Home |
Search |
Today's Posts |
|
Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
Reply |
|
LinkBack | Thread Tools | Display Modes |
#1
Posted to sci.electronics.repair
|
|||
|
|||
amplifier loss of signal strength
I recently installed a transformer (12VAC, 2Amp) in a musical
instrument amplifier (rated at 10 Watts RMS), and when I test it with a variety of input sources (guitars, keyboards, different cables), for about 30 seconds, the amplifier produces a nice healthy volume level, and then, consistently, the volume just drops about 50%. For some reason, this problem sounds like it might be heat-related. Has anyone else experienced this? Thanks Louis |
#2
Posted to sci.electronics.repair
|
|||
|
|||
amplifier loss of signal strength
"ll" wrote in message oups.com... I recently installed a transformer (12VAC, 2Amp) in a musical instrument amplifier (rated at 10 Watts RMS), and when I test it with a variety of input sources (guitars, keyboards, different cables), for about 30 seconds, the amplifier produces a nice healthy volume level, and then, consistently, the volume just drops about 50%. For some reason, this problem sounds like it might be heat-related. Has anyone else experienced this? Thanks Louis Several questions. Why were you having to 'install' the Tx in the first place ? Was it a replacement for one that had failed ? If so, in what way had it failed, and was there any apparent reason for the failure ? If this Tx is a replacement, is it an exact manufacturer's original, or a 'ringer' that you've sourced yourself ? If a ringer, is the output voltage exactly the same as the original ? Have you checked if any of the power supply rails are dropping as the output diminishes ? If the power supply uses monolithic regulator ICs ( 78xx / 79xx series ) then if these are subjected to excessive input voltage, compared to the amount of heatsinking that they have, they will rapidly overheat, which will cause them to retreat into their SOA, by reducing their output voltage. Also, if the mounting bolts to the heatsink have come loose, or the heatsink compound behind them has dried out and powdered, the same thing will happen, even with the correct level of input voltage. Does anything mounted on a heatsink ie regulators, output IC or transistors, 'feel' or even smell like it's getting too hot ? Very often in small or cheap amps, the heatsinking is only just about adequate for the job, and the devices will run very hot even in normal circumstances. Never-the-less, they should still not be so hot after 30 seconds, that they are so uncomfortable to touch that you can't keep your finger tip on for more than a couple of seconds. If you suspect that you have found a device that's getting too hot, if you can find enough exposed metal on the heatsink, you can try clipping on an office bulldog clip to temporarily increase the amount of heatsinking to se if that makes it stay on longer. Arfa |
#3
Posted to sci.electronics.repair
|
|||
|
|||
amplifier loss of signal strength
ll wrote: I recently installed a transformer (12VAC, 2Amp) in a musical instrument amplifier (rated at 10 Watts RMS), and when I test it with a variety of input sources (guitars, keyboards, different cables), for about 30 seconds, the amplifier produces a nice healthy volume level, and then, consistently, the volume just drops about 50%. For some reason, this problem sounds like it might be heat-related. Has anyone else experienced this? Are you by any chance sine wave testing it for those 30 seconds ? Graham |
#4
Posted to sci.electronics.repair
|
|||
|
|||
amplifier loss of signal strength
"Eeyore" wrote in message ... ll wrote: I recently installed a transformer (12VAC, 2Amp) in a musical instrument amplifier (rated at 10 Watts RMS), and when I test it with a variety of input sources (guitars, keyboards, different cables), for about 30 seconds, the amplifier produces a nice healthy volume level, and then, consistently, the volume just drops about 50%. For some reason, this problem sounds like it might be heat-related. Has anyone else experienced this? Are you by any chance sine wave testing it for those 30 seconds ? Graham I took it he wasn't, as he says that "when I test it with a variety of input *sources* ... " and then specifically mentions guitars and keyboards. Arfa |
#5
Posted to sci.electronics.repair
|
|||
|
|||
amplifier loss of signal strength
