Need Help Repair Tektronix TDS520B

[chihaxinh]

Hi Everyone

I have one Tek TDS520B , so when power on , all led flash , fan run click click and loop again and again.
So when search information about repair tds 5xx 6xx 7xx on internet i starting open case and check All.
1st Step : Check PSU
- A lot electrolytic capacitor fillter low vol and 24V have leaky . So i desoder all cap and note value / vol rate.

So i need to go store buy capacitor in last week . I checked Mainboard and Acq board dont have any capacitor leak ( Mother board have repair and change cap by some one ) so i decied replace all electrolytic again. ( For sure all in good conditional ).

I Have one question
Can i replace all electrolytics 33uF /10V by tantanum capacitor ? I dont want using electrolytics in acq board ( prevent cap leak in future ).

Many thank your support !
Nam Nguyen Hai

[bdivi]

I replaced all electrolitics on the digital and acquisition boards by tantalum capacitors and it's all been good for more than 3 years.

Just make sure you wash the hell out of the boards after removing the old leaky capacitors - brush and lots of IPA are your friends here.

Cheers

[chihaxinh]

I replaced all electrolitics on the digital and acquisition boards by tantalum capacitors and it's all been good for more than 3 years.

Just make sure you wash the hell out of the boards after removing the old leaky capacitors - brush and lots of IPA are your friends here.

Cheers

Many thank bdivi
Early you repair tds 520 , do you have any problem in PSU ? If you repair PSU, can you share your  experience ?

[bdivi]

I only repaired the boards not the PSU - it was ok and still is.

btw my scope is 540A but the electrolytic issue is identical.

[David Hess]

Can i replace all electrolytics 33uF /10V by tantanum capacitor ? I dont want using electrolytics in acq board ( prevent cap leak in future ).

Yes, you can replace the bulk decoupling capacitors with solid tantalums and with solid tantalums, you can use 2 to 4 times less capacitance and still get the same ESR.

[nctnico]

No, you can't use tantalums in TDS500/600/700 series scopes to replace electrolytics! In some places the circuits depend on the ESR of the capacitors (IIRC in the main oscillator circuit) so tantalum and MLCC capacitors may mess things up. Another problem with tantalums is that the rush-in current needs to be controlled. Also Tektronix made a mistake in some spots which puts a higher voltage on the capacitor than it is rated for. All in all tantalums are not a good replacement. Just go for electrolytics and don't try to improve what doesn't need improving.

[David Hess]

I am only referring to places where the small aluminum electrolytic capacitors were used for local bulk decoupling.  Other spots would have to be evaluated based on application.

[chihaxinh]

Thank for all comment !

So tantanum good choice for decoupling ( aka bypass capacitor ) for filtering power and cancer high freq noise !

New update: I checking PSU have 4 cap 1nF Y class crack . So what type capacitor i can replace , in my country Y class capacitor hard to find .

Another question : How to check PSU after repair ? ( dont connect to main board and acq board )

Many thank your support !
Nam Nguyen Hai

[David Hess]

New update: I checking PSU have 4 cap 1nF Y class crack . So what type capacitor i can replace , in my country Y class capacitor hard to find .

That is a common problem.  Y class capacitors can be used as replacements for X class capacitors but not the reverse which does not help you.  Paper, film, or ceramic capacitors with significant voltage derating can be used as replacements.  Peak voltage for a 240 volt AC line is about 370 volts so I would want a 800 volt DC or higher capacitor.  The higher the better.

The power supply will operate without these if necessary but EMI will be increased.

Another question : How to check PSU after repair ? ( dont connect to main board and acq board )

Use a dummy load on the various low voltage outputs for testing before connecting the other boards.

[chihaxinh]

New update: I checking PSU have 4 cap 1nF Y class crack . So what type capacitor i can replace , in my country Y class capacitor hard to find .

That is a common problem.  Y class capacitors can be used as replacements for X class capacitors but not the reverse which does not help you.  Paper, film, or ceramic capacitors with significant voltage derating can be used as replacements.  Peak voltage for a 240 volt AC line is about 370 volts so I would want a 800 volt DC or higher capacitor.  The higher the better.

The power supply will operate without these if necessary but EMI will be increased.

Another question : How to check PSU after repair ? ( dont connect to main board and acq board )

Use a dummy load on the various low voltage outputs for testing before connecting the other boards.

