|
Hello, I am trying to fix my Tektronix 465B Oscilloscope. I have the service manual and have been using it to try and figure out what's wrong. When turned on, there is nothing on the screen, even when pressing and holding beam finder, and the power light doesn't come on. The only control that seems to work is scale illumination, which makes the screen glow orange when turned up.
I checked the voltages at the test points on the main board A4 as described on page 4-24, and found all of them to be within spec: +55, +15, +5, and +110, EXCEPT for the -8, which measures 0.2mV. The power supply wiring diagram at the back of the manual shows two blue wires from the transformer, going through a fuse F4501 (desoldered and removed from board, it's good), then through a rectifier bridge CR4411, which is also good.
So where do I go from here? What voltage should I be seeing at the two blue wires, because I'm starting to suspect that part of the transformer? Thank you
|
Most likely a bad capacitor. It's late here and I don't have time to dig
into the manual tonight. You might take a look at the power supply schematic and
follow how the -8 volt supply is derived.
?
The -8 must be good or the scope will not work.
?
Just for the exercise, talk?your ohm meter and check that the -8 volt
test point is not shorted to ground. If so, the next most likely thing is a
shorted tantalum capacitor. Take a look at the power distribution schematic the
see where the -8 volts goes.
?
?
Have fun,
Tom
?
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----- Original Message -----
Sent: Tuesday, January 15, 2013 1:32
AM
Subject: [TekScopes] 465B Oscilloscope No
-8V
?
Hello, I am trying to fix my Tektronix 465B Oscilloscope. I have the
service manual and have been using it to try and figure out what's wrong. When
turned on, there is nothing on the screen, even when pressing and holding beam
finder, and the power light doesn't come on. The only control that seems to
work is scale illumination, which makes the screen glow orange when turned
up.
I checked the voltages at the test points on the main board A4 as
described on page 4-24, and found all of them to be within spec: +55, +15, +5,
and +110, EXCEPT for the -8, which measures 0.2mV. The power supply wiring
diagram at the back of the manual shows two blue wires from the transformer,
going through a fuse F4501 (desoldered and removed from board, it's good),
then through a rectifier bridge CR4411, which is also good.
So where
do I go from here? What voltage should I be seeing at the two blue wires,
because I'm starting to suspect that part of the transformer? Thank
you
|
you might want to check for shorted tantalum caps in -8 circuits.
see:
phil
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Show quoted text
--- In TekScopes@..., "rwdplz1" wrote:
Hello, I am trying to fix my Tektronix 465B Oscilloscope. I have the service manual and have been using it to try and figure out what's wrong. When turned on, there is nothing on the screen, even when pressing and holding beam finder, and the power light doesn't come on. The only control that seems to work is scale illumination, which makes the screen glow orange when turned up.
I checked the voltages at the test points on the main board A4 as described on page 4-24, and found all of them to be within spec: +55, +15, +5, and +110, EXCEPT for the -8, which measures 0.2mV. The power supply wiring diagram at the back of the manual shows two blue wires from the transformer, going through a fuse F4501 (desoldered and removed from board, it's good), then through a rectifier bridge CR4411, which is also good.
So where do I go from here? What voltage should I be seeing at the two blue wires, because I'm starting to suspect that part of the transformer? Thank you
|
The resistance between the -8V test point and ground (either a test point labeled ground on the board or the scope case) is 49.7 ohms.
I replaced one tantalum, C4203, 33uF, with no effect.
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Show quoted text
--- In TekScopes@..., "pdxareaid" wrote:
you might want to check for shorted tantalum caps in -8 circuits.
see:
phil
--- In TekScopes@..., "rwdplz1" wrote:
Hello, I am trying to fix my Tektronix 465B Oscilloscope. I have the service manual and have been using it to try and figure out what's wrong. When turned on, there is nothing on the screen, even when pressing and holding beam finder, and the power light doesn't come on. The only control that seems to work is scale illumination, which makes the screen glow orange when turned up.
I checked the voltages at the test points on the main board A4 as described on page 4-24, and found all of them to be within spec: +55, +15, +5, and +110, EXCEPT for the -8, which measures 0.2mV. The power supply wiring diagram at the back of the manual shows two blue wires from the transformer, going through a fuse F4501 (desoldered and removed from board, it's good), then through a rectifier bridge CR4411, which is also good.
So where do I go from here? What voltage should I be seeing at the two blue wires, because I'm starting to suspect that part of the transformer? Thank you
|
I agree with the above replies that a bad Tantalum is the most likely cause; however, I've also had a similar issue with an older 465 (non-B) where apparently a PCB via had gone bad and as a result there was a few hundred ohms in series with the output of the regulator.
Isn't there a step-by-step troubleshooting procedure for the power supply in the manual? There was one for the 465 I've been fixing.
Samuel
|
OK, good. I would think the next thing to check is C-4419, a 5000 uF can
capacitor.
?
Regards,
Tom
?
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----- Original Message -----
Sent: Tuesday, January 15, 2013 2:40
AM
Subject: [TekScopes] Re: 465B
Oscilloscope No -8V
?
The resistance between the -8V test point and ground (either a test point
labeled ground on the board or the scope case) is 49.7 ohms.
I replaced
one tantalum, C4203, 33uF, with no effect.
--- In TekScopes@...,
"pdxareaid" wrote:
>
> you might want to check for shorted
tantalum caps in -8 circuits.
