Fm multiplex (stereo) generator. - UK Vintage Radio Repair and Restoration Discussion Forum

sabaman · 8th Apr 2006, 6:37 pm

Hi,

Does anyone have an fm multiplex generator in their workshop that they no longer use? I hope an item like this comes under the term 'vintage' - they would certainly be mostly obsolete now as all fm stereo decoders in radio sets are in IC form and require no or very little adjustment. I hasten to add that this would help me align discrete component valve decoders that I have in my collection of early stereo radios. If anyone has anything like this I would be most interested in hearing from you.

Cheers,

Sabaman.

Sideband · 8th Apr 2006, 9:58 pm

If you fancy building one, I have a copy of an article from TV magazine about three years ago. It gives near-broadcast quality signals at RF and baseband. I've used mine several times for aligning old decoders and repaired my own tuner a couple of years ago using the generator to trace a faulty transistor in the decoder.

Recently I've been using it to transmit a signal from the Sky box to the Hi Fi system so that I can listen to Classic Gold in stereo....much better than having the TV on with a fixed logo on the screen and better quality as well.

The design uses easily available IC's and the two main boards are built into die-cast boxes. Most parts including the modulator should be available from Maplin. Probaly cost around £100 if you have to buy everything. Mine worked first time and needed minimal adjustment. You can test each part as you build it. OK it's not as good as a professional instrument but more than adequate for hobby work.

Rich.

Roy · 9th Apr 2006, 12:24 pm

Richard, that design from TV magazine sounds interesting.

Unfortunately my subscription ended before it was published. I would be interested to see the article, if you could make it available.

Thanks,

Roy

sabaman · 9th Apr 2006, 4:16 pm

Hi All,

Thanks for your replies. I am in the same position as Roy in that I have only just restarted my subscription to 'Television' after a long lapse. Rich, if you have a copy of the article available I too would be interested - it sounds like a very useful project. Hi Paul, I had thought about using one of those mini-transmitters, but thought it may be easier to use a device suited to the job - I would also like to have a 'proper' one to add to my small collection of test gear.

Best regards,

Sabaman.

Sideband · 9th Apr 2006, 10:37 pm

Hi all.

Hmm!! It seems I've started something with this homebuild job! I have had a couple of PM's about it as well. I will have to find the article so please give me a few days to sort it. Perhaps Paul will allow me to post it on the forum as PDF's?

It certainly works well considering a professional generator could cost £2000.

Richard.

Sideband · 12th Apr 2006, 1:15 pm

Well I've found the articles and as there seems to be no objection to posting the copies from TV mag, I've attached them here in two PDF's.

All I can add is that mine worked well first time. I would suggest that you use the specified transistors in the Master Oscillator because I tried subs and couldn't get it to oscillate. I think the ZTX transistors specified have a low input capacity. Also I couldn't guarantee that all specified specialist parts are still available. I'm sure suitable subs are around though. The OP275 IC's were available from RS.

This was a very interesting item to build over about two weeks.

Have fun

!

Rich

ppppenguin · 12th Apr 2006, 3:20 pm

There's nothing very critical in that parts list. BC109, BC549 or any ordinary transistor should work perfectly well in place of the ZTX450. The TL082 is a very ordinary dual opamp while the OPA275 is a more specialised audio opamp. Another TL082 would work (TL072 might be better) but might have slightly higher noise or distortion. Put it in a socket and it's easy to swap since virtually all dual opamps have the same 8 pin arrangement and there will be plenty of modern alternatives.

The silver mica cap can be replaced by a polystyrene part or NPO or COG ceramic. The OA91 can be replaced by any small schottky diode.

I can't comment about the RF module.

Sideband · 12th Apr 2006, 3:38 pm

Quote:

Originally Posted by ppppenguin

There's nothing very critical in that parts list. BC109, BC549 or any ordinary transistor should work perfectly well in place of the ZTX450.

