Hi everybody, I have a pair of Sansui radios, a SAX100 and a SM12A, both rather obscure but very interesting probably not only because they are single ended 6BQ5 amps. Not much difference between the two, the SAX has a transistor phono input, the other one has a simpler power supply but two AM tuners for early stereo simulcast and a dual magic eye.
Now the problem is that I have found some differences in the output transformers. I was measuring them without much care for the values, just to check they are the same in each radio and found they were not.
The SAX has one transformer with about 240 Ohm DC resistance primary (and about 7H inductance) and the other transformer has about 200KOhm DC resistance and doesn't give an inductance reading (Peak LCR meter) - I get some negative value with a question mark. Secondaries check out, they are all the same, and make sense.
The 12A exhibits the same ailment. One of the outputs has near identical parameters with the first one above (approx 240Ohm DC resistance and 7H primary) whereas the other one has 500KOhm primary DC resistance and reads gibberish for inductance. Secondaries of both replicate almost to the decimal the values of the other two.
What gives?!
Are the two trannies with high DC resistance cooked? Or is the LCR meter out of its depth?
A little bit of investigation revealed the transformers used in these radios are the same part so they should be swappable (my intention for the moment). Externally they look identical, and after a little bit of cleaning I found under the grime the same labels on the transformers (all also have "5.2K" written on the primary side plus terminal labels).
My gut feeling sez the two transformers with "normal" parameters are still good and I plan to use them to restore the 12A (the SAX is worse for wear), but I would like to know if I can recover the other two. Are they dead? I mean do they need a rewind? Not going to throw them away, but would be good to know.
Would be interesting to know how they managed to go high resistance, because I thought they would either go open or short. Not sure how a wire can increase its resistance. There are signs of heat stress, the outputs are right between the trannies, the wax has obviously run out of the coils somewhat and pooled under the casings.
Any comments would be appreciated.
Now the problem is that I have found some differences in the output transformers. I was measuring them without much care for the values, just to check they are the same in each radio and found they were not.
The SAX has one transformer with about 240 Ohm DC resistance primary (and about 7H inductance) and the other transformer has about 200KOhm DC resistance and doesn't give an inductance reading (Peak LCR meter) - I get some negative value with a question mark. Secondaries check out, they are all the same, and make sense.
The 12A exhibits the same ailment. One of the outputs has near identical parameters with the first one above (approx 240Ohm DC resistance and 7H primary) whereas the other one has 500KOhm primary DC resistance and reads gibberish for inductance. Secondaries of both replicate almost to the decimal the values of the other two.
What gives?!
Are the two trannies with high DC resistance cooked? Or is the LCR meter out of its depth?
A little bit of investigation revealed the transformers used in these radios are the same part so they should be swappable (my intention for the moment). Externally they look identical, and after a little bit of cleaning I found under the grime the same labels on the transformers (all also have "5.2K" written on the primary side plus terminal labels).
My gut feeling sez the two transformers with "normal" parameters are still good and I plan to use them to restore the 12A (the SAX is worse for wear), but I would like to know if I can recover the other two. Are they dead? I mean do they need a rewind? Not going to throw them away, but would be good to know.
Would be interesting to know how they managed to go high resistance, because I thought they would either go open or short. Not sure how a wire can increase its resistance. There are signs of heat stress, the outputs are right between the trannies, the wax has obviously run out of the coils somewhat and pooled under the casings.
Any comments would be appreciated.
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I had this case three years ago. a Sensui SM12A model... If I remember correctly, there was a problem with the sound on one channel. I checked everything...I measured the inductance of both output chokes.!!!!! The sound of that channel was faulty... I think... I opened the choke... the copper was rusty and in bad condition. I had to re-wind the chokes for both channels... Sensui had wound a 16 ohm coil on the core Separately. Your choke wires are probably rusty.
I have the bobbin of one of the transformers, the one with the correct DC ohms. I will send a picture of it tomorrow.
Ha!
I did not think of that. But that would explain the coil going high resistance. Thank you very much.
