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Topics - ilyaa

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Amplifier Discussion / yamaha CP30 repair
« on: February 08, 2017, 04:14:35 AM »

fixing up this old analog keyboard -

here's a link to the schematic:

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: with two 'discrete' diodes? say, 1N4007s?

Amplifier Discussion / MOSES mpx838 powered mixer output issue
« on: January 11, 2017, 03:13:57 PM »
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??

Amplifier Discussion / jbl es150p subwoofer current transient....?
« on: October 27, 2016, 01:13:33 PM »
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!!


Amplifier Discussion / acoustic 260 relay replacement
« on: July 18, 2016, 06:54:17 PM »
bringing this one back from the dead

looks like the reed relay died whenever and someone had rewired the bright switch to bridge the gap. unfortunately, as a result, the fuzz does not work - cannot be switched into the signal.

ive tried snooping around online for a replacement relay but really no dice!

check out page 23 of the schematic

part number GB 651C 10?

cant find it - not sure what a good way to find a replacement would be - any advice appreciated thanks!!!

Tubes and Hybrids / cathode bias question - ampeg m15
« on: July 04, 2016, 02:18:07 PM »
couple questions about this amp, just wanna get some opinions and thoughts -

here's the scheme:

it was sounding kind of honky and felt like the tubes were running really hot. i measured and got ~32VDC at the cathodes of the power tubes, so 128 mA running through the 250 ohm cathode resistor - 64 mA per tube at idle. too hot!! right? i was hesitant to f!!* with the design - almost all original parts in here, BUT they were new JJ tubes and i figured well maybe they pull more current so itd be worth it to tame it down - i raised the cathode resistor 330 ohm now im about 50 mA per tube, still hot but seems a bit better. i know this is a tried and true ampeg design - does it make sense to try and tame it like i did?

other question: even with the amp turned up all the way, its really not as loud as 2 6L6s should be - i checked the output of the PI to the power tube grids and im only getting about 30Vp-p sine wave with volume all the way up. now the voltage at the cathodes, with the new 330 ohm resistor, is about 35VDC, so shouldnt i be getting closer to 70Vp-p at the power tube grids? or is this amp designed to not push the tubes all the way? i dont have any 6SL7 tubes around, but i tried rotating the three that are in the amp and there was no change. please disillusion me if im applying grid bias principles to a cathode bias amp and therefore barking up the wrong tree.

Amplifier Discussion / LEM baby tape echo
« on: June 23, 2016, 12:16:52 PM »
trying to get this old univox/LEM tape echo working

looks like that (mine's in a bit worse shape....)

the mixer works, passes signal. the tape loop spins and looks okay, but there is no effect. it's impossible to find a schematic so i am poking around in the dark a bit.

is there a simple way to measure that there is bias on the tape heads? can i just measure AC across the heads?

Amplifier Discussion / orange crush 35B primary
« on: April 07, 2016, 01:28:58 PM »
this is a cheapo PT - maybe same brand as one that was recalled in the tiny terror?

at any rate - it's bad - not putting out anything.

any suggestions for a replacement?

Amplifier Discussion / LH1000 fuse blowing - short?
« on: March 20, 2016, 06:43:14 PM »
trying to figure out this hartke LH1000 that's blowing fuses - my light bulb limiter tells me its a dead short somewhere - bulb goes on real bright real fast.

i started measuring around the power section, wondering if it's the PT. if i disconnect CN2 from the board, i read a short (<1 ohm) across any/all of the leads (black or red or red). seems like that part of the PT should NOT be shorted, correct?

here's a schematic:

Preamps and Effects / DBX 160X
« on: March 14, 2016, 03:29:33 PM »
having some problems with this thing distorting -

wondering if anyone here has any knowledge of these units?

i know op-amps alright, but theres a lot of *s!!t* going on here......

specifically: im trying to figure out if U103 is powered okay. its got -12V at pin 4 and +17V at pin 7. shouldnt those both be +/-12V?


Amplifier Discussion / quick STK404 140S question
« on: March 01, 2016, 04:27:26 PM »
hey peeps -

long time no post!

quick question: im fixing an acoustic B200. everything looks good until the last power stage, where i get no signal on the STK404 output.

i'm getting +/- 50VDC on pins 12 and 13 (output pins). looks like they are spiking up to the rails. clear sign that the IC needs to go? should be 0VDC there, right? just not 100% sure i understand the layout of this particular chip....


