Sansui TU-X1 tuner very poor on 75 ohm input

[stevehertz]

A Sansui TU-X1 tuner works fine on the 300 ohm input but the 75 ohm input is very weak, in fact almost, but not quite like having no aerial connected at all. My aerial feeder is 75 ohm, so obviously I want to use the 75 ohm input – as I do with all my receivers and tuners. BTW, using the 75 ohm feeder with a 75/300 balun to connect to the 300 ohm input gives great results. So, somewhere along the line there’s something wrong with the 75 ohm input of the tuner. It’s basically a ‘simple’ circuit – see schematic below. Now, could someone please explain to me what is going on with that schematic as it shows two small circular ‘terminals’ for the 75 ohm input, plus it somewhat confusingly shows a representation of the chassis connector, both ‘connections’ inferring an open circuit when the plug is inserted, ie one side of the matching transformer (balun?) left unconnected?! Presumably, the 75 ohm feeder should be depicted connecting to each end of that coil? I need to get this schematic conundrum sorted before I can begin to check out the wiring to see if it is correct made, faulty etc.

[SteveCG]

A curious circuit. On all the tuner inputs I've seen a 75 Ohm input is connected between one of the 300 Ohm terminals and chassis. As seen, the circuit implies that the extra transformer is simply a 1:1 turn jobbie.

['LIVEWIRE?']

I've not seen a circuit like that, either, but, if I had good results using a 75/300 ohm Balun I'd just use it that way and ignore the 75 ohm input, although, in theory it should work if that 75 ohm i/p Tx is a 1:1 device. Be that as it may is there any reason why you can't or don't want to use a balun?

[paulsherwin]

I suspect there is just a continuity break in the 75 ohm input somewhere, maybe a dry joint or an open circuit winding.

[Graham G3ZVT]

The circuit makes no sense unless there is a jumper across the "75 ohm" points.

Is there DC continuity from outer to inner of the co-ax connector?

[stevehertz]

Originally Posted by 'LIVEWIRE?'

I've not seen a circuit like that, either, but, if I had good results using a 75/300 ohm Balun I'd just use it that way and ignore the 75 ohm input, although, in theory it should work if that 75 ohm i/p Tx is a 1:1 device. Be that as it may is there any reason why you can't or don't want to use a balun?

Firstly, it's not in my nature to leave features and functions on sets in a non-working state, I want to fix the problem with that 75 ohm input. Which is a good enough reason in itself. Secondly, like I said: "My aerial feeder is 75 ohm, so obviously I want to use the 75 ohm input – as I do with all my receivers and tuners." Basically as I change from one hifi unit to another, as I frequently do, I don't want to be bothered with unscrewing wires from the balun to the 300 ohm input screw on terminals; messing about with a balun for the sake of one set is a pain.

[cathoderay57]

I'd be inclined to test the primary of the 75 Ohm coil for continuity and if OK then just earth the lower end of it (assuming it isn't earthed already and the circuit is erroneous) then reapply the 75 Ohm feeder and see what happens

[turretslug]

I wonder if the apparent odd series connection of coaxial and 75 ohm screw terminals is a drafting error?

The Rotel RX-802 here uses a similar 75 ohm input using 1:1 transformer going to what is effectively one half of the 300 ohm center-tap primary of the input tuned circuit- I wondered if this was in accordance with some (say) Japanese or American rule/recommendation/whatever about keeping coaxial aerial feeder screen isolated from equipment chassis and earthed elsewhere for static/lightning protection or somesuch. Some territories have what seem to us slightly odd ways of doing things, but would be accepted practice there. The 1:1 balun in question is wound on a small ferrite core with polythene-insulated single strand- and all 4 connections have rather badly melted-back insulation, I think they've been like that from manufacture. Perhaps the Sansui balun in question is like this and has developed a signal-quenching short!

[ronbryan]

What happens if you temporarily ignore the 1:1 75 ohm input transformer and apply the 75 ohm aerial betwen one leg of the 300 ohm input and chassis?

Ron

[TrevorG3VLF]

That is the way it is normally done. Half the voltage, quarter the impedance.

[stevehertz]

I have physically checked out the connections of the 75 ohm input transformer. Assuming my TU-X1 was wired correctly from new (I'm pretty sure it is because it's the most logical way), then the schematic depiction of the arrangement is at best misleading, in truth plain wrong. The way it is drawn indicates an open circuit in the primary side of the transformer. The problem is, the schematic kinda suggests that there are two sets of 75 ohm input connections; a coax set and a pair of separate sockets/terminals. There is actually just the coax socket and a physical check on my TU-X1 shows that the 75 ohm primary is permanently connected to the 75 ohm input socket. So no need for links. Why they drew it that way, who knows? They should have shown just the coax socket with the primary connected across it and the words '75 ohms' to the left hand side of the socket.

