Maker Pro
Maker Pro

stereo amp cleaning

B

bz

Jan 1, 1970
0
An oscilloscope and signal generator are handy, but again is somewhat
hit or miss (for me) when I get into a transistor network as it's not
immediately obvious where the signal path is. It's almost easier in a
newer amp with IC's... what you end up doing is replacing an IC with ten
transistors in it because you know the problems's gotta' be in there
someplace if you've got signal going in and none coming out. If the IC
is not obsolete and/or proprietary and hence unavailable. In the past
I've used the scope to localize distortion which saves A LOT of time and
lifting of legs to test components.

If you have a scope (it need not be an expensive one or have high freq
response) one of the handiest trouble shooting devices I have is a curve
tracer I built that is similar to this one:
http://www.techlib.com/electronics/curvetrace.html

Mine is like the single range model.

Anyway, I find it very useful for locating bad transistors and bad gates on
IC because the 'knee' gets rounded or is absent on bad junctions.

Often, you can test 'in circuit', but sometimes you will have to cut some
traces to isolate the junction from caps or resisitors that are in parallel
with it.

A little practice with one of these, and a few 'reference junctions' to
compare with the curves you see, and you may love your curve tracer too.

--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

[email protected] remove ch100-5 to avoid spam trap
 
M

Mark D. Zacharias

Jan 1, 1970
0
bz said:
If you have a scope (it need not be an expensive one or have high freq
response) one of the handiest trouble shooting devices I have is a
curve tracer I built that is similar to this one:
http://www.techlib.com/electronics/curvetrace.html

Mine is like the single range model.

Anyway, I find it very useful for locating bad transistors and bad
gates on IC because the 'knee' gets rounded or is absent on bad
junctions.

Often, you can test 'in circuit', but sometimes you will have to cut
some traces to isolate the junction from caps or resisitors that are
in parallel with it.

A little practice with one of these, and a few 'reference junctions'
to compare with the curves you see, and you may love your curve
tracer too.


I use a very similar device and have for years. It was instrumental in
making me a halfway decent tech. Without having this available I might have
given up on being a tech and moved on to something else before I ever "got a
clue".

Once a person has learned how to use this thing it really speeds up the
process of doing quick checks on components in-circuit. A leaky transistor
junction becomes obvious. Open capacitor, no problem. You can even connect
it to a phototransistor and fire a remote at the transistor and observe it
turning on and off on your 'scope screen (though this doesn't tell how well
the device works in-circuit...)


Mark Z.
 
D

Dave

Jan 1, 1970
0
Mark D. Zacharias said:
I use a very similar device and have for years. It was instrumental in
making me a halfway decent tech. Without having this available I might
have given up on being a tech and moved on to something else before I ever
"got a clue".

Once a person has learned how to use this thing it really speeds up the
process of doing quick checks on components in-circuit. A leaky transistor
junction becomes obvious. Open capacitor, no problem. You can even connect
it to a phototransistor and fire a remote at the transistor and observe it
turning on and off on your 'scope screen (though this doesn't tell how
well the device works in-circuit...)


Mark Z.
I must say I am intrigued... sort of an all-in-one tester which costs less
than $5 to make. Doesn't get much better than that. What is the value or
the variable resistor ? 1K? 100K? Or would one use the guess and check
method for an acceptable output on the scope?

Dave
 
B

bz

Jan 1, 1970
0
I must say I am intrigued... sort of an all-in-one tester which costs
less than $5 to make. Doesn't get much better than that. What is the
value or the variable resistor ? 1K? 100K? Or would one use the guess
and check method for an acceptable output on the scope?


Yep. You want to have a good horizontal deflection. Depends on the gain on
your horizontal input.

I built my into a wallwart, brought the scope leads for the resistors
directly out through some slits in the case. I just clip the probes from my
scope to the leads.

Glued a diode to the case to use as a reference.

Used a few feet of light duty speaker wire and a couple of E-Z Hook clips
for the circuit board test end.



