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Topics - ilyaa

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Tubes and Hybrids / JMP 2100 not making power
« on: 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...)

Amplifier Discussion / that orange stuff....
« on: 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 it nicotine (i think roly maybe mentioned that) or other noxious residue? or just oxidation?

just curious.....

Tubes and Hybrids / peavey heritage vtx snow camo
« on: 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):

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....

Tubes and Hybrids / sound city clean-up
« on: 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 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.

Tubes and Hybrids / blues deville got the blues
« on: 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?

Amplifier Discussion / tubeworks mosvalve 500 killer hum
« on: March 03, 2014, 03:27:14 PM »
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?

Amplifier Discussion / roland jc120 weird woes
« on: 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......)

Amplifier Discussion / acoustic 370 buzz/noise problem
« on: 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 it probably a bad joint? its going to be such a pain in my ass to remove the boards and resolder.....

any ideas?

Amplifier Discussion / Re: ampeg ss150 supersonic/parasitic oscillation
« on: January 17, 2014, 09:15:59 PM »
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

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