Gow to resolve a "motorboating" problem

My name is León

I am restoring a 1935/6 (?) AC/DC radio built in Spain by "La Voz de su Amo" (His Master's Voice) company. The schematics is identical to an RCA model 114 (25Z5, 43, 77, 78, AF6) and, in fact, was mostly assembled from imported US parts. The circuitry of the radio was in very poor condition and I had to rebuild it almost completely. All fixed capacitors have been replaced and the tubes either checked for good condition or replaced. The ballast cable has been replaced with ballast resistors. I did not replace any other resistors but those that I checked (not all) showed the right value.

The radio is now working and receives very well AM stations. No hum but there is "motorboating" that disappears when the volume knob is turned close to maximum. When the volume is lowered "motorboating" appears again. I checked the volume control variable resistor with the ohmeter and seems to be ok. Three of the tubes (77, 78 and 6A7) have shields. To eliminate the possibility of a grid leakage I replaced suspected tubes one by one with no improvement.

Can anybody help?

Many thanks in advance. 

León

To thank the Author because you find the post helpful or well done.

Motorboating is indeed often a nasty problem. You have probably now all cndensers renewed. See that you have really condensers which don't lieak in all automatic control loops. Check if all shields are grounded well - some have to have a good connection with each other (same point).

The shielding of tubes is very essential but also some of the wires have to be kept short - and sometimes shielded.

An other fault can be that the mixer/oscillator has not correct voltages. Is the voltages are correct then you can shorten the oscillator. If the motorboating vanishes you find perhaps that the shielding of the IF-tube is not good enough.

If motorboating stays you install the normal connection again and open the grid of the mixer to introduce a modulated RF-signal. If you have now no motorboating then the problem lies in the oscillator coil.

If motorboating stays the coupling of the oscillator coils might be to high. See also trimmer and paddings. Control the voltages of the mixer and the resistor on the oscillator grid.

Please tell us about your success or remaining problem since this is at first only a general view without looking into the schematic of your set. There will be an expert who has more experience than I have - but I did not want keep you waiting ;-)

To thank the Author because you find the post helpful or well done.

There's not much to add to Mr. Erb's detailed reply, but I'd like to suggest again to double check if the shields are effectively grounded.

I recently got crazy with a radio set with an awful motorboating problem, until I found out that one of the shields had been "borrowed" from another radio set and thus couldn't grant an efficient connection to the chassis under some conditions.

Fixed the shield, gone the boat...

To thank the Author because you find the post helpful or well done.

Dear Leon,

You have read Herr Erb's careful comments and it is difficult to add.

However I note two things - you have largely rebuilt the receiver and the motorboating disappears at maximum volume.  There are therefore two other possibilities.

1.  In rewiring you did not exactly follow the earthing or HT points that were used.  This could give rise to impedance feedback paths both at RF and AF.

2. The maximum volume symptom is interesting.  At increased volume the HT will probably fall, this would then reduce the gain of all stages and cause the feedback oscillations to cease.  However it could also indicate that the HT impedance is high due to inadequate smoothing capacitance.

I suggest that when it is motorboating you try adding a much larger electrolytic capacitor of at least 100 microfarads across the HT to see if it cures it.  (This could indicate that your existing ones are poor or that there is a fault elsewhere in the ciruit)  The use of  ballast resistors in place of the cable will also have affected the supply impedance at HF.

Please let us know what you find.

Regards,

Roy 

To thank the Author because you find the post helpful or well done.

Since a reference was made to RCA 114, I would draw Your attention to the fact, that volume control is accomplished in the RF section only through cathode bias change (RF and Osc./mixer). The audio section is always at full gain. And the screen grids of stage 1 to 3 are directly connected without a common blocking capacitor.

That could be one branch of the undesired process. It is certainly not the primary cause, but a provisional test by blocking with 1 µF might help to trace the culprit.

There is another remote possibility: Is the voice coil connected with chassis ground ?

Good luck

To thank the Author because you find the post helpful or well done.

Ernst, Marco, Roy, Konrad,

Many thanks for your quick response to my question. I am really impressed by the amount of quality information that you have given to me. It will take me some time to follow your indications in full and I am not even sure if I will be able to do it accurately enough.  One thing is sure: I will learn a lot about tube radios in this forum.

I think I have now work for a couple of weekends. I will keep you informed of the results.

One thing more. I have now replaced the ballast resistor with two 22microF electrolytic capacitors back-to-back to get the correct 69 volts accross the filaments. The radio  works well but "motorboating" is still there.

Thanks again and best regards

León

To thank the Author because you find the post helpful or well done.

Hi Leon, a warning:

I would not use electrolytics in an AC circuit. You will risk depolarising and consequently change of capacity, breakthrough and eventually overloading the filaments.

Try to use metal/paper types as they are used in the classic fluorescent lamps (tubular). Such caps are available in several values, e.g. 3.4 or 4.5 µF. They are really safe. Dont risk the life of Your valuable tubes.

To thank the Author because you find the post helpful or well done.

Thanks again Konrad.

I will follow your advice.

Best regards

León

To thank the Author because you find the post helpful or well done.

Ernst, Marco, Konrad, Roy,

Problem resolved! The radio is now working wonderfully. There was more than one problem. I started with what t I thought was the easiest part of your proposed tests.

1) Following Roy's advice, I checked the groundings first and found one capacitor connection to chassis (2nd detector and AF amplifier) suspicious. Once properly welded there was an improvement (still a lower "pout pout").

2) Then, as recommended by Ernst and Marco I checked the tube shields. The one on 2nd detector looked very loose. Bending a little bit the metal I managed to improve the fit with the base.

Motorboating gone!

Following Konrad's warning, I am now into replacing the "back to back" bipolar capacitor I used instead of the ballast resistor.

Thanks to all of you. Great forum, especially for beginners like me!

Regards

León

To thank the Author because you find the post helpful or well done.