- 4pcs Infineon IRFP250M 200v 30A MOSFETs
- 4pcs Aavid Thermalloy 4180G aluminium oxide thermal pads
- 2pcs Panasonic ECQ 6.8uF 100v capacitors
- 2pcs Panasonic ECQ 3.3uF 100v capacitors
- 2pcs Nichicon LGU 5600uF 63v capacitors
The Panasonic caps, as can be seen, were siliconed to the bottom of the PCB. Those are most definitely NOT going anywhere, no matter how loud this sub will be run. For the MOSFETs i used some Prolimatech PK-3 thermal paste, simply because i had some.
As a first step, i connected the amplifier to my bench supply, turned the voltage up to max (about 31v or so) and the current limit up to around half an amp. Turned the power on, and after a few seconds of current limiting (while the big caps charged up), it settled down to roughly 100mA idle draw, no smoke, no signs of distress. A promising start, i'd dare say.
One notable (and bothersome) detail was that there was quite a bit of squeaking and (repeated) clicking upon power-down. The protection relay (a Tai-Shing TRDT 15A, 48v coil high-sensitivity SPDT), in series with the outputs, did click on and off as one might expect, but the turn-off click after switching off the mains voltage came strangely late.
In the datasheet of the uPC1237HA protection chip i discovered that pin 4 was the AC detect. Probing it during power-down revealed that it was reaching its switching threshold too late. Fair enough, let's whack in a resistor from pin 4 to ground (conveniently located at pin 5), to form a divider with the 88.7k already in line. The voltage did come down lower and quicker, but it didn't seem to affect the relay switching.
Pin 6 was connected directly to the relay, and during power-up, it crept up to about 2.32v in roughly half a second, and back to 0v on power-off in about the same time. Well that's no good now, is it?
As luck would have it, just a few days ago, while rearranging some of my part-stash, i rediscovered a ziploc with four of these chips that i had bought ages ago for some amplifier projects that never came to be. After removing the chip from the amplifier board (quite tedious, with only solder-wick, on a double-sided board), sure enough, the chips measured differently on the diode-test. After installing the new chip, the switch-off relay click is inaudible over the click of the mains switch.
At long last, this overpriced heap is back to... Actually no, scratch that - this overpriced heap is in a state that it should've left from the drawing board (not even the factory).
Hi, thanks for a helpful blog post! I have the same subwoofer, except for the fact that mine has DB5004 rev02. Sub is about 10 year old, but the amp was changed by the manufacturer during the warranty period. Now after running another 5 years with the new amp, a new problem showed up. At 'cold start' there is a buzz noise for several (maybe 5) minutes before sound get normal and sub starts to work. Any idea what might cause kind of behaviour? Could it be the output relay? I already changed the same capacitors as you've changed but that did not have any impact. At least visually rest of the caps seems to be ok. Btw, Rev02 goes not have any CapXon and also instead of uPC1237 it has a tiny PCB with some SMD components. Regards, Sami
ReplyDeleteThe relay will most likely not be the source of the noise - maybe unless the contacts are severely oxidized or whatever (but if they were, they wouldn't "come good" after 5mins).
ReplyDeleteThe source of the noise is more likely somewhere in the small-signal stages (on the other circuit board). That being said, the brown glue is a guarantee of headaches and problems down the line.
Thanks for your comment! Well, I contacted Amphion directly and sounds that we are able make this thing fixed by the manufacturer at a quite reasonable cost. For me that is a safe choice as I'm running our of skills... :(
ReplyDeleteBtw, the sound is audible on this video https://photos.app.goo.gl/wnUoAFN4bJmhPUIO2 , problem disappears around 1 min 20 sec.
That sounds... kinda scary, actually. One easy way to help determine where the noise is coming from, is with a cardboard tube to your ear (from paper kitchen-towels, for example).
DeleteIs the whole assembly, by any chance, vibrating noticeably, along with the noise it's making? Just wondering if it's maybe the power transformer.
If the relay itself was clicking on and off THAT fast and constantly, i'd expect to hear something like a piece of paper being torn sort of noise, coming from the speaker (due to the amp being connected and disconnected from the speaker when the signal isn't "zero").