Hi there,
I've got a cheap pa system ( Kempton Gt 12a ) that i use for partys etc etc
So, they are quite new, maybe they worked for 50-60 hours and i never really bringed them to the limit.
One of them now has big issues:
1 no signal from the horn (horn works)
2 overall an enourmus power loss (i can put the gain and the volume at max and talking on the phone in the same moment!)
3 though it's difficult to say, the little signal coming out is pretty much distorted
I use to repair electronic stuff, though i'm more a loudspeaker freak and don't feel really an expert of amps.
Anyway, i checked the out transistor, and some other transistor i could desold easily. They are fine.
I would like to understand what i should check first, instead of randomly desold stuff etc etc.
Unfortunately i couldn't find any circuit scheme yet, but i can post some pictures if that could help.
Any hints-comments-tips?
Thanks in advance, bye!
I've got a cheap pa system ( Kempton Gt 12a ) that i use for partys etc etc
So, they are quite new, maybe they worked for 50-60 hours and i never really bringed them to the limit.
One of them now has big issues:
1 no signal from the horn (horn works)
2 overall an enourmus power loss (i can put the gain and the volume at max and talking on the phone in the same moment!)
3 though it's difficult to say, the little signal coming out is pretty much distorted
I use to repair electronic stuff, though i'm more a loudspeaker freak and don't feel really an expert of amps.
Anyway, i checked the out transistor, and some other transistor i could desold easily. They are fine.
I would like to understand what i should check first, instead of randomly desold stuff etc etc.
Unfortunately i couldn't find any circuit scheme yet, but i can post some pictures if that could help.
Any hints-comments-tips?
Thanks in advance, bye!
It could be one of the caps, especially one in a bias position.
Have a look for any bulging electrolytics.
Check all the power rails. There may be a separate pre-amp +/- 15V supply where a regulator may have failed.
Have a look for any bulging electrolytics.
Check all the power rails. There may be a separate pre-amp +/- 15V supply where a regulator may have failed.
Kemton is an FBT Italy product.
Write them and ask for a schematic.
Otherwise you will only get generic suggestions like the one above, of the type "there must be some damaged thingie inside there"
Write them and ask for a schematic.
Otherwise you will only get generic suggestions like the one above, of the type "there must be some damaged thingie inside there"
I checked the caps, looks fine. I tried to ask Fbt for a schematic several times, unfortunately they didn't answer...
Where exactly should i measure the rail voltage, in your opinion?
I mean, there are 2 regulators, 15 v, and i checked them off the board. They are allright.
I would like to learn to measure the rail voltages.. Therefore if you know some "easy" guide , please tell me!
I appreciate for the answers... Best regards.
Where exactly should i measure the rail voltage, in your opinion?
I mean, there are 2 regulators, 15 v, and i checked them off the board. They are allright.
I would like to learn to measure the rail voltages.. Therefore if you know some "easy" guide , please tell me!
I appreciate for the answers... Best regards.
Measure the rail voltages at the two big caps with the highest voltage rating. Probably near the transformer/switcher coil. If a switcher toroid, the two highest rated are line voltage, so the ones below that.
Or if class AB, the collectors of the output transistors.
If class T, you're on your own. Posting under PA amps thread may get you scrutiny by a couple of music repair shop owners. Moderator, please move.
Or if class AB, the collectors of the output transistors.
If class T, you're on your own. Posting under PA amps thread may get you scrutiny by a couple of music repair shop owners. Moderator, please move.
Thanks a lot.
I will check as soon as i return back home, in a couple of days. It shouldn't be a class D though.
