Testing local oscillator. - UK Vintage Radio Repair and Restoration Discussion Forum

petervk2mlg · 26th Mar 2007, 4:20 am

What's the best way to see if the local oscillator in a set is working?
TIA

Peter

dave_n_t · 26th Mar 2007, 6:52 am

Peter,

A quick test is to hold a (transistor, usually) portable set up close, tuned to the middle of the medium waveband, and tune the radio under test across the MW band. If the local oscillator is working, there'll be all sorts of heterodynes [whistles] audible on the portable set as you do this.

HTH!

Dave

jjl · 26th Mar 2007, 3:15 pm

For valve sets with the typical triode / hexode type frequency changer, if the oscillator is running, the triode grid will show a negative voltage when measured with a DC meter.

Dave's suggestion is less intrusive though as there is no need to poke around under the chassis.

John

Skywave · 27th Mar 2007, 8:08 pm

The replies given to the Q. so far are fine when the Q,. is regarded in its broadest sense.

However, to me, "the best way to see if the L.O. is 'working' " implies a basic check on its frequency, purity of waveform, variation in level & freq. when tuning is changed, etc.

For this, a 'scope with a high-impedance probe is required. Or possibly a spectrum analyser.

Al / Skywave.

petervk2mlg · 28th Mar 2007, 5:13 am

Should one attach the scope probe to the oscillator grid?

Peter

Mike Phelan · 28th Mar 2007, 8:22 am

The anode is better, Peter - lower impedance.

Skywave · 28th Mar 2007, 2:34 pm

Just one small thought on this.

What Mike Phelan has said will be appropriate for the vast majority of cases.

But since we are not discussing any particular set here - therefore I'm reading this in a very general context - be aware that there are some (albeit rare) oscillator circuits that have a r.f. grounded anode. So, if you do find no signal at the anode, and the cct. diag. is available, have a close look at it.

And good luck!

Al / Skywave

petervk2mlg · 9th Apr 2007, 7:51 am

Many thanks for all the answers, guys.

Peter

26th Mar 2007, 4:20 am	#1
petervk2mlg Heptode Join Date: Jan 2007 Location: Parkes, NSW Australia Posts: 877	Testing local oscillator. What's the best way to see if the local oscillator in a set is working? TIA Peter Last edited by Darren-UK; 1st Oct 2007 at 1:00 am. Reason: Typo corrected to aid search.

26th Mar 2007, 6:52 am	#2
dave_n_t Hexode Join Date: Sep 2003 Location: Cheltenham, Gloucestershire, UK. Posts: 298	Re: testing local oscillator Peter, A quick test is to hold a (transistor, usually) portable set up close, tuned to the middle of the medium waveband, and tune the radio under test across the MW band. If the local oscillator is working, there'll be all sorts of heterodynes [whistles] audible on the portable set as you do this. HTH! Dave __________________ Dave Teague

26th Mar 2007, 3:15 pm	#3
jjl Octode Join Date: Jan 2003 Location: Ware, Herts. UK. Posts: 1,082	Re: testing local oscillator For valve sets with the typical triode / hexode type frequency changer, if the oscillator is running, the triode grid will show a negative voltage when measured with a DC meter. Dave's suggestion is less intrusive though as there is no need to poke around under the chassis. John

27th Mar 2007, 8:08 pm	#4
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: testing local oscillator The replies given to the Q. so far are fine when the Q,. is regarded in its broadest sense. However, to me, "the best way to see if the L.O. is 'working' " implies a basic check on its frequency, purity of waveform, variation in level & freq. when tuning is changed, etc. For this, a 'scope with a high-impedance probe is required. Or possibly a spectrum analyser. Al / Skywave.

28th Mar 2007, 5:13 am	#5
petervk2mlg Heptode Join Date: Jan 2007 Location: Parkes, NSW Australia Posts: 877	Re: testing local oscillator Should one attach the scope probe to the oscillator grid? Peter

28th Mar 2007, 8:22 am	#6
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: testing local oscillator The anode is better, Peter - lower impedance. __________________ Mike.

28th Mar 2007, 2:34 pm	#7
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: testing local oscillator Just one small thought on this. What Mike Phelan has said will be appropriate for the vast majority of cases. But since we are not discussing any particular set here - therefore I'm reading this in a very general context - be aware that there are some (albeit rare) oscillator circuits that have a r.f. grounded anode. So, if you do find no signal at the anode, and the cct. diag. is available, have a close look at it. And good luck! Al / Skywave

9th Apr 2007, 7:51 am	#8
petervk2mlg Heptode Join Date: Jan 2007 Location: Parkes, NSW Australia Posts: 877	Re: testing local oscillator Many thanks for all the answers, guys. Peter