Jamo SUB660 repair - Page 1
Forum section: electronics / repair

author: Raff
author_rank: Regular Contributor
timestamp: March 19, 2021, 03:14:06 am
content: Hi everyone,My much loved Jamo SUB660 subwoofer stopped working recently and I decided to investigate. The whole board was covered in "Sony bond" glue which had gone into various stages of decomposition. It had eaten the legs from a few parts and generally caused mayhem.Long story short I removed as much of the glue as I could, hot air at 100c seems to help greatly, and replaced parts that were damaged as I went along.Parts replaced included several electros, 15v regulators 7815 and 7915, and a few ceramics 100n 100v that didn't survive the glue removal and cracked.I plugged it in for testing via a light bulb current limiter, (150w) and it seemed to work, then one of the ceramics I had replaced flamed up.Testing the ceramics I had replaced showed that they had turned into resistors! I had trouble finding new 100nF 100v plus ceramics so raided the junk box and found some. They seem to work fine.After more testing and fault finding I had it working fine via the light bulb, went to full 240v input and the 6.8uF electro in the loss pass filter vented!I'll add that I don't think the speaker relay had engaged at this point as I was using a Variac and light bulb in series to test. Pretty sure I was around 150 or so volts input to the amp.Now to my question(s).I will include a circuit diagram Im pretty sure is correct for the power amp part of the subwoofer amplifier.What would cause c17 which in my amp was rated at 160 volts to vent? I assume it passes the whole 160 plus volts generated by the class D output stage, as part of the low pass filter.And as I cant find any 6.8uf 160V bipolar caps anywhere, would a film cap do assuming I can fit it in there physically?Thanks and regards,Raff,grumpy old retired tech :PS the schematics are for the SUB650, but the power supply and amplifier seem VERY similar with slight changes here and there.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: March 19, 2021, 07:01:49 pm
content: Most of the output filter caps I see are film caps such as these:https://uk.farnell.com/kemet/r60ir46805040k/cap-6-8-f-250v-10-pet-radial/dp/2495879I suspect Jamo's use of a Mickey Mouse non polorised electrolytic is meant to save a few cents off the production costs.You get what you pay for, I guess.

author: dietert1
author_rank: Super Contributor
timestamp: March 19, 2021, 07:46:00 pm
content: Anybody who knows how do DIY the capacitor replacement of electrolytic by film caps in speaker x-overs is in a very privileged position. I offered and did this mod for many friends, always with great results, especially if the speakers were older than 5 or 10 years. In my opinion electrolytic caps aren't good enough to make x-overs. But the consumer segment has lots of them in there and often not even the bipolar kind.Regards, Dieter

author: Raff
author_rank: Regular Contributor
timestamp: March 19, 2021, 11:05:26 pm
content: Thanks for the replies.Yes the bipolar electros used in speaker crossovers are not the best idea, but of course driven by cost, I guess the same holds true for using iron cores in lieu of air cored inductors in speakers as wellOver the years I have made or modified quite a few speakers systems and yes film caps and air core inductors made quite a difference in the sound quality, at least to my ears.Speaking of which, see what I did there? lol. I think I might recap my Jamo speakers with film caps as well, scored a large batch of 1, 2, 10 and 20uF film caps the other day for free.The plate amp Jamo used is I believe a modified Monacor or Yung design made in China "to Jamo's Specifications" so maybe some cost cutting went on here or there, who knows.Yes indeed, all of the class D amp schematics I can find seem to use film caps as part of the low pass filter, so I'm going with one of those, see what happens.Thanks again,Raff.

author: Raff
author_rank: Regular Contributor
timestamp: March 21, 2021, 07:50:08 am
content: As its raining rather a lot around here today I thought Id do some experimentation.If I use a computer simulation of the low pass filter of the output stage of the subwoofers amp, and leave out the 4 or 8 ohm speaker load, the output rises to almost the full rail voltage regardless of the duty cycle of any square wave used as input.Using 220kHz square wave as an input and using the values of the actual circuit, 100uH inductor and 6.8uF bi polar capacitor, I also get a sine wave out, at around 31kHz at near full rail voltage without the speaker load connected. Add the speaker load, all is well, guess that's a lesson for me, don't run a class D amp without the right load!It should according to how Class D amps are supposed to work, be zero DC volts out for a 50% duty cycle.I gather that is why the output capacitor let go, I had inadvertently connected my jug element resistive load (unwound to read about 8 ohms), AFTER the relay not before, and as I was on restricted input current via a 150 Watt lamp in series with the mains input, the speaker relay never activated, hence the output ran pretty much without a load, save for the 1k 2 Watt resistor on the board.Live and learn I guess.Raff