i think it could be in the crossover section because the high's aren't working but you have signal (even though it's distorted) on the low's so input is kinda making it through (a quick test/check using the XLR out can confirm that the input section is ok) that leaves the x-over and amps (we don't know if the high amp is dead or is just not getting signal)
yes the crossover is active it's ahead of the amps (and i guess by your way of thinking it's part of the preamp)
how are your schematic reading skills?
and yes that velleman kit would do the trick but i'd change the injector's frequency. 1khz for long periods causes me to drink too much scotch!
yes the crossover is active it's ahead of the amps (and i guess by your way of thinking it's part of the preamp)
how are your schematic reading skills?
and yes that velleman kit would do the trick but i'd change the injector's frequency. 1khz for long periods causes me to drink too much scotch!
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Scotch 😱 Drink tequila, be happy! 😀
Well, i work with passive crossovers since i am a teen, but that's really another world.. I would say my schematic reading skills are low but not inexistent.
I haven't even considered to look for the pre-xover yet, so it's kinda going to the next level with op amps and so. I have the thing in front of me, the circuit, there are a lot of op amps, 4558 as you can see on the scheme. I guess i should check them first.
Looking forward to buy such a kit anyway.
Thanks again to all of you, such a great community here. 🙂
Well, i work with passive crossovers since i am a teen, but that's really another world.. I would say my schematic reading skills are low but not inexistent.
I haven't even considered to look for the pre-xover yet, so it's kinda going to the next level with op amps and so. I have the thing in front of me, the circuit, there are a lot of op amps, 4558 as you can see on the scheme. I guess i should check them first.
Looking forward to buy such a kit anyway.
Thanks again to all of you, such a great community here. 🙂
can't drink or smell tequila any more...long story...
U3,4,5,6 (look at the schematic,then look for the silkscreen lettering on the pc board that's your crossover)
while your still into checking for voltage rails check the supply pins of all those ic's
post script:i hope your running this on some form of current limiter!
U3,4,5,6 (look at the schematic,then look for the silkscreen lettering on the pc board that's your crossover)
while your still into checking for voltage rails check the supply pins of all those ic's
post script:i hope your running this on some form of current limiter!
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I usually find it productive to check power supplies before I get into AC tracing. Especially if no output at all. If only highs are missing, obviously the power supplies on the low side are working.
I use an analog VOM to trace AC signals, with a .047 uf cap blocking the negative input probe so it wor't read "AC" on DC signals. 2 VAC and 20 VAC scales are very useful. DVM don't work on music, they read random numbers until you get to the $129 true RMS meters. I use a portable radio as a signal source, tuned to a rock station so you can see the beats of the music on the meter pointer. IF you see asteady AC signal, it is hum or oscillation.
Of course you can buy a scope, but all the old ones I've acquired broke immediately. This Simpson 260-6XLPM VOM seems to go on forever.
I use an analog VOM to trace AC signals, with a .047 uf cap blocking the negative input probe so it wor't read "AC" on DC signals. 2 VAC and 20 VAC scales are very useful. DVM don't work on music, they read random numbers until you get to the $129 true RMS meters. I use a portable radio as a signal source, tuned to a rock station so you can see the beats of the music on the meter pointer. IF you see asteady AC signal, it is hum or oscillation.
Of course you can buy a scope, but all the old ones I've acquired broke immediately. This Simpson 260-6XLPM VOM seems to go on forever.
A VOM on the AC scale, when put on a DC source, will read the DC voltage. This might think you had AC when you do not. Say a shorted output transistor, VOM on speaker terminal on 100 VAC scale, speaker voltage = 32 vac. Wrong. It was 32 VDC. Insert a blocking cap in the lead clipped to ground so you read only AC voltage on AC scales.
The Simpson VTVM has a "radio" setting of the probe with the blocking cap right there with the switch. the Simpson 260 VOM does not have switches in the probe.
Wheras my sears DVM will read any number you want on single tone music, just wait a while for the random number you are waiting for to come up.
BTW, you can check the frequency of an oscillation or hum with a VOM with different blocking caps. Hum will go through .047 uf, only RF oscillation will go through 390 pf. I found an op amp oscillation in my RA88a mixer that way.
The Simpson VTVM has a "radio" setting of the probe with the blocking cap right there with the switch. the Simpson 260 VOM does not have switches in the probe.