--
bz 73 de N5BZ k

please pardon my infinite ignorance, the set-of-things-I-do-not-know is an
infinite set.

[email protected] remove ch100-5 to avoid spam trap
 
G

GregS

Jan 1, 1970
0
I must say I am intrigued... sort of an all-in-one tester which costs less
than $5 to make. Doesn't get much better than that. What is the value or
the variable resistor ? 1K? 100K? Or would one use the guess and check
method for an acceptable output on the scope?

Seems like some cheap scopes came with a component tester. probably about
the same. Reminded me I somehow aquired a commercial unit, so i am looking at it, never used it.
It has a 555 oscillator and some circuitry, but I don'r have a schematic. Can't find
it on the web but will look some more. i should use this usefull tool.

greg
 
D

Dave

Jan 1, 1970
0
GregS said:
Seems like some cheap scopes came with a component tester. probably about
the same. Reminded me I somehow aquired a commercial unit, so i am looking
at it, never used it.
It has a 555 oscillator and some circuitry, but I don'r have a schematic.
Can't find
it on the web but will look some more. i should use this usefull tool.

greg

If the circuit is anything like the one in the link bz posted, it's a few
resistors and a transformer, shouldn't be hard to figure out schematic.
 
D

Dave

Jan 1, 1970
0
Mark D. Zacharias said:
Man.

This might be really easy. Q 405 conrols the turn-on delay. HK's don't use
relays, so they mute the signal til the amp stabilizes. On this model it
is Q405 and Q406 respectively. The transistor could be bad, but I would be
especially concerned with D401, R405, C405, C407, and D403, which is a 15
volt zener.

Could just be solder connections relating to the above, but in any case
you need to see that Q405 turns off hard a few seconds after turn-on.
The -12.5 or so volts at the base is critical.

Mark Z.
Something is still failing in my amp. As I noted, I put the original caps
(C401, 403, 405, 407) back in and the problem went away. For awhile. It's
back... I measured voltages after the right channel had cut out.

The collector of Q407 was 0V. So was the collector and base of Q409, and
the base of Q413. Makes sense as they're all tied together. The collector
of Q405 was also 0V. the collectors of Q411 and Q413 were ALSO 0V. I noted
that although R437 tested fine, it looks like it's been run hot. As in it's
dark brown and you can't really read the color bands any more.

I don't understand how Q405 works as far as the turn-on delay, but if it
were bad could it be causing no voltage where I've indicated?

I tested the other, working channel and all voltages were correct.

Thanks

Dave
 
M

Mark D. Zacharias

Jan 1, 1970
0
Dave said:
Something is still failing in my amp. As I noted, I put the original
caps (C401, 403, 405, 407) back in and the problem went away. For
awhile. It's back... I measured voltages after the right channel
had cut out.
The collector of Q407 was 0V. So was the collector and base of Q409,
and the base of Q413. Makes sense as they're all tied together. The
collector of Q405 was also 0V. the collectors of Q411 and Q413 were
ALSO 0V. I noted that although R437 tested fine, it looks like it's
been run hot. As in it's dark brown and you can't really read the
color bands any more.
I don't understand how Q405 works as far as the turn-on delay, but if
it were bad could it be causing no voltage where I've indicated?

I tested the other, working channel and all voltages were correct.

Thanks

Dave

I need the base and emitter voltages on Q405 versus Q406. I wasn't thinking
clearly before, but the basic principle is the same. As the power supply
comes up, a positive voltage is passed through D401 and D402 to turn ON Q405
and Q406. In the case of Q405, you can see that there should be a negative
voltage at the emitter, and a somewhat LESS negative voltage at the base.
This turns ON the transistor and should pass a negative voltage through the
transistor, so that the negative 7 volts or so appears at the collector. If
there's no voltage at the collector of Q405, it's likely there is no
negative voltage at the emitter either. Possibly R429 or R429 are opening
up, or there could be bad solder connections on one or both of them.

