[PSU Pic. request] Tektronix TDS 220 repair

[sbeube02]

Hi all,

I have recently acquired a Tek TDS 220. The calibration is no longer OK and the CH1 connector was loose. I fixed it with fresh SnPb solder.
This scope's firmware is v2.03
When powered up, it shows a message: CH1 CH2 Acquisition FAILED


The error log shows numerous errors "6 DI Line Trigger Diag failed":


Anyway, the scope seems to work correctly.
Something is strange, the Probe Comp. signal is at 3 kHz instead of 1 kHz:


The service manual is clear about the Probe Comp signal, it must be at 1 kHz:

I have tracked the signal on the CPU board, this probe comp signal is generated directly by one of the ASIC. There is only one tristate buffer and some resistors between the ASIC output and the probe comp output on the front panel.

I have also checked the power supply unloaded (i.e. alone, without the LCD and main board) and also loaded.

Here are the results:

Pin	Description	No load	With load
Pin 1	4.85 Vdc to 5.15 Vdc	5.01 V	4.98 V
Pin 2	4.85 Vdc to 5.15 Vdc	5.01 V	4.98 V
Pin 3	GND	GND	GND
Pin 4	-4.35 Vdc to -5.15 Vdc	-4.6 V	-4.79 V
Pin 5	-23 Vdc to -25 Vdc	-23.8 V	-24.2 V
Pin 6	GND	GND	GND
Pin 7	1,4 Vpkpk to 5 Vpkpk, 47 to 63 Hz	1 Vpkpk, 50 Hz	0.8 Vpkpk, 50 Hz

Pin 7 is for the line trigger. The problem must be on the power supply but I don't really understand how this signal can even reach 1,4 V pkpk or even 5 Vpkpk since it is made by a phototransistor and a resistor network derivated from the 5 VDC.

Any help will be appreciated.

Some people with a TDS 200 series with firmware v2.03 or higher could check the Probe Comp signal is at 1 kHz or 3 kHz?
Thanks a lot.

 

[Stray Electron]

 I'm not familiar with the Tektronix TDS scopes but can you change the comp frequency from the front panel? I know that you can on the LeCroy 937X models and I think I can on the HP that I just picked up.  LeCroy even let's you change the output waveform type!

[sbeube02]

No the comp waveform or frequency could not be changed on this model. I wonder if a wrong calibration procedure could have changed the frequency of the probe comp?

[tautech]

Check all PSU voltages for RIPPLE.
As these units are quite old now you might suspect caps in the PSU have reached the end of their lives.
Just checked a copy of the TDS210 Service manual I had and there is no ripple spec. 
I'd be looking for less than 20mV on both +& -5V supplies and beter than 50mV on the -24V rail.
It would seem the -24 is unregulated, that is the spec is -23 to -25 so you would expect some ripple to be acceptable.
Trace the source of the -24V rail for a sanity check.
These are just guesses, probably the Tek spec is lower. 

I wouldn't worry about the line trigger until you know the PSU ripple is low.

I'd be tempted to pull the PSU caps and check them against their labeled values, more than ~5% low, replace them. Especially those on the secondary side.
These are likely to be low ESR and without an ESR meter, just replace them.

[sbeube02]

Thanks for the advices. The ripple was quite high on all power rails (about 50 mV).
I have removed and checked all electrolitic capacitor on the power supply board as they are now 20 years old.
I checked them with a RLC meter for capacitance and ESR.
2 of them were leaking and several were outside tolerance. I replaced almost all of them with better and higher voltage caps (original capacitors were rated at 6.3V for the 5V rail, not a lot of margin!)
Now the ripple of the power supply loaded with all boards is about 10 to 20 mV. The -5V rail is quite noisy with some charge commutation. That's weird.
This doesn't resolved my trigger line diag problem.

[tautech]

This doesn't resolved my trigger line diag problem.

Line trigger usually originates from the PSU PCB and is transfered to the motherboard via a dedicated conductor in the supply harness group. Try to identify its source on the PSU PCB and check the connection to MB is sound. It may be a mains frequency sine or possibly square wave. 
If it is not getting to the MB this will be responsible for the errors.

Tell us what you find.

[sbeube02]

Yes you are right, the line trigger comes from the PSU.

I have probed the signal with an oscilloscope, without and with the mother board connected
Unloaded:


Loaded:


The line trigger signal is not within the specifications (1 V pkpk unloaded or 800 mV pkpk loaded). The manual says it must be from 1,4 V pkpk to 5 V pkpk.

I have traced down the diagram of this part on the power supply:

When the Phototransistor is open, the voltage should not be higher than 1.063 V. When the phototransistor is closed, output voltage must be about -0.8 V (-1 V plus the 0.2 V Vce sat).

[KJDS]

You've calculated that ok.