I will check as soon as i return back home, in a couple of days. It shouldn't be a class D though.
hi there, i am back and i've got good news…
i attached the circuit scheme and a picture of the board.
now you don't have any excuses and must help me! 😀
anyway, i checked the 3 rail voltages (power amp, preamp and..? ) pointing the negative lead of the DMM at the ground and the positive one at the + of the last capacitor of each filtering stage. is that correct?
so, starting with the biggest caps, so for the power amp section, i measured around 55 V.
at the middle stage (preamp?) i measured around 18 v.
at the third stage, the one with the smaller caps, there is just a tiny 0.1v (not steady) which is suspect.
i checked the rectifier, a Rs205: with the Dmm positive on the positive of the rectifier, and therefore the negative on the cathode, i've got an open lead.
in reversed position, that is + of Dmm on cathode and - of Dmm on anode, i've got a voltage drop of 0.54.
for my limited knowledge (until now, only teorethical) the rectifier is working.
what do you think?
feel very excited to learn this things, thanks guys.
i attached the circuit scheme and a picture of the board.
now you don't have any excuses and must help me! 😀
anyway, i checked the 3 rail voltages (power amp, preamp and..? ) pointing the negative lead of the DMM at the ground and the positive one at the + of the last capacitor of each filtering stage. is that correct?
so, starting with the biggest caps, so for the power amp section, i measured around 55 V.
at the middle stage (preamp?) i measured around 18 v.
at the third stage, the one with the smaller caps, there is just a tiny 0.1v (not steady) which is suspect.
i checked the rectifier, a Rs205: with the Dmm positive on the positive of the rectifier, and therefore the negative on the cathode, i've got an open lead.
in reversed position, that is + of Dmm on cathode and - of Dmm on anode, i've got a voltage drop of 0.54.
for my limited knowledge (until now, only teorethical) the rectifier is working.
what do you think?
feel very excited to learn this things, thanks guys.
Attachments
If that's the board, where is the schematic?
I see only opamps in the schematic but none on the board?
The schematics are all the same I assume.
Post the correct schematic for the board....
I see only opamps in the schematic but none on the board?
The schematics are all the same I assume.
Post the correct schematic for the board....
From that first page, there should be + and - 15 v on those 47 uf caps. If you got 0.1 v and the rectifier is good, I would start by reheating the preceding solder joints. Then replace the 47 uf ecaps. The 7815 7915 regulators are also likely failure parts. Your image is a blot I can't tell which part number you have.
For me the caps and regulators are under $1 each so I would buy them all because shipping is minimum $8. Farnell has a minimum order charge so I just change all the electrolytic caps on any amp over 15 years old - saves on freight and improves reliability. East electronics repair old amps thread starts off with that. http://www.diyaudio.com/forums/solid-state/136261-vintage-amplifier-repair-upgrade-manual.html
I buy caps with more than 3000 hours service life, so I don't have to do this again in 500 hours.
I'd buy the regulator(s) on the first shipment but only replace it if it was bad.
For me the caps and regulators are under $1 each so I would buy them all because shipping is minimum $8. Farnell has a minimum order charge so I just change all the electrolytic caps on any amp over 15 years old - saves on freight and improves reliability. East electronics repair old amps thread starts off with that. http://www.diyaudio.com/forums/solid-state/136261-vintage-amplifier-repair-upgrade-manual.html
I buy caps with more than 3000 hours service life, so I don't have to do this again in 500 hours.
I'd buy the regulator(s) on the first shipment but only replace it if it was bad.
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I will have to presume that either its something else that you wanted to talk about or simply you wrote something with no meaning at all ...
there is almost no capacitors in a bias circuit ( in typical class AB amplifiers ) then again if you have any bias issue can cause many trouble but not the one described while the amplifier of the high freq will continue to work without an issue .
Low bias or no bias = extensive distortion but no loss of total power
open bias = Dc offset in the output, amplifier out of balance, but this will be detected by the protection and shut the amp totally down
too much bias = extensive temperature but this also will be detected by the protection and shut the amplifier totally down
So this part of your comment is totally wrong ....Bias has nothing to do with the symptom described ...\
Think ppl :
This is an active 2 way system any problem in the amplifiers will trigger the protection but if one of them for the woofer was working there was going to be loss in the high or the opposite .
If both ways low freq and high freq present loss of power then there is an input failure
NO 1 issue is poor soldering since this is an active monitor ( broken pcb is included in your search together with the gain pots and input plugs that suffer the most )
NO 2 issue will be local power supply issues 15+15 rails and so on
op amps do not fail easily under those conditions
Kind regards
Sakis
Hi, in fact i desoldered the rs205 in order to measure it properly.
