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.

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Old November 1st 04, 07:46 PM
Tom
 
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Default Technics SE-9060 DC on input potentiometer

Hi

I am trying to repair an old Technics SE-9060 amplifier.
The input stage in one of the channels sends out DC to the input
potentiometer.
The amplifier goes in protection mode unless the potentiometer is fully off
or on.
The amplifier still plays with the potentiometer fully on, but I would like
to correct the error.
I would be happy if someone could help, e.g. with the schematic of the
amplifier.
I have checked most of the transistors in the inputstage and they all seems
ok.
I am running out of ideas, so any help would be appreciated.

/Tom



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Old November 1st 04, 08:42 PM
Sofie
 
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Default

Tom:
You say the transistors on the input stage "they all seems OK"....... how
did you check them?
Look carefully at the input stage, driver stage and output stage of the
defective channel .... likely suspects would be leaky and defective
transistors or diodes... ... bad capacitors, resistors, etc.
Since you have only one bad channel you can compare your test results and
meter readings with the good channel.
--
Best Regards,
Daniel Sofie
Electronics Supply & Repair
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -


"Tom" wrote in message
...
Hi

I am trying to repair an old Technics SE-9060 amplifier.
The input stage in one of the channels sends out DC to the input
potentiometer.
The amplifier goes in protection mode unless the potentiometer is fully

off
or on.
The amplifier still plays with the potentiometer fully on, but I would

like
to correct the error.
I would be happy if someone could help, e.g. with the schematic of the
amplifier.
I have checked most of the transistors in the inputstage and they all

seems
ok.
I am running out of ideas, so any help would be appreciated.

/Tom




  #3   Report Post  
Old November 2nd 04, 05:42 PM
Tom
 
Posts: n/a
Default

You say the transistors on the input stage "they all seems OK"....... how
did you check them?
Look carefully at the input stage, driver stage and output stage of the
defective channel .... likely suspects would be leaky and defective
transistors or diodes... ... bad capacitors, resistors, etc.
Since you have only one bad channel you can compare your test results and
meter readings with the good channel.




Thanks for your reply.

I have a transistor tester, a device that can identify if a transistor is
PNP, NPN, short or open.

The amplifier is DC coupled; so a little divagation in DC level will
propagate throughout the whole amplifier.

So if I e.g. turn the potentiometer fully on or off, there does not seems to
be a problem, but if I adjust it towards the middle, the DC level increases
throughout the amplifier, so it is difficult to locate the error.

The amplifier is 28 years old, so it is likely that some of the capacitors
no longer meet their specifications. I have changed some of them in the
input stage, but no changes.



/Tom


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Old November 2nd 04, 08:07 PM
Asimov
 
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"Tom" bravely wrote to "All" (02 Nov 04 18:42:43)
--- on the heady topic of " Technics SE-9060 DC on input potentiometer"

To The amplifier is DC coupled; so a little divagation in DC level will
To propagate throughout the whole amplifier.

To So if I e.g. turn the potentiometer fully on or off, there does not
To seems to be a problem, but if I adjust it towards the middle, the DC
To level increases throughout the amplifier, so it is difficult to locate
To the error.
To The amplifier is 28 years old, so it is likely that some of the
To capacitors no longer meet their specifications. I have changed some of
To them in the input stage, but no changes.

The input stage is not the source of the dc. Since the offset is worst
with the pot in the middle this indicates the leakage is from the
amplification stage immediately following the pot's wiper. I'll bet
there is a coupling capacitor inbetween which is leaky.

With a 28 year old piece of circuitry it is possible to have a leaky
electrolytic capacitor here and there, especially if it sat idle for
some months, years, decades, and they may just need "reforming".

If the amplifier is truly DC from input to output then the problem could
be the offset trim or balance is off. Perhaps a resistor went up in
value or the trimmer has a dirty or noisy contact. It may also be a
leaky transistor though.

A*s*i*m*o*v

.... If all else fails, hurl it across the room a few times!

