I think hooman made the point earluer the secondaries are separate windings (as per his sketch).
The Sensui 16 OHM layer is wrapped around the core and is completely separate. I opened it before.
Hang on. Am I to understand that the 16 Ohm winding is the first one to go on and the rest comes on top of it? That is surprising.
I would have expected the primary to go on first, then the secondaries (no matter what order).
I would have expected the primary to go on first, then the secondaries (no matter what order).
It's a rather basic way of interleaving, but ineffective in reality.
Best regards!
Best regards!
Mate, they are shot. Don’t over think it.
If a measurement “ is likely to be” low hundreds ohms but you are actually measuring 3 orders of magnitude higher: Houston we have a problem.
My 2 cents, ymmv etc
If you have a desire to learn about transformer winding in a practical sense you could do a lot worse than starting here: https://www.turneraudio.com.au/index.html
Mr. Turner (RIP) may well be interpreted as a grumpy old bastard. But as a grumpy old bastard myself (and not dead yet) I find his prose refreshing and honest.
Read the very well kept re-production of Patrick’s site, and report back If you still have questions.
My 2 cents, ymmv etc
Who else than grumpy old bastards do have the knowledge and experience in successfully winding output transformers?
Best regards!
Best regards!
I have experience with normal (i.e. power supply) transformer winding. I just didn't think I was going to have to wind an output one. From what I read (I have a book on vacuum tube amplifiers) interleaving is done differently. Not that I thought anyone would bother with it for a consumer grade receiver. 70 years ago.
No need to learn how to wind out put transformer ... . Sansui has done the job. The only problem is fitting the wires into this size transformer. Because each layer needs insulation .
Yeah, but now they're toast and I don't think the good ones I have will last much longer. We'll see. I'll pull one of the bad ones apart and see what's what.
As I said before, check for possible reasons, especially for shorted capacitors form plate to ground. It is very alarming that you've got two popped OT's in two different, but basically similar amplifiers, and it may point to a possible design flaw.
Are you planning to rewind the defective OT's? Then you may think of a more sophisticated interleaving scheme, see Patrick Turner's advices.
If shipping fees from Australia to Germany weren't that high for heavy goods like transformers, I'd show interest in the defective ones and rewind 'em myself.
Best regards!
Are you planning to rewind the defective OT's? Then you may think of a more sophisticated interleaving scheme, see Patrick Turner's advices.
If shipping fees from Australia to Germany weren't that high for heavy goods like transformers, I'd show interest in the defective ones and rewind 'em myself.
Best regards!
Postage is not that much this way compared to the other way around.
I will keep the transformers anyway. I rarely throw away or sell anything, especially that old.
Yes, there is a design flaw. If you find a picture of the radio you will see the transformers are right next to the output valves. That can't be good for them. Under the chassis, right under the transformers there are two massive ceramic resistors (10W each). That can't be good. One of the transformers however is on the edge of the chassis, so at least it is heated only on one side (and from below). But fear not, see my post above, this has been rectified.
If you have a look at a picture, you will see. on top two 100uF electrolytics next to the power transformer. One of these had leaked its guts on the chassis in the radio I am restoring.
But I don't think any of this has killed the outputs. Or not directly. I htink the heat just helped the sealing wax leak out and then moisture got in and corroded the wires.
Here you go:
https://www.stereonet.com/forums/topic/322089-sansui-sm-12m-vintage-tube-receiver/
Have a look down the page to see the top and bottom of the chassis.
I will keep the transformers anyway. I rarely throw away or sell anything, especially that old.
Yes, there is a design flaw. If you find a picture of the radio you will see the transformers are right next to the output valves. That can't be good for them. Under the chassis, right under the transformers there are two massive ceramic resistors (10W each). That can't be good. One of the transformers however is on the edge of the chassis, so at least it is heated only on one side (and from below). But fear not, see my post above, this has been rectified.
If you have a look at a picture, you will see. on top two 100uF electrolytics next to the power transformer. One of these had leaked its guts on the chassis in the radio I am restoring.
But I don't think any of this has killed the outputs. Or not directly. I htink the heat just helped the sealing wax leak out and then moisture got in and corroded the wires.
Here you go:
https://www.stereonet.com/forums/topic/322089-sansui-sm-12m-vintage-tube-receiver/
Have a look down the page to see the top and bottom of the chassis.
I don't like to power unknown gear before I go over it and make sure there's nothing to go pop, especially with old stuff like this with unobtainium parts. More so when I found the capacitor with its guts spilled all over the chassis. So no, I didn't check anything before hand. One note, it looks like this unit has been partially recapped at some point with film capacitors, which are still in good shape, so I left those in but I replaced all the original caps still present (Suzuki oil and paper) plus a few minor electrolytics. I checked a few of them for fun and can confirm they are all garbage. The main bulk caps (a couple of multisection electrolytics) were also replaced at some point in the past and still checked okay, but I replaced those too because I didn't trust them and also I wanted to move them under the chassis as I explained above.