And bummer. Now I have to rewind the transformers. Do you remember by any chance the winding turns and wire diameter?
I did not think of that. But that would explain the coil going high resistance. Thank you very much.
And bummer. Now I have to rewind the transformers. Do you remember by any chance the winding turns and wire diameter?
Unfortunately, I don't remember....only that the 16 ohm coil was below and completely separate. Why don't you open the one whose ohms are changing? The wire has developed resistance. It can't be used,
I have to have one of those. I'll open it and count it. I have to look, please wait. I'm sure I have it.
I will open it at some point, no doubt. Was just thinking maybe you have some notes from when you rewound yours.
Yes, the transformers showing the infinite resistance readings are popped. Are there capacitors from the output tube plates to ground? If so, check, better replace them all (!) immediately!
Best regards!
Best regards!
The resistance is not infinite, in fact it is very repeatably measured at the values mentioned above, so I don't think they are burnt out, but I do believe hooman's explanation that the wires are corroded and manky. Not unexpected, I suppose.
And no, I didn't even try to plug the radios in. I am currently replacing capacitors as you suggest and moving some parts around to get the heat out of the bottom of the chassis. The three high power resistors for instance in the power supply have been moved on top in place of the large multisection filter capacitors and these have been replaced with individual capacitors under the chassis. Found a broken resistor in the process too, so that was replaced. Some new tubes are on their way from Japan too.
And no, I didn't even try to plug the radios in. I am currently replacing capacitors as you suggest and moving some parts around to get the heat out of the bottom of the chassis. The three high power resistors for instance in the power supply have been moved on top in place of the large multisection filter capacitors and these have been replaced with individual capacitors under the chassis. Found a broken resistor in the process too, so that was replaced. Some new tubes are on their way from Japan too.
Well, your readings, compared with some hundreds of Ω with intact OPT's, resemble infinite 😉. The primaries are open. Btw, you didn't per chance measure between terminals of isolated windings?
Best regards!
Best regards!
Nah, I measured them correctly. You may be right, the primaries may be open. I mean for all intents and purposes open even if not physically. I expect the thinner wire of the primary is easier to corrode through and now there's just some leakage through whatever gunk has formed there under corrosion that gives the highly elevated resistance readings. We'll se in the autopsy.
One extra detail though. It was the "inner" transformer that was dead in both receivers. By inner I mean it is placed between the output valves and the bulk capacitors. The other one is "outside", on the other side of the output tubes and near the edge of the chassis. Coincidence?
One extra detail though. It was the "inner" transformer that was dead in both receivers. By inner I mean it is placed between the output valves and the bulk capacitors. The other one is "outside", on the other side of the output tubes and near the edge of the chassis. Coincidence?
Copper salts left from corrosion or carbonized insulators may provide a high-resistance path. But aren't necessarily a constant resistance with voltage - may arc over with B+ voltage. If it failed at a solder joint and if that solder joint is on the outside of the winding, it might be repairable. Odds are not good, though.
Yeah, I won't try to use them. I have two (possibly) good transformers, so I'll go with those. We'll see what else needs attenton and make a decision later.
One thing that irks me is they don't have a former, or at least not a modern style former with connection tabs. The top and bottom of the windings are exposed and the tabs just poke out of the winding layers. What a crap design. If I have to rewind the trannies, I will make formers out of some phenolic board, 1.5mm or so. I will probably leave off the 16 Ohm winding too, don't think I'll ever need it.
One thing that irks me is they don't have a former, or at least not a modern style former with connection tabs. The top and bottom of the windings are exposed and the tabs just poke out of the winding layers. What a crap design. If I have to rewind the trannies, I will make formers out of some phenolic board, 1.5mm or so. I will probably leave off the 16 Ohm winding too, don't think I'll ever need it.
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50 turns of wire were wound around the transformer...and then the core...I applied AC voltage to the 16 ohm coil and determined the number of turns.
Wow. I am speechless. You are properly geared to do this job. What size wire are they? And did you impregnate the transformer with anything?
I guess the secondary is continuously wound and tapped at 92 turms.
Best regards!
Best regards!