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Testing Sansui 1000A Output Transformers

Discussion in 'Tube Audio' started by danrclem, Sep 7, 2018.

  1. danrclem

    danrclem AK Subscriber Subscriber

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    Last weekend I bought a 1000A and I have read a lot about the OPTs being bad so I'd like to see if mine are OK. How do I go about checking them out? Can I take the 7591s out and then test it with power or should I test it without powering up? I haven't put any power to it yet and don't want to unless it can be done safely. The seller said that it hasn't been ran in many years and it appears to be original except for the unblown 10 amp fuse that it has in it and two output tubes. The fuse is what makes me nervous.

    It is a Dec. 1965 build with one bias pot. I can't complete this project right now because I have so many other things going on but would just like to know if it's going to be fixable without spending a great deal of money on it.
     

     

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  2. cademan

    cademan Addicted Member

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    You're probably okay in the output transformer department for now but testing them for shorts is easy. Just take ohm readings of the primary to the secondary. If there is a short, the meter will read zero ohms. If there is no short, it will read into infinity.

    FYI, there should never be any continuity between the primary and secondary windings of the output transformers or any transformers for that matter.

    Those 1000A's really need to be recapped before you worry about the transformers. Those things contain the dreaded (and very leaky) Elna capacitors so have fun rebuilding that thing! :rolleyes:

    I own one which had a good output transformer and a shorted one. I removed the shorted one along with its two 7591 tubes and ran it for a year or so as a mono amplifier/receiver. A few years later the other output transformer shorted due to the other two 7591 tubes severely red plating. I was only in my teens and never understood that it was bad coupling capacitors causing the red plating. I'm now in my upper 50's.

    I never could find a decent output transformer for it. I still have it but it is basically a parts unit with only the front face plate and knobs, original circuitry, minus both output transformers and power transformer. :(
     
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  3. gadget73

    gadget73 junk junkie Subscriber

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    Ohm test them for shorts to ground and opens from plate to plate on the output tube sockets. If it passes that, chances are very good that the transformers are fine. The fail mode seems to be one or both sides goes open.
     
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  4. danrclem

    danrclem AK Subscriber Subscriber

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    I knew that I had to change all of the capacitors before running it but just wanted to make sure the transformers were good before spending money on the caps. I know that at least one of the coupling caps are leaking and one of single value can caps is leaking. I'm going to change them all just to be on the safe side.

    Thanks guys. I'll check them out and see what I come up with.
     
  5. primosounds

    primosounds Powered with pure tube sounds. Subscriber

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    The 1000A has SS rectifiers so as long as your power supply is working you can easily test the lot with a variac. Just set it at 10 or 20 % and measure the voltage at the Anode of the 7591 sockets. All the voltages should be very close.
    The reason to use a variac instead of using full power is that if the PS caps are bad you will not risk damaging the output trannys or the mains trans from an over current situation. I have had several 1000A but none had bad OPTs.
    I did find that the 2 can caps in the voltage doubler circuit always need to be changed and the PIO coupling caps are invariably leaky by now. There are many of those coupling caps which are hard to get at in some places.
     
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  6. primosounds

    primosounds Powered with pure tube sounds. Subscriber

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    I should also warn you about the solder lugs on the Sansui, being a bit fragile, compared to USA made ones. It seems that they are just made with less material so if you try to tug on a lead to get it unsoldered from the lug you might break the lug with little effort. Breaking off a solder lug is a huge PITA, and to be avoided if at all possible. So, in lieu of trying to remove any wire leads by desoldering at the lug it is best to cut the lead, make a hook in the old lead and fasten the new lead in that new hook. Not my preferred way of installing new components but a good way for the Japanese tubed audio rebuild.
     

     

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  7. markn2wae

    markn2wae Mark T N2WAE

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    I agree! :thumbsup:

    Mark T. :music:
     
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  8. AlTinkster92

    AlTinkster92 AK Subscriber Subscriber

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    Just a quick question, a 10 amp fuse is kinda high in this isn't it?:idea:
     
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  9. danrclem

    danrclem AK Subscriber Subscriber

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    Yes 10 amps is too high and that's one reason why I am concerned even though it's not blown. It's supposed to be a 4 amp and the original probably blew. I really need to change it out while it's on my mind because I might forget about it later on.

    I still haven't tested anything yet because I still have another project on the bench which hasn't been touched for awhile because of other priorities. The 1000A won't officially get under away for awhile but I just wanted to know the condition of the transformers and not get a surprise later on when I do start it.