I think you meant capacitors from one plate to the next grid, it is usually those that destroy tubes and OPT when they start leaking DC and turn up the following stage driving the tube into redplating.
I am aware of various techniques of interleaving transformers, I mentioned above, I read the book.
Hooman, I have that diagram as well, but I don't think it is a 12A but a 12M (or some other version) because my 12A has a 6BE6 in the front end of the AM2 section. Or maybe mine is an earlier (or later?) version. The schematic glued inside the cover of the radio looks like it has been corrected by hand with an "E" over what was originally an "A". The chassis has a very clear "E" stamped next to the valve.
And I have a couple more schematics for 12M I think and some other version.
But here's the real oddity. None of the schematics above have one connection I have found in mine and have also seen online in pretty much all underside pictures of these. There is a wire that goes straight from where arrow "A" connects to pin 6 (plate of triode 2) of the front end tube in the FM section. If you follow the link I posted above, you will see that guy's radio has this wire too. What is that for? Maybe you can check your radio has it too. See picture below.
I think you meant capacitors from one plate to the next grid, it is usually those that destroy tubes and OPT when they start leaking DC and turn up the following stage driving the tube into redplating.
I am aware of various techniques of interleaving transformers, I mentioned above, I read the book.
Hooman, I have that diagram as well, but I don't think it is a 12A but a 12M (or some other version) because my 12A has a 6BE6 in the front end of the AM2 section. Or maybe mine is an earlier (or later?) version. The schematic glued inside the cover of the radio looks like it has been corrected by hand with an "E" over what was originally an "A". The chassis has a very clear "E" stamped next to the valve.
And I have a couple more schematics for 12M I think and some other version.
But here's the real oddity. None of the schematics above have one connection I have found in mine and have also seen online in pretty much all underside pictures of these. There is a wire that goes straight from where arrow "A" connects to pin 6 (plate of triode 2) of the front end tube in the FM section. If you follow the link I posted above, you will see that guy's radio has this wire too. What is that for? Maybe you can check your radio has it too. See picture below.
PS. Just remembered the other radio is not a SM12 but SAX100. Very similar but has a transistor phono preamp and a FM MPX unit. Other than that, is pretty much the same as my 12A, including the 6BE6 tube in the AM front end.
Well, I tested one channel of the final amp only with input from AUX and the amp is alive. Well, at lest one channel.
Something is wrong though because the signal is amplified nicely all the way to the output transformer and then after the transformer I get a low signal. In fact the output valve goes into clipping at about 75% volume. I suspect the transformer still has a problem but not sure what?
Haven't mentioned this, but the DC resistance of the secondaries is in the order of a few Ohm, inductance I can't remember but it seemed consistent between the two transformers that I thought were good. I think about 14-15mH? Something like that. Even the trannies with busted primaries measured very similar on the secondaries both in resistance and inductance. Do these figures sound about right?
I also went to a local shop and found some audio output transformers (100V line voltage) that measure very similarly to these (the primaries are a bit down though, given the lower voltage of 100V - about 150 Ohm; secondaries very similar, a few Ohm. Haven't measured inductance yet).
If my output trannies are good, what else can it be?
I'll check tomorrow the other channel and then I'll try to hook up a speaker.
I used an old Heathkit IO 21 scope to look at signals, basic tool with no quantitative info but fully restored by myself a couple of years ago. Running a sweep with the generator it looks like the response curve has a big bass boost up to about 3-4000Hz and then it goes pretty flat all the way to 20kHz. This is measured on the output valve plate. This seems consistent with what I have seen online in other people's tests.
Something is wrong though because the signal is amplified nicely all the way to the output transformer and then after the transformer I get a low signal. In fact the output valve goes into clipping at about 75% volume. I suspect the transformer still has a problem but not sure what?
Haven't mentioned this, but the DC resistance of the secondaries is in the order of a few Ohm, inductance I can't remember but it seemed consistent between the two transformers that I thought were good. I think about 14-15mH? Something like that. Even the trannies with busted primaries measured very similar on the secondaries both in resistance and inductance. Do these figures sound about right?
I also went to a local shop and found some audio output transformers (100V line voltage) that measure very similarly to these (the primaries are a bit down though, given the lower voltage of 100V - about 150 Ohm; secondaries very similar, a few Ohm. Haven't measured inductance yet).
If my output trannies are good, what else can it be?
I'll check tomorrow the other channel and then I'll try to hook up a speaker.
I used an old Heathkit IO 21 scope to look at signals, basic tool with no quantitative info but fully restored by myself a couple of years ago. Running a sweep with the generator it looks like the response curve has a big bass boost up to about 3-4000Hz and then it goes pretty flat all the way to 20kHz. This is measured on the output valve plate. This seems consistent with what I have seen online in other people's tests.
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