Sansui TU-X1 tuner very poor on 75 ohm input - Page 2 - UK Vintage Radio Repair and Restoration Discussion Forum

Mr 1936 · 31st Jul 2019, 11:08 am

Your 35 K reading is a puzzle, but some types of core are slightly conductive and if the windings have cut into the core at a sharp corner then there could be a conductive path. Even so, I'm surprised such a high resistance noticeably affects the loss in a 75 ohm impedance circuit.

It's not too hard to make a 1:1 impedance ratio transformer for 100 MHz. Take a small ferrite bead or toroid core (type fairly uncritical), and two strands of enamelled wire loosely twisted together. It helps if the wire has 2 different colours, even better to use bifilar wire but I appreciate that it can be hard to source. Wind the wires round the core, 3 turns is a good starting point, the original transformer will be a guide. If you have to use same-colour wires, use an ohmmeter to identify the primary start/finish and the secondary start/finish.

Alternatively, strip and rewind the existing component.

stevehertz · 31st Jul 2019, 1:32 pm

Thanks for the ongoing input guys. Mr 1936, I may be using your instructions..

In order to be able to check things out more thoroughly, I decided to remove the trannie from it's position nestled tightly between the tuner back panel and the inner chassis. That same narrow gap being occupied by a mass of tight wires and other fixtures and fittings. With those hair thin wires just asking to be snapped, nuts and bolts to undo, and 'inaccessible' wires to unsolder, it was not easy to say the least, and I will have to reverse the process at some point.. In doing so I made some thorough checks..

The small panel on which the 75 ohm input socket is bolted is partially conductive (see photo, compressed black material?). Taking measurements with sharp prods into the panel at a small distance apart I get a reading of about 600k. So that would offer a 600k path to ground from one side of the primary, simultaneously making a 600K connection between primary and secondary (due to the latter having one side tied to ground).

When disconnected, the resistance between the primary and secondary of the trannie is around 8M. So I am assuming that my original 'in circuit' 35k reading was due to a combination of the mounting panel being partially conductive and the parallel paths that may be created by that.

An 8M primary to secondary reading I would have thought should not be a problem in itself (?), but that does not mean that one of the windings may have a shorted turn. How can I test the trannie for functionality out of circuit?

Oh, BTW once the small mounting panel was removed, with some difficulty I was able to connect my aerial to the loose socket (hanging on hair thin wires!) and the FM performance was barely any better. This seems to suggest that the conductive panel was having little effect, and that the problem is largely or wholly with the trannie itself. That's why I want to test it dynamically.

Skywave · 31st Jul 2019, 1:38 pm

A quick thought. When you obtained a resistance of 35k Ohms, what type of meter were you using? I ask that because digital multi-meters, when set to resistance, can give misleading results when connected to an inductance.

Al.

stevehertz · 31st Jul 2019, 1:52 pm

Trusty Avo 8 mk3.

Graham G3ZVT · 31st Jul 2019, 2:06 pm

Before you did all that, did you read Synchrodyne's post #16 which suggests it is a known issue?

Did you have DC continuity between the outer and inner of the co-ax socket?
If not, are there a pair of 75ohm balanced terminals as well as the 300ohm ones?

If the circuit is to believed they must exist, either as actual user accessible terminals or at least internal test points. In either case something needs to be strapped before the co-ax socket will work properly.

ms660 · 31st Jul 2019, 3:00 pm

Some of the details in the schematic arn't to be believed so far as I can tell....The FA-7 antenna switch is a four pole two way job but the schematic fails to show that, instead it shows one single pole two way switch with no circuit ref. and three ganged single pole two way switches which have circuit refs.

For some of the best online photo's of the innards check the first link that shows the FA-7 antenna switch and the second link for the root page that gives links to the rest of the photo's, of which there are numerous....

FA-7 antenna switch photo:

http://amp8.com/sub/radio/tr-tuner/s...eg/tu-x1-7.jpg

Root page for the rest:

http://amp8.com/sub/radio/tr-tuner/sansui/jpeg/

Lawrence.

Mr 1936 · 31st Jul 2019, 4:55 pm

Just a thought. Far East manufacturers were fond of glueing components down with a pale brown adhesive (You can guess its colloquial name). In your photo the T01 component with the binocular ferrire crore appears to be sitting on a smudge of the stuff. Over time this seems to break down and become conductive. Thorough removal is the only cure. PCBs themselves are normally resin bonded paper and I haven't heard of any becoming conductive unless there is carbon from arcing - not usually a problem for a low level circuit unless it's had a lightning induced surge. However, the sort of leakage resistances you are seeing wouldn't cause significant signal loss, so a shorted turn seems more likely.

