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Topics - ilyaa

#21
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
#22
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

http://bmamps.com/Schematics/acoustic/Acoustic_260_Service_Manual.pdf

part number GB 651C 10?

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

#23
couple questions about this amp, just wanna get some opinions and thoughts -

here's the scheme: http://www.unofficialampeg.com/schematics/j-12-b.gif

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.
#24
Amplifier Discussion / LEM baby tape echo
June 23, 2016, 12:16:52 PM
trying to get this old univox/LEM tape echo working

https://www.google.com/search?q=lembaby+schematic&biw=1600&bih=749&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjS0_myxb7NAhWIPCYKHS61D804ChD8BQgGKAE#imgrc=df06GrRwLy3VuM%3A

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?
#25
Amplifier Discussion / orange crush 35B primary
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?

http://music-electronics-forum.com/attachments/24395d1374795161-orange-cr35b-pwr.gif
#26
Amplifier Discussion / LH1000 fuse blowing - short?
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:
http://music-electronics-forum.com/attachments/6758d1256749305-lh1000-v3.8.pdf
#27
Preamps and Effects / DBX 160X
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?

schematic: http://rdn.harmanpro.com/product_documents/documents/602_1324058489/160X%20Schematic_original.pdf
#28
Amplifier Discussion / quick STK404 140S question
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....

thanks!
#29
Tubes and Hybrids / music man hd130 opamp
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 right....im 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: http://www.music-in.de/pics/hd130a.gif
#30
Amplifier Discussion / quick impedance question
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 -
#31
Tubes and Hybrids / AIMS 6550 red-plating question
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: https://www.flickr.com/photos/73628360@N00/14628046201/
#32
Tubes and Hybrids / kay 720 short hunt
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?!

schematic: http://elektrotanya.com/PREVIEWS/63463243/23432455/egyeb/kay_720.pdf_1.png
#33
Amplifier Discussion / acoustic 150 zobel?
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...
#34
this amp is huge and cumbersome and a pain to take apart but i think its *mostly* working

i didnt do a road test but just took some measurements for now -

just a few questions (refer to attached schematic):

1) i get +300V at the PI supply, but on the transistor end of the load resistors i get ~10V everywhere i meausure - any of the transistor leads that arent at ground give me about 10.25V. one or more of the transistors bad probably? ive dealt with and fixed a bunch of these hybrid peaveys and almost ALWAYS the PI transistors have to be replaced.....

2) im getting a weird looking ripple at all points on the amp when measured with my scope. kind of a large, jagged wave. even when i touch the scope to ground its there. about 100Vp-p!! what the....??? i thought it may have been death cap related so i removed those caps but that only seemed to make it worse......any ideas?

3) related to that, what do the two caps on the power cord side of the PT actually do?

#35
Tubes and Hybrids / PCB question
February 26, 2015, 03:51:49 PM
i have a question about circuit board (potential) damage:

someone gave me a hot rod deville to fix up - one of the tubes is red plating the other barely glowing - figured just might be a shorted tube. i opened it up to take a look, though, and saw some bad looking light brown ash kind of residue over the area on the PCB where the red-plating tube is.

i cleaned it with some alcohol but there is still some dark brownish residue i cant quite get off

(see pics: http://dropcanvas.com/#68fj9h9Dk788l6)

1) any tips for a better solution to clean this crap?
2) i think Roly mentioned before that a burnt PCB can become resistive. the owner of the amp has said its had problems before and im wondering if a resistive PCB in the power amp section might be shorting tubes for him....any thoughts?
#36
Tubes and Hybrids / magnatone ancient amp
February 08, 2015, 02:56:50 PM
another one from a friend -

works - speakers are shot (cone-wise) but working - while he's getting those fixed up or getting new ones im trying to see if anything inside needs work

its all pretty old and worn/rusted (see pics) - i think the amp's from 1948!!

its got those cool tubes with the plate tap on top!

two primary concerns (as youll see in the pics):

1) someone put in a new power cable and just thought to leave the safety ground hanging!!!

2) the leads coming out of the OT are kind of weird - each side of the winding comes out as a two, unstranded, wound together, exposed copper wires....these have some green oxidation on the ends right before the terminal where they hook up to the speaker wires - thoughts? i tried to look up this OT - triad 1746 - but not much luck....whats up with these exposed leads? thicker for wattage handling? kind of a small OT, too....anyway, just curious if i should clean them/cut off the green part/replace the OT.....

http://dropcanvas.com/#7SH56kJ26C2G6C
#37
Tubes and Hybrids / fender champ line out
January 30, 2015, 03:21:30 AM
i built a champ a while ago and my buddy made me a cab for it!

sounds great - i want to put a line out on it for recording -

anyone have a definitive method??

PS its a 5F1
#38
hey!

long time no post -

fixing a music man tube/ss hybrid for a buddy

works fine sounds great for about 1 hour and then starts to lose volume and completely crap out, making some weird sounds on the way!

any ideas where to start with something like this? seems to be heat related...whats a good way to diagnoze/troubleshoot problems like that?

im guessing its the power amp or PI, because once it starts to crap out it doesnt matter which channel you plug into -
#39
amps fine but vibratos not working....

thought it was the LDR/optocoupler so i replaced that but no dice

so far the only issue i can see is that both cathodes of are at 0 V, instead of 2 and 13DVC.

but ive never really troubleshot this circuit before - good place to start?

its not actually a dual showman, but it seems to be pretty much exactly this schematic:

http://www.thevintagesound.com/ffg/schem/twin_reverb_sf_100_schem.jpg

edit: ive got -44V at the grids of the vibrato tube - is that right?

thanks!!
#40
Tubes and Hybrids / kay model 720 => AB mod
September 09, 2014, 12:39:34 AM
got this amp the other day because i was intrigued by the cabinet design - its a 1x15 but with a grill on each side and the controls and stuff on the traditional 'rear' to encourage you to face the speaker butt out - and thought itd sound good to use together with my sound city concord -

recapped everything but the main can and its sounding good but its a little quieter than i expected - took another look - didnt realize it was a class A design!

i was thinking itd be an interesting and fun learning experience/experiment to modify it to run AB instead. ive looked into tapping off a negative bias supply (http://music-electronics-forum.com/t2545/ - thats a useful thread!) and am deciding between the ampeg style capacitive divider and this simpler one: http://music-electronics-forum.com/attachments/547d1175892694-kscan_0001.gif. what kind of diode is appropriate for this - just any ole high voltage silicon style 1N400x?

but before i get into that i want to make sure im not missing something -

1) i cant find any info on the specific output transformer - if its meant to run two 6L6's Class A, should it be able to handle AB operation?
2) and a follow up to that: because of the greater range of input voltage of to AB designs, will the amp actually be making more power - larger voltage swing?
3) is this a stupid idea?