Maker Pro
Maker Pro

amplifier loss of signal strength

L

ll

Jan 1, 1970
0
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
 
A

Arfa Daily

Jan 1, 1970
0
ll said:
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
 
M

Meat Plow

Jan 1, 1970
0
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

What kind of amp? Why did you replace the transformer? Replace it with
exact replacement? Schematic?
 
E

Eeyore

Jan 1, 1970
0
ll said:
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
 
A

Arfa Daily

Jan 1, 1970
0
Eeyore said:
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
 
E

Eeyore

Jan 1, 1970
0
Arfa said:
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
 
A

Arfa Daily

Jan 1, 1970
0
Eeyore said:
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
 
M

Meat Plow

Jan 1, 1970
0
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. :)
 
A

Arfa Daily

Jan 1, 1970
0
Meat Plow said:
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
 
L

ll

Jan 1, 1970
0
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.
 
A

Arfa Daily

Jan 1, 1970
0
ll said:
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
 
E

Eeyore

Jan 1, 1970
0
Arfa said:
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
 
M

Meat Plow

Jan 1, 1970
0
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 shit about a tube :)
 
M

Meat Plow

Jan 1, 1970
0
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?
 
L

ll

Jan 1, 1970
0
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.
 
M

Meat Plow

Jan 1, 1970
0
Thanks - the tx was replaced because there was 0V at the secondaries.

So the secondary was 12vac? Just trying to picture what kind of amp this
is and what the PO is. Why don't you tell me more, you've been fairly
vague so far.
 
A

Arfa Daily

Jan 1, 1970
0
Eeyore said:
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
 
A

Arfa Daily

Jan 1, 1970
0
Meat Plow said:
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 shit 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
 
M

Meat Plow

Jan 1, 1970
0
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.
 
L

ll

Jan 1, 1970
0
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 :)


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
 
Top