Rohde & Schwarz LC Bridge BN620

[RogerWalker]

Hello everyone - been on the 'dark side of the moon' moving to Powys, mid-Wales. The move brought to light a couple of bits of R&S gear that I would dearly love to get going and restore - but no sign of a manual for either, anywhere on the web. If anyone has anything on an LC bridge type BN620 or a tuneable amplifier, I'd love to hear from you - it would be a great incentive to get going in my new abode! I also unearthed a waveform generator by Feedback Ltd - a company that has seemingly ceased to exist. It's a WG300. Again, a manual would be a good starting point.
Kind regards
Roger

[Radio Wrangler]

Feedback ltd made function generators, where an integrator and a schmitt circuit made a triangular wave which got shaped into a sinewave. The distortion is a bit high for some purposes, but they are easily swept. They didn't make much headway in the market against the likes of Wavetek. Also HP and Tektronix had their own similar products.

Old R&S gear is beautifully made, but fairly rare in the UK.

David

[Vicboduk]

Sorry, I can't help with your R&S equipment but Feedback are still in existence, see here. I had a need to contact them recently, for work purposes, with regard to their ETL range which is also obsolete. Although they seem to have gone through a few changes they did respond, with geuine interest, to my e-mail within two days. So, it may be worth contacting them.

I have a small collection of Feedback equipment (post #6 here) but I'm not aware of the WG300 - does your unit look like it is of a similar generation?

Vic

[RogerWalker]

Thanks David and thanks Vic. I wrote a [for me] long response earlier - but at the point of posting it it disappeared into the ether in a cloud of [from me] invective. Where posts go I know not - but when it just happens, it is frustrating to say the least!
Attached are some photos of the R&S items in case they trigger some distant memory in someone. I have never seen a "tunable amplifier" before - it covers a wide range from DC to 600kHz and as David commented, is built beautifully, likewise the bridge. The LC bridge is a bit of a mystery as it has no built-in null indication and as well as a direct readout in H and uF, also has a readout in the time domain.
Cautionary tale:
The amplifier needed a replacement for its rubber insulated mains cord. The cracked and useless old one had red, black and what looked like white or possibly pale green conductors. I took this to be red = live, black = neutral and white/green= earth.

I was about to substitute a modern brown, blue, green/yellow coded cord when something stopped me and told me to check. And luckily I did. Red was connected to chassis, Black was neutral and White/green was live! A close shave?

If anyone can point me to a source of a circuit for either piece, I will be most grateful. I wrote a begging letter to R&S but received no response at all. Unlike Vic with Feedback.
I will give them a go. The waveform generator I have is full of valves and dates from the mid-late sixties I reckon - well before the unit referred to by Vic.

Kind regards
Roger

[RogerWalker]

I now have some photos of the Feedback device - I have sent a request to the manufacturers for information - let's see what happens eh? This unit employs a raft of ECL82s - seven, plus 2 x ECC83 and 2 x ECC81. It seems very well built and uses solid, reliable components and seems to have reformed all its capacitors in good time. Let's see what people know about this beast and hope I can get a manual eventually.
Best
Roger

[Chaparal]

The real name of the LC precision bridge BN620 is LCB
BN is the manufacturers construction number.

I have attached a short description with specs (german)

Maybe you can make some pics from the inside ?

Peter

[Chaparal]

The real name of the "tunable display amplifier" is UBM BN12121/x

Here an application example - schering bridge
Schematic representation of the experimental arrangement
for measurement of dielectric relaxation

Freq of RC-Generator 0-300 KHz

[RogerWalker]

Thanks Chaparal - unfortunately my German lessons of 50 years ago are not enough to help me understand the LCB short-form enough to work out how / what null indicator is connected.

I have researched "dielectric relaxation" on the Internet but am frankly only a little wiser as a result. Sounds as though tuneable amplifiers are ultra sophisticated laboratory/research items of mainly academic interest and of no practical use for anything mundane.

Thanks for the input anyway - at least it gives me some background starting point.
Kind regards
Roger

[Chaparal]

Roger,

to use the bridge you need
an external AF source (RC-Generator) [INPUT]
with some power output (some Volt at 10 Ohm) 50 Hz - 20 KHz, low distortion
( like R&S SRM, SRB, SBF, SIT)
and an external Display Amplifier (Zero Beat indication) [OUTPUT]

There are different display amplifiers availible,
with fixed frequency 1000 Hz (like R&S UBK) or
tuneable (like R&S UBM) or an
AF-Voltmeter (like R&S UVN) but limited posibility

The tuneable amplifier is better, because of limiting/filtering harmonics
(sensitivity starts at 10 uV) for better measuring results.

The unknown RLC is diplayed/readout digital at the front panel.

Biasing of DUT is possible (magnetic Lx or voltage Cx).

Btw. it is not a "ultra sophisticated laboratory/research items of mainly academic interest"
it's a precision measuring equipment with a little more stuff around ;-)


Peter

[Chaparal]

Here are 2 pics of the R&S UBM tuneable display amplifier

[RogerWalker]

Ah! So I could use the UBM tunable amplifier as a zero beat indicator with the LCB bridge!
That's excellent news - they're such beautiful instruments. I have various RC audio generators - the Marconi TF1101 should have enough output with 2 x EL91 in push-pull.
I am much heartened - thank you Peter.
MfG
Roger