[tautech]

The line trigger signal is not within the specifications (1 V pkpk unloaded or 800 mV pkpk loaded). The manual says it must be from 1,4 V pkpk to 5 V pkpk.

This implies the Line trig signal is being loaded down OR the Phototransistor is faulty OR is not getting the required input signals.

You are narrowing down the likely fault nicely. 

[KJDS]

I'm wondering if the 100k is the only thing that actually pulls the transistor up to 5V, or should its destination act as a current source?

[sbeube02]

I have just tried to replace the R32 resistor by a 47k ohm to rise the top of the trigger line signal to about 1.8 V.
The oscilloscope always shouts a Line Trigger Diag fail!
The line trigger signal goes to the CPU board and disapear within the 4 layers of the PCB. It is very hard to trace and I suspect it goes directly to the acquisition ASIC.
So I think the service manual is wrong about the specifications, my line trigger signal is OK and the ASIC is fried.
I have read somewhere that performing a calibration may solve this problem.

Another thing that is weird with this DSO: the Probe Comp signal is at 3 kHz instead of 1 kHz as stated in the service manual  Is it something related to the line trigger or the firmware v2.03?

[tautech]

I have just tried to replace the R32 resistor by a 47k ohm to rise the top of the trigger line signal to about 1.8 V.

Why?
In your OP it did not meet spec UNLOADED.

IMHO the PSU Line trig circuitry is the problem.

[sbeube02]

I have tried to input the correct waveform to the CPU board to see if the diag. message would disappear. It does not.
I suspect the service manual to be wrong because with the components in place on the power supply, the line trigger could not reach more than 1V.
I will try to check more on the PSU board.

[sbeube02]

Please do someone have a high resolution picture of the PSU board or schematic? I have a broken diode on the -5V output but there is no reference on the diode.

[tautech]

Please do someone have a high resolution picture of the PSU board or schematic? I have a broken diode on the -5V output but there is no reference on the diode.

Description, package?
A pic from you might help us identify this diode at a glance. 

[JJalling]

Please do someone have a high resolution picture of the PSU board or schematic? I have a broken diode on the -5V output but there is no reference on the diode.

My father has a TDS 210, so if the PSU is the same, I can take a picture or two of the PSU.

/Jonas

[sbeube02]

Here is a detailed picture of the diode and the PSU. The diode D10 don't have any marking, not even a mark on the cathode.
If someone have a Tek PSU from a TDS 210, 220 or 224,  PSU rev. D or higher it would be great to have some pictures to compare.

[tautech]

Please do someone have a high resolution picture of the PSU board or schematic? I have a broken diode on the -5V output but there is no reference on the diode.

Here is a detailed picture of the diode and the PSU. The diode D10 don't have any marking, not even a mark on the cathode.

Any polarity indication on the underside of the PCB? Different pad shape?

[singapol]

Please do someone have a high resolution picture of the PSU board or schematic? I have a broken diode on the -5V output but there is no reference on the diode.

Here is a detailed picture of the diode and the PSU. The diode D10 don't have any marking, not even a mark on the cathode.

Any polarity indication on the underside of the PCB? Different pad shape?

I can see the silkscreen cathode marking on the component side...same orientation as D9 ( right hand side). Service manual shoild have the specification or type.

[sbeube02]

Yes the cathode marking is on the silk screen. The problem is the service manual which don't contain any schematic. It just says: in case of malfunction, replace the whole board...

[SeanB]

What does the diode do? If it is overvoltage protection it will likely be a 6V2 1W zener or a transient suppression diode of the same rating. If it is a rectifier diode a UF4004 diode will do as a replacement.

[sbeube02]

This is just a rectifier diode.
I have googled for PSU pictures and found a picture of this diode on a PSU Rev. A



D10 looks to be a UF30xxx? I will try to get a replacement diode. It must be a fast rectifier diode, about 1A.

[singapol]

In the first picture I see a SB360 a 3A schottkey diode. Most likely it's after the transformer.
They are commonly schottkey diodes. If you substitute a UF 1A type (UF400X) or a 3A( UF540X) it's not going to last long. Just look at the originals of your board they are definitely 3A sized.

[creatou]

Bonjour sbeube02
a tu reussi à trouver une solution?
j'ai le même problème, CH1 et CH2 acquisition en panne
3 khz sur la probe comp

Merci

[grumpydoc]

Bonjour sbeube02
a tu reussi à trouver une solution?
j'ai le même problème, CH1 et CH2 acquisition en panne
3 khz sur la probe comp

Merci

Bonjour et bienvenue au EEVBlog - mais pouvez-vous poster en anglais SVP, il y a plusiers gens dans le forum qui peut vous aider mais nous sommes anglophones ici.

I'm sure most people can translate - but just in case, creatou said:

Hello sbeube02
Have you suceeded in finding a slution.
I have the same problem, CH1 and CH2 acquisition not working.
3 khz on the probe compensation.

Well, more or less anyway - my French isn't great