> see:
>
>
phil
>
>
>
> --- In TekScopes@...,
"rwdplz1" wrote:
> >
> > Hello, I am trying to fix my
Tektronix 465B Oscilloscope. I have the service manual and have been using it
to try and figure out what's wrong. When turned on, there is nothing on the
screen, even when pressing and holding beam finder, and the power light
doesn't come on. The only control that seems to work is scale illumination,
which makes the screen glow orange when turned up.
> >
> >
I checked the voltages at the test points on the main board A4 as described on
page 4-24, and found all of them to be within spec: +55, +15, +5, and +110,
EXCEPT for the -8, which measures 0.2mV. The power supply wiring diagram at
the back of the manual shows two blue wires from the transformer, going
through a fuse F4501 (desoldered and removed from board, it's good), then
through a rectifier bridge CR4411, which is also good.
> >
>
> So where do I go from here? What voltage should I be seeing at the two
blue wires, because I'm starting to suspect that part of the transformer?
Thank you
> >
>
|
OK, I checked C4419, it is a 5000uF 35V capacitor. ESR is 0.08, which should be about right. However, after desoldering it from the board (which is a real pain with the two large mounting legs), and measuring capacitance, one of my meters couldn't measure it, and the other read 0.185 nano Farads. Both of these meters read the capacitance of other small capacitors I have on hand perfectly, so the meters are fine. ESR of the other large capacitors is about the same, but I'd rather not desolder them from the board if I don't have to, to prevent damage to the fragile traces.
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--- In TekScopes@..., "Tom Miller" wrote:
OK, good. I would think the next thing to check is C-4419, a 5000 uF can capacitor.
Regards,
Tom
----- Original Message -----
From: rwdplz1
To: TekScopes@...
Sent: Tuesday, January 15, 2013 2:40 AM
Subject: [TekScopes] Re: 465B Oscilloscope No -8V
The resistance between the -8V test point and ground (either a test point labeled ground on the board or the scope case) is 49.7 ohms.
I replaced one tantalum, C4203, 33uF, with no effect.
--- In TekScopes@..., "pdxareaid" wrote:
>
> you might want to check for shorted tantalum caps in -8 circuits.
> see:
>
> phil
>
>
>
> --- In TekScopes@..., "rwdplz1" wrote:
> >
> > Hello, I am trying to fix my Tektronix 465B Oscilloscope. I have the service manual and have been using it to try and figure out what's wrong. When turned on, there is nothing on the screen, even when pressing and holding beam finder, and the power light doesn't come on. The only control that seems to work is scale illumination, which makes the screen glow orange when turned up.
> >
> > I checked the voltages at the test points on the main board A4 as described on page 4-24, and found all of them to be within spec: +55, +15, +5, and +110, EXCEPT for the -8, which measures 0.2mV. The power supply wiring diagram at the back of the manual shows two blue wires from the transformer, going through a fuse F4501 (desoldered and removed from board, it's good), then through a rectifier bridge CR4411, which is also good.
> >
> > So where do I go from here? What voltage should I be seeing at the two blue wires, because I'm starting to suspect that part of the transformer? Thank you
> >
>
|
So that cap is bad. Do you have a 4700/35 radial lead cap that you could
replace it with? Be sure to note how the can of the old cap was part of the
ground circuit (look at the top side of the board carefully).
?
?
Tom
?
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----- Original Message -----
Sent: Tuesday, January 15, 2013 9:50
AM
Subject: [TekScopes] Re: 465B
Oscilloscope No -8V
?
OK, I checked C4419, it is a 5000uF 35V capacitor. ESR is 0.08, which
should be about right. However, after desoldering it from the board (which is
a real pain with the two large mounting legs), and measuring capacitance, one
of my meters couldn't measure it, and the other read 0.185 nano Farads. Both
of these meters read the capacitance of other small capacitors I have on hand
perfectly, so the meters are fine. ESR of the other large capacitors is about
the same, but I'd rather not desolder them from the board if I don't have to,
to prevent damage to the fragile traces.
--- In TekScopes@..., "Tom
Miller" wrote:
>
> OK, good. I would think the next thing to check
is C-4419, a 5000 uF can capacitor.
>
> Regards,
>
Tom
>
> ----- Original Message -----
> From: rwdplz1
> To: TekScopes@...
> Sent: Tuesday, January 15, 2013 2:40 AM
> Subject: [TekScopes]
Re: 465B Oscilloscope No -8V
>
>
>
> The resistance
between the -8V test point and ground (either a test point labeled ground on
the board or the scope case) is 49.7 ohms.
>
> I replaced one
tantalum, C4203, 33uF, with no effect.
>
> --- In TekScopes@...,
"pdxareaid" wrote:
> >
> > you might want to check for
shorted tantalum caps in -8 circuits.
> > see:
> >
>
> phil
> >
> >
> >
> > --- In TekScopes@...,
"rwdplz1" wrote:
> > >
> > > Hello, I am trying to fix
my Tektronix 465B Oscilloscope. I have the service manual and have been using
it to try and figure out what's wrong. When turned on, there is nothing on the
screen, even when pressing and holding beam finder, and the power light
doesn't come on. The only control that seems to work is scale illumination,
which makes the screen glow orange when turned up.
> > >
>
> > I checked the voltages at the test points on the main board A4 as
described on page 4-24, and found all of them to be within spec: +55, +15, +5,
and +110, EXCEPT for the -8, which measures 0.2mV. The power supply wiring
diagram at the back of the manual shows two blue wires from the transformer,
going through a fuse F4501 (desoldered and removed from board, it's good),
then through a rectifier bridge CR4411, which is also good.
