UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > General Vintage Technology > Success Stories

Notices

Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

Closed Thread
 
Thread Tools
Old 29th Aug 2020, 2:55 pm   #1
cathoderay57
Nonode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,358
Default Farnell LFM Sine/Square Wave Oscillator

Hello folks, I have just restored a Farnell LFM Sine/Square Wave Oscillator to working order. It dates from around 1965 and is the second design of its type introduced by Farnell. There were several subsequent models that included enhancements to frequency range, push-button controls and use of ICs. Mine uses discrete transistors and diodes. As you can see it has the benefit of an output level meter which the first model, the LF did not. I guess the M stands for meter! It appears to be a military surplus item (see pic #2 label indicating REME workshop possibly at Donnington). There wasn't much wrong with it. I opened it up for a look inside and nothing seemed to be amiss until I spotted a grey coax cable that had come adrift; you should just about be able to see it going to the wafer switch top-left of image #3. I was unable to find a circuit on-line and so I powered the thing up anyway with the coax cable hanging loose. I got nice clean fixed frequency sine and square wave outputs on the scope. Amplitude controls worked but the frequency dial and multiplier switch didn't. In the end, by a process of elimination, I identified which wafer switch contact the cable had come from. It then worked perfectly and I have posted a couple of images of the scope in the next post. The only other issue was the output meter didn't work. It is wired via a push-switch that doubles for internal battery test. No batteries were fitted, but a wiring harness was getting in the way and holding open the push-switch contacts. I moved them away and the meter would read, but only about 50% FSD. I tried adjusting the meter pot to no avail. The meter is fed via a diode bridge of 4 x OA90 germanium diodes with a 250uF capacitor across the dc legs. The cap was OK but one of the diodes was o/c. I looked up data for the OA90 and was surprised to find it is rated up to 0.5A peak. I checked stock and had OA71 (150mA), OA85 (50mA), GD91 (no data) and an XK31 diode made by STC. I couldn't find any data anywhere on the XK31 either, but stuck it in anyway to try it. Full scale deflection restored. Quite a handy instrument to have in the workshop providing square and sinewave outputs via attenuator from 1Hz - 100kHz.
Attached Thumbnails
Click image for larger version

Name:	P1080208.jpg
Views:	197
Size:	47.8 KB
ID:	214561   Click image for larger version

Name:	P1080207.jpg
Views:	126
Size:	62.6 KB
ID:	214562   Click image for larger version

Name:	P1080205.jpg
Views:	134
Size:	108.8 KB
ID:	214563   Click image for larger version

Name:	P1080204.jpg
Views:	147
Size:	83.5 KB
ID:	214564   Click image for larger version

Name:	P1080206.jpg
Views:	129
Size:	85.2 KB
ID:	214565  

cathoderay57 is offline  
Old 29th Aug 2020, 2:55 pm   #2
cathoderay57
Nonode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,358
Default Re: Farnell LFM Sine/Square Wave Oscillator

Here are the output waves on the scope.
P.S. Another kind Forum member has subsequently offered me a copy of the manual
Attached Thumbnails
Click image for larger version

Name:	P1080209.jpg
Views:	99
Size:	57.2 KB
ID:	214566   Click image for larger version

Name:	P1080210.jpg
Views:	72
Size:	60.0 KB
ID:	214567  
cathoderay57 is offline  
Old 30th Aug 2020, 8:55 am   #3
Electronpusher0
Nonode
 
Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
Posts: 2,287
Default Re: Farnell LFM Sine/Square Wave Oscillator

Its good to see what the inside should look like. A couple of years ago I bought 2 of these at £10 each at a car boot. It was only when I got them home and opened them up I found they had been stripped of absolutely everything except what you could see from the front.
I keep meaning to make a generator to fit, maybe even replicate the original?

Peter
Electronpusher0 is offline  
Old 30th Aug 2020, 11:52 am   #4
cathoderay57
Nonode
 
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,358
Default Re: Farnell LFM Sine/Square Wave Oscillator

Cheers, Peter. Thanks to Bryan you can now find the circuit and parts list in Post #7 here https://www.vintage-radio.net/forum/...d.php?t=170529 . Note this is the LF circuit, which is the same as the LFM, although some different transistors have been used in the LFM. Also, the LF circuit doesn't have the meter and the small driver board that you can see in Photo #5 above. The circuit for that board is the same as in the LFM2 circuit which is available from the Radio Museum.
cathoderay57 is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 9:13 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.