u73 identification and help

[ben@timberlakeaudio]

Hello everyone,

Someone brought me 2 of these u73s to get up and running for them. They are labeled u73b on the front, but the controls and circuit do not match the u73b schematic.

I'm thinking they might be u73a models, at least electronically, but I cannot find much information on that revision. I also know the connector pinout is different from a u73b. These units do not have pins for an external meter and pins 4a,b go to the coils of a relay that is before the input transformer.

The tube in the back by the power supply caps is a 150B2 voltage stabilizer tube, but the other tubes are the same as the u73b.

Can anyone provide any info on these? And would anyone be willing to send me any schematics or documentation on the u73 and u73a?

 

[rock soderstrom]

Hi Ben, here we go. This is all I can find in the kubi files about U73. Time to polish your german

Cheers

 

[rock soderstrom]

The tube in the back by the power supply caps is a 150B2 voltage stabilizer tube, but the other tubes are the same as the u73b.

Non of the U73 schematics I have seen so far shows a 150B2 voltage stabilizer. I'm curious what the experts will say.

Edit: This really seems to be a transitional device. Technically more like a U73a. Nice photos can be found here:

https://reverb.com/de/item/45528046-telefunken-tab-u73a-u73-u73b-compressor-limiter-pair

 

[ben@timberlakeaudio]

Thanks for posting the docs! I will give them a read through.

I think you're right about it being a transitional device. Or maybe it was modded for a special function at some point. The reverb listing you linked also has the 150B2 stabilizer tube, but it's up in the front behind the front panel switch. One the ones I have it's in the location of one of the power supply caps and that cap has been moved next to the rectifier.

The ones on the reverb listing also do not have the relay, so I'm not sure its exaclty a u73a either.

I also noticed the front panel does not have the TAB, or any other, logo.

 

[rock soderstrom]

The reverb listing you linked also has the 150B2 stabilizer tube, but it's up in the front behind the front panel switch. One the ones I have it's in the location of one of the power supply caps and that cap has been moved next to the rectifier.

You're right, I saw that too.

The ones on the reverb listing also do not have the relay, so I'm not sure its exaclty a u73a either.

Yep, your device is somehow in between.

I also noticed the front panel does not have the TAB, or any other, logo.

I noticed that too. Does your device also have a sticker on the inside of the release lever?

Edit: This U73a from the reverb site was originally intended for Deutsche Welle, a German state-owned radio and television broadcaster for foreign countries.

 

[C.B. - Boudio]

I cannot stress enough how I love these modules... and I have never, ever seen one (V or U, no matter which of them) in person (yet).
And they never cease to surprise me how many transformers and inductors they crammed in there. Seems almost if it was a running joke among those engineers: 'Heinz, vhere can ve place anotzer block of iron in zhis ding?' (I know, I know more or less what they are for )
How does the 6-position Kompression switch work? The schematic only has the three-position 'ohne - kompressor - begrenzer' switch.

 

[rock soderstrom]

And they never cease to surprise me how many transformers and inductors they crammed in there. Seems almost if it was a running joke among those engineers: 'Heinz, vhere can ve place anotzer block of iron in zhis ding?'

That's probably how it was Really, the old boys were throwing iron around as if it grew on trees back then.

 

[C.B. - Boudio]

Maybe it's time to set up a GroupDIY transformer factory? Then we too can get that part of the 'IRT engineer experience'. Everything wound 'in-house' for even our most demanding members!

 

[ben@timberlakeaudio]

Yes, these also have the same stickers in the release handles. However, the silver area is blank.

These also are dated 20 Apr 1963, so they predate the u73 documentation above. I guess they were making multiple revisions at the same time.

 

[Recording Engineer]

Maybe it's time to set up a GroupDIY transformer factory? Then we too can get that part of the 'IRT engineer experience'. Everything wound 'in-house' for even our most demanding members!

I motion for Moby to head that department. First task at hand is recreating all we need for the U73!

 

[telewulf]

Maybe it´s just my imagination but on Bens pictuers it looks like the letters are not engraved into the surface of the plate but just inked onto it. All modules I ever saw have their description and serial numbers engraved. Maybe these units are not made by one of the big companies like Siemens, TAB or Maihak but rather self made. On the other hand the high serial number does not account for that.

Reminds me of an EQ that I found at Radio Bremen. It was built in house but never the less named "W95" because it fitted with the Braun Buch.

greetings from Bremen,
Wulf

 

[telewulf]

This is all I can find in the kubi files about U73.

This is all I can find in the kubi files about U73.

Are those files still online somewhere??? I was devastated when I realized they had gone!

 

[rock soderstrom]

Maybe it´s just my imagination but on Bens pictuers it looks like the letters are not engraved into the surface of the plate but just inked onto it. All modules I ever saw have their description and serial numbers engraved. Maybe these units are not made by one of the big companies like Siemens, TAB or Maihak but rather self made.

Hi Wulf, I've been waiting for your input on this topic! All manufacturer logos are also missing which is very strange. I don't think it's a copy/DIY/replica, it's just too well made for that.

I believe in a pre-production/intermediate model which was sent to Deutsche Welle studios worldwide for testing. This is also supported by the 0A2 tube, which was then replaced by a Zener diode in the production model. Our ancestors were very progressive when it came to such things.

 

[rock soderstrom]

This is all I can find in the kubi files about U73.

Are those files still online somewhere??? I was devastated when I realized they had gone!

Yep, Jens (member @jensenmann ) provided us a link:

https://tradefair.audio/data/

or this backup link from the nice guy behind @KA-Electonics.com

https://proaudiodesignforum.com/forum/php/viewtopic.php?t=1341

 

[telewulf]

Marvellous!!! Thank you so much!!!

 

[telewulf]

I believe in a pre-production/intermediate model

Something like that, yes.

What strikes me as odd is that you can see lots and lots of transistos on Bens pictures but then there are the tubes as well - very curious!

 

[rock soderstrom]

What strikes me as odd is that you can see lots and lots of transistos on Bens pictures but then there are the tubes as well - very curious!

I was also surprised and wondered what they were doing.

 

[C.B. - Boudio]

Maybe they are used as diodes? In my Revox F36, there are diodes that look like transitors (and they are more or less, with one leg snipped short).

 

[rock soderstrom]

Maybe they are used as diodes? In my Revox F36, there are diodes that look like transitors (and they are more or less, with one leg snipped short).

possible

 

[Dark1]

Most photos of production U73a units (i.e. w/the correct U73a faceplate and controls, not re-stamped/modified U73 units, those are also in the wild) I have seen had the 150B2 stabilizer socketed in front of the input transformer (where, in standard U73 units, the front switch normally protrudes). This one seems to be modified (hence also the different pinout to accomodate the added relais?), and tracing the schematics would be the only option, since U73a diagram is not available (yet). The semiconductors are all diodes (just older packaging). The circuitry and layout between stock U73 (and U73b) and stock U73a seems to be rather different (judging by the photos), and it would be very interesting to see a schematics/leaflet that would explain this version.