> > >
> > > So where do I go from here? What voltage should I be seeing
at the two blue wires, because I'm starting to suspect that part of the
transformer? Thank you
> > >
>
>
>
|
Another example of why it ?makes no sense to measure ESR in circuit ( or you must be sure from the schematics there is nowhere, on any place in the scope an other cap or other conducting thing parralel to this one)
The most caps fail on low capacitance or DC leakage, ?most times in smps ESR can become a problem. But i just repaired the batterycharger of my heated motorjacket. This is a normal transformer psu and only thing I could find was to much ripple. Two electrolitic caps. Not leaking, good capacitance but a rather high D (so also higher ESR) according my GR1608 bridge. My peak esr meter thought it was fine, my home made esr meter however agreed with the GR. I replaced them and the ripple was gone. A bit odd, i think there was some chemical intbility in the cap. I have seen that once before. Monitoring the ESR that was good, and then suddenly starts to rise, wiggles a bit then suddenly falls back. I have done a lot of tests on that cap and see it o this several times. I can stil not explain what happens.
If you measure a value in the pF range or een nF for such a big cap the problem is probably electrode erosion. If you open the cap you will see the anode most times is corroded in two and you measure the capacitance between the remaining strip and the kathode ( outside can)
Fred PA4TIM
Op 15 jan. 2013 om 15:50 heeft "rwdplz1" < rwdplz1@...> het volgende geschreven:
?
OK, I checked C4419, it is a 5000uF 35V capacitor. ESR is 0.08, which should be about right. However, after desoldering it from the board (which is a real pain with the two large mounting legs), and measuring capacitance, one of my meters couldn't measure it, and the other read 0.185 nano Farads. Both of these meters read the capacitance of other small capacitors I have on hand perfectly, so the meters are fine. ESR of the other large capacitors is about the same, but I'd rather not desolder them from the board if I don't have to, to prevent damage to the fragile traces.
--- In TekScopes@..., "Tom Miller" wrote:
>
> OK, good. I would think the next thing to check is C-4419, a 5000 uF can capacitor.
>
> Regards,
> Tom
>
> ----- Original Message -----
> From: rwdplz1
> To: TekScopes@...
> Sent: Tuesday, January 15, 2013 2:40 AM
> Subject: [TekScopes] Re: 465B Oscilloscope No -8V
>
>
>
> The resistance between the -8V test point and ground (either a test point labeled ground on the board or the scope case) is 49.7 ohms.
>
> I replaced one tantalum, C4203, 33uF, with no effect.
>
> --- In TekScopes@..., "pdxareaid" wrote:
> >
> > you might want to check for shorted tantalum caps in -8 circuits.
> > see:
> >
> > phil
> >
> >
> >
> > --- In TekScopes@..., "rwdplz1" wrote:
> > >
> > > Hello, I am trying to fix my Tektronix 465B Oscilloscope. I have the service manual and have been using it to try and figure out what's wrong. When turned on, there is nothing on the screen, even when pressing and holding beam finder, and the power light doesn't come on. The only control that seems to work is scale illumination, which makes the screen glow orange when turned up.
> > >
> > > I checked the voltages at the test points on the main board A4 as described on page 4-24, and found all of them to be within spec: +55, +15, +5, and +110, EXCEPT for the -8, which measures 0.2mV. The power supply wiring diagram at the back of the manual shows two blue wires from the transformer, going through a fuse F4501 (desoldered and removed from board, it's good), then through a rectifier bridge CR4411, which is also good.
> > >
> > > So where do I go from here? What voltage should I be seeing at the two blue wires, because I'm starting to suspect that part of the transformer? Thank you
> > >
> >
>
|
I'm thinking the cap is NOT bad, and your meters simply cannot measure large caps. Try measuring a know-good 4700uF or so cap with it. Did you discharge it before measuring? My Fluke 87 III will not measure large uF caps. Not sure what the spec on it is, but I'm thinking anything above 10 uF won't measure.
Why don't you troubleshoot by checking voltages with the scope turned on. That's the best way sometimes. For example, what's the unregulated Vdc (i.e, the volts DC across C4419) when you get one re-installed?
Jimmy
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--- In TekScopes@..., "rwdplz1" wrote:
OK, I checked C4419, it is a 5000uF 35V capacitor. ESR is 0.08, which should be about right. However, after desoldering it from the board (which is a real pain with the two large mounting legs), and measuring capacitance, one of my meters couldn't measure it, and the other read 0.185 nano Farads. Both of these meters read the capacitance of other small capacitors I have on hand perfectly, so the meters are fine. ESR of the other large capacitors is about the same, but I'd rather not desolder them from the board if I don't have to, to prevent damage to the fragile traces.
--- In TekScopes@..., "Tom Miller" wrote:
OK, good. I would think the next thing to check is C-4419, a 5000 uF can capacitor.
Regards,
Tom
----- Original Message -----
From: rwdplz1
To: TekScopes@...
Sent: Tuesday, January 15, 2013 2:40 AM
Subject: [TekScopes] Re: 465B Oscilloscope No -8V
The resistance between the -8V test point and ground (either a test point labeled ground on the board or the scope case) is 49.7 ohms.
I replaced one tantalum, C4203, 33uF, with no effect.
--- In TekScopes@..., "pdxareaid" wrote:
>
> you might want to check for shorted tantalum caps in -8 circuits.
