U 434

Dear Radiophiles,

Can anyone help me to find the trouble I am having with my Ultra U434.It does not oscilate in any of the the bands on the half lower frequency,when the tunning capacitor is half rotated.The oscilator grid current apears at half rotation of tunning,R2,C10,R3,R4,C22 seem OK,Continuity and resistances of coils according to manufacturer´s specifications.Could it be that the use of a UCH 42 instead of a 10C1 should be the cause for this fault?And beeing so,what could be done to overcome this problem using the UCH42 once the 10C1 is not available here? Sugestions wellcome with thanks,A.B.Regada

Dear Antonio,

first of all, you should load your question entering from the model page. You should select the 'Forum info or question' from the 'Upload and question' tab.

Going to your problem, you should check the clearance between the plates of the tuning capacitor, stator and rotor, through the tuning range. It looks as either some rotor plates are bent somewhere around the midle of their travel or the ball bearings that usually support the rotor have some backlash. You can do this check with an ohmeter across C21.

Regards, Emilio

Hello Mr. Ragada,

Has resolved itself.
Regards.

In first Place I go to the Mr. Arpad,

To ask him if by chance our friend member António is known already solved the problem of Ultra on which asked for help? To me I think that no..... Without comments.

Hello Radiophilo António Regada,

Following the reasoning of the member Emílio, I don't know if you understood what he wants to say, it is that should measure with the ohmmeter the continuity among each group of elements of sheets of the variable condenser. In other words, the continuity between the movable sheets and the fixed correspondents, they should have an infinite continuity moving them in the complete cycle, for such you can make this measurement turning off the thread that is going to each group of fixed sheets, in that terminal it puts the tip of pointer the ohmmeter, (in scale direct, Rx1), and the ground, (chassis). Make the same in the other group of sheets; it cannot have any passage. There are radios that the variable condensers have three groups of sheets, the procedure is the same.

If it exists in certain position of the condenser a continuity, it is symptom that the movable sheets clear in the fixed ones, and if this happens or the radio doesn't detect R.F. or the oscillator doesn't work in those points that touch each other, it depends on the group of sheets with problems.

Good work

Júlio Branco

This thread should be moved to the matching model Ultra U 434. Thanks.

Regards, Bernhard Nagel

Thread is moved to model.

Regards

Hello Emilio,

Thanks for your prompt answer,also to Julio.As a matter of fact,I had not been able to made me completly understood.The problem is not because of any short or small passage among the plates of the tunning condenser.The oscilation slowly disminishes,until at a certain point.This can be seen on the oscilator grid current fading until zero and with the osciloscope the same.This is why I was asking if the use of the UCH42 instead of the 10C1 could bring this problem.I found no difference changing the grig/cathod resistance from 47K to 22K.Only changing the capacitance of padder J,the range of oscilation frequency could be extended,but now the tunning was impossible.One thing is sure:there is no short or even high resistance between the fixed and moving plates of the tunning condenser. Both padders and trimmers conections to chassis are riveted,not soldered,but I don´t find any bad resistance there.The schematic and parts list does not show the capacitance of both the padders and the trimmers,so I am unable to check wether or not they are OK.Is there any more ideas about this problem and how can I fix it?Wellcome help in advance.A.B.Regada

Dear Antonio,

Some parameters of the oscillating circuit influence the oscillation but cannot be changed when using a different tube, as the values of tuning C and L and the mutual inductance. Other conditions for the triode to oscillate include the tube amplification factor, its mutual conductance and the series resistance of the tuned circuit. Unfortunately the data sheet of the 10C1 is not available, but the data sheet of the UCH42 gives a value of 22K for R3. Even if it not sure that the substitution can work, to use the UCH42 you should first adjust the grid bias installing a 22K grid resistor. Then, if the problem is still present, you could try to find a proper value for R4.

I hope that this solves the problem.

Emilio
 

Hello radiophilo António,

 What can tell him and I make it willingly it is to lend him a valve 10C1, to remove doubts if the oscillator works out of frequency to be or not to be provoked by the valve UCH42.

I have one 10C1 in stock and I can send him for the mail.

 Good work

Júlio Branco