Thank for your help , Let me repair in weekend and report for you . Let go to find dummy load ! 

[chihaxinh]

I desolder all tin in hole so very hard and take 4hour for complate

Tektronix layout very complex , one pin have big track on top and other in bottom ! So next time it's solder all new cap ! 

[chihaxinh]

Hi All

After replace all capacitor, i turn on PSU with Bulk Lamp 500W series limit current . Lamp flash at a rate of about once per second , likely protect overload current it's active .

I extract 2 page PSU and attach in post . Next time i checking all diode in secondary size SMPS 

[singapol]

Hi All

After replace all capacitor, i turn on PSU with Bulk Lamp 500W series limit current . Lamp flash at a rate of about once per second , likely protect overload current it's active .

I extract 2 page PSU and attach in post . Next time i checking all diode in secondary size SMPS 

After replace all capacitor, i turn on PSU with Bulk Lamp 500W series limit current . Lamp flash at a rate of about once per second , likely protect overload current it's active .

This is because power supply is in Hiccup mode. Please read pg.1-29 low voltage supply for better understanding.

From pg. 1-30 TDS520b service manual:

Six signals are ORed together into the ONE-SHOT TRIG signal. They are the
24 V current limit, 24 V over voltage, the main 5.1 V over voltage, main 5.1 V
current limit, and the output voltage summing comparator. Any of these faults
will start hiccup mode. In hiccup mode the power supply cycles on and off at a
low repetition rate until the fault is removed.

Your problem is similar to this TDS684 although the circuit is different.
https://www.eevblog.com/forum/testgear/fixing-tek-tds684-psu/

Refering to TDS520b A17 circuit U12 outputs +15V Bias voltage for PWM ic U5 (SG3525A) See circuit A18 pg.1 & 2. If +15 Bias is missing then U5 will not work. So check A17 U12 other components like VR24 (p6ke150),
CR46 ( BYM26C). C117 ( 47uf 35V). Good luck.

[chihaxinh]

Refering to TDS520b A17 circuit U12 outputs +15V Bias voltage for PWM ic U5 (SG3525A) See circuit A18 pg.1 & 2. If +15 Bias is missing then U5 will not work. So check A17 U12 other components like VR24 (p6ke150),
CR46 ( BYM26C). C117 ( 47uf 35V). Good luck.

Hi Singapol

When i checked my PSU have primary like TDS544A PSU , but secondary side like TDS520B

Primary Side schematics in attachment
In primary side i checked Q8 - BU508A it's OK

Secondary Side in attackment
In secondary side i checked all Diode after T6 , It's OK , and i checked  CR29 CR33 CR27 in output size it's ok .

Everything look like ok , no crack cap and no burn active componant.

What should i do ? 

[singapol]

Refering to TDS520b A17 circuit U12 outputs +15V Bias voltage for PWM ic U5 (SG3525A) See circuit A18 pg.1 & 2. If +15 Bias is missing then U5 will not work. So check A17 U12 other components like VR24 (p6ke150),
CR46 ( BYM26C). C117 ( 47uf 35V). Good luck.

Hi Singapol

When i checked my PSU have primary like TDS544A PSU , but secondary side like TDS520B

Primary Side schematics in attachment
In primary side i checked Q8 - BU508A it's OK

Secondary Side in attackment
In secondary side i checked all Diode after T6 , It's OK , and i checked  CR29 CR33 CR27 in output size it's ok .

Everything look like ok , no crack cap and no burn active componant.

What should i do ?

By this you mean output voltages at CR29 = +15V, CR33 =  +5V and CR27 = -5.1V . There is also CR34.
In fact there are 5 voltages namely -15V  and +24V. If any one is missing it will trigger hiccup mode untill
it is fixed. There is also stand by circuit.

Below is the discription of how the power supply works. Good luck.

From page 1-29 of TDS 520B Mod CM service manual:

Low Voltage Power Supply A16

The low voltage power supply generates voltages used throughout the oscilloscope.
It is a switching power converter with active power factor control.
The AC Input includes an EMI filter (C5–C7, C11, L2, L3, and L7) and bleeder
resistor. Also, thermistors and varistors protect against excessive current (RT1),
voltage (RV1), and temperature (RT7).