> see:
>
> phil
>
>
>
> --- In TekScopes@..., "rwdplz1" wrote:
> >
> > Hello, I am trying to fix my Tektronix 465B Oscilloscope. I have the service manual and have been using it to try and figure out what's wrong. When turned on, there is nothing on the screen, even when pressing and holding beam finder, and the power light doesn't come on. The only control that seems to work is scale illumination, which makes the screen glow orange when turned up.
> >
> > I checked the voltages at the test points on the main board A4 as described on page 4-24, and found all of them to be within spec: +55, +15, +5, and +110, EXCEPT for the -8, which measures 0.2mV. The power supply wiring diagram at the back of the manual shows two blue wires from the transformer, going through a fuse F4501 (desoldered and removed from board, it's good), then through a rectifier bridge CR4411, which is also good.
> >
> > So where do I go from here? What voltage should I be seeing at the two blue wires, because I'm starting to suspect that part of the transformer? Thank you
> >
>
|
Given the initial problem you described, I believe you’re looking for capacitors that check a consistent relatively low ohm with a standard ohm meter (i.e. do not charge to show open or build towards open). Open or bad caps (high esr wise) would cause you to have high ripple but not lack of DC. ? Unless I missed something along the way. Hopefully helpful Rob
toggle quoted message
Show quoted text
--- In TekScopes@..., "rwdplz1" wrote: > > OK, I checked C4419, it is a 5000uF 35V capacitor. ESR is 0.08, which should be about right. However, after desoldering it from the board (which is a real pain with the two large mounting legs), and measuring capacitance, one of my meters couldn't measure it, and the other read 0.185 nano Farads. Both of these meters read the capacitance of other small capacitors I have on hand perfectly, so the meters are fine. ESR of the other large capacitors is about the same, but I'd rather not desolder them from the board if I don't have to, to prevent damage to the fragile traces. > > --- In TekScopes@..., "Tom Miller" wrote: > > > > OK, good. I would think the next thing to check is C-4419, a 5000 uF can capacitor. > > > > Regards, > > Tom > > > > ----- Original Message ----- > > From: rwdplz1 > > To: TekScopes@... > > Sent: Tuesday, January 15, 2013 2:40 AM > > Subject: [TekScopes] Re: 465B Oscilloscope No -8V > > > > > > > > The resistance between the -8V test point and ground (either a test point labeled ground on the board or the scope case) is 49.7 ohms. > > > > I replaced one tantalum, C4203, 33uF, with no effect. > > > > --- In TekScopes@..., "pdxareaid" wrote: > > > > > > you might want to check for shorted tantalum caps in -8 circuits. > > > see: > > > > > > phil > > > > > > > > > > > > --- In TekScopes@..., "rwdplz1" wrote: > > > > > > > > Hello, I am trying to fix my Tektronix 465B Oscilloscope. I have the service manual and have been using it to try and figure out what's wrong. When turned on, there is nothing on the screen, even when pressing and holding beam finder, and the power light doesn't come on. The only control that seems to work is scale illumination, which makes the screen glow orange when turned up. > > > > > > > > I checked the voltages at the test points on the main board A4 as described on page 4-24, and found all of them to be within spec: +55, +15, +5, and +110, EXCEPT for the -8, which measures 0.2mV. The power supply wiring diagram at the back of the manual shows two blue wires from the transformer, going through a fuse F4501 (desoldered and removed from board, it's good), then through a rectifier bridge CR4411, which is also good. > > > > > > > > So where do I go from here? What voltage should I be seeing at the two blue wires, because I'm starting to suspect that part of the transformer? Thank you > > > > > > > > > >
|
If the fuse F4501, the rectifier CR4411, the capacitors C4419
and C4203 and the test point TP4439 only shows a tiny voltage while showing 50
Ohms to ground in the off condition, it looks like something around U4206B,
Q4102, Q4105, and Q4106 has failed.
The schematic gives you some test voltages to check in that
area.
The last thing I would suspect is the transformer, but then
anything is possible.
Common sense is your best friend.
tom jobe...
?
?
?
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----- Original Message -----
Sent: Tuesday, January 15, 2013 5:40
PM
Subject: RE: [TekScopes] Re: 465B
Oscilloscope No -8V
?
Given the
initial problem you described, I believe you’re looking for capacitors that
check a consistent relatively low ohm with a standard ohm meter (i.e. do not
charge to show open or build towards open). Open or bad caps (high esr wise)
would cause you to have high ripple but not lack of DC.
Unless I
missed something along the way. Hopefully helpful
Rob
--- In TekScopes@...,
"rwdplz1" wrote:
>
> OK, I checked C4419, it is a 5000uF 35V
capacitor. ESR is 0.08, which should be about right. However, after
desoldering it from the board (which is a real pain with the two large
mounting legs), and measuring capacitance, one of my meters couldn't measure
it, and the other read 0.185 nano Farads. Both of these meters read the
capacitance of other small capacitors I have on hand perfectly, so the meters
are fine. ESR of the other large capacitors is about the same, but I'd rather
not desolder them from the board if I don't have to, to prevent damage to the
fragile traces.
>
> --- In TekScopes@..., "Tom
Miller" wrote:
> >
> > OK, good. I would think the next
thing to check is C-4419, a 5000 uF can capacitor.
> >
> >
Regards,
> > Tom
> >
> > ----- Original Message
-----
> > From: rwdplz1
> > To: TekScopes@...
> > Sent: Tuesday, January 15, 2013 2:40 AM
> > Subject:
[TekScopes] Re: 465B Oscilloscope No -8V
> >
> >
>
>
> > The resistance between the -8V test point and ground
(either a test point labeled ground on the board or the scope case) is 49.7
ohms.
