Farnell LFM2 sine / square oscillator - no sine. - UK Vintage Radio Repair and Restoration Discussion Forum

Nanozeugma · 16th Sep 2020, 4:03 pm

Hi all

Just got round to powering this (recently acquired) specimen for the first time.
It was allegedly working but with a complaint of "low output."

What I found was that I can't get sine wave out of it, the output is square wave whether the squaring button is engaged or not.
Engaging said button just seems to reduce output amplitude by about 50% on the internal meter, and a scope corroborates this.

The "squaring" switch needs a flick to release as the mechanism is tired. I've Super10'ed the contacts and oiled the mech without improvement but it does release if you flick it.

I have the user guide and circuit diagram but **no board layout, voltages or oscillograms**.

The collector of the last stage in the oscillator section (feedback amplifier) shows the waveform attached - with the sine / square switch in sine. I would have expected a sine wave at this point.
The circuit uses an STC R54 in the negative feedback loop, I'm hopeful it isn't duff. it reads about 2k in circuit although there are several resistive paths in parallel and I haven't tried it out of circuit.

The circuit uses a split rail +/- 10v supply and these are as expected.
I've included a circuit extract.

I wondered if anything springs to mind...?

Electronpusher0 · 16th Sep 2020, 7:36 pm

Unfortunately it does sound like the thermistor is open circuit.
Acording to this thread
https://www.vintage-radio.net/forum/...d.php?t=107227
the R54 is 50kohms at 20 degC, 620 ohms at 3mW dissipation.
You could try replacing the thermistor with a 50k pot and varying it.
The oscillator will go from square wave at 50K to no oscillation at about 500 ohms.
Some where in between you will get something approaching a sine wave but very unstable, thats the thermistors function.

I have a spare R54 (used) if you need one. PM me.

Peter

Nanozeugma · 16th Sep 2020, 10:13 pm

Hi Peter,
[Edited]
I've isolated the R54 and it reads infinity - completely o/c.
I'll P.M. you.
Many thanks.

Nanozeugma · 22nd Sep 2020, 11:53 am

With grateful thanks to Peter (Electronpusher0) who provided the replacement NTC thermistor which I have now fitted...

We have sine wave, yay!

Mods: Thread may be closed as desired.

16th Sep 2020, 4:03 pm	#1
Nanozeugma Heptode Join Date: Nov 2017 Location: London, UK. Bury, Lancashire quite regularly :) Posts: 611	Farnell LFM2 sine / square oscillator - no sine. Hi all Just got round to powering this (recently acquired) specimen for the first time. It was allegedly working but with a complaint of "low output." What I found was that I can't get sine wave out of it, the output is square wave whether the squaring button is engaged or not. Engaging said button just seems to reduce output amplitude by about 50% on the internal meter, and a scope corroborates this. The "squaring" switch needs a flick to release as the mechanism is tired. I've Super10'ed the contacts and oiled the mech without improvement but it does release if you flick it. I have the user guide and circuit diagram but no board layout, voltages or oscillograms. The collector of the last stage in the oscillator section (feedback amplifier) shows the waveform attached - with the sine / square switch in sine. I would have expected a sine wave at this point. The circuit uses an STC R54 in the negative feedback loop, I'm hopeful it isn't duff. it reads about 2k in circuit although there are several resistive paths in parallel and I haven't tried it out of circuit. The circuit uses a split rail +/- 10v supply and these are as expected. I've included a circuit extract. I wondered if anything springs to mind...? Attached Thumbnails __________________ Thermionic Emission, warms the cockles of your tubes.

16th Sep 2020, 7:36 pm	#2
Electronpusher0 Nonode Join Date: Jun 2016 Location: Bognor Regis, West Sussex, UK. Posts: 2,294	Re: Farnell LFM2 sine / square oscillator - no sine. Unfortunately it does sound like the thermistor is open circuit. Acording to this thread https://www.vintage-radio.net/forum/...d.php?t=107227 the R54 is 50kohms at 20 degC, 620 ohms at 3mW dissipation. You could try replacing the thermistor with a 50k pot and varying it. The oscillator will go from square wave at 50K to no oscillation at about 500 ohms. Some where in between you will get something approaching a sine wave but very unstable, thats the thermistors function. I have a spare R54 (used) if you need one. PM me. Peter

16th Sep 2020, 10:13 pm	#3
Nanozeugma Heptode Join Date: Nov 2017 Location: London, UK. Bury, Lancashire quite regularly :) Posts: 611	Re: Farnell LFM2 sine / square oscillator - no sine. Hi Peter, [Edited] I've isolated the R54 and it reads infinity - completely o/c. I'll P.M. you. Many thanks. __________________ Thermionic Emission, warms the cockles of your tubes. Last edited by Nanozeugma; 16th Sep 2020 at 10:39 pm.

22nd Sep 2020, 11:53 am	#4
Nanozeugma Heptode Join Date: Nov 2017 Location: London, UK. Bury, Lancashire quite regularly :) Posts: 611	Re: Farnell LFM2 sine / square oscillator - no sine. With grateful thanks to Peter (Electronpusher0) who provided the replacement NTC thermistor which I have now fitted... We have sine wave, yay! Mods: Thread may be closed as desired. Attached Thumbnails __________________ Thermionic Emission, warms the cockles of your tubes. Last edited by Nanozeugma; 22nd Sep 2020 at 12:02 pm.