With the negative lead of DMM on the + of the 205:
+DMM on ac1 0.67 V
+DMM on ac2 0.60 V
+DMm on -205 1.10 V
If i put the positive lead of the DMM on the - of the 205, and swap the negative lead on the ac and positive pins of the 205, i've got the same results of above.
In any other position i've got OL.
I think this should be fine, right?
I mean... I don't bother buying new pieces, and i surely will anyway (caps included) but i have to continue the troubleshooting cause there has to be some other problem...
Or maybe the meter reading shows that the component is damaged and i should quit doing all this! 😀
The Kempton schematic power page is too fuzzy to tell what a RS205 is or refers to. I don't think I see component number on te schematic. Perhaps you are reading the component numbers from the circuit board which we don't have?
I don't check anything more complicated than a transistor with a DMM anyway, I check them in circuit. You did prove it is not shorted across, that is something, anyway. If RS205 is a regulator and you don't want to put it back, you can take a wall transformer , put in 19-30 vdc in the in, and see if you get 15 v on the capacitor on the out. The wall transformer (if linear) will have a capacitor on the input. You can load a regulator-capacitor combination with a junk resistor. heat sinks on the tab of a regulator help them last longer. I use sawed up and drilled door frame aluminum for heat sinks on things that don't need that much heat flow.
Putting parts in and taking them out too much tends to lift traces off the board. If a bad solder joint was the problem, taking the part out doesn't help that.
Observe polarity in your test jig, 7815 and 7915 are different. Alligator clip leads are useful. You can download datasheets from datasheetcatalog.com.
If you don't want to buy new pieces, give up amp repair. Unless you have a huge pile of scrap amps. Then you may still have all the wrong parts. If you are on a South Sea Island and have mail service only three times a year, you can cut up an old TV KCRTor PC power supply, salvage a 14 v zener diode and a transistor, take a couple of resistors and a bit if insulated scrap sheet, and build a 7815 replacement. If not a $.75 new IC is usually more productive.
I don't check anything more complicated than a transistor with a DMM anyway, I check them in circuit. You did prove it is not shorted across, that is something, anyway. If RS205 is a regulator and you don't want to put it back, you can take a wall transformer , put in 19-30 vdc in the in, and see if you get 15 v on the capacitor on the out. The wall transformer (if linear) will have a capacitor on the input. You can load a regulator-capacitor combination with a junk resistor. heat sinks on the tab of a regulator help them last longer. I use sawed up and drilled door frame aluminum for heat sinks on things that don't need that much heat flow.
Putting parts in and taking them out too much tends to lift traces off the board. If a bad solder joint was the problem, taking the part out doesn't help that.
Observe polarity in your test jig, 7815 and 7915 are different. Alligator clip leads are useful. You can download datasheets from datasheetcatalog.com.
If you don't want to buy new pieces, give up amp repair. Unless you have a huge pile of scrap amps. Then you may still have all the wrong parts. If you are on a South Sea Island and have mail service only three times a year, you can cut up an old TV KCRTor PC power supply, salvage a 14 v zener diode and a transistor, take a couple of resistors and a bit if insulated scrap sheet, and build a 7815 replacement. If not a $.75 new IC is usually more productive.
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Do please bear in mind that at that stage in the thread we were working completely blind. With nothing to work with I was only trying to be helpful.
Indianajo:
Thanks.
Ihave no problems in buying new components. The ones i checked so far seems to be ok, though. I am looking forward to measure them as you do, it's kinda a new territory for me, i love it.
By the way i was wrong about the third rail voltage, the schematic is not really complete as you said, but i was wrong anyway.
The third filtered voltage, the 15V one, is present. I pointed the DMM at the wro spot.
I checked on the other amp, the one of the other speaker, and found again 0.1V. So i thougt i was measuring wrong. I measured it initially after the last cap (on the 4th page C34) and it showed 0.1v, but i found the 15v on the diode next to it (D6).
Now, the schematic IS fuzzy, i know. But i expect coming more DC out from that cap, right?
Sorry if these are silly questions for you, i appreciate any help of yours.
🙂
Thanks.
Ihave no problems in buying new components. The ones i checked so far seems to be ok, though. I am looking forward to measure them as you do, it's kinda a new territory for me, i love it.
