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Topics - ilyaa

#41
Tubes and Hybrids / carvin x-amp
August 03, 2014, 06:45:37 PM
heres a weird hybrid one -

R132 was (literally) burnt out - i replaced it and the amp is working fine, but im trying to understand what would make that resistor fry -

i checked the diodes in the bridge that feed it and they seem fine - where does the V- that its filtering/providing even go? i assume to the opamps for a +/-15V swing but i cant quite find evidence of that on the schematic -

what could make that particular resistor burn up? its in a lowish voltage part of the amp so im a bit confused.

note: R132 is in the lower right part of the schematic - off the transformer, through the lower voltage bridge -
#42
Amplifier Discussion / another acoustic 370
July 30, 2014, 03:03:22 AM
another one of these -

complaint was "farty/buzzy" - i tried it out and heard no such thing, but it shocked the hell out of me so i figured i may as well take care of that and see what else might be the matter in the process.

odd thing was, it had a three prong cord, but jokes on me for thinking that meant it was actually grounded in a safe manner: someone had just put a three prong adaptor onto a two-lead power cord (leaving the earth lead open!). what a dangerously strange move - ill take care of that.

otherwise, visual inspection just yielded one thing: L301 (the inductor on the output) looks a bit charred - there is a burnt looking black spot on the windings....bad?? any way to measure it? what does that inductor and its parallel resistor (R328) do, anyway?
#43
Amplifier Discussion / ampeg 350H got wet
July 09, 2014, 01:45:07 PM
okay as promised heres a SS amp -

ampeg 350H that was salvaged from a flooded basement. smells a bit musty in there and some of the chalk power resistors look like theyve absorbed a bit of water (the chalk looks darkish greenish kind of).

the good news is it powers up - my limiter tells me no shorts - and things actually look okay in there. the power resistors measure fine (should i be concerned about them) and im getting +/-60 and +/-16 on the main power rails.

edit: i had my scope set wrong. its giving me close to 20V p-p when cranked (into 8 ohms).

im not quite sure where to start - mosfet output stage but its hard to get to them because of an enormous heat sink they are glued to.

anyone give me some good advice for beginning to diagnose this kind of thing? (ive been in tubeland for a while and i have to be honest - my solid state theory needs some fleshing out)

i guess splitting the amp in half power/pre would be a good first step - ill do that and report back!
#44
Tubes and Hybrids / ampeg vt-22 PI
July 08, 2014, 04:15:37 AM
once again - fixing this one up for a friend.....

symptom was no/low signal and weird sound when turned on.

quick preliminary looked showed that the screen grid resistor on V5 (R42) was fried (low voltage there - resistor tested open and looked a bit burnt). ill change that.

BUT upon looking around the rest of the amp i also found that the PI voltages were bad. ~150V on pin 1 and ~330V on pin 6. now i remember having a similar issue on a deville i worked on. i also know more about long-tailed pair configurations now, enough to say that it looks like pin 6 is not conducting! both heaters are lit. i tried a different tube - same symptoms (a 12ax7, admittedly, not a 12au7 as it says on the schematic, but that shouldnt matter for this issue, correct?). ive resoldered all the pins and adjoining connections and checked the region and it all looks good. does NOT seem mechanical.......any other things i should check or am i just being sloppy with the mechanical stuff most likely?
#45
Tubes and Hybrids / peavey classic B coupling caps
June 11, 2014, 03:57:37 AM
fixing this one up for a friend -

incidentally i at one point had 3 of these identical amps in my garage -  a popular one in my milieu, i guess....

anyway, this one was doing something weird and i traced the problem to the phase inverter (solid state differential amp), which was giving me wonky voltages. replaced the transistors and all voltages are back to normal.

amp sounded fine - nice and loud - but had an unpleasant kind of congestion and loss of volume still when cranked up.

noticed the bias voltage for the PI (which should have been 330V) was only at 260 or so. saw that the last resistor in the PS filter network on the main cap was not, in fact, 68K but 87K, for some reason. change that out and voltage is closer (now at about 300V) - the first resistor in the HT filtering is 1K (instead of 400), so that might be it, but ill leave that as is for now.