The Standby Supply is a flyback converter. When an integral transistor in U12 is
on, the voltage on C12 is applied to T7. This voltage (at C22) is monitored to
provide the power fail warning signal ~PF (W4 pin 1).

The Boost Converter provides a pre-regulated bulk supply for the Main
Converter. Two parallel FETs (Q5 and 6) form the boost converter switch. When
pulsed on, energy is stored in boost inductor L1. When the FETs turn off, the
inductor energy is delivered to bulk cap C12. The U10 Power Factor Control
controls the on-time of the FETs to regulate the bulk voltage. The control circuit
also provides for near unity power factor at the mains input.

The PWM Control (A18
) references the secondary of T6 (+5.1 V
FEED BACK). It turns Main Converter switching FETs Q7 and Q8
(A17
) on and off by way of gate drive transformer T4. With Q7 and Q8 on,
the bulk cap voltage is applied to the primary winding of transformer T6.

The Pulse Width Modulator (A18
) width-regulates the GATE DRIVE pulse
(Q10/Q11 base) regulating the main output voltage (+5.1 V).

The +24 output voltage is regulated from +5.1 through T6 turns ratio. The other
four outputs are series pass regulated by VR10, VR11, Q14, and Q15. An output
voltage summing comparator (U5) starts hiccup mode if one of these four
outputs go outside of the 50% voltage window.

Pulse Width Modulator U5 is the heart of the PWM Control . Resistor R20 and
capacitor C4 set the switching frequency. R41, VR3, and C15 provide a 1%
reference. Capacitor C5 is a soft-start capacitor that slowly starts up the Boost
Converter.

Pin 10 of U5 is a shutdown input, and it is also a pulse-by-pulse current limit.
Pulling pin 10 high stops the output pulse within 200 ns. If pulled high for a
short time, the soft-start cap [ pin 8 ] will not be discharged. If the soft-start cap
discharges, it causes a normal soft-start when the voltage at pin 10 is removed.
Six signals are ORed together into the ONE-SHOT TRIG signal. They are the
24 V current limit, 24 V over voltage, the main 5.1 V over voltage, main 5.1 V
current limit, and the output voltage summing comparator. Any of these faults
will start hiccup mode. In hiccup mode the power supply cycles on and off at a
low repetition rate until the fault is removed.

The On/Standby Control debounces the switch. Relay K1 ensures that the power
supply powers up in the same state (On or Standby) it was in when the oscilloscope
powered down.

The low bulk fault circuit monitors the bulk voltage by way of the peak detector
on the Standby Supply. It warns the processor system and shuts down the Main
Converter when the bulk voltage drops to about 300 V.

[nctnico]

Hi All

After replace all capacitor, i turn on PSU with Bulk Lamp 500W series limit current . Lamp flash at a rate of about once per second , likely protect overload current it's active .

I extract 2 page PSU and attach in post . Next time i checking all diode in secondary size SMPS 

You need to load the 24V rail of the PSU with at least the fan otherwise you'll get this behaviour.

[chihaxinh]

Hi all

Many thank for support !
I Have free time and go back to repair TDS520B

I checked follow your guide but dont help ...  in second side of T6 - A17 (2) page 359 i dont have any AC out.
Next step i checked pinout of second side , i have missing oscillator in +-5V Rail-2-Rail

Missing osicllator in:

- Pin 7-8 T6 for +5V
- Pin E12-E13 T6  for -5V
- Pin  E10 - E11 T6 - for 5V and 5.1V feedback

Other pin in second side have oscillator but don't have any AC output .
I active componant in second side i checked ok . No Mosfet burn, short, no diode fail or open , all cap and opamp replace.

I thing T6 have short winding in second side or burn in case . So if Transformer short , everything go to hiccup mode. 

Many thank your support !
Nam.

[singapol]

chihaxinh,

if you look at A17 schematic the low voltage supply mainly go to A10 Acquisition board.
Like +5V - 12A, -5 -12A, -15V - 1.5A.  :oSo the only way to know whether secondary coil is short or not is to unload or disconnect supply to A10. Unplug the respective connectors to A10 and test AC  voltage on T6 again.

If there is AC voltage and correct Dc voltage at A17 then suspect A10 board.

