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Old 15th Jun 2018, 9:08 pm   #1
david winter
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Default Problem converting Loewe Super 32 to FM

Hello,

I'm used to restore and convert tube radios to FM but for the first time I have a quite unusual one which design makes things more complicated.

The radio is a Loewe Super 32, which is almost same as the Thule.
The schematic from the link below shows two particularities:
- three different heating circuits (basically the rectifier, one single directly heated triode at 4V, and the other 4V tubes)
- a speaker coil with multiple windings form which various voltages are obtained. The three-part coil is split as 5K, 6K and 7K just over the mains transformer on the original schematic.

The main problem is as follows. As you will see in the links below, the radio section (AM / IF) is grounded to the chassis whereas the amplifier (preamplifier triode and output tube) is not, hence around -190V between the Pick-Up inputs and the chassis.

The FM tuner is powered from the 4V circuit and grounded to the chassis. Since the pick-up input is at -190V or so from the chassis, I inserted a 10nF 630V capacitor between the FM output and the PU input.
So far the FM works but only at very moderate volume, after what I get some serious motor boating.

Of course all bad capacitors are replaced except the "vacuum" ones housed in vaccuum glass tubes. Those tested with no visible leakage current at 300V and 1mA meter scale. Very impressive for such old components !

My question is: how is it possible to inject the FM sound to the preamplifying triode (actually to the hot PU input) and fix the motor boating problem ? Can the triode circuit be changed to as to have the PU input grounded to the chassis ?

Any hint will be much appreciated.

I am including two links there. One is for the basic original schematic. The other is from a great site which has an extensive page about the restoration of this particular radio, but without converting it to FM.

Original schematic is here.

Restoration page with detailed schematics including the special power supply is here.


Thanks,

David
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Old 16th Jun 2018, 8:19 am   #2
Herald1360
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Default Re: Problem converting Loewe Super 32 to FM

Use a small audio isolating transformer to couple the FM audio to the PU input?
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Old 16th Jun 2018, 11:08 am   #3
david winter
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Default Re: Problem converting Loewe Super 32 to FM

Excellent idea but none of the transformers I have seem to have 200V insulation. Both windings are not isolated with paper. I would need a small model with enough insulation between the two windings.

Since only the two last amplifying tubes are kept, all tubes grounded to the chassis are removed. Maybe there is a way to have both audio tubes also grounded to the chassis, but I don't know what to do for this. Part of this modification is to shunt the 5K coil between the chassis and the audio "ground". But then some resistors would have to be changed to accomodate the grid, screen and plate voltages. I would prefer this option if someone can advise on what to do.
Thanks.
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Old 16th Jun 2018, 11:34 am   #4
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Default Re: Problem converting Loewe Super 32 to FM

Shunting the 5k winding would affect the operation of the field coil because all the output stage current flows through it, referencing the 4 volt heater winding that supplies your FM tuner to HT -ve instead of chassis might put extra strain on the heater-cathode insulation of the valves, I would consider a separate PSU for the FM tuner, 6 volt transformer, bridge rectifier, regulator etc.

Lawrence.
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Old 16th Jun 2018, 11:43 am   #5
crackle
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Default Re: Problem converting Loewe Super 32 to FM

Quote:
Originally Posted by david winter View Post
Hello,
The main problem is as follows. As you will see in the links below, the radio section (AM / IF) is grounded to the chassis whereas the amplifier (preamplifier triode and output tube) is not, hence around -190V between the Pick-Up inputs and the chassis.
If you are injecting an audio signal into the triode preamp, then surely you need only be concerned with the common negative of the output amp. This will be your "ground" for your input signal.
Is the point labelled TA the gram input.

Mike
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Old 16th Jun 2018, 12:31 pm   #6
ms660
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Default Re: Problem converting Loewe Super 32 to FM

The problem is the source for the FM tuners power, it's referenced to chassis not HT -ve.

Lawrence.
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Old 16th Jun 2018, 12:45 pm   #7
crackle
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Default Re: Problem converting Loewe Super 32 to FM

Use a small 6 volt mains transformer to power the FM tuner.
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Old 16th Jun 2018, 1:27 pm   #8
david winter
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Default Re: Problem converting Loewe Super 32 to FM

The real problem was that considering the 10nF capacitor at the FM output, the triode grid was virtually not connected, so would not discharge as with a ground resistor.
To replace that missing resistor, I took a 2M pot across the TA input and found the position where the motor boating disappeared even at high volume. It happens to be at 57K so I used a 56K resistor and the problem was gone.
The most important is the 10nF capacitor between the FM audio output and the TA input, otherwise the tuner would get -190V, and its 100V output capacitor would not make it (a 400V capacitor would have allowed a direct connection).

So far so good ! Thanks for hints.
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Old 16th Jun 2018, 2:12 pm   #9
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Default Re: Problem converting Loewe Super 32 to FM

Cheers for that, I can see now, no grid return resistor when switched to pickup, just a change of bias resistance to reduce the bias from detector function (looks like anode bend) to pickup function....it's a bit easier to figure out using the Thule schematic.

Interesting receiver design.

Lawrence.
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