By the way i was wrong about the third rail voltage, the schematic is not really complete as you said, but i was wrong anyway.
The third filtered voltage, the 15V one, is present. I pointed the DMM at the wro spot.
I checked on the other amp, the one of the other speaker, and found again 0.1V. So i thougt i was measuring wrong. I measured it initially after the last cap (on the 4th page C34) and it showed 0.1v, but i found the 15v on the diode next to it (D6).
Now, the schematic IS fuzzy, i know. But i expect coming more DC out from that cap, right?
Sorry if these are silly questions for you, i appreciate any help of yours.
🙂
I can actually see C34 and C35 on the + and - 15 v supplies respectively. The plus terminal of C34 should have +15 v on it. The minus terminal of C35 should have -15v on it. The caps should have a minus in balls on a stripe pointing to the appropriate lead. Older caps had a plus pointing at one, but 4558's are post 1980 I believe. So your 0.1 v reading on the cap might be you are reading the ground terminal, not the outer one. The D numbers are all blotches, i can tell the two digit blotches from the one digit blotches. So I can't really spot D6.
I debug DC problems on the goes inta doesn't come outa technique. If the transformer has mains AC going in, ~17 AC will come out to the right places to the bridge rectifier "436?". then the plus and minus terminals of the bridge rectifier or 4 diode sub board should have +~17 and -~16 coming out. Then C31 should have +~17, and C32 should have -~17 on the minus terminat. Ther are two caps at each place because electrolytics filter low frequencies out (like 60 hz) , and ceramic or small plastic film caps filter the higher frequencies (like RF from a lamp dimmer coming in the aC line). Then goes into the 7815 regulator and comes outa, and minus on the 7915 if that is what the negative regulator is. The solder joints and circuit board traces are just as suspect as the components, perhaps even more so. If the circuit board trace has the voltage, but the leg of the IC doesn't going in, that means a bad solder joint at that place.
So voltage goes inta 7815 one pin, comes out another pin, the middle connects to ground (the center point of C34-35 where the C34 - is tied to the C35 plus. You should check this point at 0 v against a more convenient for alligator clip ground point, for debugging).
So you work through looking for where the power supply stops. it eventually gets to pin 8 (+15) of the 4558, and pin 4 (-15) if those are DIP packages, but I don't recommend using a meter probe on those because it is easy to slip, short two pins, and burn the 4558 up. To actually look there, you need a Pamona DIP clip (if you have dip packages, not SIP) and a Pamona grabber to put on the pin you are measureing, before you turn the power on. No probe slipping that way.
It is a great puzzle, more fun than crossword puzzles to me, and who needs recreational drugs when you have broken electronics or cars to fix? I've been attempted raccoon proofing my garage roof eave this week; now that is a real puzzle. **** animal vandals even wear masks to show which side they are on, the evil side.
Have fun.
I debug DC problems on the goes inta doesn't come outa technique. If the transformer has mains AC going in, ~17 AC will come out to the right places to the bridge rectifier "436?". then the plus and minus terminals of the bridge rectifier or 4 diode sub board should have +~17 and -~16 coming out. Then C31 should have +~17, and C32 should have -~17 on the minus terminat. Ther are two caps at each place because electrolytics filter low frequencies out (like 60 hz) , and ceramic or small plastic film caps filter the higher frequencies (like RF from a lamp dimmer coming in the aC line). Then goes into the 7815 regulator and comes outa, and minus on the 7915 if that is what the negative regulator is. The solder joints and circuit board traces are just as suspect as the components, perhaps even more so. If the circuit board trace has the voltage, but the leg of the IC doesn't going in, that means a bad solder joint at that place.
So voltage goes inta 7815 one pin, comes out another pin, the middle connects to ground (the center point of C34-35 where the C34 - is tied to the C35 plus. You should check this point at 0 v against a more convenient for alligator clip ground point, for debugging).
So you work through looking for where the power supply stops. it eventually gets to pin 8 (+15) of the 4558, and pin 4 (-15) if those are DIP packages, but I don't recommend using a meter probe on those because it is easy to slip, short two pins, and burn the 4558 up. To actually look there, you need a Pamona DIP clip (if you have dip packages, not SIP) and a Pamona grabber to put on the pin you are measureing, before you turn the power on. No probe slipping that way.