Herald1360 · 31st Jul 2019, 5:08 pm

Sounds like you'd be better off junking the isolation TX and just connecting the 75R coax input to chassis and one side of the 300R input winding.

stevehertz · 31st Jul 2019, 5:14 pm

Why is the isolating trannie there? Otherwise that could be a way forward if all else fails.

cathoderay57 · 31st Jul 2019, 5:31 pm

Worth providing a wired connection between the outer shell of the coax socket and chassis and see if it makes a difference. It seems like the panel on which the socket is mounted is giving a high resistance between plug shell and chassis. I appreciate the chassis connection is not shown explicitly on the circuit diagram, but would still be interested in what difference it makes by providing one.

cathoderay57 · 31st Jul 2019, 5:45 pm

Also what resistance are you measuring across the primary winding of the input transformer and also the secondary? If either is o/c that obviously won't be good

stevehertz · 31st Jul 2019, 7:05 pm

They're not o/c, checked. Actual reading is very small, so hard to define.

stevehertz · 31st Jul 2019, 7:57 pm

How can I test the input trannie? (apart from DC tests which appear ok)

Can I just apply say 98MHz to one side via suitable capacitor and check to see what is coming out of the other side? Trouble is, I haven't got a scope that goes that high. Anyway, would that be the best procedure?

Graham G3ZVT · 31st Jul 2019, 9:37 pm

OK, at last I have found a picture of the back of the TU-X1

Click image for larger version

Name: sansui.jpg
Views: 80
Size: 47.4 KB
ID: 187658

Does yours have this mysterious "FA-7 antenna" F-type connector with a switch operated by the barrel of the connector? Is that where the problem is?

Here is the Youtube video that I screen-grabbed the image above

https://youtu.be/8pwvtqo5Hwg?t=351

The antenna "routing" of the TU-X1 is mentioned at the bottom of this page.

http://www.goodsoundclub.com/Forums/...px?postID=4649

cathoderay57 · 31st Jul 2019, 10:24 pm

If both input tfmr windings have continuity and you are getting 8M resistance between pri and sec (possibly via some path as yet unknown) then the tfmr is probably OK. You could test it using it between an FM aerial and another FM receiver ideally with a signal strength meter if you can connect to the tfmr without damaging it. I'm tending to go along with rambo1152 and beginning to suspect the FA-7 switch. Not sure how this works -was there a special plug on the AM/FM antenna lead that actuates the plunger on the switch? what does the switch do when it is actuated?

TrevorG3VLF · 31st Jul 2019, 11:06 pm

I have made a vertical aerial from 300 ohm feeder, pinned up in the loft. 300 ohm cable is then fed to a distribution amplifier. To match the cable to the amplifier I used a balun which came from a portable TV with a loop aerial, this worked well. If you do not want to wind your own balun, then one of these will do to input to the 300 ohm connections.

stevehertz · 1st Aug 2019, 12:51 pm

For now I have put the isolating trannie to one side and made connections directly to one side of the 300 ohm input and earth (the 300 ohm trannie centre tap). Interestingly, each side of the 300 ohm trannie gives different results. The side that is also connected to F7 (the FM/AM hybrid antenna input socket) gives poor results. This is the side that is also used when the isolating trannie is used - as per normal. The other, 'free' side gives good results, akin to when I use a 300 ohm feeder/balun into the 300 ohm input. That suggests two things..

1) That Sansui 'chose' to connect the 75 ohm input to that (F7) side of the trannie even though intrinsically it was not the best choice - a design error basically.

2) That there is a loading issue with the F7 socket/circuit. Previously I have cleaned and double cleaned and lubricated the plunger switch associated with F7 and it appears to be working ok as it connects and disconnects the signal coming in from the normal FM sockets.

Anyway, I have to spend time doing other things so I may not report back further today.

Radio Wrangler · 1st Aug 2019, 1:14 pm

Looking back at the diagram in the first post...

There is such a thing as 75 Ohm twin feeder. The wires are much closer than 300 Ohm and it looks rather like figure-of-eight mains cable.

