Philips 794AN alignment - UK Vintage Radio Repair and Restoration Discussion Forum

Paul Mathew · 22nd Oct 2018, 6:00 am

I have restored this set made in the US of A. The reception is great for 2 of the local AM band stations on 828kHz and 729kHz. The dial gauge reads around 670kHz when I tune in to the 828kHz station. I can't get down to the lower end of the AM band at 567kHz where one would find a third station. The schematic is based on the All American Five set, but the front end has got a far more elaborate design with two elongated cans on the left of pic, with 4 trimmer caps for the one can and 3 trimmer caps for the rear (behind the front can).
I'm reluctant to start fiddling with the trimmer caps, as I was unable to find any technical specs on this model, and in particular, alignment procedure.
This is uncharted territory for me!
Is there a safe way forward?

TrevorG3VLF · 22nd Oct 2018, 10:13 am

The circuit you show is MW only whereas the Philips 794AN is LMS. Circuit available top right and includes the rat's nest of the wiring.
If you have no band switches, things will be easier.

The alignment is a long way out so it is not just a matter of a slight adjustment of a trimmer, perhaps a bad connection or bad trimmer. The one you are after is the one across the oscillator coil.

Boater Sam · 22nd Oct 2018, 10:21 am

Has the phantom twiddler been at it? The oscillator is miles out, if its not been twiddles I would expect to find a faulty component or two. It cannot have just drifted so far on its own.

Paul Mathew · 22nd Oct 2018, 11:53 am

Thanks for the replies. I thought a slight adjustment was wishful thinking. I will get the schematic, as this one has a 5-position waveband selector switch, and the associated rats nest of wiring!

Paul Mathew · 22nd Oct 2018, 5:10 pm

Well, I couldn't find the schematic on the site, but I tried some ideas around the trimmer caps that I thought might be associated with the oscillator circuit, and I hit the jackpot when I soldered a 3.9nF cap in parallel with the trimmer cap in the attached pic. All the discussion around this topic seemed to be about increasing capacitance. I now have all 3 AM local stations. Not entirely on the correct spot on the dial gauge, but close enough!

Paul Mathew · 24th Oct 2018, 5:29 pm

The (almost) final product polished and with new grill cloth.

22nd Oct 2018, 6:00 am	#1
Paul Mathew Pentode Join Date: Jan 2014 Location: Cape Town, South Africa Posts: 214	Philips 794AN alignment I have restored this set made in the US of A. The reception is great for 2 of the local AM band stations on 828kHz and 729kHz. The dial gauge reads around 670kHz when I tune in to the 828kHz station. I can't get down to the lower end of the AM band at 567kHz where one would find a third station. The schematic is based on the All American Five set, but the front end has got a far more elaborate design with two elongated cans on the left of pic, with 4 trimmer caps for the one can and 3 trimmer caps for the rear (behind the front can). I'm reluctant to start fiddling with the trimmer caps, as I was unable to find any technical specs on this model, and in particular, alignment procedure. This is uncharted territory for me! Is there a safe way forward? Attached Thumbnails

22nd Oct 2018, 10:13 am	#2
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: Philips 794AN alignment The circuit you show is MW only whereas the Philips 794AN is LMS. Circuit available top right and includes the rat's nest of the wiring. If you have no band switches, things will be easier. The alignment is a long way out so it is not just a matter of a slight adjustment of a trimmer, perhaps a bad connection or bad trimmer. The one you are after is the one across the oscillator coil.

22nd Oct 2018, 10:21 am	#3
Boater Sam Banned Join Date: Dec 2013 Location: Middlewich, Cheshire, UK. & Winter in the Philippines. Posts: 3,897	Re: Philips 794AN alignment Has the phantom twiddler been at it? The oscillator is miles out, if its not been twiddles I would expect to find a faulty component or two. It cannot have just drifted so far on its own.

22nd Oct 2018, 11:53 am	#4
Paul Mathew Pentode Join Date: Jan 2014 Location: Cape Town, South Africa Posts: 214	Re: Philips 794AN alignment Thanks for the replies. I thought a slight adjustment was wishful thinking. I will get the schematic, as this one has a 5-position waveband selector switch, and the associated rats nest of wiring!

22nd Oct 2018, 5:10 pm	#5
Paul Mathew Pentode Join Date: Jan 2014 Location: Cape Town, South Africa Posts: 214	Re: Philips 794AN alignment Well, I couldn't find the schematic on the site, but I tried some ideas around the trimmer caps that I thought might be associated with the oscillator circuit, and I hit the jackpot when I soldered a 3.9nF cap in parallel with the trimmer cap in the attached pic. All the discussion around this topic seemed to be about increasing capacitance. I now have all 3 AM local stations. Not entirely on the correct spot on the dial gauge, but close enough! Attached Thumbnails

24th Oct 2018, 5:29 pm	#6
Paul Mathew Pentode Join Date: Jan 2014 Location: Cape Town, South Africa Posts: 214	Re: Philips 794AN alignment The (almost) final product polished and with new grill cloth. Attached Thumbnails