Yes I would agree in most cases...I'm using BC548's in the audio stages. However they don't seem to work for some reason in the master oscillator so I used the specified transistors. It then worked perfectly.

I wondered about the RF module as well because a lot of the older modules seem to be discontinued. However again it's not a critical item and a simple 'bug' type RF oscillator could probably be made to work just as well.

Rich.

ppppenguin · 12th Apr 2006, 3:42 pm

Quote:

Originally Posted by Richard_Newman

Yes I would agree in most cases...I'm using BC548's in the audio stages. However they don't seem to work for some reason in the master oscillator so I used the specified transistors. It then worked perfectly.

I suppose I would take the experimental approach of looking through my parts box and trying different transistors. If an audio class device doesn't work I would try a 2N2369A or similar switching/RF device.

sabaman · 23rd Apr 2006, 7:03 pm

Hi Igranic,

Thanks for the info. I've now bought a Radiometer SMG1 from eBay, but I'll keep a watch on the IG-37. I have seen a few on eBay previously but let them get away. I downloaded a manual for the IG-37 a few days ago. If it's still cheap I might place a bid.

Cheers,

Sabaman.

Brased · 24th Apr 2006, 8:10 pm

Quote:

Originally Posted by igranic

There is presently a Heathkit FM Stereo Generator IG-37 on eBay UK here.
This is a valve-based design.

I have one of these which I saved from the skip. Unfortunately it didn't have any connecting leads and I'm not sure what connectors I should be looking for to match the two coaxial outlets. The handbook just describes them as 'male chassis connector'. The outer part seems to be the same as a normal '259' type but the inner connector is almost flush with the insulating sleeve. Please can someone identify it and tell me what is the correct type of plug to fit?

Edward

igranic · 24th Apr 2006, 10:48 pm

The connectors shown in Brased's post were common on 50s/60s/70s American equipment, and can sometimes be found on cheap handheld dynamic microphones as the cable connector. One of my IG-37 units came fitted with these, but it was a simple job to remove them and replace with single-hole BNC connectors, which are a surprisingly good fit in the existing holes.

Purists may decry this modification, but the later model I also have, had BNCs supplied by Heathkit as part of the kit.

Brased · 25th Apr 2006, 5:19 pm

Thanks Igranic

That does explain why I've also seen similar sockets on the US equivalent of the UK supplied RF-1U signal generator whereas my UK version has standard tv type coaxials. I had wondered about using BNC connectors as I find these are a convenient standard socket fitted on most of my other testgear. It seems that I can now fit these with an almost clear conscience.

Edward

8th Apr 2006, 6:37 pm	#1
sabaman Pentode Join Date: Mar 2005 Location: Portsmouth, Hampshire, UK. Posts: 116	Fm multiplex (stereo) generator. Hi, Does anyone have an fm multiplex generator in their workshop that they no longer use? I hope an item like this comes under the term 'vintage' - they would certainly be mostly obsolete now as all fm stereo decoders in radio sets are in IC form and require no or very little adjustment. I hasten to add that this would help me align discrete component valve decoders that I have in my collection of early stereo radios. If anyone has anything like this I would be most interested in hearing from you. Cheers, Sabaman.

8th Apr 2006, 9:58 pm	#2
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,578	Re: Fm multiplex (stereo) generator wanted. If you fancy building one, I have a copy of an article from TV magazine about three years ago. It gives near-broadcast quality signals at RF and baseband. I've used mine several times for aligning old decoders and repaired my own tuner a couple of years ago using the generator to trace a faulty transistor in the decoder. Recently I've been using it to transmit a signal from the Sky box to the Hi Fi system so that I can listen to Classic Gold in stereo....much better than having the TV on with a fixed logo on the screen and better quality as well. The design uses easily available IC's and the two main boards are built into die-cast boxes. Most parts including the modulator should be available from Maplin. Probaly cost around £100 if you have to buy everything. Mine worked first time and needed minimal adjustment. You can test each part as you build it. OK it's not as good as a professional instrument but more than adequate for hobby work. Rich. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

