Echoes from a distant past
Part three - The Rohde & Schwarz NGU laboratory power supply unit for vacuum tubes
Edit:
Part one - Siemens Rel. send. 22bPart two - The Two TransformersPart three - The Rohde & Schwarz NGU laboratory power supply unit for vacuum tubes
Part four - The Rohde & Schwarz Vacuum Tube Meter URI BN 1050Part five - The Rohde & Schwarz UVN BN 12001 NF MillivoltmeterPart six - The Nordmende digital multimeter DIVO 3355 (with Nixie tubes!)Hello,
today I'd like to introduce you to the Rohde & Schwarz lab PSU, type NGU, developed for designing vacuum tube circuits.
This devices comes in nearly mint condition. No rust visible on the outside nor on the inside. The mains cable is stiff and brittle,
but this will be replaced anyway. It weighs ca. 12kg and can produce several outputs of DC and AC voltages.
The main regulated DC output voltage (R&S called this the "U") goes from 100V up to 300+V with a max current of 100mA.
This is lethal territory so better be careful!
Let's start with some pictures from the outside.
Front with the lid cover on, top and bottom view, front without the lid cover.
The brittle mains cable and the glowing eye of an HAL9000 (probably).
So, let's see what's in the inside.
Top view, bottom view. And we have a date code: January, 27th, 1961
Beautiful wiring as always with this old R&S devices. No sign for leakage or rust but some corrosion (we'll see that later).
There are five vacuum tubes in it and all are located in one corner of the NGU.
Easy to access and to replace. All tubes can be exchanged without adjustments except of the two 85A2.
Those are the voltage refrences of the NGU. Kind of the vacuum tube version of modern Zener diodes.
EL34 (6CA7)- power pentodeEF80 (6BX6) - RF pentodeECC82 (12AU7) - AF twin triode85A2 (0G3) - voltage regulator tube (two of them)
Some Bosch electrolytic capacitors from October 1960. They are huge and must have cost a fortune back in the days.
A strange construction to hold another capacitor in place. Looks expensive (the holder).
The mains transformer. Lots of windings, lots of tappings. This must have been an expensive beast as well!
Not only the transformer itself but also the assembling of it.
There are some stains on the outside of the transformer but they can be removed easily with a dry paper towel.
Mains input and the fuses. The red cable is the PEN, another sign, how old this device is.
The fuses are corroded which I'm finding a bit odd. They are inside of a plastic cover and the corrosion looks like,
if a battery or an electrolytic cap has spewed its guts on them. I've removed the fuses (they look as they were the original ones),
cleaned the fuse holders with lots of IPA and replaced the fuses with new ones. This blue-greenish stuff was very crusty,
so I used a small piece of paper towel wrapped around a small screwdriver. I wasn't able to remove all of the corrosion but most of it.
The rotary switch for the coarse setting of the U voltage. The potentiometers are Rosenthal branded. Germans know Rosenthal as well for their
excellent porcelain tableware, but their potentiometers are also top quality. These ones work smoothly as they were brand new. Well done!
Some detailed views of the wiring and parts being used.
You may have noticed those blue dots on all those solder joints. I'm not sure what's their purpose. Some kind of quality control?
Here are some measures of the outputs.
BM235 - 6.3VAC for heating the vacuum tubes
121GW - measuring E
1 (goes from 0V down to -10V DC)
U1252B - measuring E
2 (goes from 0V down to -100V DC)
BM869s - measuring the DC and AC of the U output (DC goes from +100V up to +311V)
I'm not sure, why the U output is that much off. Perhaps some adjustments needs to be done, perhaps the 85A2 needs to be replaced. I'll find out.
(please also note, how crappy the display of the 121GW compared to the others is. I'm a bit disappointed in this respect (sorry Dave).)
Here are some pictures from the manual, including a hi-res photo of the schematics.
Features page one, features page two, example for how to connect two NGUs together, schematics.
I do really enjoy, how much effort and love for details R&S putted into their manuals. It is not only the pictures
but also their clear explanations on how their devices are working. It is really a pity, that such things aren't
available anymore. In every single line one can read all the pride of the people which have designed this device.
I hope, you've enjoyed this teardown and report of the R&S NGU.
Please stay tuned for part four, the Rohde & Schwarz URI vacuum tube multimeter.
Thank you for watching.