Maker Pro
Maker Pro

stereo amp cleaning

D

Dave

Jan 1, 1970
0
Mark D. Zacharias said:
Dave wrote:
The bias diode is essentially two diodes in series. (Sometimes three,
actually). In this case two, looks like. A diode check function of a DMM
would be OK for a static check, and if it became necessary you could use
the DMM to help match a replacement pair of diodes to replace it with.
However, using regular diodes might not work as well in terms of correctly
responding to heat changes.
The proof is in the operation, and in this case that means the actual
voltage drop across the device in-circuit, while the amp is running. Bias
diodes opening up is uncommon but it does happen, and when it does,
catastrophe is the result.

In your case the resistors may have burned because of a momentary short at
the speaker wires, etc although this would ordinarily have destroyed the
output transistors as well.

If the bias diode is two diodes in series, why does the parts list call it a
varistor? It's a funky-looking thing that looks like one of those
connectors you put at the end of a wire to connect it to a screw. Don't
think I'd likely find another one. It's physically screwed to the heatsink
the outputs use.

Has technology evolved so that a momentary crossing of speaker wires could
not destroy expensive output transistors? You see all sorts amps with
"protection" circuits in them nowadays and lots of amp module IC's which
boast both thermal and overcurrent resettable protection...

I measured D405 vs D406 in circuit with no power applied and the value,
although useless quantitatively, was the same for both channels. I'll check
it in operation when I get all the resistors replaced (and Q417).

Bonus for me: the one transistor used in that amp which I've got in stock
is 2SC945, which matches Q417 the bias transistor. I like BD Enterprises
which you directed me to, they seem to stock all sorts of transistors nobody
else does, or their search facility immediately suggests an alternative if
they don't have the original.

Dave
 
S

Smitty Two

Jan 1, 1970
0
Has technology evolved so that a momentary crossing of speaker wires could
not destroy expensive output transistors?

It hasn't as much to do with technological evolution as it has to do
with intelligent design. Hasn't there always been good stuff and bad
stuff? A good design would allow you to hard wire the speaker leads
together and go fishing.
 
D

Dave

Jan 1, 1970
0
Smitty Two said:
It hasn't as much to do with technological evolution as it has to do
with intelligent design. Hasn't there always been good stuff and bad
stuff? A good design would allow you to hard wire the speaker leads
together and go fishing.

Yes but for the same money, wouldn't you rather have a cool-looking volume
knob that glows blue in the dark or a built-in function to make it sound
like Britney is playing in a gothic cathedral than a protection-oriented
design? Sadly it appears most people would.
 
M

Mark D. Zacharias

Jan 1, 1970
0
Smitty said:
It hasn't as much to do with technological evolution as it has to do
with intelligent design. Hasn't there always been good stuff and bad
stuff? A good design would allow you to hard wire the speaker leads
together and go fishing.

Sorry. From a technical standpoint, that is an idiotic statement, one that
has been made before, and disproven time and again. People like to make
statements like "This amp will drive a nail!" etc but it simply isn't true.
It is the nature of bipolar transistors, in fact any semiconductor, that
they cannot carry current much beyond their rating for more than a
split-second. The instantaneous current demanded by a short-circuit will
destroy ANY semiconductor output device unless that device is being so
limited that it cannot produce music, which is dynamic in nature. Any
protection circuit is there simply to protect the speakers from an amp
failure, and to protect the amp only if the overload isn't too severe. This
presumes that the sensing circuit will sense a large (but not TOO large)
current surge prior to total failure. Any circuit which would really protect
the amp from a short circuit would also protect it from the music.


Mark Z.
 
M

Mark D. Zacharias

Jan 1, 1970
0
Dave said:
If the bias diode is two diodes in series, why does the parts list
call it a varistor? It's a funky-looking thing that looks like one
of those connectors you put at the end of a wire to connect it to a
screw. Don't think I'd likely find another one. It's physically
screwed to the heatsink the outputs use.

Has technology evolved so that a momentary crossing of speaker wires
could not destroy expensive output transistors? You see all sorts
amps with "protection" circuits in them nowadays and lots of amp
module IC's which boast both thermal and overcurrent resettable
protection...
I measured D405 vs D406 in circuit with no power applied and the
value, although useless quantitatively, was the same for both
channels. I'll check it in operation when I get all the resistors
replaced (and Q417).
Bonus for me: the one transistor used in that amp which I've got in
stock is 2SC945, which matches Q417 the bias transistor. I like BD
Enterprises which you directed me to, they seem to stock all sorts of
transistors nobody else does, or their search facility immediately
suggests an alternative if they don't have the original.

Dave

I was going by the schematic, and had not seen the physical mounting. Of
course the best choice to replace the "diode" would be to order it from
Harmon. I would be comfortable using diodes out of my tool box if necessary
though, using the diode check function of a DMM to help match the voltage
drop of the original (from the other channel, that is).

As I state in another post, output transistors cannot be fully protected
from shorts without also protecting them from the music.


Mark Z.
 