In my case, further investigation and measurement has highlighted a DC resistance reading of 35k ohms between the primary and secondary of the 75 ohm input transformer. There should be no such path between the two. So, it looks like I have a partial short (35k) on my TU-X1's 75 ohm input trannie. Unless I can find a replacement (seems unlikely), then for now I will use the 300 ohm input with my 75-300 ohm balun and at some point remove the transformer from the chassis and attempt a repair/re-wire. It shouldn't be too difficult as these things tend to have very few windings.

I'm wondering, re similar problems that others have had, far from this issue being the oft described need for a link etc (a red herring), this might be a manufacturing problem and maybe a lot of these trannies were manufactured either with, or in such a way as for faults to appear over time, eg windings wrapped too tightly on the ferrite core or poor insulation of the wire etc.

[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.

[Nuvistor]

From:-
http://www.fmtunerinfo.com/sansui.html#sansui-tux1

Which makes sense to me, also see post #5



Following up on a discussion in our FMtuners group about poor sensitivity in some TU-X1s, our panelist Ray theorizes that there needs to be a jumper used across the second 75-ohm input, which may have been furnished with the TU-X1 when new. Without the jumper, the 75-ohm input is much less sensitive than using a 75-ohm to 300-ohm balun on the 300-ohm input. Ray explains further: "A review of the tuner's service manual schematic shows the 75-ohm coax jack to only connect one side of the input's primary, leaving the other end to float. However, if one were to put a strap across the 75-ohm alternate terminals, the circuit would be complete. Without that connection, one will get the reduced signal that Hank reported.

Edit. Missed the post about the leak from primary to secondary, the primary may only be a few turns, perhaps rewind the primary?

[ms660]

Is the FA-7 antenna switch in the correct position, eg: not stuck.

Lawrence.

[michamoo]

possibly passed lightning damage to the transformer

[Synchrodyne]

What result do you get if you use the FA-7 aerial input? This was intended for use with Sansui’s own FA-7 combined FM-AM aerial, which appears to have had a coaxial feeder, impedance unknown but perhaps not too far from 75 R. But there is no obvious reason why it could not be used otherwise.






The FM signal from the FA-7 appears to pass through a high-pass filter (highlighted in yellow) and a transformer (highlighted in mauve), and thence to one half of the 300 R input transformer primary, thus bypassing the 75 R input transformer. That highlighted transformer might be there for impedance matching purposes. If so, then that in turn implies that the Sansui FA-7 feeder is other than 75 R, although why so is not immediately obvious.


Cheers,

[stevehertz]

Originally Posted by Nuvistor

From:-
http://www.fmtunerinfo.com/sansui.html#sansui-tux1

Which makes sense to me, also see post #5



Following up on a discussion in our FMtuners group about poor sensitivity in some TU-X1s, our panelist Ray theorizes that there needs to be a jumper used across the second 75-ohm input, which may have been furnished with the TU-X1 when new. Without the jumper, the 75-ohm input is much less sensitive than using a 75-ohm to 300-ohm balun on the 300-ohm input. Ray explains further: "A review of the tuner's service manual schematic shows the 75-ohm coax jack to only connect one side of the input's primary, leaving the other end to float. However, if one were to put a strap across the 75-ohm alternate terminals, the circuit would be complete. Without that connection, one will get the reduced signal that Hank reported.

Edit. Missed the post about the leak from primary to secondary, the primary may only be a few turns, perhaps rewind the primary?

I've been through all that stuff Nuvistor - it doesn't work for me! Plus, as you refer to in your edit, my trannie has that partial short primary to secondary. Thanks for your efforts, appreciated.

[stevehertz]

FA7 input is/was for a special, combined FM/AM aerial that Sansui sold.

The signal level meter is calibrated in dBf. Here's the results tuning in to a strong station using various inputs and links etc:

60dBf - using the 300 ohm input with 300/75 balun
55dBf - using one side of the 300 ohm input and ground
35dBf - using F7 socket
30dBf - using 75 ohm socket

But to my mind these figures are now academic given that I have discovered a partial short between the primary and secondary of the 75 ohm isolating trannie? Can I ask a straight question, surely that trannie should not exhibit a resistance of 35k ohms between primary and secondary? If the answer is no, then there's my problem!

[cathoderay57]

Well - it depends. If the primary as well as the secondary of the 75 Ohm aerial input transformer are both connected to chassis at one end, clearly there will be continuity between the 2 windings. If you have measured resistance with both ends of the primary isolated at both ends then if there is 35k resistance, you have an interwinding short circuit. Maybe I'm stating the obvious in which case I apologise. Cheers, Jerry

[stevehertz]

As per the schematic shown, the primary does not have an earth connection. So in my book there's no reason to read 35k ohms primary to secondary.