Arfa Daily wrote: "Eeyore" wrote ll wrote: I recently installed a transformer (12VAC, 2Amp) in a musical instrument amplifier (rated at 10 Watts RMS), and when I test it with a variety of input sources (guitars, keyboards, different cables), for about 30 seconds, the amplifier produces a nice healthy volume level, and then, consistently, the volume just drops about 50%. For some reason, this problem sounds like it might be heat-related. Has anyone else experienced this? Are you by any chance sine wave testing it for those 30 seconds ? I took it he wasn't, as he says that "when I test it with a variety of input *sources* ... " and then specifically mentions guitars and keyboards. I was a bit dozy at the time. I've seen that effect though. Graham |
#6
Posted to sci.electronics.repair
|
|||
|
|||
amplifier loss of signal strength
"Eeyore" wrote in message ... Arfa Daily wrote: "Eeyore" wrote ll wrote: I recently installed a transformer (12VAC, 2Amp) in a musical instrument amplifier (rated at 10 Watts RMS), and when I test it with a variety of input sources (guitars, keyboards, different cables), for about 30 seconds, the amplifier produces a nice healthy volume level, and then, consistently, the volume just drops about 50%. For some reason, this problem sounds like it might be heat-related. Has anyone else experienced this? Are you by any chance sine wave testing it for those 30 seconds ? I took it he wasn't, as he says that "when I test it with a variety of input *sources* ... " and then specifically mentions guitars and keyboards. I was a bit dozy at the time. I've seen that effect though. Graham Likewise. I often deliberately test group amps with a 400Hz tone for a few minutes, just to get the output stages thoroughly thrashed up to temperature. Arfa |
#7
Posted to sci.electronics.repair
|
|||
|
|||
amplifier loss of signal strength
"Meat Plow" wrote in message ... On Tue, 03 Jul 2007 08:28:35 +0000, Arfa Daily wrote: "Eeyore" wrote in message ... Arfa Daily wrote: "Eeyore" wrote ll wrote: I recently installed a transformer (12VAC, 2Amp) in a musical instrument amplifier (rated at 10 Watts RMS), and when I test it with a variety of input sources (guitars, keyboards, different cables), for about 30 seconds, the amplifier produces a nice healthy volume level, and then, consistently, the volume just drops about 50%. For some reason, this problem sounds like it might be heat-related. Has anyone else experienced this? Are you by any chance sine wave testing it for those 30 seconds ? I took it he wasn't, as he says that "when I test it with a variety of input *sources* ... " and then specifically mentions guitars and keyboards. I was a bit dozy at the time. I've seen that effect though. Graham Likewise. I often deliberately test group amps with a 400Hz tone for a few minutes, just to get the output stages thoroughly thrashed up to temperature. I made a looped test CD with me playing some leads, licks and riffs on my Les Paul through my Boss GT-6 just to test guitar amps. You should send me the files !! Arfa |
#8
Posted to sci.electronics.repair
|
|||
|
|||
amplifier loss of signal strength
On Jul 3, 8:18 am, "Arfa Daily" wrote:
"Meat Plow" wrote in message ... On Tue, 03 Jul 2007 08:28:35 +0000, Arfa Daily wrote: "Eeyore" wrote in message ... Arfa Daily wrote: "Eeyore" wrote ll wrote: I recently installed a transformer (12VAC, 2Amp) in a musical instrument amplifier (rated at 10 Watts RMS), and when I test it with a variety of input sources (guitars, keyboards, different cables), for about 30 seconds, the amplifier produces a nice healthy volume level, and then, consistently, the volume just drops about 50%. For some reason, this problem sounds like it might be heat-related. Has anyone else experienced this? Are you by any chance sine wave testing it for those 30 seconds ? I took it he wasn't, as he says that "when I test it with a variety of input *sources* ... " and then specifically mentions guitars and keyboards. I was a bit dozy at the time. I've seen that effect though. Graham Likewise. I often deliberately test group amps with a 400Hz tone for a few minutes, just to get the output stages thoroughly thrashed up to temperature. I made a looped test CD with me playing some leads, licks and riffs on my Les Paul through my Boss GT-6 just to test guitar amps. You should send me the files !! Arfa Yes, guitars, lots of guitars (a variety) were used to test the amp. The tx was an exact replacement. It seems we have narrowed this down to heat-related. |
#9
Posted to sci.electronics.repair
|
|||
|
|||
amplifier loss of signal strength