Let me know what you find.

Mark Z.
 
D

Dave

Jan 1, 1970
0
Mark D. Zacharias said:
I need the base and emitter voltages on Q405 versus Q406. I wasn't
thinking clearly before, but the basic principle is the same. As the power
supply comes up, a positive voltage is passed through D401 and D402 to
turn ON Q405 and Q406. In the case of Q405, you can see that there should
be a negative voltage at the emitter, and a somewhat LESS negative voltage
at the base. This turns ON the transistor and should pass a negative
voltage through the transistor, so that the negative 7 volts or so appears
at the collector. If there's no voltage at the collector of Q405, it's
likely there is no negative voltage at the emitter either. Possibly R429
or R429 are opening up, or there could be bad solder connections on one or
both of them.

Let me know what you find.
Oops I guess I omitted information... had typed it then deleted it as
insignificant. I did test the negative power rail at R417 where the schemo
shows -14.2V. It was, I think around -14.9V. In the zone anyways which
would rule out R427 or R429 as culprits. If one of these had opened up
would I not likely see a positive voltage at the base of Q405?

Base Collector Emitter
Q405 -14.2 -0.4 -14.7
Q406 -12.7 -6.9 -13.5

As you can see the base vs. emitter of the two transistors has a comparable
delta... say a half-volt lower at the base which is what you'd expect I
think.

So, if I have the correct voltages at the base and emitter of Q405, but the
wrong collector voltage, I should be looking hard at Q405? I would think
that the -0.4V seen at the collector would be derived from the 2 x -0.6V
coming out of Q401 and Q403 attenuated by R413/R415/R483 with no
contribution from Q405.

Dave
 
M

Mark D. Zacharias

Jan 1, 1970
0
Dave said:
Oops I guess I omitted information... had typed it then deleted it as
insignificant. I did test the negative power rail at R417 where the
schemo shows -14.2V. It was, I think around -14.9V. In the zone
anyways which would rule out R427 or R429 as culprits. If one of
these had opened up would I not likely see a positive voltage at the
base of Q405?
Base Collector Emitter
Q405 -14.2 -0.4 -14.7
Q406 -12.7 -6.9 -13.5

As you can see the base vs. emitter of the two transistors has a
comparable delta... say a half-volt lower at the base which is what
you'd expect I think.

So, if I have the correct voltages at the base and emitter of Q405,
but the wrong collector voltage, I should be looking hard at Q405? I
would think that the -0.4V seen at the collector would be derived
from the 2 x -0.6V coming out of Q401 and Q403 attenuated by
R413/R415/R483 with no contribution from Q405.

Dave


I do think I'd replace Q405 at this point. You say the B-E voltage is
comparable but I don't think so. There's a big difference between 0.5 volts
and 0.8 volts when it comes to turning on a transistor. If the transistor
isn't the problem I'd still say it's not turning on sufficiently. Could be a
resistor is not open but the value has changed, for example.


Mark Z.

Mark Z.
 
D

Dave

Jan 1, 1970
0
Mark D. Zacharias said:
I do think I'd replace Q405 at this point. You say the B-E voltage is
comparable but I don't think so. There's a big difference between 0.5
volts and 0.8 volts when it comes to turning on a transistor. If the
transistor isn't the problem I'd still say it's not turning on
sufficiently. Could be a resistor is not open but the value has changed,
for example.
Well, I'll lift the legs of R427 and R429... and, I suppose, R407, R417 and
R405 just to rule out the resistors as I likely have replacements kicking
around for them whereas the transistor will need to be procured. Something
has got to be causing the ~1.5V differential between the R and L amp
channels in the first place.

I could add a small ~40-ohm resistor to the base, too and see if Q405 turns
on with a 0.8V B-E drop... FYI the spec drop is only 0.6V and I doubt the
designer would cut it so close that a 0.1V difference causes failure... but
them I'm often surprised.

Will post results, might be awhile before I get my hands on the transistor.

