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Left channel dead till crank

K

Kermit

Jan 1, 1970
0
I have an old Vector Research VRX 9100 receiver that will (usually)not
play the left channel until you really turn it up. Once you do, you
can back the volume down to a much lower level and it will work fine.
Sometimes I turn it on and it's fine out of the chute. I assumed that
the protection relay was causing the problem so I opened a little
access port in the side of it since it was too well sealed to get open
and cleaned up the contacts for that side. Still same problem. I can
actually get in there with a pick and screw around with the contacts
when it doing this and still no dice. Thats exhausts this rookie's
limited diagnostic ability. I assume it could be an output transistor
but i don't know if they fail this way.

Thanks,

Robb
 
M

Mark D. Zacharias

Jan 1, 1970
0
The best way to check the relay is with an oscilloscope. Either the signal
is there at the input side when your problem occurs or it's not. The problem
may be almost anywhere in the circuit, but likely at some point after the
volume control, since as you noted, increasing the volume fixes the problem
momentarily.


Mark Z.
 
B

Bob Shuman

Jan 1, 1970
0
I'd have guessed, like you that this symptom was being caused by a defective
relay contact. If you are certain it is not the relay, I'd trace the
channel's output to look for a cold solder joint or a high resistance
connection that gets overcome when the voltage is increased. Make sure you
also check the speaker wire and the connections in the speaker as well.
This can be done by temporarily swapping the left and right channels at the
back of the amp and seeing if the problem follows the speaker or stays with
the left channel.

Good Luck!

Bob
 
N

Norm Dresner

Jan 1, 1970
0
Kermit said:
I have an old Vector Research VRX 9100 receiver that will (usually)not
play the left channel until you really turn it up. Once you do, you
can back the volume down to a much lower level and it will work fine.
Sometimes I turn it on and it's fine out of the chute. I assumed that
the protection relay was causing the problem so I opened a little
access port in the side of it since it was too well sealed to get open
and cleaned up the contacts for that side. Still same problem. I can
actually get in there with a pick and screw around with the contacts
when it doing this and still no dice. Thats exhausts this rookie's
limited diagnostic ability. I assume it could be an output transistor
but i don't know if they fail this way.

If you're willing to open the unit up, look for "cold solder joint(s)"
in and around the protection circuit.
 
K

Kermit

Jan 1, 1970
0
Thanks Bob and Mark,

I neglected to mention that the signal disappears at the input side of
the protection relay when this happens so I know it's not the relay
itself. I've visually checked the circuit path back a fair distance
from there and have poked and prodded a fair amount to see if I could
get it to come alive when this happens. I've also done the standard
percussive maintenance of slamming the entire unit on the counter but
still nothing :)

Can an ouput transistor fail like this? I can't imagine any of the
discretes acting this way as these are not SMT parts, so intermittents
don't seem likely but then again I don't have enough experience to
make this call. Any additional help would be greatly appreciated.

Thanks,

Robb
 
M

Mark D. Zacharias

Jan 1, 1970
0
You could try manipulating various controls and switches and see if any of
them affect your problem, aside from the volume control that is, since we
already know it does.

It's pretty unlikely that the outputs are bad on that channel., but not
impossible. Flat-pack transistors sometimes go open-circuit on the
emitter-base junctions. Goosing the volume might make them work temporarily,
though in my experience it's usually the opposite - increasing the load
makes the transistor fail.

You say you've done "percussive maintenance", but does this include
specifically tapping around on the various boards, etc?


Mark Z.
 
R

Robert and Kristi Hubbard

Jan 1, 1970
0
Robb,

I have seen this happen when dust or 'fuzz' has gotten inside the volume
control 'POT'. It will not work till the control is turned well up, and
then works when down to the regular volume, often intermittently. Dust can
also cause a crackling sound when the volume or balance is adjusted. Then
the affected control must be cleaned.

There are contact and tuner cleaners that can be used. Just find a open
hole on the control, usually where it's crimped together. Also, I came
across a service kit for one of the well known oscilloscope brands and they
said to use WD-40 to do the job. You just need to use what you think will
work best, IF this is the cause.

If you determine that it is the POT control, if all else fails, replace the
pot.

Rob H
 
K

Kermit

Jan 1, 1970
0
Hi Mark,

I don't see any correlation between manipulating controls other than,
as you mentioned, turning the volume way up, but I will check again.
I have poked around different components in the area to see if I could
get anything to change but no luck. With your suggestion though, and
the fact that you doubt that the transistors could be causing this
problem, I will check this again also. BTW, is it bad etiquette to
repost this if it becomes a little dated and I still don't get it
resolved?

Thanks,

Robb
 
M

Mark D. Zacharias

Jan 1, 1970
0
Not bad etiquette as long as nobody acts like a jerk along the way. So far
we're in good shape there :)

You need to try to break this down into blocks. Is the signal reaching the
main amp input, for example. Does this piece have pre / main jumpers at the
rear panel?

Often one can king of drag a finger across coupling caps etc and induce a
"buzz" that would tell you if signal is getting through from that part of
the circuit. The nice thing about stereo is there's an identical channel
there, usually laid out in a very similar manner. This can be most helpful
for comparison.


Mark Z.
 
K

Kermit

Jan 1, 1970
0
Thanks guys,

I've checked the volume and balance pots and nothing there
contributing to the problem. They were not crackly as dirty ones
usually are but I cleaned them anyway and no improvement. I may give
up on this but I have one more question. In a receiver are the volume
pots located in the pre-amp or amp circuit. I should know this but
don't.

