W28 (German)

[dsk]

This circuit may be drawn differently, I have tried to redraw after Ralp Meyers book translate.
Please comment.
dsk

[dsk]

A picture of my telephone:

(The transmitter cap is not the right one, it should look more like a ball)

dsk

[countryman]

The above diagram is normal for a Modell 26 and W28.
The original Modell 26 designed by Siemens & Halske had sound slots on the base. This is a very rare version! It was never approved for official use but was restricted to private systems.
The W28 then was officially approved and made in large numbers by Siemens and many others.
Simultaneously, private versions were still made that more resemble the Modell 26, but without the sounding slots in the base. Yours seems to be such a phone, usually referred to as "Modell 28". Frequent differences compared with "official" W28 are:
-Handset cord enters the shell on the back
-metal cradle and upper part of shell
-nickel finger wheel
-card holder is not a separate part screwed to the front, but an oval window right in the shell.
These are often stamped VSatist66.. on the bottom.

Electrically, all these versions are alike, or so I thought until I came across this diagram with the addition "Na.v-Nachbildung veränderlich": https://www.youtube.com/watch?v=XjGmFMzX6JM&list=LLRI5YE17geu5JArErr8a9nw&index=3&t=0s 1:44 min.
Unusual for German phones of that time it optionally uses a second capacitor for the anti-sidetone circuit in parallel with a 600 Ohm resistor. Normally a 200 Ohm resistor alone is used (400 Ohm on later German models).
The phone in the video seems to be a W28 made by Stöcker & Co., Leipzig. Unfortunately, the youtuber does not show the underside.

The low DC resistance in parallel with the transmitter (200+95 Ohm) is the culprit for the low microphone voltage available on these phones. Transistor capsules will often fail on them, as well as aged carbon mics that still work fair on other phones.

[RB]

DSK, if that top pic is your drawing, it is Infinitely better than the scan pic!!!
were you a school teacher in a past life?
I can read that top pic. Thank you!

[dsk]

DSK, if that top pic is your drawing, it is Infinitely better than the scan pic!!!
were you a school teacher in a past life?
I can read that top pic. Thank you!

Regarding the german phones changig that 200 or 400 ohms resistor to a resistor in series with a capacitor of 1-5 uF you will get a slightly different side tone damping, but a transmitter will get all the DC current and that will make it easier to put in a transmitter from e.g. a WE 2500.
or a transistorized capsule. 

dsk
Thank you, it was a test to try to translate the way of drawing schematics.

Yes I'm a teacher, and will probably continue with that for a few years more.
The last 3 moths has been a little different where teaching has been digitalized and the lessons has been Teams meetings. Now it seems to slowly turn back to somthing closer to the traditional school. We have leearned a lot and the school system is hopefully developping to a better school than before.

dsk

[dsk]

More pictures, I'm still wondering what I have, the circuitry seems to follow the w28 regarding the 200 ohm balance resistor.
This is actually a problem for the voltage measured over the transmitter are so low that a LB transmitter should be the right one to use.

dsk

[RB]

DSK, your rendering of the schematic, is awesome!
The original schematics, not so much

[countryman]

More pictures, I'm still wondering what I have, the circuitry seems to follow the w28 regarding the 200 ohm balance resistor.
This is actually a problem for the voltage measured over the transmitter are so low that a LB transmitter should be the right one to use.

dsk

It's a V.Sa.tist 66... also called Modell 28. It's a PABX version of the official Post W28. The PABX models retained some characteristics of the earlier Modell 26, which was not accepted by the Post and is rare (easily identified by the sounding slots in it's base). The dial is a type N30
The German Post used 60 Volt central batteries, most other countries used 48 Volt and modern equipment often only provides 24 Volt. That's also part of the trouble with transmitters in this type of phones.

[dsk]

Great to learn such things. 60V and as remember they also used 60V for ringing?

Here is my wall version, that one is one I always have called W28 but maybe its not.
This one is a gift from a fellow collector, and has been his fathers.
dsk

[countryman]

Here are some pics I stole from this auction: https://www.ebay.de/itm/124345211925
This is a true Modell 26 with the slots in the base. They were omitted on later V Sa tist 66... models, making this exact version a rarity.
The S&H logo in the transmitter cap is also unusual, the cherry on the cake.
The ADo27 plug ("Walzenstecker") is another nice detail.
Not my kind of price tag though (300 € BIN, ~ 350 $ US).

[countryman]

auction pic & diagram