"ll" wrote in message ups.com... On Jul 3, 8:18 am, "Arfa Daily" wrote: "Meat Plow" wrote in message ... On Tue, 03 Jul 2007 08:28:35 +0000, Arfa Daily wrote: "Eeyore" wrote in message ... Arfa Daily wrote: "Eeyore" wrote ll wrote: I recently installed a transformer (12VAC, 2Amp) in a musical instrument amplifier (rated at 10 Watts RMS), and when I test it with a variety of input sources (guitars, keyboards, different cables), for about 30 seconds, the amplifier produces a nice healthy volume level, and then, consistently, the volume just drops about 50%. For some reason, this problem sounds like it might be heat-related. Has anyone else experienced this? Are you by any chance sine wave testing it for those 30 seconds ? I took it he wasn't, as he says that "when I test it with a variety of input *sources* ... " and then specifically mentions guitars and keyboards. I was a bit dozy at the time. I've seen that effect though. Graham Likewise. I often deliberately test group amps with a 400Hz tone for a few minutes, just to get the output stages thoroughly thrashed up to temperature. I made a looped test CD with me playing some leads, licks and riffs on my Les Paul through my Boss GT-6 just to test guitar amps. You should send me the files !! Arfa Yes, guitars, lots of guitars (a variety) were used to test the amp. The tx was an exact replacement. It seems we have narrowed this down to heat-related. In what way had the original Tx failed ? Might have some relevance to the problem, which although may be heat-related, might not be a 'heat' problem - if you see what I mean ... For instance, it may be that one of the supply rails is dropping due to a monolithic regulator IC going into heat foldback, but that could be because something on the end of it is drawing too much current. I have also had this occur due to open circuit decoupling caps on the regulators, allowing them to hoot, and even open circuit decouplers around the output IC, or open circuit Zobel networks on the output, either of which can allow the stage to hoot ultrasonically. Arfa |
#10
Posted to sci.electronics.repair
|
|||
|
|||
amplifier loss of signal strength
Arfa Daily wrote: In what way had the original Tx failed ? Might have some relevance to the problem, which although may be heat-related, might not be a 'heat' problem - if you see what I mean ... For instance, it may be that one of the supply rails is dropping due to a monolithic regulator IC going into heat foldback, but that could be because something on the end of it is drawing too much current. Or the screw holding it to its heatsink may have come loose. Basically it needs proper fault finding ! Monitoring the supply rail(s) is the obvious starting place. Graham |
#11
Posted to sci.electronics.repair
|
|||
|
|||
amplifier loss of signal strength
On Jul 3, 11:57 am, Meat Plow wrote:
On Tue, 03 Jul 2007 14:09:20 +0000, ll wrote: On Jul 3, 8:18 am, "Arfa Daily" wrote: "Meat Plow" wrote in message ... On Tue, 03 Jul 2007 08:28:35 +0000, Arfa Daily wrote: "Eeyore" wrote in message ... Arfa Daily wrote: "Eeyore" wrote ll wrote: I recently installed a transformer (12VAC, 2Amp) in a musical instrument amplifier (rated at 10 Watts RMS), and when I test it with a variety of input sources (guitars, keyboards, different cables), for about 30 seconds, the amplifier produces a nice healthy volume level, and then, consistently, the volume just drops about 50%. For some reason, this problem sounds like it might be heat-related. Has anyone else experienced this? Are you by any chance sine wave testing it for those 30 seconds ? I took it he wasn't, as he says that "when I test it with a variety of input *sources* ... " and then specifically mentions guitars and keyboards. I was a bit dozy at the time. I've seen that effect though. Graham Likewise. I often deliberately test group amps with a 400Hz tone for a few minutes, just to get the output stages thoroughly thrashed up to temperature. I made a looped test CD with me playing some leads, licks and riffs on my Les Paul through my Boss GT-6 just to test guitar amps. You should send me the files !! Arfa Yes, guitars, lots of guitars (a variety) were used to test the amp. The tx was an exact replacement. It seems we have narrowed this down to heat-related. Ok one last time, why was the transformer replaced? What led up to that diagnosis? Thanks - the tx was replaced because there was 0V at the secondaries. |
#12
Posted to sci.electronics.repair
|
|||
|
|||
amplifier loss of signal strength