9th Apr 2006, 12:24 pm	#3
Roy Hexode Join Date: May 2003 Location: Hertfordshire, UK Posts: 258	Re: Fm multiplex (stereo) generator wanted. Richard, that design from TV magazine sounds interesting. Unfortunately my subscription ended before it was published. I would be interested to see the article, if you could make it available. Thanks, Roy

9th Apr 2006, 4:16 pm	#4
sabaman Pentode Join Date: Mar 2005 Location: Portsmouth, Hampshire, UK. Posts: 116	Re: Fm multiplex (stereo) generator wanted. Hi All, Thanks for your replies. I am in the same position as Roy in that I have only just restarted my subscription to 'Television' after a long lapse. Rich, if you have a copy of the article available I too would be interested - it sounds like a very useful project. Hi Paul, I had thought about using one of those mini-transmitters, but thought it may be easier to use a device suited to the job - I would also like to have a 'proper' one to add to my small collection of test gear. Best regards, Sabaman.

9th Apr 2006, 10:37 pm	#5
Sideband Dekatron Join Date: Oct 2003 Location: Croydon, Surrey, UK. Posts: 7,578	Re: Fm multiplex (stereo) generator wanted. Hi all. Hmm!! It seems I've started something with this homebuild job! I have had a couple of PM's about it as well. I will have to find the article so please give me a few days to sort it. Perhaps Paul will allow me to post it on the forum as PDF's? It certainly works well considering a professional generator could cost £2000. Richard. __________________ There are lots of brilliant keyboard players and then there is Rick Wakeman.....

12th Apr 2006, 3:20 pm	#7
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: Fm multiplex (stereo) generator wanted. There's nothing very critical in that parts list. BC109, BC549 or any ordinary transistor should work perfectly well in place of the ZTX450. The TL082 is a very ordinary dual opamp while the OPA275 is a more specialised audio opamp. Another TL082 would work (TL072 might be better) but might have slightly higher noise or distortion. Put it in a socket and it's easy to swap since virtually all dual opamps have the same 8 pin arrangement and there will be plenty of modern alternatives. The silver mica cap can be replaced by a polystyrene part or NPO or COG ceramic. The OA91 can be replaced by any small schottky diode. I can't comment about the RF module.

23rd Apr 2006, 7:03 pm	#10
sabaman Pentode Join Date: Mar 2005 Location: Portsmouth, Hampshire, UK. Posts: 116	Re: Fm multiplex (stereo) generator wanted. Hi Igranic, Thanks for the info. I've now bought a Radiometer SMG1 from eBay, but I'll keep a watch on the IG-37. I have seen a few on eBay previously but let them get away. I downloaded a manual for the IG-37 a few days ago. If it's still cheap I might place a bid. Cheers, Sabaman.

24th Apr 2006, 10:48 pm	#12
igranic Rest in Peace Join Date: Jan 2005 Location: North Bedfordshire, UK. Posts: 395	Re: Fm multiplex (stereo) generator wanted. The connectors shown in Brased's post were common on 50s/60s/70s American equipment, and can sometimes be found on cheap handheld dynamic microphones as the cable connector. One of my IG-37 units came fitted with these, but it was a simple job to remove them and replace with single-hole BNC connectors, which are a surprisingly good fit in the existing holes. Purists may decry this modification, but the later model I also have, had BNCs supplied by Heathkit as part of the kit.

25th Apr 2006, 5:19 pm	#13
Brased Hexode Join Date: Mar 2005 Location: London, UK. Posts: 318	Re: Fm multiplex (stereo) generator wanted. Thanks Igranic That does explain why I've also seen similar sockets on the US equivalent of the UK supplied RF-1U signal generator whereas my UK version has standard tv type coaxials. I had wondered about using BNC connectors as I find these are a convenient standard socket fitted on most of my other testgear. It seems that I can now fit these with an almost clear conscience. Edward