> >
> > I replaced one tantalum, C4203, 33uF, with no
effect.
> >
> > --- In TekScopes@...,
"pdxareaid" wrote:
> > >
> > > you might want to check
for shorted tantalum caps in -8 circuits.
> > > see:
> >
>
>
> > phil
> > >
> > >
> > >
> > > --- In TekScopes@...,
"rwdplz1" wrote:
> > > >
> > > > Hello, I am
trying to fix my Tektronix 465B Oscilloscope. I have the service manual and
have been using it to try and figure out what's wrong. When turned on, there
is nothing on the screen, even when pressing and holding beam finder, and the
power light doesn't come on. The only control that seems to work is scale
illumination, which makes the screen glow orange when turned up.
> >
> >
> > > > I checked the voltages at the test points on
the main board A4 as described on page 4-24, and found all of them to be
within spec: +55, +15, +5, and +110, EXCEPT for the -8, which measures 0.2mV.
The power supply wiring diagram at the back of the manual shows two blue wires
from the transformer, going through a fuse F4501 (desoldered and removed from
board, it's good), then through a rectifier bridge CR4411, which is also good.
> > > >
> > > > So where do I go from here?
What voltage should I be seeing at the two blue wires, because I'm starting to
suspect that part of the transformer? Thank you
> > > >
>
> >
> >
>
|
There could also be a bad bridge. This is not a difficult supply to trouble
shoot, being completely linear and using an op-amp for the control loop. But we
are just going through the process of check most likely failure first. In most
of the cases where the -8 is very low, either a shorted tantalum cap is dragging
the supply down, the bulk capacitor is open, the bridge has opened one diode, or
the op-amp has failed. This assumes the +15 and +55 volt supplies are good, as
stated by the OP.
?
I am somewhat confused with the ESR and Capacitance readings reported as
they conflict. Of course this assumes the test equipment is good :).
?
?
Best,
Tom
?
toggle quoted message
Show quoted text
----- Original Message -----
Sent: Tuesday, January 15, 2013 8:40
PM
Subject: RE: [TekScopes] Re: 465B
Oscilloscope No -8V
?
Given the
initial problem you described, I believe you’re looking for capacitors that
check a consistent relatively low ohm with a standard ohm meter (i.e. do not
charge to show open or build towards open). Open or bad caps (high esr wise)
would cause you to have high ripple but not lack of DC.
Unless I
missed something along the way. Hopefully helpful
Rob
--- In TekScopes@...,
"rwdplz1" wrote:
>
> OK, I checked C4419, it is a 5000uF 35V
capacitor. ESR is 0.08, which should be about right. However, after
desoldering it from the board (which is a real pain with the two large
mounting legs), and measuring capacitance, one of my meters couldn't measure
it, and the other read 0.185 nano Farads. Both of these meters read the
capacitance of other small capacitors I have on hand perfectly, so the meters
are fine. ESR of the other large capacitors is about the same, but I'd rather
not desolder them from the board if I don't have to, to prevent damage to the
fragile traces.
>
> --- In TekScopes@..., "Tom
Miller" wrote:
> >
> > OK, good. I would think the next
thing to check is C-4419, a 5000 uF can capacitor.
> >
> >
Regards,
> > Tom
> >
> > ----- Original Message
-----
> > From: rwdplz1
> > To: TekScopes@...
> > Sent: Tuesday, January 15, 2013 2:40 AM
> > Subject:
[TekScopes] Re: 465B Oscilloscope No -8V
> >
> >
>
>
> > The resistance between the -8V test point and ground
(either a test point labeled ground on the board or the scope case) is 49.7
ohms.
> >
> > I replaced one tantalum, C4203, 33uF, with no
effect.
> >
> > --- In TekScopes@...,
"pdxareaid" wrote:
> > >
> > > you might want to check
for shorted tantalum caps in -8 circuits.
> > > see:
> >
>
>
> > phil
> > >
> > >
> > >
> > > --- In TekScopes@...,
"rwdplz1" wrote:
> > > >
> > > > Hello, I am
trying to fix my Tektronix 465B Oscilloscope. I have the service manual and
have been using it to try and figure out what's wrong. When turned on, there
is nothing on the screen, even when pressing and holding beam finder, and the
power light doesn't come on. The only control that seems to work is scale
illumination, which makes the screen glow orange when turned up.
> >
> >
> > > > I checked the voltages at the test points on
the main board A4 as described on page 4-24, and found all of them to be
within spec: +55, +15, +5, and +110, EXCEPT for the -8, which measures 0.2mV.
The power supply wiring diagram at the back of the manual shows two blue wires
from the transformer, going through a fuse F4501 (desoldered and removed from
board, it's good), then through a rectifier bridge CR4411, which is also good.
> > > >
> > > > So where do I go from here?
What voltage should I be seeing at the two blue wires, because I'm starting to
suspect that part of the transformer? Thank you
> > > >
>
> >
> >
>
|
Just randomly desoldering caps will not bring you anywhere (or just with luck). Have you checked the voltage and ripple before the regulator? If this is fine you know that the bug's in the regulator or later in the chain. If not you know it's in the transformer/rectifier/filtering cap.
Samuel
|
Yesterday I left out some words in my post about the missing
-8 volt problem, shame on me!
Let me add the words "all test good".
tom jobe...
?
If the fuse F4501, the rectifier CR4411, the capacitors C4419
and C4203 all test good and the test point TP4439 only
shows a tiny voltage while showing 50 Ohms to ground in the off condition, it
looks like something around U4206B, Q4102, Q4105, and Q4106 has
failed.
