Fixing a small French radio set.

[Geordie McBoyne]

The subject of this topic is a very small superhet I picked up in France many tears ago.  The set also appeared as a topic on the UKVRRR in the summer of 2012.
Today, I decided to tough it out and attempt to make the set work.   The receiver is designed for operation on 110 volts so for the time being the variac will be used until some other supply arrangement can be employed.
First fault found was the 25Z5 HT rectifier was found to have an OC heater.   Couldn't find one in the shop, the 25Z5 wasn't used often in UK made radios.  This valve has the 6 pin American base. The 25Z6 is electrically similar but has the octal base.  For the time being I've bodged in a PY88 TV booster diode, the B9A base has had three wires connected to it and these are passed through the holes of the old valve socket.  
That's got the audio amplifier of the radio working, good hum heard when the top cap of the type 75 amplifier valve is touched.   Eventually, I got the set to work but apart from bad sound quality because of an off centre 'speaker speech coil the IF amplifier is unstable, whistles and buzzing sounds.
Low intermediate frequency, 130Kc/s and it's known these low IFs can be a bit unstable.  Let's see if I can tame it.

Geordie McBoyne.

[AlanBeckett]

I picked up in France many tears ago.

I know the feeling well

Alan

[Geordie McBoyne]

Absolument!

Geordie McBoyne.

[Geordie McBoyne]

The cause of the instability was traced to an open circuit cathode bypass capacitor which is shared between the IF amplifier and frequency changer valves. The value of the capacitor is 500,000cm, work that out in microfarads.
It was discovered when the volume control was turned down the instability disappeared. I was expecting the usual method of volume control, that is a 500Kohm potentiometer across the detector load resistor tapping off the desired audio to the AF triode amplifier.
Not so, in this receiver the volume control adjusts the cathode bias of the FC and IF amplifier valves, it's not a nice arrangement but that's how it is.
It's all very strange because the AVC loop operates within the gain controlled valves, so it happens when the volume control is turned up the negative AVC voltage goes up and reduces the gain of the FC and IF stages.
The set operates on a 100 volt HT supply. The screen grids of the type 6A7 frequency changer and type 78 IF amplifier are supplied 37 volts from a common potential divider across the HT supply. The bottom end of the potential divider is returned to the cathode of the valves. 37volts might be a bit low.
Because the set has a low IF a three gang tuning capacitor is employed for bandpass tuning to reduce image interference.
Other valve types used in this set are a type 75 double-diode-triode as the detector and audio amplifier and a type 43 output pentode.

Geordie McBoyne.

[ppppenguin]

This is a wonderful website where you can waste a few minutes.

http://www.unc.edu/~rowlett/units/dictC.html

centimeter (cm) [1]the basic unit of distance in the CGS version of the metric system, equal to 0.01 meter. One centimeter is about 0.393 700 787 inch.

centimeter (cm) [2]an obsolete name for the statfarad (approximately 1.11 picofarad), the CGS electrostatic unit of capacitance.

centimeter (cm) [3]an obsolete name for the abhenry, the CGS electromagnetic unit of inductance. The abhenry is the same as the nanohenry.

centimeter (cm) [4]one of several traditional units of pressure, including the centimeter of mercury and the centimeter of water (next two entries).

I'm just so glad we don't use CGS unit any more.

[Mike Watterson]

Capacitors in centimeters or centimetres?
Anyway I've only encountered those a couple of times reading actual old documents.

[pwdrive]

Here's some capacitors pictures I took in an old French radio I have, 2nd & 3rd pictures in the third group down:

http://golbornevintageradio.co.uk/forum/...p?tid=5249&highlight=recepteur

Lawrence.

[Geordie McBoyne]

Here's a picture of the set.

Geordie McBoyne.

[ed731pdh]

Looks like something Ren'e would use. :D

[Geordie McBoyne]

It certainly would look perfect on a shelf in Rene's café. It is an unusual looking set, quite stylish, n'est ce par?
According to the No. 6 Brimar Valve and Teletube book the 25Z5 can be replaced by the more common 1D6. The 25Z5 is in fact a double diode for use in voltage doubling circuits and in half wave rectifier applications both sections are connected in parallel.

Geordie Mcboyne.