I wonder if Sansui expect you to short the 75 ohm twin feeder terminals if you use 75 Ohm coax and to shove a shorting plug in the coax socket if using the 75 Ohm twin terminals?

This would explain the purpose of the 1:1 transformer

David

stevehertz · 1st Aug 2019, 1:35 pm

There are no 'separate' 75 ohm terminals, just the 75 ohm F connector. That is why the schematic is confusing, it infers that there is, as well as showing an open circuit input that's begging to be linked. Truth is, there is just the 75 ohm F connector and there is no open circuit to worry about. It's just the confusing way it is drawn. My problem is something else as with my last post, I think I'm getting to the bottom of it.

Hartley118 · 1st Aug 2019, 2:04 pm

Quote:

Originally Posted by paulsherwin

The 75 ohm input is often a bit of an afterthought with Japanese tuners of the 70s and 80s, as 75 ohm downleads were rarely used in Japan and North America. I can quite believe that it left the factory like that.

Manufacturing defects are unfortunately not unknown. A few years ago, I bought a very reputable Cambridge Audio FM/DAB tuner. It was fine on DAB, but hopelessly noisy on FM, not unlike the symptom described here. That was despite my 4-element external aerial. So I returned it to the dealer, Richer Sounds, who exchanged it for another without question. But that had the identical noise problem.

Fortunately, Richer willingly then agreed to swap it for a slightly more pricey Denon tuner with no penalty to me. That worked, and continues to work, totally faultlessly. The Cambridge Audio units were clearly defective. Perhaps the whole batch had a problem. The factory test spec obviously didn’t pick up the fault: perhaps the 75 ohm input wasn’t in the test spec.

Martin

31st Jul 2019, 11:08 am	#21
Mr 1936 Heptode Join Date: Mar 2017 Location: Romsey, Hampshire, UK. Posts: 524	Re: Sansui TU-X1 tuner very poor on 75 ohm input Your 35 K reading is a puzzle, but some types of core are slightly conductive and if the windings have cut into the core at a sharp corner then there could be a conductive path. Even so, I'm surprised such a high resistance noticeably affects the loss in a 75 ohm impedance circuit. It's not too hard to make a 1:1 impedance ratio transformer for 100 MHz. Take a small ferrite bead or toroid core (type fairly uncritical), and two strands of enamelled wire loosely twisted together. It helps if the wire has 2 different colours, even better to use bifilar wire but I appreciate that it can be hard to source. Wind the wires round the core, 3 turns is a good starting point, the original transformer will be a guide. If you have to use same-colour wires, use an ohmmeter to identify the primary start/finish and the secondary start/finish. Alternatively, strip and rewind the existing component.

31st Jul 2019, 1:32 pm	#22
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,835	Re: Sansui TU-X1 tuner very poor on 75 ohm input Thanks for the ongoing input guys. Mr 1936, I may be using your instructions.. In order to be able to check things out more thoroughly, I decided to remove the trannie from it's position nestled tightly between the tuner back panel and the inner chassis. That same narrow gap being occupied by a mass of tight wires and other fixtures and fittings. With those hair thin wires just asking to be snapped, nuts and bolts to undo, and 'inaccessible' wires to unsolder, it was not easy to say the least, and I will have to reverse the process at some point.. In doing so I made some thorough checks.. The small panel on which the 75 ohm input socket is bolted is partially conductive (see photo, compressed black material?). Taking measurements with sharp prods into the panel at a small distance apart I get a reading of about 600k. So that would offer a 600k path to ground from one side of the primary, simultaneously making a 600K connection between primary and secondary (due to the latter having one side tied to ground). When disconnected, the resistance between the primary and secondary of the trannie is around 8M. So I am assuming that my original 'in circuit' 35k reading was due to a combination of the mounting panel being partially conductive and the parallel paths that may be created by that. An 8M primary to secondary reading I would have thought should not be a problem in itself (?), but that does not mean that one of the windings may have a shorted turn. How can I test the trannie for functionality out of circuit? Oh, BTW once the small mounting panel was removed, with some difficulty I was able to connect my aerial to the loose socket (hanging on hair thin wires!) and the FM performance was barely any better. This seems to suggest that the conductive panel was having little effect, and that the problem is largely or wholly with the trannie itself. That's why I want to test it dynamically. __________________ A digital radio is the latest thing, but a vintage wireless is forever..

