Anyone have bv.1784 Transformer Info?
« on: July 12, 2018, 08:16:58 PM »
Hi there!

A quick call for a bit of help.

I've been looking over the Siemens U273b schematics (for the approx 100th time) and I've figured out that the bv.2558 listed on the schematic as the input transformer is a 10K:2.4K 2:1 input, the bv.2557 is wired 2:1 and is approx 100R:100R but wired as approx 100R:50R. This is by looking at the TAB U374a, which has those transformers also, and I found some notes from someone who measured the transformers in that compressor.

The interstage bv.1784 in the U273b, however, is a bit more mysterious. Does anyone have a U273b handy to please measure what the two primaries and one secondaries are?

A simulation suggests that the bv.1784 is probably a 2.4K:10K 1:2 wired before the output amplifier. I am pretty sure it has to be 1:2 due to the gain staging after it, but I'm not sure about the impedance. I could really be any 1:2, possibly even 150R:600R, or possibly even a much higher impedance to avoid loading the transistor voltage-controlled gm in the inter-stage VCA.

If anyone has any info it would be greatly appreciated.

Also, as far as I can tell the bv.2559 used in the side-chain is approx 1:4, or 1:3, 75R:1.2K or maybe 150R:1.2K (probably fine for either of those), it looks like a mic transformer most likely.

If anyone has a U273b I'd be greatly appreciative if you could get out your LCR meter and take one for the team!

I promise to give back something interesting for whatever help you give.

Does anyone even know which manufacturer made it? I've tried Haufe, Beyer and Telefunken internet searches but got nothing.

Thanks again!


Re: Anyone have bv.1784 Transformer Info?
« Reply #1 on: July 13, 2018, 05:48:48 AM »
Maybe ask haufe if they have a datasheet? They probably made them, so always a good idea to ask first :)


Re: Anyone have bv.1784 Transformer Info?
« Reply #2 on: July 13, 2018, 06:03:50 AM »
I gave that a go and they say they can't find any info about them:

"Hello Luke,

thank you for your mail.

Under this descriptions we can’t find anything.

We need our description for that trafos to check, if we find something.

We have not been producing for Siemens for more than 30 years, so I assume that we have no data left."


Re: Anyone have bv.1784 Transformer Info? New
« Reply #3 on: July 13, 2018, 06:39:27 AM »
Turns out I actually have a TAB U374a here and it has two of those transformers in it:

Here are my measurements (mind you, this is in-circuit, so no doubt it's far from perfect, but it's a start, I didn't want to desolder them for this test, but I might do this later on for comparison):

BV.2557 - primary 86.82mH, DCR 7.2R, secondary 86.08mH, DCR 7.2R
In the U374a it is the output transformer wired on the output as two windings in series so each of the two output windings is obviously half the inductance and resistance. Hence turns ratio as used in the U374a is 1:1, but in the U273b where the output windings are in parallel, the turns ratio would be 2 : 1.

BV.2559 - primary 86.03mH, DCR 14.7R and secondary 2.216H, DCR 116.0R, so turns ratio as used in both the U374a and U273b is 1 : 5.

I still can't find any info on the BV.1784 however :'(


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