"Eeyore" wrote in message ... Arfa Daily wrote: In what way had the original Tx failed ? Might have some relevance to the problem, which although may be heat-related, might not be a 'heat' problem - if you see what I mean ... For instance, it may be that one of the supply rails is dropping due to a monolithic regulator IC going into heat foldback, but that could be because something on the end of it is drawing too much current. Or the screw holding it to its heatsink may have come loose. Basically it needs proper fault finding ! Monitoring the supply rail(s) is the obvious starting place. Graham I did detail the mounting bolt problem in my initial reply, Graham Arfa |
#13
Posted to sci.electronics.repair
|
|||
|
|||
amplifier loss of signal strength
"Meat Plow" wrote in message ... On Tue, 03 Jul 2007 13:18:52 +0000, Arfa Daily wrote: "Meat Plow" wrote in message ... On Tue, 03 Jul 2007 08:28:35 +0000, Arfa Daily wrote: "Eeyore" wrote in message ... Arfa Daily wrote: "Eeyore" wrote ll wrote: I recently installed a transformer (12VAC, 2Amp) in a musical instrument amplifier (rated at 10 Watts RMS), and when I test it with a variety of input sources (guitars, keyboards, different cables), for about 30 seconds, the amplifier produces a nice healthy volume level, and then, consistently, the volume just drops about 50%. For some reason, this problem sounds like it might be heat-related. Has anyone else experienced this? Are you by any chance sine wave testing it for those 30 seconds ? I took it he wasn't, as he says that "when I test it with a variety of input *sources* ... " and then specifically mentions guitars and keyboards. I was a bit dozy at the time. I've seen that effect though. Graham Likewise. I often deliberately test group amps with a 400Hz tone for a few minutes, just to get the output stages thoroughly thrashed up to temperature. I made a looped test CD with me playing some leads, licks and riffs on my Les Paul through my Boss GT-6 just to test guitar amps. You should send me the files !! Arfa Probably could in MP3 form. Would have to dig it up tho. These days when someone brings one I just get out a guitar . I won't work on any SS stuff these days, just vintage Marshall, Fender, Ampeg, Traynor and a few stray oddball Silvertone, Alamo, Gibson, etc...Back in the day my services were pretty heavily sought. Tried to train a couple young apprentices because I had so much work but it was too hard to train and do top quality repair at the same time. Nobody wanted to do tube repair and I didn't want to do SS guitar, still don't unless you come groveling on your knees with a wad of green in your hand. I used to do my fair share SS of high power pro audio amps like Soundcraftsman, BGW, Crown, SCS, Peavey. Those I did have some help from a friend who did Yamaha warranty work but he didn't know **** about a tube Know what you mean. None of the kids seem to have any aptitude for high voltage stuff any more. I far prefer working on tubed stuff to SS. At least it's well behaved, and fixes - and better yet *stays* fixed - with a minimum of fuss. The circuitry is straightforward, and understandable. At a pinch, you can pretty well fix any tube amp without schematics, just by having an understanding of the principles of operation. Unfortunately, the music shop in my village only has one person doing their service work - me ! So I have to take on all-comers, although it is a proper music shop, always having several long-haired 'musos' hanging about talking, or jamming in the play test room, so a large proportion of the gear is tubed anyway. Back in the old days, I knew Jim Marshall. Their works is just down the way a few miles from where I live. Jim himself custom built an amp and console for a very good friend of mine who ran a mobile disco, when such things were new. I used to run the light show for him. We were out on the road 6 nights a week, and I reckon that it's got to be one of the best periods in my life .... Arfa Arfa |
#14
Posted to sci.electronics.repair
|
|||
|
|||
amplifier loss of signal strength
On Jul 3, 9:55 pm, Meat Plow wrote:
On Wed, 04 Jul 2007 01:51:27 +0000, Arfa Daily wrote: "Meat Plow" wrote in message ... On Tue, 03 Jul 2007 13:18:52 +0000, Arfa Daily wrote: "Meat Plow" wrote in message ... On Tue, 03 Jul 2007 08:28:35 +0000, Arfa Daily wrote: "Eeyore" wrote in message ... Arfa Daily wrote: "Eeyore" wrote ll wrote: I recently installed a transformer (12VAC, 2Amp) in a musical instrument amplifier (rated at 10 Watts RMS), and when I test it with a variety of input sources (guitars, keyboards, different cables), for about 30 seconds, the amplifier produces a nice healthy volume level, and then, consistently, the volume just drops about 50%. For some reason, this problem sounds like it might be heat-related. Has anyone else experienced this? Are you by any chance