Out of curiousity, what does the Q5/Q6 circuit which biases Q405 do? As
near as I can tell, it takes 30VAC prior to rectification, runs it through a
diode to separate out the negative half of the wave, then past a 12V zener
to drop the voltage. Not sure what the function of D9 is, it is oriented
the reverse of D8. you end up with -1.1V at the base of Q5. The net result
being -0.56V out of the collector of Q6 to bias Q405/6. Seems like a lot of
work to derive a half-volt of regulated power but I guess they wanted it
fully independent of the amp channel power rails.

Thanks again.

Dave
 
M

Mark D. Zacharias

Jan 1, 1970
0
Dave said:
Well, I'll lift the legs of R427 and R429... and, I suppose, R407,
R417 and R405 just to rule out the resistors as I likely have
replacements kicking around for them whereas the transistor will need
to be procured. Something has got to be causing the ~1.5V
differential between the R and L amp channels in the first place.

I could add a small ~40-ohm resistor to the base, too and see if Q405
turns on with a 0.8V B-E drop... FYI the spec drop is only 0.6V and I
doubt the designer would cut it so close that a 0.1V difference
causes failure... but them I'm often surprised.

Will post results, might be awhile before I get my hands on the
transistor.
Out of curiousity, what does the Q5/Q6 circuit which biases Q405 do? As
near as I can tell, it takes 30VAC prior to rectification, runs it
through a diode to separate out the negative half of the wave, then
past a 12V zener to drop the voltage. Not sure what the function of
D9 is, it is oriented the reverse of D8. you end up with -1.1V at
the base of Q5. The net result being -0.56V out of the collector of
Q6 to bias Q405/6. Seems like a lot of work to derive a half-volt of
regulated power but I guess they wanted it fully independent of the
amp channel power rails.
Thanks again.

Dave

The function of Q5 and 6 is just to provide a more positive bias to Q405
after a delay (mainly determined by R24 and C22), enabling the amp channel
to "un-mute". It neen't be actually "positive" just more positive than the
negative rail at the emitter 0f Q401. Diode D9 is a question mark to me
also, but I suspect that since the capacitor C21 is only 33uF then some
ripple would still be present, and the diode might then act similar to a
zener and clamp the voltage drop across R24. I have to admit that since I'm
no engineer I can lose it when trying to figure the theory aspects. I just
fix them when they break.

You can swap Q405 and Q406 and see if the problem moves to the other
channel. Resistor R417 (560 ohm) can be checked in circuit. R405, a 68K,
would probably need to be checked out-of-circuit. Experience tells me that
high-value resistors can change value or go open-circuit for no good reason.
Worth keeping in the back of your mind sometimes as you're troubleshooting
problems such as this.

Not sure how you would use a 40 ohm added to the base of Q405.... in series
with R405 (68K) would do nothing, and in parallel with the BE junction of
Q405 would just keep the transistor from ever turning on in this
application.

At this point I think R427 and 429 are probably OK, or you wouldn't see the
voltage at the emitter of Q405. Our problem seems to involve getting 405 to
turn on, which is to say, getting the base of Q405 a bit more positve (less
negative) than it is right now.


Mark Z.
 
D

Dave

Jan 1, 1970
0
Mark D. Zacharias said:
Dave wrote:
You can swap Q405 and Q406 and see if the problem moves to the other
channel. Resistor R417 (560 ohm) can be checked in circuit. R405, a 68K,
would probably need to be checked out-of-circuit. Experience tells me that
high-value resistors can change value or go open-circuit for no good
reason. Worth keeping in the back of your mind sometimes as you're
troubleshooting problems such as this.
R417 and R418 both test around 560 in circuit

R405 and R406 both test 66k in circuit

I'll try swapping Q405 and Q406 and see if the problem follows the
transistor. I found them relatively close (4 hour drive) in stock, or I can
order them in to my local supplier and wait a couple of days/weeks to
receive. How many do you think I'd have to order to find a pair with DC
gain matched to within, say, 15%? The 2SC2603 crosses to NTE289, which is
available as NTE289AMP, a matched pair for use in amplifiers...

