wish i could tell you!

it blew up - charred.

hey!

i know this probably is not a popular topic, as i just spent a bit too long looking for this schematic in vain.

wtf. i found one schematic for this amp and i was trying to work off of it for a bit until i started to notice some strange discrepancies - the IC used for compression in the schematic has 14 pins - the one im working on has 16! - the input IC on the schematic i was looking at was a dual op-amp - the one i'm working on is a 5534!

this is the schematic i was able to find: http://www.prowessamplifiers.com/schematics/sunn/Coliseum_300_Bass.html

anyone have one for the OTHER version, with the 16 pin compression IC and the single op-amp input IC? can't tell what the other differences are...

the power amp in working and im suspecting its the compression section that's the problem (seeing as the IC is fried beyond recognition) but i cant really work on it with this other schematic that doesn't match up at all....

interestingly enough i think this very amp has been asked about in forums through the years.......we'll see if i become just one of many!

any tips on tracking something like that down in a long, complicated circuit like this one?

yeah i had traced it to the bridge -

replaced it with some 1n4007s and we were ALMOST okay. still a lowish voltage (like -3V). disconnected one card at a time - traced it to one of the lower octaves - there was some bad connection there - found a weird errant solder hair - put it back together and we are back in business keyboard works.

now im trying to decide if the noisiness of this thing is normal or not - its not a hummy noise more staticy kind of 'ssssssssss', quieter than the signal but the signal to noise ratio seems a bit low for my liking. thinking to just try recapping the power supply and take it from there - not too many components in there so wont be crazy.

hey!

fixing up this old analog keyboard -

here's a link to the schematic: http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&ved=0ahUKEwicodKMlIDSAhUH_4MKHfZIAzAQFgglMAE&url=http%3A%2F%2Fwww.bleeps-and-peeps.com%2Fimages%2Fstories%2Fhardware%2Fyamaha%2Fcp30%2Fservice_manual%2Fyamaha%2520cp30%2520servicemanual.pdf&usg=AFQjCNE9RQwQYVl8lBlOrCX86TcliSv4Zg&sig2=KwFP1a67oFJjWmA7ISqsdw&cad=rja

im not getting the -30V from the DC supply (see page 12). other voltages look fine - the AC coming into that section (the one that feeds TR7 and TR8) looks okay - about 30VAC - but im only getting 0.8V coming out of the bridge. thats my first suspect.

just a quick question: is there anything wrong with replacing this: http://www.sansui-vintage.com/product_info.php?products_id=98 with two 'discrete' diodes? say, 1N4007s?

whoa interesting result -

it says 4 ohms on the back - but it motorboats into a 4 ohm load. into an 8 ohm load its all good!

it says 250W into 4 ohms on the back, but there are two jacks per side. i was plugging one 4 ohm load into just one of the jacks on just one of the sides.

hey everyone

working on this solid state powered mixer - main issue is i cant find a schematic anywhere so its a bit hard to troubleshoot it. its having a strange problem:

the output is nice and powerful, but once it gets to a certain volume, it starts to experience a periodic drop - every 1.5 seconds or so, the volume will dip between full power and 1/8 power - almost like a very slow tremolo. ive traced the problem to the power amp - it does not appear in the main preamp out, but does appear throughout the power amp. it only happens with a load plugged in - if i scope throughout the amp, all the way to the output transistors, when its unloaded, it works perfectly. but once a load is plugged in, the power amp seems unable to maintain full power.

now whats hard is that without a schematic i cant quite trace it properly - i was just wondering if anyone has a thought what might cause such a strange symptom?

its a long shot but anyone seen a schematic for this thing around??

enzo,

do you mean that there may well be a turn-on transient issue?

lemme try and explain it again.

this is what's strange:

after removing the power amp PCB - i turned the amp on with a bulb in series: no short.

i took the bulb OUT of series and just turned it on with the recommended 3 amp fuse: fuse blew.

i turned the amp back on with a bulb in series IMMEDIATELY after: no short.