the amp is making power. the slightly low HT voltages seem an unlikely suspect. i have noticed, looking through the amp, that many of the smaller coupling/DC blocking caps have a white residue on them as if they have suffered a tiny explosion. is this a sure sign of age/wear? should any caps with this kind of sign of stress be replaced? just wondering if that's a clear indicator or one of those 'maybe you can change it but it's probably okay' kind of things.

thanks!
#46
Amplifier Discussion / two prong to three prong
May 30, 2014, 01:48:59 AM
im fixing up an acoustic 150b head

works great - just had some caps that had desoldered themselves and other ones that seem leaky - just doing a basic clean up and recap

this amp actually sounds awesome! a real growler

anyway, the power cord is kind of deteriorated and also only 2 prong

just wondering: what is standard procedure for retrofitting an older SS amp with a three prong cable?

im particularly confused because the cable it has now is polarity indifferent (equal sized prongs) -

guidance please!
#47
Amplifier Discussion / acoustic 370 rustle rustle
April 28, 2014, 12:31:48 PM
this amps giving me guff again - have to say im getting a little tired of it.

when on (no input signal) the amp makes a low end kind of rustling noise - almost sounds like rats have made a home in the speaker cones. its, unfortunately, an intermittent problem. sometimes the amp will be on for a while and not make the noise and sometimes the noise will get louder and louder as time goes on.

what ive managed to figure out/try so far:

1) i think its coming after the volume, because the volume has no effect on it (it can even be all the way down and the noise still persists)
2) it might come before the main EQ, because the treble/middle/bass knobs seem to have a *slight* impact on it
3) it definitely comes before the graphic EQ, because that has a huge impact on it. if i turn the graphic EQ all the way down the noise *almost* disappears and if i crank it the noise becomes thunderous
4) pretty sure its in the preamp, because if i send the Line Out to another amp the noise comes with it
5) scoping around in there i dont get too much info.
    scope on the speaker out when its happening: what you'd expect, just some low frequency looking     noise
    DC volts on the speaker out when its happening: a little bit, jumping around between like 15 and 70 mV
   scoping around inside the preamp when its happening: i dont get anything at all really UNTIL the collector of Q105, when the flat line on the scope starts to move up and down (not AC-style, but looking like shifting DC) - it moves along with the rustle. as i move further along the preamp the movement gets more significant but stays the same - just that flat line moving up and down.

my thoughts:

poor filtering feeding the preamp transistors an unsteady/fluctuating bias thats leaking into things like a low frequency AC? only thing is ive checked the main big caps and they seem pretty solid - although of course they are old.....

bad/leaky caps in the preamp? particularly C108, C112, C115, C116, C118, C119, C122, C124, C125, and C127.

am i going down the right path? i dont want to disappoint everyone and resort to shotgunning these caps around but in the absence of a clear indicator of the fault im not sure what to do. unless the shifting DC on the scope points to something specific.

on a side note: im curious about the speaker outs on this amp. with the speaker unplugged, the amp shows like 25V on the speaker outs, but once i plug it in that goes down to near 0. that normal?
#48
Tubes and Hybrids / JMP 2100 not making power
March 26, 2014, 10:52:05 PM
so im trying to put some of my new skills to use -

someone gave me this amp to look at  - "sounds quiet"

B+/plate voltage - ~385VDC

tubes are EL34s
i did a quick bias, trying to get 14 watts or so dissipation.
started with one tube - looking for about 37mA. got it there with about ~-31VDC on the grids.
checked the other tube - it was pulling about 50mA.
switched them around to see if tubes or sockets and the readings moved with the tubes.
time for new tubes?

i then checked the power output with a 100mV sine wave into my (brand new) dummy load - it was giving me 12V p-p - so 4V RMS, so it's only making about 4 watts (into 4 ohms)?!?!?

this is a 50 watt amp...

only other thing that looked suspicious is the ripple on the main filter cap - about 8V p-p. is that acceptable? seems high....i thought the plate voltage seemed a bit low....

how am i doing?

(as far as i can tell the attached schematic corresponds to this amp...)