sine wave testing it for those 30 seconds ? I took it he wasn't, as he says that "when I test it with a variety of input *sources* ... " and then specifically mentions guitars and keyboards. I was a bit dozy at the time. I've seen that effect though. Graham Likewise. I often deliberately test group amps with a 400Hz tone for a few minutes, just to get the output stages thoroughly thrashed up to temperature. I made a looped test CD with me playing some leads, licks and riffs on my Les Paul through my Boss GT-6 just to test guitar amps. You should send me the files !! Arfa Probably could in MP3 form. Would have to dig it up tho. These days when someone brings one I just get out a guitar . I won't work on any SS stuff these days, just vintage Marshall, Fender, Ampeg, Traynor and a few stray oddball Silvertone, Alamo, Gibson, etc...Back in the day my services were pretty heavily sought. Tried to train a couple young apprentices because I had so much work but it was too hard to train and do top quality repair at the same time. Nobody wanted to do tube repair and I didn't want to do SS guitar, still don't unless you come groveling on your knees with a wad of green in your hand. I used to do my fair share SS of high power pro audio amps like Soundcraftsman, BGW, Crown, SCS, Peavey. Those I did have some help from a friend who did Yamaha warranty work but he didn't know **** about a tube Know what you mean. None of the kids seem to have any aptitude for high voltage stuff any more. I far prefer working on tubed stuff to SS. At least it's well behaved, and fixes - and better yet *stays* fixed - with a minimum of fuss. The circuitry is straightforward, and understandable. At a pinch, you can pretty well fix any tube amp without schematics, just by having an understanding of the principles of operation. Unfortunately, the music shop in my village only has one person doing their service work - me ! So I have to take on all-comers, although it is a proper music shop, always having several long-haired 'musos' hanging about talking, or jamming in the play test room, so a large proportion of the gear is tubed anyway. Sounds like one of the shops I did work for. I wasn't the resident but was the only Fender and Marshall specialist around. I really tried to stick with a couple brand names but always got tossed others. Back then I had long hair tied in a pony and a face full of beard so I had the rocker image plus I was that rocker too Back in the old days, I knew Jim Marshall. Their works is just down the way a few miles from where I live. Jim himself custom built an amp and console for a very good friend of mine who ran a mobile disco, when such things were new. I used to run the light show for him. We were out on the road 6 nights a week, and I reckon that it's got to be one of the best periods in my life I used to run lights and sound in between gigs for friends or customers caught in a jamb. Lots of times I would get up and fill in on a set too. Those were some great days of my life. That sounds great. Did Jim do much custom work, outside of Marshall? I would love to learn about tube repair - my father did quite a bit of that, as well as in organs. Speaking of that, Emerson's organ technicians must get quite a workout! Back to the amp in question - it's rated at 10watts RMS. I did a few more 'listening' tests with it yesterday, and made more comparisons. I was able to have the amp on for about 15 minutes before the volume 'cut out' occurred (no variation in the input source - same guitar). I then turned it off and then back on - the signal was strong again. This isn't consistent, though, as I could turn off, wait about 10 seconds and then turn on again, and it might or might not have the full volume. I tested the line out and headphone jacks, and they each produced a healthy audio output. One item of note: when plugging into the line out, the amp's internal speaker went back to that 'cut out'/ half volume sound. This would occur, even if just plugging in an unconnected guitar cable. When the internal speaker was producing that weak output, the amp into which the line out was plugged was producing a healthy signal. The headphone jack had the same effect. After I had turned it off and on several times, I would hear a 'scratchy' sound (like turning dirty pots) just after turning back on. Now I'm wondering if speaker wires/connections might be suspect? Thanks |
#15
Posted to sci.electronics.repair
|
|||
|
|||
amplifier loss of signal strength
ll wrote:
On Jul 3, 9:55 pm, Meat Plow wrote: On Wed, 04 Jul 2007 01:51:27 +0000, Arfa Daily wrote: "Meat Plow" wrote in message ... On Tue, 03 Jul 2007 13:18:52 +0000, Arfa Daily wrote: "Meat Plow" wrote in message ... On Tue, 03 Jul 2007 08:28:35 +0000, Arfa Daily wrote: "Eeyore" wrote in message ... Arfa Daily wrote: "Eeyore" wrote ll wrote: I recently installed a transformer (12VAC, 2Amp) in a musical instrument amplifier (rated at 10 Watts RMS), and when I test it with a variety of input sources (guitars, keyboards, different cables), for about 30 seconds, the amplifier produces a nice healthy volume level, and then, consistently, the volume just drops about 50%. For some reason, this problem sounds like it might be heat-related. Has anyone else experienced this? Are you by any chance sine wave testing it for those 30 seconds ? I took it he wasn't, as he says that "when I test it with a variety of input *sources* ... " and then specifically mentions guitars and keyboards. I was a bit dozy at the time. I've seen that effect though. Graham Likewise. I often deliberately test group amps with a 400Hz tone for a few minutes, just to get the output stages thoroughly thrashed up to temperature. I made a looped test CD with me playing some leads, licks and riffs on my Les Paul through my Boss GT-6 just to test guitar amps. You should send me the files !! Arfa Probably could in MP3 form. Would have to dig it up tho. These days when someone brings one I just get out a guitar . I won't work on any SS stuff these days, just vintage Marshall, Fender, Ampeg, Traynor and a few stray oddball Silvertone, Alamo, Gibson, etc...Back in the day my services were pretty heavily sought. Tried to train a couple young apprentices because I had so much work but it was too hard to train and do top quality repair at the same time. Nobody wanted to do tube repair and I didn't want to do SS guitar, still don't unless you come groveling on your knees with a wad of green in your hand. I used to do my fair share SS of high power pro audio amps like Soundcraftsman, BGW, Crown, SCS, Peavey. Those I did have some help from a friend who did Yamaha warranty work but he didn't know **** about a tube Know what you mean. None of the kids seem to have any aptitude for high voltage stuff any more. I far prefer working on tubed stuff to SS. At least it's well behaved, and fixes - and better yet *stays* fixed - with a minimum of fuss. The circuitry is straightforward, and understandable. At a pinch, you can pretty well fix any tube amp without schematics, just by having an understanding of the principles of operation. Unfortunately, the music shop in my village only has one person doing their service work - me ! So I have to take on all-comers, although it is a proper music shop, always having several long-haired 'musos' hanging about talking, or jamming in the play test room, so a large proportion of the gear is tubed anyway. Sounds like one of the shops I did work for. I wasn't the resident but was the only Fender and Marshall specialist around. I really tried to stick with a couple brand names but always got tossed others. Back then I had long hair tied in a pony and a face full of beard so I had the rocker image plus I was that rocker too Back in the old days, I knew Jim Marshall. Their works is just down the way a few miles from where I live. Jim himself custom built an amp and console for a very good friend of mine who ran a mobile disco, when such things were new. I used to run the light show for him. We were out on the road 6 nights a week, and I reckon that it's got to be one of the best periods in my life I used to run lights and sound in between gigs for friends or customers caught in a jamb. Lots of times I would get up and fill in on a set too. Those were some great days of my life. That sounds great. Did Jim do much custom work, outside of Marshall? I would love to learn about tube repair - my father did quite a bit of that, as well as in organs. Speaking of that, Emerson's organ technicians must get quite a workout! Back to the amp in question - it's rated at 10watts RMS. I did a few more 'listening' tests with it yesterday, and made more comparisons. I was able to have the amp on for about 15 minutes before the volume 'cut out' occurred (no variation in the input source - same guitar). I then turned it off and then back on - the signal was strong again. This isn't consistent, though, as I could turn off, wait about 10 seconds and then turn on again, and it might or might not have the full volume. I tested the line out and headphone jacks, and they each produced a healthy audio output. One item of note: when plugging into the line