31st Jul 2019, 1:38 pm	#23
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Sansui TU-X1 tuner very poor on 75 ohm input A quick thought. When you obtained a resistance of 35k Ohms, what type of meter were you using? I ask that because digital multi-meters, when set to resistance, can give misleading results when connected to an inductance. Al.

31st Jul 2019, 1:52 pm	#24
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,835	Re: Sansui TU-X1 tuner very poor on 75 ohm input Trusty Avo 8 mk3. __________________ A digital radio is the latest thing, but a vintage wireless is forever..

31st Jul 2019, 2:06 pm	#25
Graham G3ZVT Dekatron Join Date: May 2010 Location: Greater Manchester, UK. Posts: 18,725	Re: Sansui TU-X1 tuner very poor on 75 ohm input Before you did all that, did you read Synchrodyne's post #16 which suggests it is a known issue? Did you have DC continuity between the outer and inner of the co-ax socket? If not, are there a pair of 75ohm balanced terminals as well as the 300ohm ones? If the circuit is to believed they must exist, either as actual user accessible terminals or at least internal test points. In either case something needs to be strapped before the co-ax socket will work properly. __________________ -- Graham. G3ZVT

31st Jul 2019, 3:00 pm	#26
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Sansui TU-X1 tuner very poor on 75 ohm input Some of the details in the schematic arn't to be believed so far as I can tell....The FA-7 antenna switch is a four pole two way job but the schematic fails to show that, instead it shows one single pole two way switch with no circuit ref. and three ganged single pole two way switches which have circuit refs. For some of the best online photo's of the innards check the first link that shows the FA-7 antenna switch and the second link for the root page that gives links to the rest of the photo's, of which there are numerous.... FA-7 antenna switch photo: http://amp8.com/sub/radio/tr-tuner/s...eg/tu-x1-7.jpg Root page for the rest: http://amp8.com/sub/radio/tr-tuner/sansui/jpeg/ Lawrence.

31st Jul 2019, 4:55 pm	#27
Mr 1936 Heptode Join Date: Mar 2017 Location: Romsey, Hampshire, UK. Posts: 524	Re: Sansui TU-X1 tuner very poor on 75 ohm input Just a thought. Far East manufacturers were fond of glueing components down with a pale brown adhesive (You can guess its colloquial name). In your photo the T01 component with the binocular ferrire crore appears to be sitting on a smudge of the stuff. Over time this seems to break down and become conductive. Thorough removal is the only cure. PCBs themselves are normally resin bonded paper and I haven't heard of any becoming conductive unless there is carbon from arcing - not usually a problem for a low level circuit unless it's had a lightning induced surge. However, the sort of leakage resistances you are seeing wouldn't cause significant signal loss, so a shorted turn seems more likely.

31st Jul 2019, 5:08 pm	#28
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: Sansui TU-X1 tuner very poor on 75 ohm input Sounds like you'd be better off junking the isolation TX and just connecting the 75R coax input to chassis and one side of the 300R input winding. __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

31st Jul 2019, 5:14 pm	#29
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,835	Re: Sansui TU-X1 tuner very poor on 75 ohm input Why is the isolating trannie there? Otherwise that could be a way forward if all else fails. __________________ A digital radio is the latest thing, but a vintage wireless is forever..

31st Jul 2019, 5:31 pm	#30
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,389	Re: Sansui TU-X1 tuner very poor on 75 ohm input Worth providing a wired connection between the outer shell of the coax socket and chassis and see if it makes a difference. It seems like the panel on which the socket is mounted is giving a high resistance between plug shell and chassis. I appreciate the chassis connection is not shown explicitly on the circuit diagram, but would still be interested in what difference it makes by providing one.

31st Jul 2019, 5:45 pm	#31
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,389	Re: Sansui TU-X1 tuner very poor on 75 ohm input Also what resistance are you measuring across the primary winding of the input transformer and also the secondary? If either is o/c that obviously won't be good

31st Jul 2019, 7:05 pm	#32
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,835	Re: Sansui TU-X1 tuner very poor on 75 ohm input They're not o/c, checked. Actual reading is very small, so hard to define. __________________ A digital radio is the latest thing, but a vintage wireless is forever..

31st Jul 2019, 7:57 pm	#33
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,835	Re: Sansui TU-X1 tuner very poor on 75 ohm input How can I test the input trannie? (apart from DC tests which appear ok) Can I just apply say 98MHz to one side via suitable capacitor and check to see what is coming out of the other side? Trouble is, I haven't got a scope that goes that high. Anyway, would that be the best procedure? __________________ A digital radio is the latest thing, but a vintage wireless is forever..