The schematic gives you some test voltages to check in that
area.
The last thing I would suspect is the transformer, but then
anything is possible.
Common sense is your best friend.
tom jobe...
?
?
?
?
toggle quoted message
Show quoted text
----- Original Message -----
Sent: Tuesday, January 15, 2013 5:40
PM
Subject: RE: [TekScopes] Re: 465B
Oscilloscope No -8V
?
Given the
initial problem you described, I believe you’re looking for capacitors that
check a consistent relatively low ohm with a standard ohm meter (i.e. do not
charge to show open or build towards open). Open or bad caps (high esr wise)
would cause you to have high ripple but not lack of DC.
Unless I
missed something along the way. Hopefully helpful
Rob
--- In TekScopes@...,
"rwdplz1" wrote:
>
> OK, I checked C4419, it is a 5000uF 35V
capacitor. ESR is 0.08, which should be about right. However, after
desoldering it from the board (which is a real pain with the two large
mounting legs), and measuring capacitance, one of my meters couldn't measure
it, and the other read 0.185 nano Farads. Both of these meters read the
capacitance of other small capacitors I have on hand perfectly, so the meters
are fine. ESR of the other large capacitors is about the same, but I'd rather
not desolder them from the board if I don't have to, to prevent damage to the
fragile traces.
>
> --- In TekScopes@..., "Tom
Miller" wrote:
> >
> > OK, good. I would think the next
thing to check is C-4419, a 5000 uF can capacitor.
> >
> >
Regards,
> > Tom
> >
> > ----- Original Message
-----
> > From: rwdplz1
> > To: TekScopes@...
> > Sent: Tuesday, January 15, 2013 2:40 AM
> > Subject:
[TekScopes] Re: 465B Oscilloscope No -8V
> >
> >
>
>
> > The resistance between the -8V test point and ground
(either a test point labeled ground on the board or the scope case) is 49.7
ohms.
> >
> > I replaced one tantalum, C4203, 33uF, with no
effect.
> >
> > --- In TekScopes@...,
"pdxareaid" wrote:
> > >
> > > you might want to check
for shorted tantalum caps in -8 circuits.
> > > see:
> >
>
>
> > phil
> > >
> > >
> > >
> > > --- In TekScopes@...,
"rwdplz1" wrote:
> > > >
> > > > Hello, I am
trying to fix my Tektronix 465B Oscilloscope. I have the service manual and
have been using it to try and figure out what's wrong. When turned on, there
is nothing on the screen, even when pressing and holding beam finder, and the
power light doesn't come on. The only control that seems to work is scale
illumination, which makes the screen glow orange when turned up.
> >
> >
> > > > I checked the voltages at the test points on
the main board A4 as described on page 4-24, and found all of them to be
within spec: +55, +15, +5, and +110, EXCEPT for the -8, which measures 0.2mV.
The power supply wiring diagram at the back of the manual shows two blue wires
from the transformer, going through a fuse F4501 (desoldered and removed from
board, it's good), then through a rectifier bridge CR4411, which is also good.
> > > >
> > > > So where do I go from here?
What voltage should I be seeing at the two blue wires, because I'm starting to
suspect that part of the transformer? Thank you
> > > >
>
> >
> >
>
|
In this case, it is not?the straight forward measurement you envision.
The -8 volt supply does not directly reference to ground. Also, it is NOT a
random process. The first step I recommended was to take an ohmmeter reading to
ground of the -8 volt test point. Since he measured 50 ohms or so, that meant
that there were no directly shorted tantalums.
?
Again, it is a simple linear supply and easiest to check the items most
likely to fail. The bulk capacitor, the bridge rectifier, and the op-amp. Next
would be the pass transistors and the current limit circuit.
?
?
Regards,
Tom
?
toggle quoted message
Show quoted text
----- Original Message -----
Sent: Wednesday, January 16, 2013 4:30
AM
Subject: [TekScopes] Re: 465B
Oscilloscope No -8V
?
Just randomly desoldering caps will not bring you anywhere (or just with
luck). Have you checked the voltage and ripple before the regulator? If this
is fine you know that the bug's in the regulator or later in the chain. If not
you know it's in the transformer/rectifier/filtering
cap.
Samuel
|
Sorry it's been a few days since my last reply, here is what I have found:
I did desolder and check the bridge rectifier, and it was good.
Voltages according to diagram:
Where voltage should be 3.3, it's 7.6mV, at the capacitor C4419 and at Q4102
op amp leg 5, instead of 0V, it's 8.34V
op amp leg 7, instead of 2.4, it's 0.93V
Q4102 emitter, instead of 1.0V, it's 0.49V at the base, and 0 at the emmiter and collector
R4312 does read near +55, at 54.8V
Replacing the op amp U4206B made no difference in any of the readings.
Q4105 and Q4106: both are 2N2222 NPN transistors, both had hfe within spec, 122 and 189. Replaced them anyway since I had some and they just slide in and out of their sockets, and of course no change.
C4419: ESR of new capacitor measured 0.03, and neither of my meters could read it's capacitance, so my meters probably just can't read capacitance that high. So the original Sprague may have been OK, BUT after putting the new capacitor in, the -8V test point now reads, 0.75 Volts, and that was the only part I had changed out at the time. I checked continuity between C4419 and all the other parts it connects to in the diagram, R4311, R4204, and C4203 on the negative side, and Q4102 on the positive side.