out, the amp's internal speaker went back to that 'cut out'/ half volume sound. This would occur, even if just plugging in an unconnected guitar cable. When the internal speaker was producing that weak output, the amp into which the line out was plugged was producing a healthy signal. The headphone jack had the same effect. After I had turned it off and on several times, I would hear a 'scratchy' sound (like turning dirty pots) just after turning back on. Now I'm wondering if speaker wires/connections might be suspect? Thanks Sounds like a dodgy 'insert jack' Are there contacts on the line out jack? maybe they are dirty or one of the soldered joints is fractured. Ron(UK) -- Lune Valley Audio Public Address Systems Hire Sales Maintenance www.lunevalleyaudio.com |
#16
Posted to sci.electronics.repair
|
|||
|
|||
amplifier loss of signal strength
"Ron(UK)" wrote in message ... ll wrote: On Jul 3, 9:55 pm, Meat Plow wrote: On Wed, 04 Jul 2007 01:51:27 +0000, Arfa Daily wrote: "Meat Plow" wrote in message ... On Tue, 03 Jul 2007 13:18:52 +0000, Arfa Daily wrote: "Meat Plow" wrote in message ... On Tue, 03 Jul 2007 08:28:35 +0000, Arfa Daily wrote: "Eeyore" wrote in message ... Arfa Daily wrote: "Eeyore" wrote ll wrote: I recently installed a transformer (12VAC, 2Amp) in a musical instrument amplifier (rated at 10 Watts RMS), and when I test it with a variety of input sources (guitars, keyboards, different cables), for about 30 seconds, the amplifier produces a nice healthy volume level, and then, consistently, the volume just drops about 50%. For some reason, this problem sounds like it might be heat-related. Has anyone else experienced this? Are you by any chance sine wave testing it for those 30 seconds ? I took it he wasn't, as he says that "when I test it with a variety of input *sources* ... " and then specifically mentions guitars and keyboards. I was a bit dozy at the time. I've seen that effect though. Graham Likewise. I often deliberately test group amps with a 400Hz tone for a few minutes, just to get the output stages thoroughly thrashed up to temperature. I made a looped test CD with me playing some leads, licks and riffs on my Les Paul through my Boss GT-6 just to test guitar amps. You should send me the files !! Arfa Probably could in MP3 form. Would have to dig it up tho. These days when someone brings one I just get out a guitar . I won't work on any SS stuff these days, just vintage Marshall, Fender, Ampeg, Traynor and a few stray oddball Silvertone, Alamo, Gibson, etc...Back in the day my services were pretty heavily sought. Tried to train a couple young apprentices because I had so much work but it was too hard to train and do top quality repair at the same time. Nobody wanted to do tube repair and I didn't want to do SS guitar, still don't unless you come groveling on your knees with a wad of green in your hand. I used to do my fair share SS of high power pro audio amps like Soundcraftsman, BGW, Crown, SCS, Peavey. Those I did have some help from a friend who did Yamaha warranty work but he didn't know **** about a tube Know what you mean. None of the kids seem to have any aptitude for high voltage stuff any more. I far prefer working on tubed stuff to SS. At least it's well behaved, and fixes - and better yet *stays* fixed - with a minimum of fuss. The circuitry is straightforward, and understandable. At a pinch, you can pretty well fix any tube amp without schematics, just by having an understanding of the principles of operation. Unfortunately, the music shop in my village only has one person doing their service work - me ! So I have to take on all-comers, although it is a proper music shop, always having several long-haired 'musos' hanging about talking, or jamming in the play test room, so a large proportion of the gear is tubed anyway. Sounds like one of the shops I did work for. I wasn't the resident but was the only Fender and Marshall specialist around. I really tried to stick with a couple brand names but always got tossed others. Back then I had long hair tied in a pony and a face full of beard so I had the rocker image plus I was that rocker too Back in the old days, I knew Jim Marshall. Their works is just down the way a few miles from where I live. Jim himself custom built an amp and console for a very good friend of mine who ran a mobile disco, when such things were new. I used to run the light show for him. We were out on the road 6 nights a week, and I reckon that it's got to be one of the best periods in my life I used to run lights and sound in between gigs for friends or customers