[chihaxinh]

chihaxinh,

if you look at A17 schematic the low voltage supply mainly go to A10 Acquisition board.
Like +5V - 12A, -5 -12A, -15V - 1.5A.  :oSo the only way to know whether secondary coil is short or not is to unload or disconnect supply to A10. Unplug the respective connectors to A10 and test AC  voltage on T6 again.

If there is AC voltage and correct Dc voltage at A17 then suspect A10 board.

Many thank singapol

Now i have free time and go back to repair TDS520 . After checking schematics , i find out my version PSU it's mixing with TDS540 PSU and TDS520B PSU.

My PSU like version in link below.
http://jvgavila.com/tds524a.htm



My PSU not using MC34262 Power Factor Controllers . By using MC14093 ( ~ CD4093 logic) create pulse for Boost Converter ( TDS540 PSU Schematics).
it's point measurement voltage .

In Bulk Cap in have only 280V ( Have 408V if boost converter working ). I thing i have problem in PFC Board.

I thing Q5 and Q6 not switching and we dont have much 408V DC in bulk caps.

I Don't know why not running, T1 it's current sensor and return CUR SENS+ and CUR SENS- to PFC Board. I Checking it's ok, all specs like datasheet.

Standby supply worked .

I have some attach picture in thread. Pls advice me solution to check and fix.







Many tks so much !
Nam Nguyen Hai

[nctnico]

What is the voltage across the mains rectifier and what is your mains voltage?
Also don't forget to load the 24V rail with the fan because the power supply will go into hickup-mode without it. I also spend a couple of hours trying to troubleshoot one of these power supplies only to find out it was perfectly fine but it just needed the fan to be connected!

[chihaxinh]

What is the voltage across the mains rectifier and what is your mains voltage?
Also don't forget to load the 24V rail with the fan because the power supply will go into hickup-mode without it. I also spend a couple of hours trying to troubleshoot one of these power supplies only to find out it was perfectly fine but it just needed the fan to be connected!

I Try load 24V rail with Fan but not help ! In Bulk Cap allway below 408V ( 280V )

My Main voltage ( 220V ) .

And when attach fan to 24v rail ! Fan sound click click and vibaration ! Not runing Using dummy load not help

[nctnico]

If the bulk capacitor is below 400V then the problem is between the mains plug and that point.

[chihaxinh]

Hi All

Now i have 440V in Bulk capacitor . and 5.1V for microprocessor board and 5.1V feedback

But all voltage not like schematics . I place PSU Stand alone with connect FAN to 24V rail

24V i connect to Fan have only 5V
+15V LM317 i have only 2.7V
Other i have Only 1.7V

I using VOM Analog to check pin 4-8 of MC34802 . Have peak 15V  , and pin 2 of Voltage reference LT1009 have 1.6 and go down to 0V.

All diode i desolder, and check it's OK ( Red Highlight)
All trans and regulator ic check it's OK ( Red Highlight )
All Opamp i replace exclude MC34082 ( Yellow Highlight) .


I have on question . Can i check PSU stand alone ( not go back to case and connecto to microprocessor board and acq board ). Because i checked on internet said PSU like ATX Power , have Standby switch . and 5V Standby Signal ?

Many thank so much !
Best regards
Nam Nguyen Hai

[chihaxinh]

Hi All

I connect PSU to ACQ , Main board and CRT Board , now CRT Board fail and go to hiccup mode. I detach connection to CRT and running.

In Led DS1 display like 1 - A11 DRAM Processor/Display - Troubleshooting Procedure

Bus Control Register
Video
https://goo.gl/photos/RTkaLwcu4L1vkukt9

And i read can setting bypass check CRT , i want signal go to VGA display . Searching on Internet i see can setting S1001 . But see on table i don't understand how to bypass CRT test.

You have no display, which is almost certainly the processor board related. To deal with the lack of display, you can rule out the other boards by unplugging them. You can prevent the scope from recognizing problems and continuing to boot by flipping the S0001 6/7 pins; if the processor board is capable of producing video it will in this mode.

https://forum.tek.com/viewtopic.php?f=568&t=136381&hilit=TDS520+not+boot

Many tks so much !
Best Regards
Nam Nguyen Hai

[chihaxinh]

Hi All

I turn switch S1001  pin 6 - 7 on - Connect cable VGA to LCD . Running display in LCD but DS1 display .E ( FlashROM Checksum ) .