For sure Q405 is not turning on. I watched the output of Q406 and, about 5
seconds after turning power on, it goes from -0.4 to -7.1V. Q405 just stays
at -0.4V.

Dave
 
D

Dave

Jan 1, 1970
0
Mark D. Zacharias said:
Not sure how you would use a 40 ohm added to the base of Q405.... in
series with R405 (68K) would do nothing, and in parallel with the BE
junction of Q405 would just keep the transistor from ever turning on in
this application.
I meant in series with R407 to drop another 0.2V at the base of Q405 to try
to get it to turn on.
 
M

Mark D. Zacharias

Jan 1, 1970
0
Dave said:
R417 and R418 both test around 560 in circuit

R405 and R406 both test 66k in circuit

I'll try swapping Q405 and Q406 and see if the problem follows the
transistor. I found them relatively close (4 hour drive) in stock,
or I can order them in to my local supplier and wait a couple of
days/weeks to receive. How many do you think I'd have to order to
find a pair with DC gain matched to within, say, 15%? The 2SC2603
crosses to NTE289, which is available as NTE289AMP, a matched pair
for use in amplifiers...
For sure Q405 is not turning on. I watched the output of Q406 and,
about 5 seconds after turning power on, it goes from -0.4 to -7.1V. Q405
just stays at -0.4V.

Dave

If they are using a 2SC2603 then it is not critical. The 2603 is a pretty
standard signal transistor - nothing special. A 2SC945 would be fine, heck,
even a ECG / NTE 123AP. Just make sure the basing is correct. An American
type transistor will usually go EBC rather than ECB as viewed from the
front.

Mark Z.
 
M

Mark D. Zacharias

Jan 1, 1970
0
Dave said:
I meant in series with R407 to drop another 0.2V at the base of Q405
to try to get it to turn on.

Ah. I get it.

40 ohms wouldn't get it. I'd try maybe about 1 kOhm to about 2.2K.

Mark Z.
 
D

Dave

Jan 1, 1970
0
Mark D. Zacharias said:
Ah. I get it.

40 ohms wouldn't get it. I'd try maybe about 1 kOhm to about 2.2K.

Mark Z.
I figured that if 3.6K dropped the negative rail voltage from -42.6 to -14.2
(through R427/R429) then that's about 125 ohms/V. 40 ohms ~= 1/3V.
 
D

Dave

Jan 1, 1970
0
Mark D. Zacharias said:
If they are using a 2SC2603 then it is not critical. The 2603 is a pretty
standard signal transistor - nothing special. A 2SC945 would be fine,
heck, even a ECG / NTE 123AP. Just make sure the basing is correct. An
American type transistor will usually go EBC rather than ECB as viewed
from the front.

OK. I have lots of 2SC945's, they're in everything. But... I swapped Q405
and Q406 and the problem DID NOT follow the transistor.

I kept thinking about C405 and C407. If either one leaked, even a little
bit, that would throw off the base voltage of Q405, maybe enough to keep it
from turning on. So I threw in a new C405 and, voila, Q405 now shows -5.9V
at the collector. Great, I says to myself, I am so smart. (Note to self,
find new source for electrolytic caps, two bad ones out of 30 so far). Set
about checking other voltages in the channel, but HEY! what the ????
Something else is not right.

The voltages that were out were:

Base Collector Emitter
Q409 +34.2 +34.2
Q411 +33.0
Q413 +33.9 +33.8
Q401 +0.2
Q403 +0.4

I didn't check the collector voltages of Q401/Q403, I'll do that next but...
I'd be surprised if they weren't +40.5 given that all the other positive
voltages check out.

The negative power rail is fine at -43.5V, and I am getting my -12.5 at the
base of Q405 so it's not the negative supply.