caught in a jamb. Lots of times I would get up and fill in on a set too. Those were some great days of my life. That sounds great. Did Jim do much custom work, outside of Marshall? I would love to learn about tube repair - my father did quite a bit of that, as well as in organs. Speaking of that, Emerson's organ technicians must get quite a workout! Back to the amp in question - it's rated at 10watts RMS. I did a few more 'listening' tests with it yesterday, and made more comparisons. I was able to have the amp on for about 15 minutes before the volume 'cut out' occurred (no variation in the input source - same guitar). I then turned it off and then back on - the signal was strong again. This isn't consistent, though, as I could turn off, wait about 10 seconds and then turn on again, and it might or might not have the full volume. I tested the line out and headphone jacks, and they each produced a healthy audio output. One item of note: when plugging into the line out, the amp's internal speaker went back to that 'cut out'/ half volume sound. This would occur, even if just plugging in an unconnected guitar cable. When the internal speaker was producing that weak output, the amp into which the line out was plugged was producing a healthy signal. The headphone jack had the same effect. After I had turned it off and on several times, I would hear a 'scratchy' sound (like turning dirty pots) just after turning back on. Now I'm wondering if speaker wires/connections might be suspect? Thanks Sounds like a dodgy 'insert jack' Are there contacts on the line out jack? maybe they are dirty or one of the soldered joints is fractured. Ron(UK) I think that the key here is that he says it works ok from the headphone jack, when the speaker level has gone low. On a small amp like this, the headphone jack is almost certainly straight across the output, with the internal speaker switched by the socket's contacts, so that would leave only bad contacts on the jack, bad wiring to the speaker, or of course, the speaker itself. My next move would be to shove a good squib of switch cleaner / lubricant straight into the headphone jack entry, and then work a plug in and out vigourously for 10 seconds, then try again. If it now stays on, then you either need to get at the contacts and clean them properly, or better, replace the jack. As far as Jim doing custom work goes, I really don't know if he did a lot. This is going back like 35 years, when Marshall was a very small company. I met him as a result of going there a number of times with the friend who knew him well. At that time, mobile discos were a very new thing, and there was perhaps 4 or 5 only in my town and surrounding area, which is quite large. Jim built a custom semi-circular shaped console for my friend, with a preamp / mixer set in the middle between the two decks. This preamp had such innovative features ( for the time ) as slider level controls, separate mic input and gain control, headphone socket with pre fade listen and so on. The decks were a pair of Garrard semi autos, with the auto mech wedged, such that they would unload at the end of a record, when you could just lift off the current record, and drop the next one on. The deck would carry on to self cue, at which point you flicked the deck power off, then used the PFL to set your 'ready to roll' cue point. It took quite a bit of skill to run it, but it was a very smooth operation when you got used to doing it. The amp that Jim built him was a pretty meaty thing - 100 watts as I recall. It used four output tubes in parallel push pull. I seem to recall that they were KT66's or maybe even 88's. They are the true beam tetrodes over the cross substitutable EL34's, which are normally accepted as being pentodes, but are sometimes actually beam tetrodes inside. The amp definitely had the standard gold front, and Marshall logo on it. The preamp face plate was also gold, but I can't remember whether or not it had the logo. I would guess that they were probably built through the factory, at a time when any business was good business, and custom designs were taken on and built in that little factory. That console was mighty heavy I can tell you, and many was the time that I cursed it at two in the morning in the rain, as we loaded it back on that old 15cwt Ford Thames van ... Arfa |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
aerial signal strength | UK diy | |||
Aerial Signal Strength | UK diy | |||
measure signal strength? | Home Repair | |||
Measuring Cable signal strength | UK diy | |||
Poor WIFI Signal strength | UK diy |