    I do appreciate all of the good information that has been posted here already. Thanks to all.
     
  10. Sam Cogley

    Sam Cogley Last of the Time Lords Subscriber

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    I disagree. If an OPT is blown, there's not really much point to recapping the unit until a replacement is obtained. I have a project 1000A that was built out of two early units. Between the two receivers, they'd gone through 5 OPTs. I had one working OPT on each unit. One had one replacement OPT, the other had two replacements, one of which was dead.

    From my non-scientific observations, the later OPTs with the script "Sansui" logo are a lot more robust than the earlier ones with the block "Sansui" printing.
     
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  11. EastPoint

    EastPoint Factory Code No. 4200 Subscriber

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    Can Heyboer or someone else re-wind these transformers?
     
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  12. gadget73

    gadget73 junk junkie Subscriber

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    Agreed. I always do basic life tests on the iron before going any further with things. The iron is hard to replace compared to the passive parts.
     
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  13. primosounds

    primosounds Powered with pure tube sounds. Subscriber

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    So true. The trannys are the heart and soul of any tube amp and by far the hardest thing to replace. Testing the transformers with a variac is a fairly simple and quick thing to do.
    BTW, whenever you find a fuse that is very different from the stock one. It is probably covering up a problem.
     
    Last edited: Sep 9, 2018
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  14. Sam Cogley

    Sam Cogley Last of the Time Lords Subscriber

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    Probably, if they had one to take apart.
     
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  15. danrclem

    danrclem AK Subscriber Subscriber

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    I finished my other project and got the 1000A on the bench. I set the variac to 12.5 VAC and had the 7591 tubes in place. The readings in DCV to pin 3 on the right channel were 48.1 and 48.3. The left channel was 48.6 and 48.6. There were some very small fluctuations in both channels but I figured that wasn't anything to worry about. I then set the variac to about 20% and the voltages pretty much doubled. I'm guessing that the transformers are ok but I'll await the expertise of some of you guys that know a boatload more than I do.

    If everything is a go I'll make up a parts list. I do have a couple questions about parts. The can caps are all plastic and clamped in. I have looked but not too hard for the voltage doubler caps of the same size and haven't found any. Can anybody point me to a place that sells them or let me know what you've used in place of them.

    If I empty the contents of the plastic multi-cap can caps to restuff will the clamp still hold it in place and not crush it. Are there any other good alternatives to the multi-cap cans besides buying new stuffed cans or mounting them underneath? I really don't want to have the added expense of cans already stuffed or mount the caps underneath because it's so busy down there already.

    I read in one of the many threads about the 1000A that the diodes should be replaced. Any thoughts on that. One is labeled SE-05 and two of them are labeled SW-05.

    Any other suggestions.
     
  16. Shadowdog

    Shadowdog Super Member

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    You can buy new replacement output transformers from Hashimoto and they make transformers for Sansui amps also.
     
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  17. bimasta

    bimasta Super Member

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    Has anyone had this kind of work done? What kind of cost am I looking at? It seems like a ton of work — unwinding miles of wires, then rewinding with miles of the same gauge wire. Sounds pricey.

    I have a Cary 2A3 power amp I adore. One channel is dead. It could be a cheap and easy fix, like a cap. Or it MIGHT have a blown OPT, not easy or cheap.

    But I don't know. I'm just trying to get a ballpark idea of the OPT scenario, before proceeding.
     
  18. Shadowdog

    Shadowdog Super Member

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    Re: Sansui 1000A

    There are very detailed threads on AK from Mark & Larry that have rebuilt a few! Hopefully they can still be found with the pictures.
     
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  19. Shadowdog

    Shadowdog Super Member

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    Usually it's not the OPT and and as mentioned above one can test it with an ohmmeter. If it's a complicated transformer, it will cost more to rewind as it will take more time. Maybe you can get a new replacement from Cary if needed or they will tell you where to get one.
     
  20. primosounds

    primosounds Powered with pure tube sounds. Subscriber

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    If you are not getting anywhere with Cary, which would be the first place to look for a new OPT there are a few transformer winders who might have a replacement you could use. Edcor for example , along with Transcendar, Lundahl, Sophia, and Bill in Nevada. Here is the Sophia in an epay ad.
    /www.ebay.com/itm/Sale-Sophia-Electric-flagship-transformers-best-upgrade-for-2A3-tube-amplifiers/153150828368?hash=item23a8802f50:g:iLYAAOSwzgRWv7wL
    No connection , what so ever!
     

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