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Old November 3rd 04, 09:53 PM
Tom
 
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Default


The input stage is not the source of the dc. Since the offset is worst
with the pot in the middle this indicates the leakage is from the
amplification stage immediately following the pot's wiper. I'll bet
there is a coupling capacitor inbetween which is leaky.

With a 28 year old piece of circuitry it is possible to have a leaky
electrolytic capacitor here and there, especially if it sat idle for
some months, years, decades, and they may just need "reforming".

If the amplifier is truly DC from input to output then the problem could
be the offset trim or balance is off. Perhaps a resistor went up in
value or the trimmer has a dirty or noisy contact. It may also be a
leaky transistor though.


The amplifier is truly DC.
There is a switch just after the potentiometer. It says "DC or Low cut".
If the switch is in "Low cut" the signal goes from the potentiometer through
a capacitor to the input of the amplifier. If the switch is in DC, the
signal goes directly from the potentiometer to the amplifier.
If the switch is in "Low cut" the amplifier goes in protection mode, because
then the potentiometer can no longer influence on the DC level.
I have now replaced all 5 electrolytic capacitors in that channel, except
for those in the power supply just after the bridge. They are a bit special,
and their physical form needs to match for an easy replacement. I have
checked some additional transistors in the middle section of the amplifier
they were also fine.
Since I have DC out on the input, I expect the error to be in the very first
stage of the amplifier. Don't know if an error further back, can cause that.

/Tom




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Old November 4th 04, 03:58 AM
Asimov
 
Posts: n/a
Default

"Tom" bravely wrote to "All" (03 Nov 04 22:53:27)
--- on the heady topic of " Technics SE-9060 DC on input potentiometer"

To From: "Tom"
[,,,]
If the amplifier is truly DC from input to output then the problem could

[,,,]
To The amplifier is truly DC.
To There is a switch just after the potentiometer. It says "DC or Low
To cut". If the switch is in "Low cut" the signal goes from the
To potentiometer through a capacitor to the input of the amplifier. If the
To switch is in DC, the signal goes directly from the potentiometer to the
To amplifier. If the switch is in "Low cut" the amplifier goes in
To protection mode, because then the potentiometer can no longer influence
To on the DC level. I have now replaced all 5 electrolytic capacitors in
To that channel, except for those in the power supply just after the
To bridge. They are a bit special, and their physical form needs to match
To for an easy replacement. I have checked some additional transistors in
To the middle section of the amplifier they were also fine.
To Since I have DC out on the input, I expect the error to be in the very
To first stage of the amplifier. Don't know if an error further back, can
To cause that.
To /Tom

The only other thing that could cause an offset is if an output
transistor is very leaky though not totally shorted. The feedback loop
would then inject the offset current into the stage after the pot. The
point here is that it is worse with the pot near the middle but gets
better at each end. Either it is a stage offset or the output pulling.
The low cut switch really confirms the serious offset.

Try to trace the feedback loop and see if anything else connects to
it. The stage after the pot is probably differential with one side to
the pot's wiper. The other side likely has a balancing adjustment or a
fixed resistor network. Perhaps even a pot across both emitters or
sources (if JFETS). Something is upsetting the comparison voltage and
causing a large error input. Maybe even the pot wiper side device is
simply leaky. I may have the schematics of a similar Technics model but
it is on a shelf behind some speaker bins... heavy buggers!

BTW a truly DC amp is not very kind to speakers because of the danger of
damage from infrasonic signals. The speakers will pump in and out with
every wobble on an LP and may even pop out if the tonearm is
accidentally dropped. Thumps and wind noise on mics are murder too. Then
there is aliasing from digital signals which might get in. Erk!

The point of the DC amp is to have zero phase shift at bass frequencies.
In fact true DC coupling is unecessary because an amp with capacitors in
the chain can be made to have zero phase shift by adding a simple
network to correct the phase error at low audio frequencies. However,
some amps with a capacitor in the chain have a problem with damping at
low frequencies but this is rare, for example Dynaco's would motorboat.

A*s*i*m*o*v

.... A stereo system is the altar to the god of music.



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