001 is not given (due to protection) 1
Luckiestmanalive Posted April 28 Author Posted April 28 Brilliant! Thanks, very much, MBZ! Yes, sorry, I should have specified the components on the schematic and you are right - I have been referring to D7001 and RL7001. I did a little reflowing of some of the dodgier-looking solder joints on the mainboard and spent some of the weekend checking voltages as the fault/switch protection had become intermittent, random and very occasional. So I left it on for a couple of hours last night (playing music through it but without preouts connected) to see if it would get better or worse over time. I would go and check the voltage across D7001 every 30 min or so to see if it was still working or not. I turned it off when I went to bed last night and voltage at D7001 was fine. I switched it on again this morning and then read your post so I went back to the unit to do some more voltage testing. Unfortunately, I found the heatsinks for IC5002 and Q1005 and Q1006 were very hot and while I was feeling one for the second time (heatsink for Q1006) there was a spark and a crack between Q1004 and Q1006 so I immediately switched it off. Inspecting the damage, it appears the back of transistor Q1003 is missing (occurred earlier while I was out of the room?), while Q1004 is undamaged! I'm gutted. I had checked resistor values and DC voltages in this section before I started soak testing going but couldn't see anything out of spec. They seemed perfectly symmetrical and as expected from voltages in the schematic. My parts order from Digikey has just expanded - I have sourced replacement 2SA1015s and 2SC1815s. I will also install the missing capacitors C1003 and C1004 and recheck every component in this section of the board before I switch it back on.
Luckiestmanalive Posted April 28 Author Posted April 28 (edited) Checking components in the immediate vicinity, ZD1001 has failed while ZD1002 seems to measure OK. I think I will replace them both so will have to find a suitable replacement for HZ6A3Ls - any advice? At the risk of trying to make the evidence fit the crime, perhaps the missing C1003-1004 caps, which are in parallel with ZD1003-1004, have stressed the circuit and zener diodes are the weakest components most prone to failure? Maybe replacing them and the capacitors will fix the unit!? I've checked Q1005-1006 and they at least measure OK. Cheers Greg Edited April 28 by Luckiestmanalive
SonicArt Posted April 28 Posted April 28 (edited) just on your missing cap thing, could be the original used smaller bypass caps across the original main supply caps on the left, you wouldn't necessarily use bypass with the Nichicon FG caps so they left them out. Personally I wouldn't use FG as main supply caps, their ripple current ratings are not high enough and they can make things sound a bit soft. Panasonic FC or FK would be my first choice in quality affordable caps, or the nichicon eqv', I usually use the FG after regulation. Edited April 28 by SonicArt 1
Luckiestmanalive Posted April 29 Author Posted April 29 That's well appreciated advice, thanks, SA. The 3300uf/50V Nichicon FG caps are the main filter caps after the bridge diode rectification stage. On your advice, I might add replacements for for them to my order - I also might be able to find more size-appropriate replacements as the leads for these ones are narrow and make the caps sit over a centimetre above the board. The missing caps (C1003-1004) are supposed to be 22uf/100V and they don't appear to be connected in a way that suggest they are bypass caps for the main filter caps C1001-1002. They are connected between ground and the output(?) of transistors Q1013-1014, though, so they would help further smooth the voltage ripple?
Luckiestmanalive Posted April 29 Author Posted April 29 (edited) Here is the block of the schematic (power supply and regulation) with the offending parts and pic of the board itself, including the blown transistor (caution: it contains damaged components so please turn away if you are squeamish): Edited April 29 by Luckiestmanalive
mbz Posted April 29 Posted April 29 12 hours ago, Luckiestmanalive said: have to find a suitable replacement for HZ6A3Ls - any advice? Only had a quick look, NZX5V6C appears to be a good match. The HZ6A3 are 500mW 5.4-5.7V 12 hours ago, Luckiestmanalive said: perhaps the missing C1003-1004 caps, which are in parallel with ZD1003-1004, have stressed the circuit and zener diodes are the weakest components most prone to failure? The caps provide additional smoothing. With caps missing I'd be a little concerned about stress on the zeners at power ON/OFF, these transients are not very "graceful" and the zener could be exposed to some abnormally high voltages for a short time. Once the amp is powered up the zener would do it's job but the voltage would be up and down a little due to missing cap, don't see that aspect causing the failure. Looking closer at Q1003,4 these appear to be voltage regulation for +/-22.5V rails where as the zeners/missing cap is more about getting a stable voltage into the diff pair Q1013, Q1014. A long shot would be some sort of oscillation/pulsing in the power supply because Q1013,4 base voltages are unstable. Suggest replace all failed/suspect parts then DBT power up. 1
Luckiestmanalive Posted April 30 Author Posted April 30 (edited) Thanks, MBZ - I wonder if the ungraceful ons and offs and heat have stressed the zeners and transistors so I will replace at least the zeners. What makes things even more interesting is that if you look to the far right of the pic above, you'll see that half of the bridge rectifiers (D1001-1002) for each rail are missing (the other two are there)! This means it is only half wave rectification! I think I will replace those diodes, too... Edited April 30 by Luckiestmanalive
mbz Posted April 30 Posted April 30 1 hour ago, Luckiestmanalive said: This means it is only half rectification! I think I will replace those diodes, too... Having a "blonde moment". Was going to "warn" that the DC voltage after smoothing from a full wave will be higher than the half wave case, so you need to check if this higher voltage can be tolerated downstream... Thinking about it, the voltage will be the same however the ripple will be higher for the halfwave since it has half the recharge rate. Seems odd that Luxman skimped on 2x5cent parts (bulk purchase) and accepted the increase ripple/noise....
Luckiestmanalive Posted April 30 Author Posted April 30 Yeah, I agree. I don't think it was a Luxman move. Perhaps, the guy who did the recap found one of the diode pairs was faulty and removed both, gave it a listen and liked what he heard so he didn't replace them?
Luckiestmanalive Posted October 26 Author Posted October 26 Hi folks - I know it has been a while but we moved house and everything DIY audio had to be shelved for a while. Now I'm back to it I've had to reacquaint myself of the problem - never a bad thing in this situation. I've copied the bit of the schematic that is relevant and highlighted in green the parts I've replaced: Dead BJT (Q1003) that exploded and split in half (see photo above) Zener diodes (ZD1001-1002) because ZD1002 was showing as a resistor on my T7 multi-function tester. The failure of this diode might have caused the BJT to fail? The big filter caps (C1001-1002) under advisement - the new Nichicon replacements (UVR1J332MRD6) fit perfectly and are more appropriate for the job. The parts highlighted in red in the schematic are not present on my board: Two double diode T220 packages (D1002-1003) The two second stage power supply filter caps (C1003-1004) Two 3.9Kohm resistors (R1007-1008) that are supposed to be in parallel with two 560ohm resistors (R1005-1006). The effect of this absence is that the resistance is 560ohm instead of 490ohm. Maybe not a big deal? I have the first two components but not the last but I will be ordering those resistors. Which of these three would you do? All three, right?
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