S

Smitty Two

Jan 1, 1970
0
Mark D. Zacharias said:
Sorry. From a technical standpoint, that is an idiotic statement, one that
has been made before, and disproven time and again. People like to make
statements like "This amp will drive a nail!" etc but it simply isn't true.
It is the nature of bipolar transistors, in fact any semiconductor, that
they cannot carry current much beyond their rating for more than a
split-second. The instantaneous current demanded by a short-circuit will
destroy ANY semiconductor output device unless that device is being so
limited that it cannot produce music, which is dynamic in nature. Any
protection circuit is there simply to protect the speakers from an amp
failure, and to protect the amp only if the overload isn't too severe. This
presumes that the sensing circuit will sense a large (but not TOO large)
current surge prior to total failure. Any circuit which would really protect
the amp from a short circuit would also protect it from the music.


Mark Z.

I stand corrected. Maybe my ignorance is the reason I'm still listening
to tubes.
 
M

Mark D. Zacharias

Jan 1, 1970
0
Smitty said:
I stand corrected. Maybe my ignorance is the reason I'm still
listening to tubes.

Yeah, I was going to mention tubes but I'd already gone on too long...

Tube amps don't like short circuits either, but due to the physical
constuction and current limitations of the tubes themselves, plus the ohmic
resistance of the transformers (small though it is) they won't ordinarily be
damaged by a short, especially one of short duration. McIntosh solid state
models using output transformers are also somewhat less likely to be damaged
by a short.

I need to stop here - I'm a repair tech, not an engineer. If I go on too
long, I'm liable to stray into engineering theory, which I try not to do,
since I'm not qualified, and the real engineering types around here will be
forced to waste time correcting me...

Enjoy your tube stuff ! Have a good one...

Mark Z.
 
G

g

Jan 1, 1970
0
I was going by the schematic, and had not seen the physical mounting. Of
course the best choice to replace the "diode" would be to order it from
Harmon. I would be comfortable using diodes out of my tool box if necessary
though, using the diode check function of a DMM to help match the voltage
drop of the original (from the other channel, that is).

As I state in another post, output transistors cannot be fully protected
from shorts without also protecting them from the music.

Mark Z.

If the amp is beefy enough, protection circuits will NOT affect the
music.

greg
 
G

GregS

Jan 1, 1970
0
If the amp is beefy enough, protection circuits will NOT affect the
music.

Well, the music wil stop if the protection circuit opperates.

greg
 
M

Marra

Jan 1, 1970
0
Smitty said:
Sorry. From a technical standpoint, that is an idiotic statement, one that
has been made before, and disproven time and again. People like to make
statements like "This amp will drive a nail!" etc but it simply isn't true.
It is the nature of bipolar transistors, in fact any semiconductor, that
they cannot carry current much beyond their rating for more than a
split-second. The instantaneous current demanded by a short-circuit will
destroy ANY semiconductor output device unless that device is being so
limited that it cannot produce music, which is dynamic in nature. Any
protection circuit is there simply to protect the speakers from an amp
failure, and to protect the amp only if the overload isn't too severe. This
presumes that the sensing circuit will sense a large (but not TOO large)
current surge prior to total failure. Any circuit which would really protect
the amp from a short circuit would also protect it from the music.

Mark Z.

This is not always true.
The power supply can only supply so much current.
Any of my designs have MOSFETs that can take more than a shorted PSU.
By the time the MOSFETS have got anywhere near warm the fuses will
have gone.

www.ckp-railways.talktalk.net/pcbcad21.htm
 
G

g

Jan 1, 1970
0
There used to be a spray on the market - can't remember who made it at
the time, might have been GC - it was called "Blue Stuff". I worked
in astereoservice department thru college, and that spray was the
best we could find forcleaningstubbornly noisy pots and switches.
When you sprayed it on it formed a mildly abrasive blue paste, which
would actually scrub the tarnish and dirt off of switch contacts. I
never saw it fail, even with the noisiest, most tarnished switches and
controls. I think it has diatomaceous earth in it.

Anyway, I think TechSpray carries it - might be worth a try
(www.techspray.com)

I just investigated this stuff, in fact I think I ordered some and
have not yet received it. Its been discontinued I think. Yes, I
ordered it two months ago, and it said they had the quanity i ordered.
Glad you brought it up. A recording studio asked me about this stuff,
and they said they depended on it. I know of no other cleaner other
than Cramolin Contaclean, that actually has something in it to
dissolve oxides.

greg
 
M

Mark D. Zacharias

Jan 1, 1970
0
g said:
If the amp is beefy enough, protection circuits will NOT affect the
music.

greg

Not necessarily affecting the sound, just triggering and shutting the amp
down when the over-current condition is detected. It would only affect the
sound if the amp driving signal is reduced or "soft-clipped" to deal with
the overload, an approach I have never been much impressed with as a
technician. The amps seem to blow up all the same, often along with the
transistors etc used to perform the limiting (old Marantz amps for example).

Mark Z.
 
M

Mark D. Zacharias

Jan 1, 1970
0
Marra said:
This is not always true.
The power supply can only supply so much current.
Any of my designs have MOSFETs that can take more than a shorted PSU.
By the time the MOSFETS have got anywhere near warm the fuses will
have gone.

www.ckp-railways.talktalk.net/pcbcad21.htm


I have an Adcom 5802 in the shop right now with shorted Mosfets which
resulted from just a momentary short at the speaker terminals.

Mark Z.
 
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