Wheras my sears DVM will read any number you want on single tone music, just wait a while for the random number you are waiting for to come up.
BTW, you can check the frequency of an oscillation or hum with a VOM with different blocking caps. Hum will go through .047 uf, only RF oscillation will go through 390 pf. I found an op amp oscillation in my RA88a mixer that way.
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no like a "dim bulb tester" google that.there's also a thread in one of the forums here on the topic.
after looking over the schematic they're using an LM 1875 chip amp to power the high frequency horn it's about 15 watts and if this thing saw feedback from an open mic i don't doubt that it's cooked. you did say that the driver in there was ok?
after looking over the schematic they're using an LM 1875 chip amp to power the high frequency horn it's about 15 watts and if this thing saw feedback from an open mic i don't doubt that it's cooked. you did say that the driver in there was ok?
Yeah the horn is ok.
Sorry i can't understand what you mean for "if this thing saw feedback from an open mic".
I desoldered the 1875 and checked the piece with my Dmm on diode mode, and it didn't showed any problems.. I've got the whole evening today to work on these.. 🙂
Sorry i can't understand what you mean for "if this thing saw feedback from an open mic".
I desoldered the 1875 and checked the piece with my Dmm on diode mode, and it didn't showed any problems.. I've got the whole evening today to work on these.. 🙂
Repairing electronics is about method and fault finding ...when 10 ppl are telling you that you have a preamp issue why the hell you are removing 1875 ?
Did you report back on checking soldering especially in the input stage and IC or generally the board ???
Did you check for any cracks on the pcb can happen to portable speakers
Did you measure the obvious ? is voltage present in the IC that have the problem like pin 4 and pin 8 of all op amps ?
and guys come on !!! don mess up the rookie with bulb tester in a working amplifier cause this is a working amplifier it has input problem
Did you report back on checking soldering especially in the input stage and IC or generally the board ???
Did you check for any cracks on the pcb can happen to portable speakers
Did you measure the obvious ? is voltage present in the IC that have the problem like pin 4 and pin 8 of all op amps ?
and guys come on !!! don mess up the rookie with bulb tester in a working amplifier cause this is a working amplifier it has input problem
these kind of powered speakers are often used for karaoke and when a live microphone (for singing ) is brought to close to the speaker it will "screech" or "howl" that "feedback" can hurt amps and speakers!
Hi,
I checked the 1875 as first thing, before i switched the the pre! Obviously it didn't make any sense, knowing now that the problem shouldn't be at the power amp stage.
I looked for cold solder joints, couldn't find any of them.
Now i measured the voltages at every op amp and they all are ok.
Btw , how is it with op amps.. How can i know if one of them (or more) have signal problems knowing that the voltage is present? Do i need a signal tracer?
Oki doki, now i understand what you mean! It could be... I borrow these speakers to friends when i can... I hate karaoke so it cannot be me! 😀
ok the 4558 have power any DC on input or output pins?
the signal tracer would be helpful but not a necessity.
in checking the LM 1875 a multimeter test out of circuit is going to tell you anything.
the signal tracer would be helpful but not a necessity.
in checking the LM 1875 a multimeter test out of circuit is going to tell you anything.
Op amps produce A(V+-V-)=Vo Where Vo is Vout (pin 1 ) V+ is regular input (pin 3) and V- is inverting input (pin 2) in DIP package. A is Rf/Rin where Rfeedback is between pins 1 and 2.
So if your op amp output is near one power supply rail it has a problem. If the V+ and V- are far apart in DC voltage, you have a problem, unless unity gain. If the DC voltage on the trace going to an op amp input is different than the pin on the IC side of the board, you have a solder problem. If the DC voltage on a output pin is different than the voltage on the receiving pin, you have a solder or broken trace problem. As far as AC analysis, i music goes into, doesn't come outa, the problem is right around here. You can use a capacitor protected (series) amp input as a sound probe, if you clamp the input with back to back zener diodes (2.7v or parallel back to back LED's) to keep from blowing it up your amp with too much input voltage.
If you have a speaker on the broken amp output, and you are probing the op amp inputs, and you here a big pop, you might have found the bad solder joint or trace.
If this device was carried around to gigs, broken traces or loose solder joints are that much more likely.