It is a great puzzle, more fun than crossword puzzles to me, and who needs recreational drugs when you have broken electronics or cars to fix? I've been attempted raccoon proofing my garage roof eave this week; now that is a real puzzle. **** animal vandals even wear masks to show which side they are on, the evil side.
Have fun.
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O god i just worked on a roof last week.. Not funny at all! 🙂
So what you do is actually to start tracing the voltage at the beginning and looking where it stops, uh? I've read some people do the exact reverse.. Anyway, both solutions makes sense to me and i think it's a good way of understanding circuits either way. Maybe "yours" is simplier to me.
You refer at the beginning of your post to C34/35, but they are the ceramic ones.. Maybe you mean C44/45?
The schematic is blurry, i am sorry for that.. Couldn't do any better.
The D6 is in the amp schematic page on the top right of the paper, it comes after R24 and eventually goes into the delay relé circuit of the power amp.
This is by the way good information for me, so the 15V rail is used for that.
Don't know what you mean for "436", sorry.
Let me repeat, the problem with this amp is that i got no hi freq output and a very low (distorted) low freq out. The 21V rail is what i'll check first, cause it feeds the 1875.
Every day i learn something new about this circuit, thank you a lot.
Btw, i've read several times the topic of east electronics.. Great information my friend, i think i will print that down and hang it on my bedroom wall. Hope my girlfriend approves.. 😀
So what you do is actually to start tracing the voltage at the beginning and looking where it stops, uh? I've read some people do the exact reverse.. Anyway, both solutions makes sense to me and i think it's a good way of understanding circuits either way. Maybe "yours" is simplier to me.
You refer at the beginning of your post to C34/35, but they are the ceramic ones.. Maybe you mean C44/45?
The schematic is blurry, i am sorry for that.. Couldn't do any better.
The D6 is in the amp schematic page on the top right of the paper, it comes after R24 and eventually goes into the delay relé circuit of the power amp.
This is by the way good information for me, so the 15V rail is used for that.
Don't know what you mean for "436", sorry.
Let me repeat, the problem with this amp is that i got no hi freq output and a very low (distorted) low freq out. The 21V rail is what i'll check first, cause it feeds the 1875.
Every day i learn something new about this circuit, thank you a lot.
Btw, i've read several times the topic of east electronics.. Great information my friend, i think i will print that down and hang it on my bedroom wall. Hope my girlfriend approves.. 😀
well i'd start by injecting signal and using an old school signal tracer to at least locate the fault area. from the description and a brief look at the schematic i think the fault is probably in the crossover section.
i'd also inject signal at the input of each amp after the crossover to determine if both amps and associated drivers are ok.
i'd also inject signal at the input of each amp after the crossover to determine if both amps and associated drivers are ok.
Hi turk,
I am a rookie here, so i have to ask you:
1 why you think it could be a crossover associate problem?
2 the crossover is active, so are you talking in a more generic way as "the problem is probably in the preamp section"?
3 i don't have any signal tracer (yet). You mean something like this, right? https://www.velleman.eu/downloads/0/illustrated/illustrated_assembly_manual_k7000.pdf
Btw, i am testing right now and all the rails voltages of the power amp SEEMS to be fine..
EDIT
i tried the incriminated power amp with the pre of the other speaker, it works fine. So it has to be the pre of the faulty speaker. Can't wait open that thing up.
I am a rookie here, so i have to ask you:
1 why you think it could be a crossover associate problem?
2 the crossover is active, so are you talking in a more generic way as "the problem is probably in the preamp section"?
3 i don't have any signal tracer (yet). You mean something like this, right? https://www.velleman.eu/downloads/0/illustrated/illustrated_assembly_manual_k7000.pdf
Btw, i am testing right now and all the rails voltages of the power amp SEEMS to be fine..
EDIT
i tried the incriminated power amp with the pre of the other speaker, it works fine. So it has to be the pre of the faulty speaker. Can't wait open that thing up.
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