If Q409 fails to turn on, then that would make Q413 not turn on either.
There would be then be no negative voltage applied to the collector of Q411
to get down to the +1.1V we want to see. I'll check out R439.

I wouldn't have expected to see a positive voltage at the collector of
either Q409 OR Q413 given that they're basically connected to the negative
power rail through a small value resistor. Almost makes you think that if
R439 is open, R445 would be too.

But then we're back to my basic lack of understanding of how the transistor
works...

I think I'm learning something here.

Dave
 
D

Dave

Jan 1, 1970
0
Dave said:
OK. I have lots of 2SC945's, they're in everything. But... I swapped
Q405 and Q406 and the problem DID NOT follow the transistor.

I kept thinking about C405 and C407. If either one leaked, even a little
bit, that would throw off the base voltage of Q405, maybe enough to keep
it from turning on. So I threw in a new C405 and, voila, Q405 now
shows -5.9V at the collector. Great, I says to myself, I am so smart.
(Note to self, find new source for electrolytic caps, two bad ones out of
30 so far). Set about checking other voltages in the channel, but HEY!
what the ???? Something else is not right.

The voltages that were out were:

Base Collector Emitter
Q409 +34.2 +34.2
Q411 +33.0
Q413 +33.9 +33.8
Q401 +0.2
Q403 +0.4

I didn't check the collector voltages of Q401/Q403, I'll do that next
but... I'd be surprised if they weren't +40.5 given that all the other
positive voltages check out.

The negative power rail is fine at -43.5V, and I am getting my -12.5 at
the base of Q405 so it's not the negative supply.

If Q409 fails to turn on, then that would make Q413 not turn on either.
There would be then be no negative voltage applied to the collector of
Q411 to get down to the +1.1V we want to see. I'll check out R439.

I wouldn't have expected to see a positive voltage at the collector of
either Q409 OR Q413 given that they're basically connected to the negative
power rail through a small value resistor. Almost makes you think that if
R439 is open, R445 would be too.

But then we're back to my basic lack of understanding of how the
transistor works...

I think I'm learning something here.

Dave
My kids have a game which is a 4 x 4 grid of holes. At any given time,
there is a mouse poking out of one hole. One pounds the mouse into the hole
with a hammer and this forces another mouse out of another hole. Sort of
how I feel here... fix one thing and it appears to break another.
 
D

Dave

Jan 1, 1970
0
Dave said:
OK. I have lots of 2SC945's, they're in everything. But... I swapped
Q405 and Q406 and the problem DID NOT follow the transistor.

I kept thinking about C405 and C407. If either one leaked, even a little
bit, that would throw off the base voltage of Q405, maybe enough to keep
it from turning on. So I threw in a new C405 and, voila, Q405 now
shows -5.9V at the collector. Great, I says to myself, I am so smart.
(Note to self, find new source for electrolytic caps, two bad ones out of
30 so far). Set about checking other voltages in the channel, but HEY!
what the ???? Something else is not right.

The voltages that were out were:

Base Collector Emitter
Q409 +34.2 +34.2
Q411 +33.0
Q413 +33.9 +33.8
Q401 +0.2
Q403 +0.4

I didn't check the collector voltages of Q401/Q403, I'll do that next
but... I'd be surprised if they weren't +40.5 given that all the other
positive voltages check out.

The negative power rail is fine at -43.5V, and I am getting my -12.5 at
the base of Q405 so it's not the negative supply.

If Q409 fails to turn on, then that would make Q413 not turn on either.
There would be then be no negative voltage applied to the collector of
Q411 to get down to the +1.1V we want to see. I'll check out R439.

I wouldn't have expected to see a positive voltage at the collector of
either Q409 OR Q413 given that they're basically connected to the negative
power rail through a small value resistor. Almost makes you think that if
R439 is open, R445 would be too.

But then we're back to my basic lack of understanding of how the
transistor works...

I think I'm learning something here.

Dave
What would happen if C415 were open?
 
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