I don't take parts out to measure with a dmm, except resistors, which I usually only lift one end. Shorted caps you can find by lifting one end, but open caps, I don't have a cap tester, just a junk box of old caps and some clip leads.
Before you took the 1875 out, did you measure the +- voltages going in? The datasheet is on datasheetcatalog.com, but I downloaded it last week, power in + is 5 and power in - is 3. Pin 5 is on the right looking at the step side of the IC (the side with the letters on it) with the tab up. You really need to look at the picture.
So if your op amp output is near one power supply rail it has a problem. If the V+ and V- are far apart in DC voltage, you have a problem, unless unity gain. If the DC voltage on the trace going to an op amp input is different than the pin on the IC side of the board, you have a solder problem. If the DC voltage on a output pin is different than the voltage on the receiving pin, you have a solder or broken trace problem. As far as AC analysis, i music goes into, doesn't come outa, the problem is right around here. You can use a capacitor protected (series) amp input as a sound probe, if you clamp the input with back to back zener diodes (2.7v or parallel back to back LED's) to keep from blowing it up your amp with too much input voltage.
If you have a speaker on the broken amp output, and you are probing the op amp inputs, and you here a big pop, you might have found the bad solder joint or trace.
If this device was carried around to gigs, broken traces or loose solder joints are that much more likely.
I don't take parts out to measure with a dmm, except resistors, which I usually only lift one end. Shorted caps you can find by lifting one end, but open caps, I don't have a cap tester, just a junk box of old caps and some clip leads.
Before you took the 1875 out, did you measure the +- voltages going in? The datasheet is on datasheetcatalog.com, but I downloaded it last week, power in + is 5 and power in - is 3. Pin 5 is on the right looking at the step side of the IC (the side with the letters on it) with the tab up. You really need to look at the picture.
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The opamp is good if it has near zero volts across the two input terminals(VIN+ and VIN-)
Gajanan Phadte
Gajanan Phadte
I've partially understood what you mean, cause it's actually the first time i deal with op amp. I'm digging into the Horowitz and Hill book these days. I've got some theoritical difficulties to fully understand the role and the functioning of an op amp.
Staying in topic, what i just did was measuring DC on the op amps:
1 checked the Vouts (pin 1/7) on red Dmm probe, with the black one on ground, and they were all around zero BUT the 4558 marked as U7, which is the last one before the xlr out. I measured 14.7 V there, which should be a rail voltage.
This isn't good if i understood you well.
2 measured across Vin+ and Vin- and they were all around zero BUT U7 (this time only the "1" part, pin 3 and 2) which had again 14.7 V.
I guess that even if this particular 4558 would be bad or damaged, it wouldn't affect the functioning of the speaker 'cause it's another output independent of the rest of the speaker. Am i right?
AC analysis? Sorry, i haven't really understood what kind of circuit i should build up in order to do it. And probably if i don't understand this, i shouldn't do it anyway.
I checked the V on the 1875, and as i always do i wrote down the results.
I had -17.93 on -Vee and +17.89 on Vcc. I measured it again right now, it's basically the same value on both of them.
Don't know why we're back on the power amp section, though...
Mate, thanks a lot for your time. I wish i could do something for you one day.
Yeah, well your rail voltage into the 1875 is fine, I would never have pulled it out. Not without listening to the music in versus the music out, or seeing it on the pointer of a fast VOM. (rock music beats).
Your op amp with rail on input pins 3 and 2, something is wrong, not necessarily the op amp itself. Like I said voltages under the board and on the pin of the op amp can be different due to a bad solder joint. Or a resistor could be burned, or a capacitor shorted, or something. Or a trace broken from the previous circuit element going in pins 2 or 3. 6 and 5 too are inputs, on a DIP package.
SO you can solve some problems with the DVM on DC volts scale. pins 2 & 3 should be about the middle of the power supply rails, in DC.