Thanks,

Robb
 
S

sbell

Jan 1, 1970
0
Rob:

I, too, have this exact same problem with my VRX-9100. If you solve this
issue, please inform me.

sbell
 
T

TCS

Jan 1, 1970
0
I have an old Vector Research VRX 9100 receiver that will (usually)not
play the left channel until you really turn it up. Once you do, you
can back the volume down to a much lower level and it will work fine.
Sometimes I turn it on and it's fine out of the chute. I assumed that

sounds like a worn out volume potentiometer. When the wiper gets past
a certain point it makes contact again.
 
S

Sam Goldwasser

Jan 1, 1970
0
TCS said:
sounds like a worn out volume potentiometer. When the wiper gets past
a certain point it makes contact again.

No, the symptoms don't match. If it were a bad control, going back down
would cause the audio to drop out again.

Could be a bad speaker relay, some other switch or bad connection
in the left audio.

--- sam | Sci.Electronics.Repair FAQ Home Page: http://www.repairfaq.org/
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Important: The email address in this message header may no longer work. To
contact me, please use the feedback form on the S.E.R FAQ Web sites.
 
N

Norm Dresner

Jan 1, 1970
0
Sam Goldwasser said:
No, the symptoms don't match. If it were a bad control, going back down
would cause the audio to drop out again.

Could be a bad speaker relay, some other switch or bad connection
in the left audio.

We have a similar problem with a separate Stereo amplifier whose left
channel occasionally goes missing until I crank up the volume on the PreAmp.
I believe it's a relay in the amp's overload protection circuit that's the
culprit but I've never taken the time to take it apart since the cure is so
simple.

Norm
 
A

Asimov

Jan 1, 1970
0
"Norm Dresner" bravely wrote to "All" (04 Jun 04 20:41:53)
--- on the heady topic of "Re: Left channel dead till crank"

ND> From: "Norm Dresner" <[email protected]>
ND> We have a similar problem with a separate Stereo amplifier whose left
ND> channel occasionally goes missing until I crank up the volume on the
ND> PreAmp. I believe it's a relay in the amp's overload protection circuit
ND> that's the culprit but I've never taken the time to take it apart since
ND> the cure is so simple.

ND> Norm

It's not exactly a cure if it happens again, now is it? ;-)

A*s*i*m*o*v

.... Just a little force field zap.
 
N

Norm Dresner

Jan 1, 1970
0
Asimov said:
"Norm Dresner" bravely wrote to "All" (04 Jun 04 20:41:53)
--- on the heady topic of "Re: Left channel dead till crank"

ND> From: "Norm Dresner" <[email protected]>
ND> We have a similar problem with a separate Stereo amplifier whose left
ND> channel occasionally goes missing until I crank up the volume on the
ND> PreAmp. I believe it's a relay in the amp's overload protection circuit
ND> that's the culprit but I've never taken the time to take it apart since
ND> the cure is so simple.

ND> Norm

It's not exactly a cure if it happens again, now is it? ;-)

I guess I have too much of the medical view of "cure" in mind and not enough
of the engineering view, huh?

;-) Norm
 
K

Kermit

Jan 1, 1970
0
I think Norm is right. I finally got this unit to work by cleaning
the volume pot. I steered away from this as a possible culprit
because, as some others have suggested, it did not exhibit any of the
characteristics of this type of problem. The volume control was not
scratchy in the least and it would play in the full range of volume
once the circuit was established by cranking it up to 8-10 or so.
Anyway, must have been some sort of contaminant that was causing this,
as cleaning fixed it.

Robb
 
A

Asimov

Jan 1, 1970
0
"Kermit" bravely wrote to "All" (22 Jun 04 14:07:20)
--- on the heady topic of "Re: Left channel dead till crank"

That is a first for me too. Like the rest I would have suspected an
electro or a semi with a bad internal connection. Live and learn!

A*s*i*m*o*v


Ke> From: [email protected] (Kermit)

Ke> I think Norm is right. I finally got this unit to work by cleaning
Ke> the volume pot. I steered away from this as a possible culprit
Ke> because, as some others have suggested, it did not exhibit any of the
Ke> characteristics of this type of problem. The volume control was not
Ke> scratchy in the least and it would play in the full range of volume
Ke> once the circuit was established by cranking it up to 8-10 or so.
Ke> Anyway, must have been some sort of contaminant that was causing this,
Ke> as cleaning fixed it.

Ke> Robb

.... I worked hard to attach the electrodes to it.
 
G

gothika

Jan 1, 1970
0
"Kermit" bravely wrote to "All" (22 Jun 04 14:07:20)
--- on the heady topic of "Re: Left channel dead till crank"

That is a first for me too. Like the rest I would have suspected an
electro or a semi with a bad internal connection. Live and learn!

A*s*i*m*o*v


Ke> From: [email protected] (Kermit)

Ke> I think Norm is right. I finally got this unit to work by cleaning
Ke> the volume pot. I steered away from this as a possible culprit
Ke> because, as some others have suggested, it did not exhibit any of the
Ke> characteristics of this type of problem. The volume control was not
Ke> scratchy in the least and it would play in the full range of volume
Ke> once the circuit was established by cranking it up to 8-10 or so.
Ke> Anyway, must have been some sort of contaminant that was causing this,
Ke> as cleaning fixed it.

Ke> Robb

... I worked hard to attach the electrodes to it.

Oft' times the grease used to lubricate the shaft of the pot "sweats"
out onto the graphite of the potentiometer causing a loss of
conduction.
Cleaning with a mild contact cleaner will remove this waxy buildup.
 
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