Tubes and Hybrids / music man hd130 opamp
« on: August 04, 2015, 05:59:40 PM »
oh boy - this amp is so confusing! at least starting with the PI onwards....

brief rundown: amp was working OK but had some weird distortion. opened it up and saw one of the caps across the plates had a broken lead - figured there was maybe some HF oscillation or something - replaced that cap - amp was working but had quite a large very high frequency (100KHz) oscillation on the output. after some soul searching i saw someone had put an additional 68k resistor across the existing one (R49) coming back for NFB from the load. not sure why.....removed it and the oscillation was gone! amp worked great and biased up fine - ~25mV across those little emitter resistors on the transistors feeding the tubes.

all good - lots of play for a while - then one day signal dies very low power....?? open it up and see that the voltages look good but the signal looks weird around the LM1458 - the little chip feeding the PI. i replace it with a 5532 and we are back up to power! EXCEPT (THIS IS THE CURRENT ISSUE) now the amp won't bias up getting no less than 80mV across those little emitter resistors....ive tried a couple diff chips - the TL082 got me down to like 60mV but still tooo high. is it possible i need to replace it with an LM1458? thought any of those dual opamps would be about the same.......

other details is there is a very small about 1 Vp-p very high freq oscillation that disappears by the tube plates but is definitely present around that last opamp section....figure that might be related.......this amp is a bit confusing to me - anyone able to demystify??

ive been using this schematic:

Amplifier Discussion / quick impedance question
« on: July 03, 2015, 04:13:40 PM »
i know with tube amps 2:1 impedance mismatch is OK -

too high stresses transformer a bit more
too low stresses tubes a bit more

what about with SS amps? if it can be turned on with no load at all, seems like going too high with the load impedance is not a problem? what about too low? stresses output transistors risks overcurrent?

just curious -

Tubes and Hybrids / AIMS 6550 red-plating question
« on: June 30, 2015, 04:17:25 PM »
is any red-plating bad? always?

this amp sounds great but when its cranked that evil red glow starts to come out a bit from deep within one of the tubes.....

tubes are brand new!

the story: when i got it the amp had EL34s - the stock is 6550s, so the owner asked me to return it to stock. i did - not much had been changed by whoever put the EL34's in, except they had changed the 47K resistor in the bias network to a 23K and a 10K trimpot, and they had 470 ohm CONTROL grid resistors (??) on both tubes -

(also the tremolo wasnt working but i figured that out and its not relevant to my question here)

i returned things to stock (i took off those 470 grid resistors.....whats their deal, anyway?) and put in 6550s - with a 47K resistor in the bias network they were running like 80+ mA's each though, and the B+ is like 550V (!!) so i tamed them down to about 55mA (by putting a 68K resistor in series with a 20K trimpot instead of the 47K resistor in the bias network) each so like 30W each - that should be fine for 6550s, riight????

BUT i saw some red-plating - noticed there was a large-ish 20Vp-p ripple on the B+ so i replaced the first filter caps but the ripple is still there....hmm?? could that cause the red-plating?

someone added 220k bleeder resistors to the big caps, too, but i dont think those are related......

any thoughts?

oh yeah:

Tubes and Hybrids / kay 720 short hunt
« on: June 23, 2015, 04:42:51 AM »
this old kay has a short -

blew a fuse -

with all tubes except rectifier 5u4 pulled, lightbulb shows a short -

the rectifier tube lights up bright on one side only and then goes dim as lightbulb starts to glow bright -

tried diff rectifier tube - same light show

all power resistors, etc look and test OK - could main filter cap be shorted?! or PT rectifier coils....??

no DC anywhere except some negative 14 and 3 Volts on the rectifier plates?!


Amplifier Discussion / acoustic 150 zobel?
« on: April 15, 2015, 09:20:04 PM »

am i write to say that R404 and C404 make a zobel network across the output?

what would cause R404 to blow? my guess: amp plugged in without a load puts the output wattage across the puny 1W R404 and blows it up -

but then isnt that resistor just asking to be destroyed? ive seen one in another acoustic 150 that was blown literally in half...

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