grundig: What is a "Monozelle" ?

My Grundig UKW-Concert Boy 59 seems to have once had a DEAC 3.5 ah cell after the rectifier and 35 ohm dropping resistor in the filament circuit.  The battery itself is gone, but the corrosion damage suggests it was once there.  Previoulsy, I had restored a 1955 Telefunken which had a similar arrangement.  It seems that the presence of these rechargable batteries act as a excellent voltage regulator for that sensitive 1.4 volt filament circuit.  I observed this over a wide range of input voltage with the Telefunken.

My question concerns a notation on the Concert-Boy schematic, following the DEAC battery, that has two identical parallel plates (that is, not a long one indicating positive and not a short one indicating negative like a standard battery) deployed between the 1.35 volt positive buss and ground.  Between those plates is the word "Monozelle".  

Simply looking up "Monozelle" only gives pictures of single battery cells.  I simply cannot figure what is intended to be here.  The radio itself seems to have a metal bracket attached to the chassis that looks like it could bear against the negative side of a battery, and some sort of retaining clip several inches above.  There is some corrosion damage, so even this is a bit unclear.

So what is this missing component, called "Monozelle" in the circuit diagram?

Best,

Frank

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"Monozelle" means simply a primary 1,5 volts "D-Cell", see please here

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Thank you.  That fits what I found, but I can't quite understand why it is needed if the DEAC 3,5 battery preceeds it.  At 1.5 volts, it should back-discharge into the DEAC battery when the radio is off, attempting to keep it charged to about 1.4 volt.  When the radio is on battery, it should help supply the filaments along with the DEAC battery, but what is the advantage considering there is a 3500 maH battery behind it?  And when the radio is Mains powered, it will try to charge both the DEAC battery and the 1.5 volt battery; driving the voltage to 1.5 or a bit more, assuming it was originally a carbon-zinc Monozelle.  So I remain a bit puzzled, but you have at least confirmed the meaning of Monozelle.  Again, thanks.

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Dear Frank,

the different cells are not used all together. DEAC cell or "Monozelle".

Best regrads

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The Philips sets also relied on the DEAC for 1.35V regulation. Without it the filament voltage is excessive. However the running time on the DEAC battery is poor, maybe 1/3 to 1/4 of an Alkaline D size cell. The Monozelle (D cell also called U2 in UK) is for added running time during portable use as the HT pack lasts maybe 4x to 6x longer.  On the Philips sets it is essentially in parallel, so must not be left in as the DEAC would eventually discharge it and cause it to leak if the set was left in battery mode (power cable docked on the Philips models).

I'm not sure of the Grundig wiring details, but I'd be surprised if it is much different. I think the mains/battery switch disconnects the monozelle (= single cell), often a D size on many sets world wide from 1941. Mostly it was the larger USA sets and UK models that didn't use D cells, but combo packs or specialist LT packs.

You can use an Alkaline cell (gives nearly x2 life and less prone to leakage if left in and in battery mode).

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OK, this helps unwind the mystery.  Thank you.

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Michael:

In this radio, there does not appear to be any mechanism for disconnection:  the batteries seem hard-wired in parallel.   For this radio, Rudiger may be correct, that is, it is the DEAC or the Monozelle.  But your explanation makes complete sense if there is a way to disconnect.

This leads me to another thought.  In the restoration, it may make sense to use a modern NiCad only (say 10,000 maH) rather than an alkaline D cell.  My experience with my Telefunken leads me to believe the regulation of a NiCad is very good, and may be better than that provided by an alkaline D cell (although I made no tests with an alkaline cell under these circumstances).  I am worried about the 8 old, filament-voltage-sensitive tubes, which will be hard and expensive to replace.  The NiCad should level off at about 1.4 to 1.45 when on AC, and this should be just under the 1.5 volts maximum for these tubes.  On the downside, these tubes seem unhappy with 1.2 volts as the AC cord is disconnected and the voltage slides down, never really reaching the capacity of the battery.  In the Telefunken I notice markedly decreased audio output and increased distortion.

So, do you have experience with the behavior of a 1.5 volt alkaline cell under AC operation?

I'd like to hear your thoughts and those of Rudiger and your other colleagues, and I really appreciate the conversation.

Best,

Frank

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The primary cells must not ever be used on mains operation on the LT supply. They can explode if charged and the voltage on an Alkaline cell would rise to a little over 1.6V so any use of a primary cell, (Zinc Carbon, Zinc Chloride or Alkaline Manganese) can only be when the radio is on battery only.

I use a C size 4000mAH NiMH to replace the DEAC, I cut a case from coffee can and put the C cell inside. I print a label. The D size cells with 10000mAH are only online (the local shop versions are just C cells in a D cell case, 4000mAH) and it's better to stick to close to original specification.

An actual NiCd would be better as the NiMH has a slightly higher voltage on charge, but on the other hand the main part of discharge is nearly 0.5V more.

The tubes SHOULD all work on 1.2V. The worst case is the frequency changer which may stop oscillating, but originally the circuit should have worked to 1.1V which is the Zinc Carbon endpoint chosen by designers. On motors and lamps, the running time assumes a 0.9V end point. The design of 6V, 9V and 12V transistor sets assumes a 0.9V per cell endpoin. Also most layer 90V HT packs radio design assume 60V or even  55V end point which is 1V or less per cell. Below 1.1V the Zinc Carbon capacity falls off a cliff!

See this thread about the DEAC and circuit on a Philips

See the comments on the D6 model

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Michael:

Thank you for the explicit warning against attempting to recharge primary batteries.  I have never done this for the reasons you explained, but I realize in speculating about the possibilites with this radio a risky message may have inadvertently been sent.  

I like your DEAC "can" and the rechargable battery inside and the fancy label outside.  As I do better with wood than metal, I think I'll make a small wooden box to hold the battery.  I may need to revisit my work on filament voltage, as the logic you present makes a lot of sense.

Great fun this litle project.  Thanks again.

Best,

Frank

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The can is only scissors and a coffee tin! A few solder tacks. The battery might get too warm in a nice wooden sleeve. I do use wooden innards and card outers for working replica battery packs that are not rechargeable.

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