Defective Rohde & Schwarz NGM 15 - Fixed

[StefanKrupop]

Hello forum!

A long time ago when I was a child, I had a nice R&S power supply which my father provided me to power my small electronic projects.
Sadly, one day it became defective, IIRC due to a prolonged (almost-)short circuit on it's output. I do not remember what we did try to fix it, but apparently we could not fix it. But I also never wanted to throw it away, so it lived in a box for many years. Recently I found it again and decided I wanted to give fixing it another try. So much for the history.

The current state of the supply is that it turns on, but only gives a small negative output voltage of ca. -0.6 V, regardless of the voltage pot's position. The left LED (CV mode?) is lit, the right one (CC mode?) not. The current limit pot has no effect. The meter in voltage mode shows the negative DC voltage (offset to the left), in current mode it shows 0.

After opening up the case I discovered that we back then replaced one of the resistors (I suppose the current shunt) with a self-wound piece of simple isolated wire... Whose value most probably would be wrong. The missing/wrong resistor is labeled "R310".

Sadly, I could not find any schematics or even a manual for the power supply online, so I have no idea what the value of the original resistor once was. Could anyone help me with that? I suppose it's not the only fault in the unit (i think at least one semiconductor will be faulty), but would be a start. Of course having schematics would greatly help fixing the supply...

Sorry for the long text and thanks for reading!

Kind regards,
Stefan

[StefanKrupop]

Update: I found a blown fuse and replaced it (wonder why we did not do that back then...). Now the voltage regulation part of the supply seems to work again, but the current measurement and limit is way out - no wonder with that self-made (supposed) current shunt. So it seems I only need the correct value for the resistor.
When no one has the schematics with the correct value maybe I would be able to try different values and find one where the displayed current matches the drawn current...

Stefan

[nctnico]

You could start with a 0.1 Ohm resistor to find out what the ratio is between the resistor and the reading on the meter. If 0.1 Ohm causes a reading which is two times too high you need 0.05 Ohm. Vice versa if the reading is two times too low you need 0.2 Ohm.

[StefanKrupop]

Finally!

After I had not much luck with the repair the last time (there are 8 pots in the unit that all could have been dejusted, plus the unknown missing resistor), I tried again today.
This time I simply asked the Rhode & Schwarz support whether they could help me with some kind of documentation. And they did help: On the next day I had the complete service manual with schematics and calibration instructions as PDF!

So the missing resistor R310 in fact is a 0.51 Ohms, 2.25 W 5%wire-wound resistor (for the NGM 15, other models have 2x 0.51, 1 and 2 Ohms)!

The correct resistor is ordered, but with my 9x4.7 Ohms parallel test resistor the unit works fine and I was able to calibrate it.
Now after >15 years the unit is finally operational again!

Kind regards,
Stefan

[crispy_tofu]

Congratulations, hopefully it will work for years to come. 

[StefanKrupop]

Ah, I yesterday forgot that I wanted to post the calibration instructions (shortened), so this thread becomes more useful:

• Set meter movement to zero while unit is powered off (set screw on the meter movement itself)
• Adjust voltage limit pot so that after 10 turns the marking on  the knob is again on scale zero
• Connect voltmeter, adjust R201 to rated output voltage
• Adjust R208 for full scale on the meter movement
• Adjust current limit pot so that after 10 turns the marking on  the knob is again on scale zero
• Short output with amperemeter, adjust R203 to rated output current
• Adjust R206 for full scale on the meter movement

The pot on the PCB with the triac and heat sink is for the overvoltage limiter. If the voltage becomes too high, the PCB shorts out the output.
The two pots on the regulation PCB (the one with the references) are for offset correction of the voltage- and current opamps.

Kind

[Theboel]

Ah, I yesterday forgot that I wanted to post the calibration instructions (shortened), so this thread becomes more useful:

• Set meter movement to zero while unit is powered off (set screw on the meter movement itself)
• Adjust voltage limit pot so that after 10 turns the marking on  the knob is again on scale zero
• Connect voltmeter, adjust R201 to rated output voltage
• Adjust R208 for full scale on the meter movement
• Adjust current limit pot so that after 10 turns the marking on  the knob is again on scale zero
• Short output with amperemeter, adjust R203 to rated output current
• Adjust R206 for full scale on the meter movement

The pot on the PCB with the triac and heat sink is for the overvoltage limiter. If the voltage becomes too high, the PCB shorts out the output.
The two pots on the regulation PCB (the one with the references) are for offset correction of the voltage- and current opamps.

Kind

Just wondering if You could share the service manual maybe We can learn something from this power supply

[StefanKrupop]

I first thought about doing so. But as it seems not to be available anywhere online and they sent me a personal download link, I am not sure how happy they would be if I upload it publicly...
That's why I only paraphrased the calibration instructions.

Kind regards,
Stefan