Repair Power supply TDS 420 Tektronix scope

[excalibur]

Hello

I'm new on this very nice forum. My english is not perfect sorry, I'm french.

I like electronics since long ago ! My scope TDS 420 didn't want to restart and I suspected the power supply.

Severals cracked solders, caps ok.

And I find the 99% solutions to my problems with this very good topic :

https://www.eevblog.com/forum/repair/tektronix-tds410-power-supply/

At the end of the video Mrs Carlson lab (28 mn 30s ) show that he stabilize the 6,4V output by puting a resistor. OK but what is the value of this resistor. 100 ohms or less ?

Other question : Is it a good idea to leave this resistor soldered on for furthr control ?

I join the drawing of the power supply. It is the same as Lecroy

Thanks for your help.

Sincerly.

[excalibur]

Hello

Ok I tested with a 560 ohms and the 6,4V get stable. Good.

When I put the supply in the TDS and start nothing happen ! Just a little noice "tic" every 2 S.

I disconect the R503 (1 Kohm) who is connected to the led of the optocopler as proposal by others. Andthe scope start and work fine.

I believe that the opto have a problem. I ask for a new one : Vishay CNY17-2 for few cents.

When I receive I will test it.

Question is it possible to work with R503 open ? What is the risk ?

Thanks for your help.

[manicdoc]

most regulators require some current draw to regulate. the 560 ohm will be pulling 11mA . Had the same issue on an old Fluke, without that little bit of draw, the regulator didn't run and the input side voltage floated up on the transformer, not healthy for the caps..  Fine when in circuit, not good when bench testing it.  I solved two problems at once by using an LED and resistor per supply rail (+12v,+5v, -12v if memory serves) so I also had a visual aid to show it was working, and more importantly, powered up - this made it safer to work with - as they would also discharge all the caps within a few seconds.

[excalibur]

Hello

Thanks for your answer and your solution. I will put permanent  LED and resistor per supply rail (+12v,+5v, -12v and-6.5V) So if their is a problem I can immediately check ! 

The other question is the change of the opto is it enough. I have not this piece yet and mail it with lot of capacitors (Main board, supply, and rail) Long life 8000/10000H 105°C Nichicon).

In some topics one change LM 339 N without success and one have connected R414 and R415 !!! This resistors are allraidy soldered on the drawing of Lecroy. ! Strange ? I don't understand why? I give you the link to check :

https://forum.tek.com/viewtopic.php?f=568&t=142223&p=288525&hilit=TDS+420#p288525 => look anti-doom reply on may 15 2020 at 12:53

If somebody understand ?

[excalibur]

Hello

Can someone help me about my second question about the supply of the TDS 420 ? :

"I disconect the R503 (1 Kohm) who is connected to the led of the optocopler as proposal by others. Andthe scope start and work fine.

I believe that the opto have a problem. I ask for a new one : Vishay CNY17-2 for few cents.

When I receive I will test it.

Question is it possible to work with R503 open ? What is the risk ?"

Thanks for your help

[shakalnokturn]

The original schematic has errors, don't take everything for granted.
The opto is a common problem now (with age) the internal insulation degrades.
It is used for secondary over voltage and over temperature shutdown. All else being good there is not much risk running without for a short time. I would not recommend completely removing it.
This is one of the supply's where a full secondary recap is useful. Make sure to clean any capacitor leakage before fitting the new ones.
While you're in there I also recommend adding a preventive 15V zener or TVS between primary ground (anode) and green module pin N°8 (cathode). Missing pin counts as N°4.

[excalibur]

Thank you shakalnokturn for your answers. So I'm on the good drive! Fine.

OK I receive the pieces, opto, caps, aloccol isoporpylic for cleaning... I will do the job on a rainy day.

I notice that the original schematic has errors.

You recommand me to put a zener 15V between primary groung and green module pin n°8. Ok I believe that you speek about the A1 100004 ?

Should I protect the zener with a serial resistor ? Zener of 1W or more ?

Or a TVS what reference should I put ? Sorry I have few knowlegde on TVS.

Thanks for all.

To read you.

[shakalnokturn]

Yes the A1 100004.
A number of these get destroyed, they can be replicated but it's less work keeping them alive.
If you look at the schematic you attached:
If for some reason the primary switching transistor (Q1) shorts out it should blow fuse F1. But Murphy being around F1 isn't always fast enough and there is plenty of energy left in C1/C2 so current sense resistor R7 can also fuse leaving much too much voltage on R10.
The problem here is that I think this is one of the schematic errors: R10 isn't 470k but 470 Ohms! (Check for yourself) Allowing plenty of current to get through A1 100004's pin 8 destroying it.

No need to add a series resistor as R10 will fuse in such conditions, it can be a good idea elevating it from the PCB though.

1W Zener should be good enough, the idea is that it clamps the voltage on pin 8 or goes short in case of catastrophic failure, same idea with TVS 15V approx. It should be fine with a much lower voltage, I just went for 15V as that's the A1 100004 supply voltage. No idea for a TVS reference, I've always used Zener diodes I had at hand 

[excalibur]

Thanks shakalnokturn for your complete explanation.

I will save the A1 hybrid and much more with your explaination.

I will check the R10 resistor if it is a 470 ohms !!! Amazing mismatch.

Should I replace the 470 ohms with a 470 kohms ?

Sincerly

[shakalnokturn]

470 Ohms is the correct value. With 470k the time constant with C7 (the one on the right on the schematic not "rise time option C7") would be so high that there would be no point in the current sense at all.

[excalibur]

Thanks for your answer I will check it when I receive all the capacitors (some missings).

A question : When I look to the drawing I see that their is another C7 capacitor in a rectangle named "Rise time option".
What is the use of this one ?

Thanks

[shakalnokturn]

That one is for "soft start".