grundig: 3055WF/3D; High plate voltage on EABC80

Hello all, I am working on restoration of a Grundig 3055 3D radio. Plate voltage of EABC80 should be 51V per schematics but i am reading ~75V. Value of R47, R250 and R51 are correct (in 10% range). G2 of EL84 is ~222V. Filament voltage of EABC80 is ~6.25V. Grid voltage of EABC80 is ~0.350V. I thought that EABC80 became weaker due to it's age therefore plate current is reduced. I tried 4 different EABC80 but each time plate voltage was more or less the same. Any idea is welcomed. Thanks in advance, Cuneyt

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Hi,

after a look at the schematic, the grid voltage is only defined from the 10MEG resistor. If this resistor is open, the grid is to negative, and the plate is to high. If you measure at the grid, possibly you can't see the right value.

For test, short the 10MEG and watch the plate voltage.

 

Another check is, if the cathode contact to ground is ok, perhaps there is a weak contact in the socket.

Good Luck

 

Georg

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Thanks Georg.

Grid bias resistor of EAB80 is around ~11Mohms and I am reading -0.350V from the grid. Looking at the  chracteristhics of EABC80, this should give ~0.5 mA plate current with 50V plate voltage, which is close to current value in the schematics (0.4 mA).

There is something not clear in the schematics, probably I am missing something: Plate voltage of EABC80 should be :

V_plate_EABC80 = V_G2_EL84 - I_plate_EABC80 * (R51 + R250 + R47)

V_plate_EABC80 = 211V - 0.4 mA * 211Kohms = 126V

But it should be 51V per schematics. What is wrong in above equitation?

I will check the socket connections.

Thanks,

Cuneyt

 

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Hi Cuneyt,

 

possibly the reason for the deviation is the measuring instruments used by the radio repair people.

The values in the schematic are the values measured with the instruments currently used.

look this one of course here

The impedance is about 100k in the 300V range. I estimate, that your modern instrument has 10Meg oder 100Meg.

Edit: In the schematic it is mentioned, that the voltages are measured with a tube voltage meter, this has an high input impedance of 10 Meg or more.

Georg

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Hi Cuneyt,

Now, you get the answer directly from the former designer of this radios by Grundig, Mr. Hans Knoll:

->

Hallo Herr Beckmann.

Phänomen ist da nicht viel.

Wenn Sie nur kurz  gerechnet haetten Ub(schiene) = 221V. Ra 220+10Kohm =230 Kohm und 0,4 mA Ia macht einen Abfall von 80Volt

220V - 80 V = 140 Volt  Da stimmen keine 51, 60, oder 75 Volt. 

Das Schaltbild ist falsch.
Der 3055W -3D also ohne Fernbedienung hat einen Ra 200K + 200K zur Siebung.
Der 3055WF-3D hat wegen der FB einen Ra = 200k +10K zur Siebung.
Was das ergibt? 75 volt. Ua.
Anbei eine Kurve in die ich eine 200kohm Gerade eingezeichnet habe.
Bei einem Ia = 0,725 mA, liegen die 75 Volt and der Anode.

Das -Ug1 liegt bei ca. 0,65 Volt.

Also alles O.K.

Gruss Knoll 

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short summary: The schematic is wrong. There are two types, the 3055W-3D without remote with

Ra 200k + 200k

The type 3055WF-3D  with remote Ra = 200k + 10k for decoupling.

With a plate current of 0.725 mA is the plate voltage 75V

grid voltage is about -0.65V

 

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Many thanks Georg and Mr. Knoll for your helps. Cuneyt

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