surface mount (guitar amp hisses) question...

[Mike S]

I\'ve got a Stagg 10 GA 10W RMS Guitar Amp, it has a serious hiss that
comes and goes even with nothing plugged into it. So far I can\'t find a
schematic for it, asked Stagg for one but no reply yet. I don\'t see any
swollen caps, cleaned the pots with contact cleaner, the only things I
can find so far (with no o-scoe) - are one component, I believe it\'s a
zener diode, gets hot to the touch, and two surface mount components may
have fallen off, I don\'t see them anywhere but they may have fallen out
and been eaten by the carpet during disassembly.

This is the amp.
https://imgur.com/jIXc5Be

These are the two locations on the ct. bd. where there is a little glob
of red stuff, which might mean that a component lived there at one time.
https://imgur.com/CzGJ1LS
https://imgur.com/jIXc5Be

What I think are Zeners, one of which gets pretty hot.
https://imgur.com/NQu2dDS

A terrible shot of the whole ct. board from the top, there\'s one output
transistor, there are two preamp chips (I believe) on the bottom of the
board.
https://imgur.com/zBnbbjR

Do those red globs mean anything?

 

[ohg...@gmail.com]

On Friday, January 14, 2022 at 11:41:59 PM UTC-5, Mike S wrote:

I\'ve got a Stagg 10 GA 10W RMS Guitar Amp, it has a serious hiss that
comes and goes even with nothing plugged into it. So far I can\'t find a
schematic for it, asked Stagg for one but no reply yet. I don\'t see any
swollen caps, cleaned the pots with contact cleaner, the only things I
can find so far (with no o-scoe) - are one component, I believe it\'s a
zener diode, gets hot to the touch, and two surface mount components may
have fallen off, I don\'t see them anywhere but they may have fallen out
and been eaten by the carpet during disassembly.

This is the amp.
https://imgur.com/jIXc5Be

These are the two locations on the ct. bd. where there is a little glob
of red stuff, which might mean that a component lived there at one time.
https://imgur.com/CzGJ1LS
https://imgur.com/jIXc5Be

What I think are Zeners, one of which gets pretty hot.
https://imgur.com/NQu2dDS

A terrible shot of the whole ct. board from the top, there\'s one output
transistor, there are two preamp chips (I believe) on the bottom of the
board.
https://imgur.com/zBnbbjR

Do those red globs mean anything?

The fact that the adhesive exists in those locations doesn\'t mean those locations were ever populated. I\'ve worked on thousands of smd boards and most have locations that were never populated from the factory, but every location had the adhesive applied anyway. This could be because it was decided those components weren\'t needed, there was a running change of other values of components loaded, or there may be circuit \"refinements\" that are only used on some more featured boards.

Because they are designed with \"ZD\" and you can see \"12V\", those diodes are zeners. What is the voltage across the zeners? The fact that there are two of them tell me they\'re not just crowbars, but are active devices that are meant to provide a more or less regulated 12V all the time. If the voltage across them is 12V, they\'re still good most likely. If the voltage is low, one or both are probably leaky. If they read 12V across them, you want to verify the voltage fed to them on other side of the current limit resistor. For instance, if they were being fed by a 15V regulated source that has failed and is now feeding 20 volts to them, they will get hot and may not short for some time.

 

[ohg...@gmail.com]

On Friday, January 14, 2022 at 11:41:59 PM UTC-5, Mike S wrote:

What I think are Zeners, one of which gets pretty hot.
https://imgur.com/NQu2dDS

Oops, those two zeners aren\'t paralleled. Still, the advice to check the voltage across the hot one and the feed voltage across the feed resistor is the same.

 

[Peter W.]

My observations are as follows, with the caveat that I know nothing at all about this amp specifically.

a) I see a lot of what looks like corrosion at the solder points. Check to see if any of those are cracked or otherwise corroded beyond reliability.
b) Intermittent hissing is more often than not thermal in nature, and typically a semi-conductor (diode, transistor or similar) goes short with heat. Your first answer is directly on-point for testing the zeners.
c) I would not hit a glass 1/2-watt diode with freeze spray - but do check upstream of those diodes and try to determine if they are being fed too high a voltage, or downstream for a short such that they are passing too much current.

Good luck with it!

Peter Wieck
Melrose Park, PA

 

[legg]

On Fri, 14 Jan 2022 20:41:53 -0800, Mike S <mscir@yahoo.com> wrote:

>https://imgur.com/jIXc5Be

None of the imgur links will display on my browsers, but
surplus epoxy glue dots shouldn\'t be an issue, unless they
are obviously flattened/distorted from actual use.
Depopulated board positions likely still use full population
adhesive dot location programming.

The hiss might be self-oscillation - possibly visible on a
scope. Check input / output signal filters, zobel network
components on the output. Possibly sensitive to temperature.

Could be problems with simple frequency comp components -
intermittent cap or rc network component, or bad switch
contents in the equalisation bypass (if present).

Mechanical grounding connections for good components also
suspect.

RL

 

[Three Jeeps]

On Friday, January 14, 2022 at 11:41:59 PM UTC-5, Mike S wrote:

I\'ve got a Stagg 10 GA 10W RMS Guitar Amp, it has a serious hiss that
comes and goes even with nothing plugged into it. So far I can\'t find a
schematic for it, asked Stagg for one but no reply yet. I don\'t see any
swollen caps, cleaned the pots with contact cleaner, the only things I
can find so far (with no o-scoe) - are one component, I believe it\'s a
zener diode, gets hot to the touch, and two surface mount components may
have fallen off, I don\'t see them anywhere but they may have fallen out
and been eaten by the carpet during disassembly.

This is the amp.
https://imgur.com/jIXc5Be

These are the two locations on the ct. bd. where there is a little glob
of red stuff, which might mean that a component lived there at one time.
https://imgur.com/CzGJ1LS
https://imgur.com/jIXc5Be

What I think are Zeners, one of which gets pretty hot.
https://imgur.com/NQu2dDS

A terrible shot of the whole ct. board from the top, there\'s one output
transistor, there are two preamp chips (I believe) on the bottom of the
board.
https://imgur.com/zBnbbjR

Do those red globs mean anything?

I suggest that you carefully inspect solder joints and traces looking for crystalline (cold) joints or micro-cracks in the ckt board. Touch up any suspected joint with soldering iron, and fluxed solder. Also check wired connections in/out of the board.
Random/variable hissing on the audio output can often be attributed to \'noisy\' semiconductors, e.g. transistors, diodes. Output transistors are a likely suspect as they experience a fair amount of heat that can cause degradation.
If you know how to use a scope you could inject and trace a signal and see where it acquires a lot of noise. Another approach is to change any transistor in the audio signal path (s).

Good luck
J

 

[Phil Allison]

Three Jeeps wrote:
==============

Random/variable hissing on the audio output can often be attributed to \'noisy\' semiconductors, e.g. transistors, diodes.

** Not diodes.

> Output transistors are a likely suspect as they experience a fair amount of heat that can cause degradation.

** ROTFL - you are just making shit up !!

> If you know how to use a scope you could inject and trace a signal and see where it acquires a lot of noise.

** Or just turn the vol pot down and see if that changes things.

> Another approach is to change any transistor in the audio signal path (s).

Piss off fool.

...... Phil