I've not taken measurements while it's switched on before, besides when doing the idle current adjustment, and I'm not sure where's safe to put the probes without potentially causing more damage. The twin PSUs are kicking out about 33v, and the relay clicks as normal so I suppose the voltages can't be too crazy. I might try taking the FK smoothing caps out and put the original Elna caps back in there temporarily, just to eliminate any doubts about the leads. They were the last thing I changed.
While I'm on the subject of those caps, I was playing about with a circuit simulator looking at the smoothing effects of different values there. The design calls for 12,000uF but my FKs are only 10,000uF, and 22,000uF seems like the sweet spot for low ripple. If I do end up fixing this and later decide to increase the value there, where do you guys think would be the most effective place to put those FKs in my system? (CD63 MkII KI included).
Hmm, first place that springs to mind is smoothing for the output stage, +-20V in the CD63 - if they'll fit. I guess they won't so they'd have to go on leads or something. I don't know what else you could do with them.
R764 (330ohm/2w) went to the great circuit in the sky with an impressive fireworks display. Now, if I measure its resistance with my DMM, the value gradually climbs and then goes infinite. Relay no longer clicks, of course.
I tried measuring the voltage across it very for a couple of seconds with the power on and the reading climbed past 100v (really? seems like a lot) and the resistor lit up like a bulb. I suppose R764 has taken a bullet because of a fault elsewhere.
I tried measuring the voltage across it very for a couple of seconds with the power on and the reading climbed past 100v (really? seems like a lot) and the resistor lit up like a bulb. I suppose R764 has taken a bullet because of a fault elsewhere.
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I replaced the resistor, swapped the original smoothing caps back in but now the fuse blows immediately instead :-/
I really miss listening to music.
I really miss listening to music.
I think it's fried, and I'm concerned about pursuing it further in case I blow the thermal fuses in my PSUs. I've got a main board from a spares/repair amp coming, and I've found a local parts supplier. I've swapped out my FKs and tested them and they seem fine, although the left one was holding a worrying negative charge when I first pulled it. I think when I get this new board, assuming there's nothing wrong with it, this time I'm going to concentrate on getting the small things right first, in more than one sense. I'm going to start with precision trimpots, small caps and resistors, keeping the leads short so I can get the big caps closer to the PCB.
While I wait for a new board, I'm building a Lightspeed LDR attenuator kit from Uriah Dailey at BuildAnAmp.com to try instead of the standard Alps pot. It seems to be working perfectly so far except the resistance at min volume is only measuring 12.5k per channel, but if I can get that to up to 50k then my hope is that I will be able to install it directly at the main PCB input and run a very short signal wire down from the input selection switch.
I heard the Lightspeed attenuator once compared to two other preamps and it seemed to produce a much bigger soundstage. Very interesting volume control.
Great! Well that's exciting. Lots of people seem to rave about it. I think it's supposed to add some low order harmonic distortion. If my DMM readings are to be believed then the channel matching should be better than my Alps at normal listening volume too, which should help with imaging. I think I got the wrong end of the stick about the resistance when I posted earlier, and the Lightspeed board is probably good to go in as it is.
I tinkered some more with my broken amp and found that the transistors have failed, which explains why the fuse has been blowing immediately on power up. Now I'm concerned for my 805s. I hope the output protection circuitry saved them from a dose of DC.
As it happens, since I removed my DC blocking caps, my CD63 appears to have a DC offset of ~-16mv in the right channel and ~60mv in the left. I thought I had measured it, but I did it while it was paused rather than playing. I wonder if the left channel offset contributed to the demise of my amp, as it was the left channel that I was initially having problems with. I thought the BG AC coupling caps should've protected it though... :-/
It may have been the cause! The amp should have protected your speakers and they will be ok.
Brent
Brent
Hi Brent! I think you're probably right - I tested the impedance across the speaker binding posts and they both read exactly 4.7 Ohms, which seems reasonable. Phew!
I spent the evening putting all the original components back in, removed the second PSU and replaced the transistors and it still fries itself immediately on power-up. I think I've run out of ideas now, so I'm going to cut my losses with this board and wait for the replacement.
Cheers,
Ben
I spent the evening putting all the original components back in, removed the second PSU and replaced the transistors and it still fries itself immediately on power-up. I think I've run out of ideas now, so I'm going to cut my losses with this board and wait for the replacement.
Cheers,
Ben
It sounds like your woofers are ok and the tweeters are protected from DC by a series cap anyway.
Simon
Simon
Thanks Simon, I felt physically sick at the idea of frying them. I did wonder why the tweeters measured completely open, but I got the same result with my trusty old 601-S2s (their woofers don't match so well, mind you).
If you're measuring DC resistance, at the binding posts, it could measure open and be normal. If those tweeters do get broken a dealer will be able to get you replacement voice coil / dome assemblies. I've replaced one on a 602 s2 and I'd hazard a guess it's a similar procedure on a higher end B&W.
Yep, I had to do my 601 tweeters 3 times due to the pokey fingers of visiting toddlers. The 800 series parts are a LOT more expensive to replace, but at least they're not the diamonds. I think they'll be fine. I miss listening to them - I need to work on that time machine.
You need to rebuild the whole channel. All those small transistors. They usually grenade when the output goes bang.
Brent
Brent
I've just done the above mod of bypassing the PS01 board which has made a dramatic difference to the clarity of the sound, however I can hear a low frequency bumping when turning the vol pot. Any ideas, something I've missed, or is there a better way to do this mod?
Thanks in advance,
Adam 🙂
Have you removed all three ribbons completely? I had no problem wiring just L,R,LG,RG directly from the out pins on the input select switch to the pins of the pot and then down to the main PCB with Kimber TCSS twist, skipping the PCB and ground, but I got all kinds of noise and hum if I used the ribbon.