Voltages at the test points are as follows:
+110: 110.9
+5: 4.96
+55: 54.8
+15: 14.95
-8: 0.75 after replacing capacitor.
Replacing the op amp and Q4105 and Q4106 made no difference.
SO this seems to leave Q4102. It's a Motorola 852, with 034900 below the 852 number. Looking at the labeled face, it's emitter, collector, base, and is screwed into the heatsink. I can't seem to find a datasheet for it, anyone know what the modern equivlent would be?
Thank you for all the input so far
toggle quoted message
Show quoted text
--- In TekScopes@..., "rwdplz1" wrote:
Hello, I am trying to fix my Tektronix 465B Oscilloscope. I have the service manual and have been using it to try and figure out what's wrong. When turned on, there is nothing on the screen, even when pressing and holding beam finder, and the power light doesn't come on. The only control that seems to work is scale illumination, which makes the screen glow orange when turned up.
I checked the voltages at the test points on the main board A4 as described on page 4-24, and found all of them to be within spec: +55, +15, +5, and +110, EXCEPT for the -8, which measures 0.2mV. The power supply wiring diagram at the back of the manual shows two blue wires from the transformer, going through a fuse F4501 (desoldered and removed from board, it's good), then through a rectifier bridge CR4411, which is also good.
So where do I go from here? What voltage should I be seeing at the two blue wires, because I'm starting to suspect that part of the transformer? Thank you
|
Is the 5 Amp?fuse F-4501 blown? Check that the collector of Q4102 is
not shorted to ground.
?
Tom
?
toggle quoted message
Show quoted text
----- Original Message -----
Sent: Friday, January 18, 2013 5:26
PM
Subject: [TekScopes] Re: 465B
Oscilloscope No -8V
?
Sorry it's been a few days since my last reply, here is what I have
found:
I did desolder and check the bridge rectifier, and it was good.
Voltages according to diagram:
Where voltage should be 3.3,
it's 7.6mV, at the capacitor C4419 and at Q4102
op amp leg 5, instead of
0V, it's 8.34V
op amp leg 7, instead of 2.4, it's 0.93V
Q4102 emitter,
instead of 1.0V, it's 0.49V at the base, and 0 at the emmiter and
collector
R4312 does read near +55, at 54.8V
Replacing the op amp
U4206B made no difference in any of the readings.
Q4105 and Q4106:
both are 2N2222 NPN transistors, both had hfe within spec, 122 and 189.
Replaced them anyway since I had some and they just slide in and out of their
sockets, and of course no change.
C4419: ESR of new capacitor measured
0.03, and neither of my meters could read it's capacitance, so my meters
probably just can't read capacitance that high. So the original Sprague may
have been OK, BUT after putting the new capacitor in, the -8V test point now
reads, 0.75 Volts, and that was the only part I had changed out at the time. I
checked continuity between C4419 and all the other parts it connects to in the
diagram, R4311, R4204, and C4203 on the negative side, and Q4102 on the
positive side.
Voltages at the test points are as follows:
+110:
110.9
+5: 4.96
+55: 54.8
+15: 14.95
-8: 0.75 after replacing
capacitor.
Replacing the op amp and Q4105 and Q4106 made no difference.
SO this seems to leave Q4102. It's a Motorola 852, with 034900 below
the 852 number. Looking at the labeled face, it's emitter, collector, base,
and is screwed into the heatsink. I can't seem to find a datasheet for it,
anyone know what the modern equivlent would be?
Thank you for all the
input so far
--- In TekScopes@...,
"rwdplz1" wrote:
>
> Hello, I am trying to fix my Tektronix 465B
Oscilloscope. I have the service manual and have been using it to try and
figure out what's wrong. When turned on, there is nothing on the screen, even
when pressing and holding beam finder, and the power light doesn't come on.
The only control that seems to work is scale illumination, which makes the
screen glow orange when turned up.
>
> I checked the voltages at
the test points on the main board A4 as described on page 4-24, and found all
of them to be within spec: +55, +15, +5, and +110, EXCEPT for the -8, which
measures 0.2mV. The power supply wiring diagram at the back of the manual
shows two blue wires from the transformer, going through a fuse F4501
(desoldered and removed from board, it's good), then through a rectifier
bridge CR4411, which is also good.
>
> So where do I go from
here? What voltage should I be seeing at the two blue wires, because I'm
starting to suspect that part of the transformer? Thank
you
>
|
F4501 is not blown. The resistance between Q4102's collector and the ground point on the board is 4.4 MegaOhms
toggle quoted message
Show quoted text
--- In TekScopes@..., "Tom Miller" wrote:
Is the 5 Amp fuse F-4501 blown? Check that the collector of Q4102 is not shorted to ground.
Tom
----- Original Message -----
From: rwdplz1
To: TekScopes@...
Sent: Friday, January 18, 2013 5:26 PM
Subject: [TekScopes] Re: 465B Oscilloscope No -8V
Sorry it's been a few days since my last reply, here is what I have found:
I did desolder and check the bridge rectifier, and it was good.
Voltages according to diagram:
Where voltage should be 3.3, it's 7.6mV, at the capacitor C4419 and at Q4102
op amp leg 5, instead of 0V, it's 8.34V
op amp leg 7, instead of 2.4, it's 0.93V
Q4102 emitter, instead of 1.0V, it's 0.49V at the base, and 0 at the emmiter and collector
R4312 does read near +55, at 54.8V
Replacing the op amp U4206B made no difference in any of the readings.
Q4105 and Q4106: both are 2N2222 NPN transistors, both had hfe within spec, 122 and 189. Replaced them anyway since I had some and they just slide in and out of their sockets, and of course no change.
