Bruel & Kjaer Type 2111 Audio Frequency Spectrometer

[NOON]

I picked up one of these units recently because I was intrigued by the possibilities. It's a ridiculously well designed and expensive piece of test equipment from 1960 that can basically be replicated by a cheap soundcard and free analyser software these days (if you're not worried about calibration to absolutes) but I'm considering converting it for studio use. The first step is to get it restored and working as intended, then move onto converting it into a mic preamp and possibly filters / EQ. I'll document it as I go.

I have the user manual for it and a schematic, which I posted here. https://groupdiy.com/index.php?topic=77169.0

Like most of us on here, I've been naturally talented at pulling things apart since I was a toddler and I've gradually learned how to put them back together. This thing's got me pretty much stumped at step one though!

The unit removes from the case easily enough and there is a sheet metal frame with various levels attached to the front panel with a kind of cosmetic panel attached on the front. (will post some photos later) The levels need to be pulled apart to access components, but you can reach in and change valves easily enough. From what I can work out though, you need to remove the cosmetic panel to be able to unscrew the bolts holding the chassis levels together, but you need to remove sockets etc that are buried deep inside the chassis to be able to remove the front panel. I have absolutely no idea how they assembled it!

Does anyone have hands-on experience with disassembling these things? I'm sure there's some easy trick to it or something that I'm missing but I've spent days staring at it, poking at bits, unscrewing things to see if they jiggle, screwing them back together again etc, repeat....

I've emailed the company to see if they have any information on units this old but no luck so far. They're made in Denmark, maybe Gyraf has pulled some apart before?

 

[NOON]

FINALLY worked it out. It looked like it was kind of built from the top down in layers, but I couldn't figure out how to get the bottom section off. I'll document here to save future people from the trouble of working it out again.

Here's a picture of the intact chassis.

 

[NOON]

There's various wires between the levels that need to be de-soldered. Some are obvious and easy, there's a couple of more hidden ones. Make sure you document them!

There's a circlip that holds the main frequency switch to the indicator dial. It was ridiculously tight on mine, I bent my circlip pliers without even moving it! I ended up hacksawing it in half. If I end up reassembling this section I'll have to source a new circlip.

 

[NOON]

There are nine nuts that need to be removed around the outside of the filter chassis. It was the inner two that mostly had me stumped. There's a line of bolts along the inside top, only the outermost two are attached to the front panel and need to be removed. You can then very carefully slide the front panel off the filter module switch shaft.

I have no idea how to line up the indicator dial with the correct switch position if I decide to reassemble that section, but that's a challenge for another day. Now onto documenting and sourcing replacement caps where required, finding a suitable rectifier bridge to replace the selenium bridges (used for DC heaters) and checking for out of tolerance resistors etc.