Dead Tektronix TDS-620B

[Adrian Glaubitz]

Hello folks,

I retrieved a Tektronix TDS-620B digital oscilloscope from the dust-bin of my university today. The unit
is completely dead. Fuse (6A/250V) is ok. The power-supply is really sophisticated and I think servicing
it without a service-manual is impossible.

Maybe some of you guys has an idea where to get the manual or what could be the culprit. There is a sticker
which shows all output voltages of the power-supply, but I have no idea where to test them.

Any hints ?

Adrian

 

[Jim Yanik]

Hello folks,

I retrieved a Tektronix TDS-620B digital oscilloscope from the
dust-bin of my university today. The unit is completely dead. Fuse
(6A/250V) is ok. The power-supply is really sophisticated and I think
servicing it without a service-manual is impossible.

Maybe some of you guys has an idea where to get the manual or what
could be the culprit. There is a sticker which shows all output
voltages of the power-supply, but I have no idea where to test them.

Any hints ?

Adrian

the TEK "service" manual is for module exchange only.
there will be no component-level schematics or parts lists.
AFAIK,the power supply was a purchased part.

yur model is out of support;that is why they threw it away;parts or
exchange assemblies are no longer available.

http://www.tek.com/service/product-support/levels.html
doesn't even have it listed for support anymore.

(their site sucks,too.they don't even tell you anymore when it went out of
support)

 

[[email protected]]

Hello folks,

I retrieved a Tektronix TDS-620B digital oscilloscope from the dust-

bin of my university today. The unit

is completely dead. Fuse (6A/250V) is ok. The power-supply is

really sophisticated and I think servicing

it without a service-manual is impossible.

Maybe some of you guys has an idea where to get the manual or what

could be the culprit. There is a sticker

which shows all output voltages of the power-supply, but I have no idea where to test them.

Any hints ?

Adrian

Just this morning I had a Tek PAL sync generator that refused to start
because of a high ESR 100uF cap in the power supply. Also, early 1700
series waveform and vectorscopes will fail to start when a 10 uF cap
goes bad. Since it isn't blowing fuses, it doesn't have blown up power
devices - though they still may be bad. Checking power supply caps for
ESR may be a good start.

G²

 

[Adrian Glaubitz]

Hello folks,

Hello folks,

I retrieved a Tektronix TDS-620B digital oscilloscope from the dust-bin of my university today. The unit
is completely dead. Fuse (6A/250V) is ok. The power-supply is really sophisticated and I think servicing
it without a service-manual is impossible.

Maybe some of you guys has an idea where to get the manual or what could be the culprit. There is a sticker
which shows all output voltages of the power-supply, but I have no idea where to test them.

Any hints ?

Adrian

ok, I got somewhat further. I found a fusible resistor to be blown,
R176, 18 Ohms. It was completely open, I tested it outside circuit. The
unit still doesn't power up though. I already checked the transistors at
the heat-sinks with a diode-tester and they appear to be okay.
Furthermore I can measure 300V DC at the output of the rectifier. Any
more hints ?

Thanks !

Adrian

 

[JW]

Hello folks,



ok, I got somewhat further. I found a fusible resistor to be blown,
R176, 18 Ohms. It was completely open, I tested it outside circuit. The
unit still doesn't power up though. I already checked the transistors at
the heat-sinks with a diode-tester and they appear to be okay.
Furthermore I can measure 300V DC at the output of the rectifier. Any
more hints ?

Are you sure it's *completely* dead? I'd probe it's outputs with another
scope to be sure. You may be able to see it pulse it's outputs in an
attempt to power up before shutting down. If you do see pulses on the
outputs, it could just be a matter of high ESR capacitors.

Also, try disconnecting other boards/modules in the unit. One of them
could be overloading the power supply.

 

[Adrian Glaubitz]

Hi there,

Are you sure it's *completely* dead? I'd probe it's outputs with another
scope to be sure. You may be able to see it pulse it's outputs in an
attempt to power up before shutting down. If you do see pulses on the
outputs, it could just be a matter of high ESR capacitors.

Also, try disconnecting other boards/modules in the unit. One of them
could be overloading the power supply.

Yes, it is completely dead. I followed the troubleshooting procedure
step by step from the "service-manual" (they don't have schematics) from
Tek and from the flowchart it said "replace PSU".

I did some more testing today. It appears, that after the blown resistor
there is an PWM-circuit with a TOP200YAI PWM-Switch. After replacing the
resistor I can read the 300V at the input of the PWM-circuit, but there
are *no* 5V DC at the output. I did a web-search for the PWM-Switch
datasheet and found it. It actually contains a schematic for a typical
application and the PWM-circuit in the PSU appears to be completely
identical.

I tested all diodes, resistors and the windings of the transformer in
the circuit and all parts indicate ok. So I presume the PWM-Switch
itself to be bad. The thingy costs about 8 US-Dollars and I only can
find a TOP201YAI and TOP204YAI at my local dealer. Can I use those or
does it have to be the 200 ? Anyone knows where I could rip one out ?

I'd be interesting if anyone could comment on this .


Regards,

Adrian

 

[Jim Yanik]

Hi there,


Yes, it is completely dead. I followed the troubleshooting procedure
step by step from the "service-manual" (they don't have schematics) from
Tek and from the flowchart it said "replace PSU".

I did some more testing today. It appears, that after the blown resistor
there is an PWM-circuit with a TOP200YAI PWM-Switch. After replacing the
resistor I can read the 300V at the input of the PWM-circuit, but there
are *no* 5V DC at the output. I did a web-search for the PWM-Switch
datasheet and found it. It actually contains a schematic for a typical
application and the PWM-circuit in the PSU appears to be completely
identical.

I tested all diodes, resistors and the windings of the transformer in
the circuit and all parts indicate ok. So I presume the PWM-Switch
itself to be bad. The thingy costs about 8 US-Dollars and I only can
find a TOP201YAI and TOP204YAI at my local dealer. Can I use those or
does it have to be the 200 ?

check their datasheets.I suspect the TOP201 will work fine.

Anyone knows where I could rip one out ?

I'd be interesting if anyone could comment on this .


Regards,

Adrian

high ESR of the electrolytics is a big problem area in switchers.

 

[[email protected]]

I tested all diodes, resistors and the windings of the transformer in
the circuit and all parts indicate ok. So I presume the PWM-Switch
itself to be bad. The thingy costs about 8 US-Dollars and I only can
find a TOP201YAI and TOP204YAI at my local dealer. Can I use those or
does it have to be the 200 ? Anyone knows where I could rip one out ?

I'd be interesting if anyone could comment on this .

Regards,

Adrian

Just for the records. It was the TOP200YAI that was shot. Replacing
that one and the Tek would fire up again ;-). Now it's just
complaining about some calibration error.

Adrian