31st Jul 2019, 9:37 pm	#34
Graham G3ZVT Dekatron Join Date: May 2010 Location: Greater Manchester, UK. Posts: 18,725	Re: Sansui TU-X1 tuner very poor on 75 ohm input OK, at last I have found a picture of the back of the TU-X1 Does yours have this mysterious "FA-7 antenna" F-type connector with a switch operated by the barrel of the connector? Is that where the problem is? Here is the Youtube video that I screen-grabbed the image above https://youtu.be/8pwvtqo5Hwg?t=351 The antenna "routing" of the TU-X1 is mentioned at the bottom of this page. http://www.goodsoundclub.com/Forums/...px?postID=4649 __________________ -- Graham. G3ZVT Last edited by Graham G3ZVT; 31st Jul 2019 at 9:50 pm.

31st Jul 2019, 10:24 pm	#35
cathoderay57 Nonode Join Date: Oct 2017 Location: Bristol, UK. Posts: 2,389	Re: Sansui TU-X1 tuner very poor on 75 ohm input If both input tfmr windings have continuity and you are getting 8M resistance between pri and sec (possibly via some path as yet unknown) then the tfmr is probably OK. You could test it using it between an FM aerial and another FM receiver ideally with a signal strength meter if you can connect to the tfmr without damaging it. I'm tending to go along with rambo1152 and beginning to suspect the FA-7 switch. Not sure how this works -was there a special plug on the AM/FM antenna lead that actuates the plunger on the switch? what does the switch do when it is actuated?

31st Jul 2019, 11:06 pm	#36
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: Sansui TU-X1 tuner very poor on 75 ohm input I have made a vertical aerial from 300 ohm feeder, pinned up in the loft. 300 ohm cable is then fed to a distribution amplifier. To match the cable to the amplifier I used a balun which came from a portable TV with a loop aerial, this worked well. If you do not want to wind your own balun, then one of these will do to input to the 300 ohm connections.

1st Aug 2019, 12:51 pm	#37
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,835	Re: Sansui TU-X1 tuner very poor on 75 ohm input For now I have put the isolating trannie to one side and made connections directly to one side of the 300 ohm input and earth (the 300 ohm trannie centre tap). Interestingly, each side of the 300 ohm trannie gives different results. The side that is also connected to F7 (the FM/AM hybrid antenna input socket) gives poor results. This is the side that is also used when the isolating trannie is used - as per normal. The other, 'free' side gives good results, akin to when I use a 300 ohm feeder/balun into the 300 ohm input. That suggests two things.. 1) That Sansui 'chose' to connect the 75 ohm input to that (F7) side of the trannie even though intrinsically it was not the best choice - a design error basically. 2) That there is a loading issue with the F7 socket/circuit. Previously I have cleaned and double cleaned and lubricated the plunger switch associated with F7 and it appears to be working ok as it connects and disconnects the signal coming in from the normal FM sockets. Anyway, I have to spend time doing other things so I may not report back further today. __________________ A digital radio is the latest thing, but a vintage wireless is forever..

1st Aug 2019, 1:14 pm	#38
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,902	Re: Sansui TU-X1 tuner very poor on 75 ohm input Looking back at the diagram in the first post... There is such a thing as 75 Ohm twin feeder. The wires are much closer than 300 Ohm and it looks rather like figure-of-eight mains cable. I wonder if Sansui expect you to short the 75 ohm twin feeder terminals if you use 75 Ohm coax and to shove a shorting plug in the coax socket if using the 75 Ohm twin terminals? This would explain the purpose of the 1:1 transformer David __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

1st Aug 2019, 1:35 pm	#39
stevehertz Dekatron Join Date: Sep 2011 Location: Rugeley, Staffordshire, UK. Posts: 8,835	Re: Sansui TU-X1 tuner very poor on 75 ohm input There are no 'separate' 75 ohm terminals, just the 75 ohm F connector. That is why the schematic is confusing, it infers that there is, as well as showing an open circuit input that's begging to be linked. Truth is, there is just the 75 ohm F connector and there is no open circuit to worry about. It's just the confusing way it is drawn. My problem is something else as with my last post, I think I'm getting to the bottom of it. __________________ A digital radio is the latest thing, but a vintage wireless is forever..