#49
Amplifier Discussion / that orange stuff....
March 19, 2014, 01:52:36 PM
this is just a question:

i just got a furman spring reverb rack thing and it wasnt working right and i opened it up and there is that amber-looking gunk all over the PCB solder joints.

an amp i fixed recently had the same stuff that was causing intermittent noise and when i cleaned it, it got all better.

now what IS that gunk? and why sometimes when i re-flow solder on an older board does it seem to appear?

is it rosin? but if it is, is it bad? it certainly seemed to be in the case of the amp....is it nicotine (i think roly maybe mentioned that) or other noxious residue? or just oxidation?

just curious.....
#50
Tubes and Hybrids / peavey heritage vtx snow camo
March 17, 2014, 04:14:49 AM
ill keep this one short (schematic attached)

this amp has a weird hum. higher pitched than 60 Hz. i checked power supply - looks okay - except plates are only getting ~363 volts (according to schematic should be 525). other DC stuff looks okay, but i didnt go through everything yet.

i scoped the hum (with a 1K test tone....ignore the shuffling sounds, they are unrelated. the hum kind of blends in with background noise, but you can hear it):

http://dropcanvas.com/#9i6BMjZ3oBMzr4

any ideas?

with no input signal: the amp hums with the controls all the way down. the controls in that case have no effect on the hum. if i scope that (with scope leads across speaker tabs), though, the scope seems to load it down (or ground it?) and it disappears....if i crank the amp up the scope picks up a (quieter hum) that just looks like noise (no discernible waveform) - cant tell if thats just noise floor of amp then.

the bigger problem is that this amp has a snow camo design. i wish there was a repair for that....


#51
Tubes and Hybrids / sound city clean-up
March 10, 2014, 04:46:46 PM
this might be a long one....

its a Sound City Concord combo

(schematic attached)

i inherited this amp from a friend - "if you can fix it, you can have it!" - another mutual friend had worked on it once upon a time with no success.

its already a complicated, cramped mess in there, but add to that the fact that i think the previous 'tech' may have tried out some voodoo of his own, and i really have no idea what's going on.

with a limiter in-line, the pilot lamp lights (dimly) but that's about it. no heaters no nothing. so im stuck at the first step trying to figure out: is the amp getting power? the light bulb does NOT light at all.

here are some brief measurements/observations so far:

1) something weird with the mains switch: the pilot light is hooked up DIRECTLY to the same pin as the "N" mains connection (rather than being on the other side of the switch, as it says in the schematic). on the other side of the switch is a blue wire that goes to a node that goes somewhere into the PT.

2) with the amp turned on (but on standby, of course), and one DMM lead hooked up to the "N" side of the mains, i can read ~120VAC all over the place: both main filter cap terminals, tube heater pins....??

3) the main filter caps have little lumps on the bottom with some white looking flaky residue. i assume that's bad!

4) there seems to be some kind of modification on the output of the bridge....an additional 47nF cap and 22K resistor and another diode....doing something i dont know.

i will go in and do a more careful trace to see what's going on and report back. its pretty confusing and hard to pick apart but ill start with the power section and see if i can at least figure that out.
#52
Tubes and Hybrids / blues deville got the blues
March 07, 2014, 03:43:05 AM
i like this amp a lot (the 2x12 combo) and it sounds good (usually)

this is a 93-ish deville (schematic attached)

lately ive noticed a sort of muffled/congested/sometimes even grainy quality to the drive channel. normally i have no complaints with this amp's tone, but this one is bugging me. it's sometimes crackly verging on staticy. why is it so hard to explain?! bottom line is, i keep wanting to turn it up but it just doesn't seem to break through the graininess. on top of that, sustained notes in particular will kind of crap out into a cheap sounding fuzz.

i got new power tubes. put them in. its fixed bias but i checked it out anyway and they seem fine. running about 37 mA at 474 on the plates so 17.5 watts. that's not too cold is it? (6L6GCs).

control grid voltage is only at -43 or so - should it be more like -50? im deliberating whether or not to put a bias pot in....

most voltages around the amp seem okay EXCEPT:

1) TP26 on the schematic (cathodes of the third pre-amp tube) should read 34 VDC but only reads 22 VDC
2) point Y, off of big filter caps, going to plates of third pre-amp tube and control grid of power tubes read 440 VDC should be 413
3) point X (going to plates of first two pre-amp tubes) reads over 400 VDC and should be only 364 VDC

now i know the high voltages have some leeway, but what about 1) ?? i cant help but feel like the problem is preamp related, seeing as new power tubes that seem to be running at the right bias didnt do much. if so, would a low voltage at preamp cathodes maybe cause the issue? ive swapped around preamp tubes with no luck yet (but i only have one known good one that im switching in and out)

any thoughts?

should i put in a bias pot and try to run the power tubes a bit hotter, see if that fixes it?
#53
got one of these from a friend to fix

its the mosvalve 500 (942)

i havent done anything yet but the problem as my friend has described it is:

turn it on, VERY loud hum, turning gain all the way down does nothing - hum does not stop.

would checking DC volts on the output be the right place to start, assuming everything inside looks normal once i open it up? is that what normally produces an unstoppable hum? or would DC getting to the output create other symptoms?
#54
Amplifier Discussion / roland jc120 weird woes
February 13, 2014, 03:21:05 AM
okay ive had this little combo for a bit - bought it cheap and knew it had problems - figured id get around to them sometime and here i am!

but a frustrating evening, after all.....something seems to be wrong with my scope - the screen keeps flickering and spiking even with nothing plugged in....i dont have the patience to troubleshoot my troubleshooting equipment!!! argh

anyway, heres whats up with the amp:

works great EXCEPT

1) if bright switch is NOT engaged, there is a significant loss of volume at louder levels
2) distortion circuit does not work at louder volumes either. there is signal and it's distorted, but i cant turn it up without it getting all quiet and congested

now i was ready to start figuring these things out on my own when i was foiled by

1) my scope crapping out and
2) i just cant seem to find the right schematic for this amp!!

there are a few floating around the internet - one for a 1982 jc120, one for a 1979 jc120, and one for a 1984. i think mine is like 1981 - its made in japan with a s/n something like 141xxx (i dont remember the last three now).....the 1982 schematic seems close but the part of the amp i started focusing in on (around the bright switch) does not match up......

in my amp, the bright switch either connects or disconnects the treble pot wiper to the volume pot wiper via a 22k resistor and a smallish cap. something like that.

at this stage, because i havent done any real testing, i wont ask for any help other than advice on the whole schematic issue.......are there other ones floating around?? i looked and looked but came up empty-handed. im not really confident enough with this stuff to get into it without a schematic to help me out....

(side note: taking some measurements (not carefully) i fried a non-component part of the board - a green trace was touching the chassis and got really hot and just blew up!! so i had to jump the blown out part with a fat wire. is there a good way to securely fix a broken trace issue like that??? im a moron......)
#55
Amplifier Discussion / acoustic 370 buzz/noise problem
January 17, 2014, 09:31:15 PM
anyone used these amps before? great sounding and a really great build inside EXCEPT its really hard to take it apart if you want to work on it. things are connected all over the place with straight up leads so you have to desolder stuff to take the boards out. lame. anyway, heres my problem:

works great, sounds great, BUT makes a loud (sometimes very loud) buzzing sound at different times:

1) when turned on
2) when turned off
3) when jostled, even a little bit, like if i knock it or the cab its on
4) when its on but idle itll sometimes randomly buzz, perhaps a result of outside vibration

opened up, nothing looks bad. the weird thing is, when i poke around in there, no matter where i poke it makes the sound, so its hard to isolate where the problem is (im assuming its a bad joint somehwere). also, the noise is loud at first poke, then gets quieter as i poke more until it goes away completely. if i wait a bit and poke again it comes back, sometimes very loud.is it probably a bad joint? its going to be such a pain in my ass to remove the boards and resolder.....

any ideas?
#56
okay some progress, but we might need to reassess our theories.

i DONT think its an oscillation issue. looking at a larger amplitude waveform from my guitar, there is NO discernible fuzz or distortion other than what the amp naturally provides. it looks pretty clean, in other wo