For AC analysis I can't give you a part # of a VOM, Simpson is out of business, but I'd buy a VOM, an analog one with a scale from 20 VAC to 50 VAC. My VOM has another AC scale useful around the input area, 2 VAC. I don't even know what country you are in. Panasonic was reputed to make something analog still, but they don't sell them here. Then put some rock music in the input of the preamp with a transistor radio, and look for AC with a beat, ie rock music, on the VOM pointer at various points. Sources of analog VOMs these days are boating supply areas and auto parts areas, they are *****y low ohms per volt ones but better than DVM's for chasing music. Or some people sell $80 scopes, but nobody I deal with regularly or trust. The sync circuit on a scope has to be really good for you to be able to understand anything, and eico's heathkits and televideo scopes had ****y sync circuit, everything was always a blur. My B&K 2120 scope had great sync for the one day it worked before the horizontal sweep e-cap wore out. The boards are glued in the chassis, !@#$#$%#$%. Any old used scope probably is chock full of bad e-caps, I don't really recommend them. The Simpson 260-6XLPM VOM has NO e-caps to go bad in 40 years. Just a used one has probably been dropped and won't work for that reason. 260-6 VOM are 5000 ohms/volt VOM's on AC which is about as good as you can get without going to a digital random number generator (on music) DVM or a rare as hen's teeth VTVM. I've got a working Simpson VTVM from the fifties too, no e-caps to kill that one either.
Your op amp with rail on input pins 3 and 2, something is wrong, not necessarily the op amp itself. Like I said voltages under the board and on the pin of the op amp can be different due to a bad solder joint. Or a resistor could be burned, or a capacitor shorted, or something. Or a trace broken from the previous circuit element going in pins 2 or 3. 6 and 5 too are inputs, on a DIP package.
SO you can solve some problems with the DVM on DC volts scale. pins 2 & 3 should be about the middle of the power supply rails, in DC.
For AC analysis I can't give you a part # of a VOM, Simpson is out of business, but I'd buy a VOM, an analog one with a scale from 20 VAC to 50 VAC. My VOM has another AC scale useful around the input area, 2 VAC. I don't even know what country you are in. Panasonic was reputed to make something analog still, but they don't sell them here. Then put some rock music in the input of the preamp with a transistor radio, and look for AC with a beat, ie rock music, on the VOM pointer at various points. Sources of analog VOMs these days are boating supply areas and auto parts areas, they are *****y low ohms per volt ones but better than DVM's for chasing music. Or some people sell $80 scopes, but nobody I deal with regularly or trust. The sync circuit on a scope has to be really good for you to be able to understand anything, and eico's heathkits and televideo scopes had ****y sync circuit, everything was always a blur. My B&K 2120 scope had great sync for the one day it worked before the horizontal sweep e-cap wore out. The boards are glued in the chassis, !@#$#$%#$%. Any old used scope probably is chock full of bad e-caps, I don't really recommend them. The Simpson 260-6XLPM VOM has NO e-caps to go bad in 40 years. Just a used one has probably been dropped and won't work for that reason. 260-6 VOM are 5000 ohms/volt VOM's on AC which is about as good as you can get without going to a digital random number generator (on music) DVM or a rare as hen's teeth VTVM. I've got a working Simpson VTVM from the fifties too, no e-caps to kill that one either.
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Credo che gli Italiani chiamano "Larsen" il "Feedback" .
I think Italians call feedback "Larsen" or "Larsen effect"
I think Italians call feedback "Larsen" or "Larsen effect"
The "Larsen" resistor would be between pins 1 (output) and 2 (Input-) and 7 (output) and 6 (Input -).
I take it back, the Simpson analog VOM useful for AC signal tracing is still in production: 12226 (270-5) - SIMPSON 12226 (270-5) - MULTIMETER, ANALOG, 5 FUNCTIONS | Newark element14 US
It still has a 2.5 VAC scale, and others not listed on the website.
$316 and two are in stock in North Carolina. In Europe the name of newark.com is farnell.com; they have warehouses in most countries.
My Simpson 260-6 is a 1985 model, still works fine except I burned out something in the ohms circuit by plugging it in the wall on ohms.
I take it back, the Simpson analog VOM useful for AC signal tracing is still in production: 12226 (270-5) - SIMPSON 12226 (270-5) - MULTIMETER, ANALOG, 5 FUNCTIONS | Newark element14 US
It still has a 2.5 VAC scale, and others not listed on the website.
$316 and two are in stock in North Carolina. In Europe the name of newark.com is farnell.com; they have warehouses in most countries.
My Simpson 260-6 is a 1985 model, still works fine except I burned out something in the ohms circuit by plugging it in the wall on ohms.
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