i wouldnt be puzzled if it wasnt puzzling......

that's why i thought there might be a possibility of a current transient that was blowing the fuse or the FETs - but im not sure what could cause that or where itd come from

hmmm

but is there a possibility that initial charge is problematic?

like i said - this amp was working fine and then just blew a fuse instantly upon turning on again.

shorted power amp was the likely culprit - BUT i removed the power amp and it blew a fuse then, too...but when i plugged it into a limiter after that: no short!

this one was blowing fuses -

i was getting a short across V+/V- - traced it to the MOSFETS on the little breakaway power amp PCB. fortunately or not, the FETs are the pretty much the only non-SMD/miniature parts on that board - so it was possible (although hard) to replace them. i did. amp worked! phew. put it back together - BAM - another fuse blown.

checked the FETs, two of them - q10B and q11 - had blown, but not q10.

took the power amp board out, turned the amp on with a limiter in series - all good. at this point i figured it must be something else on the power amp board - SMD bullshit - so maybe time to give up.

BUT, there is one weird sympton that id like your guys' help with: even with the bad FETs out of there/the power amp PCB unplugged, when i turn the amp on, my limiter will sometimes glow really bright (like full on short bright) for a split second when i turn it on. it blew a 3A slow blow fuse (with the power amp/shorted FETs out of circuit) once, even. it does not do this every time. sometimes i turn it on and i get a normal dull bulb for a second that gets even duller after the initial switching on. but, like i said, SOMETIMES the bulb will light like there's a short but just for a second.

what's going on here? something is causing the current to spike but only sometimes and just for a second.........any thoughts?? bad rectifiers? bad filter caps? so hard to tell because otherwise it works! voltages/ripples,etc look fine!!

hmmmm

tested the LDR i was using - it was going from about 1K to 2M. found another one i had around that went from about 200 ohms to 4M. better range.

on top of that, i replaced the incandescent bulb, which wasn't too bright, with a yellow LED. didn't change anything in the circuit - seems to work fine.

with the new LDR and the LED the tremolo sounds much better! deeper and with a more discernible range.

so far so good - thanks, enzo!

yeah c41 was a bit leaky i had already replaced it -

as far as choosing a photocell that's good for this kind of application, what are the useful criteria? large dark/light disparity?

whoops ya dono why i said neon. incandescent bulb.

yeah the original can be taken apart - i did that and the photocell was bad, bulb OK.

what i was saying was that my homemade rig is probably not as sensitive/effective as a real one - as far as photocell speed and correct light/dark resistances, etc.....

it DOES get hot just idling. fiddled with the bias and i think we're doing much better now. as it was before, it was idling at like 1.5A mains draw. got it down to 0.3A - much nicer, just gets a little warm idling. and at full power its at about 1.5A. sound about right?

on another interesting note: whoever worked on this amp before did a 'quick-fix' that's new to me! the mains fuse was blown, but instead of replacing it, they wrapped it in tin foil! so it was just an unfused straight short!!! wow what a dangerous corner to cut!

yay!

had a suitable 12v relay around. popped a 10K resistor across the 8.2k so i could get the right voltage and we're back in business! fuzz is working and bright switch is working.

two last issues:

1) the first main issue in the amp was that the LDR/lamp had failed in the tremolo section and was causing output problems. it was one of these: http://www.ebay.com/itm/Sigma-301R1-24-Datacell-Optical-Relay-24-v-lamp-1-LDR-5-Pieces-/281251030611. troubleshot it, determined that the LDR was bad as were Q18/17 darlington pair (the LFO signal as not getting to the neon). replaced those, got the neon to flash. then put it an LDR i had around and wrapped it all up together with the neon. tremolo works BUT the depth/intensity is not quite as rad as it could be. tried funking with the bias but no dice. figure that sigma datacel probably rules when it works right. worth it to get another one?

2) the output transistors are getting pretty damn hot - as is their heatsink and the chassis with it. ive ran it for about 20 minutes at nearish to full power and no problems but the heat.....should i be concerned?