C4419: ESR of new capacitor measured 0.03, and neither of my meters could read it's capacitance, so my meters probably just can't read capacitance that high. So the original Sprague may have been OK, BUT after putting the new capacitor in, the -8V test point now reads, 0.75 Volts, and that was the only part I had changed out at the time. I checked continuity between C4419 and all the other parts it connects to in the diagram, R4311, R4204, and C4203 on the negative side, and Q4102 on the positive side.
Voltages at the test points are as follows:
+110: 110.9
+5: 4.96
+55: 54.8
+15: 14.95
-8: 0.75 after replacing capacitor.
Replacing the op amp and Q4105 and Q4106 made no difference.
SO this seems to leave Q4102. It's a Motorola 852, with 034900 below the 852 number. Looking at the labeled face, it's emitter, collector, base, and is screwed into the heatsink. I can't seem to find a datasheet for it, anyone know what the modern equivlent would be?
Thank you for all the input so far
--- In TekScopes@..., "rwdplz1" wrote:
>
> Hello, I am trying to fix my Tektronix 465B Oscilloscope. I have the service manual and have been using it to try and figure out what's wrong. When turned on, there is nothing on the screen, even when pressing and holding beam finder, and the power light doesn't come on. The only control that seems to work is scale illumination, which makes the screen glow orange when turned up.
>
> I checked the voltages at the test points on the main board A4 as described on page 4-24, and found all of them to be within spec: +55, +15, +5, and +110, EXCEPT for the -8, which measures 0.2mV. The power supply wiring diagram at the back of the manual shows two blue wires from the transformer, going through a fuse F4501 (desoldered and removed from board, it's good), then through a rectifier bridge CR4411, which is also good.
>
> So where do I go from here? What voltage should I be seeing at the two blue wires, because I'm starting to suspect that part of the transformer? Thank you
>
|
What is the resistance across the AC terminals of the bridge?
?
Tom
?
toggle quoted message
Show quoted text
----- Original Message -----
Sent: Friday, January 18, 2013 6:10
PM
Subject: [TekScopes] Re: 465B
Oscilloscope No -8V
?
F4501 is not blown. The resistance between Q4102's collector and the ground
point on the board is 4.4 MegaOhms
--- In TekScopes@..., "Tom
Miller" wrote:
>
> Is the 5 Amp fuse F-4501 blown? Check that the
collector of Q4102 is not shorted to ground.
>
> Tom
>
> ----- Original Message -----
> From: rwdplz1
> To: TekScopes@...
> Sent: Friday, January 18, 2013 5:26 PM
> Subject: [TekScopes]
Re: 465B Oscilloscope No -8V
>
>
>
> Sorry it's
been a few days since my last reply, here is what I have found:
>
> I did desolder and check the bridge rectifier, and it was good.
>
> Voltages according to diagram:
>
> Where
voltage should be 3.3, it's 7.6mV, at the capacitor C4419 and at Q4102
>
op amp leg 5, instead of 0V, it's 8.34V
> op amp leg 7, instead of 2.4,
it's 0.93V
> Q4102 emitter, instead of 1.0V, it's 0.49V at the base, and
0 at the emmiter and collector
> R4312 does read near +55, at
54.8V
>
> Replacing the op amp U4206B made no difference in any
of the readings.
>
> Q4105 and Q4106: both are 2N2222 NPN
transistors, both had hfe within spec, 122 and 189. Replaced them anyway since
I had some and they just slide in and out of their sockets, and of course no
change.
>
> C4419: ESR of new capacitor measured 0.03, and
neither of my meters could read it's capacitance, so my meters probably just
can't read capacitance that high. So the original Sprague may have been OK,
BUT after putting the new capacitor in, the -8V test point now reads, 0.75
Volts, and that was the only part I had changed out at the time. I checked
continuity between C4419 and all the other parts it connects to in the
diagram, R4311, R4204, and C4203 on the negative side, and Q4102 on the
positive side.
>
> Voltages at the test points are as
follows:
> +110: 110.9
> +5: 4.96
> +55: 54.8
> +15:
14.95
> -8: 0.75 after replacing capacitor.
>
> Replacing
the op amp and Q4105 and Q4106 made no difference.
>
> SO this
seems to leave Q4102. It's a Motorola 852, with 034900 below the 852 number.
Looking at the labeled face, it's emitter, collector, base, and is screwed
into the heatsink. I can't seem to find a datasheet for it, anyone know what
the modern equivlent would be?
>
> Thank you for all the input so
far
>
> --- In TekScopes@...,
"rwdplz1" wrote:
> >
> > Hello, I am trying to fix my
Tektronix 465B Oscilloscope. I have the service manual and have been using it
to try and figure out what's wrong. When turned on, there is nothing on the
screen, even when pressing and holding beam finder, and the power light
doesn't come on. The only control that seems to work is scale illumination,
which makes the screen glow orange when turned up.
> >
> >
I checked the voltages at the test points on the main board A4 as described on
page 4-24, and found all of them to be within spec: +55, +15, +5, and +110,
EXCEPT for the -8, which measures 0.2mV. The power supply wiring diagram at
the back of the manual shows two blue wires from the transformer, going
through a fuse F4501 (desoldered and removed from board, it's good), then
through a rectifier bridge CR4411, which is also good.
> >
>
> So where do I go from here? What voltage should I be seeing at the two
blue wires, because I'm starting to suspect that part of the transformer?
Thank you
> >
>
|
