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dbx 386 Power Transformer

Discussion in 'Electronic Repair Modifications DIY' started by johnknag, Oct 18, 2016.

  1. johnknag

    johnknag Active Member

    Hi there! I have a dbx 386 dual channel tube preamp. It was out of service for some time and when I recently put it back in the rack only a couple of LEDs turn on. On inspection it seems that the larger of the two transformers supplying power to the PCB has transformed into a Gandalf No-Pass Filter ("None shall pass!"), i.e. voltage in to the transformer is good, voltage out is 0.

    After much searching I cannot find a reference for this part and what its output voltage is supposed to be. The transformer is printed "Ault Inc. 26-0290A 687-3510G", but my google-fu turns up next to nothing on this, except this excellent thread in this forum.

    Does anyone know what voltage this is supposed to output, or anyone who owns this product be willing to measure the output voltage of the larger transformer?

    Thanks for any help you can give.
  2. Kurt Foster

    Kurt Foster Distinguished Member

  3. johnknag

    johnknag Active Member

    Thanks for your response, Kurt. I actually linked to that same thread in my post. Unfortunately, the schematics posted in that thread are no longer hosted on Imageshack. I'd love to get a look at them! As far as the google image search, I did that already also. I can't find any images in that search which are actually schematics for the 386. I think because it is still in production, dbx protects it from getting out.
  4. Boswell

    Boswell Moderator Distinguished Member

    The traced-out schematics that were on Imageshack did not have anything about the power supply on them - they were purely of sections of the signal path, so would have been little help in tracking down a problem with your mains transformer.

    I don't own a DBX386, but my hazy memory of their power circuitry is that the transformers have thermal cut-outs wound into them that break the primary circuit if the windings go over-temperature. If you really are getting zero volts a.c. from each of the secondaries, that would be a good place to start looking. I would disconnect one end of the primary winding and measure the winding resistance with a multimeter. If the winding and cutout are intact, it will probably show a few tens of ohms; if the cutout has blown, you will get several megohms.

    If the primary is open circuit, then you would either have to get a replacement transformer or have your one re-wound by a specialist transformer repair outfit. However, another avenue is that DBX is one of the Harman group of companies, and they operate a part request system. I have never come across anyone who has used it, so can't comment on whether they are helpful or not.
  5. DonnyThompson

    DonnyThompson Distinguished Member

    LOLOL!!!! says the guy who has a "hazy" memory of the power circuitry of the 386.
    You're a FREAK Bos... but I sure do love ya! :love:
  6. Boswell

    Boswell Moderator Distinguished Member

    Well, maybe, but the next freakish question to ask is: if the thermal cut-out is tripped, why did it go over temperature? There could be a short or other fault such as a blown rectifier on the secondary side that caused the trip, and a new transformer would go the same way.

    It's the old fault-finder's motto: what and why?
    pcrecord likes this.
  7. DonnyThompson

    DonnyThompson Distinguished Member

    well... uhm... of course, I mean... that would have been my next question as well... LOLOL... :ROFLMAO::p

    It's only logical that one would want to know if the testicular shmiglet would overthrow the ovarian ya-ya, thereby overloading the thermo-coupler of the uterine whats-a-hooz-it.

    I wish there was a way we could download files from Bos's brain.
    I wouldn't doubt that he has every schematic and spec stored for every piece of audio gear ever made, along with all subsequent mods, many of which he's probably designed himself.
  8. dvdhawk

    dvdhawk Well-Known Member

    You may come to a point very soon, when you say, 'even if I can get a replacement transformer… is it worth it?' (considering there is one currently on eBay for $250) Boswell, as usual, brings up a good point not to be overlooked among the testing procedures. Replacing the transformer may just be the tip of the iceberg. It's a problem you find in lightning damaged equipment, and anything else that fries the power supply. You have to consider how far downstream the damage goes. It's always a gamble, even if you can test the rest of the power section at the component level. The rest of the unit's analog circuitry might be more forgiving to a certain point, but the digital section of the 386 might not be able to endure a single surge. You never know until you get the unit powered up.

    I'd recommend following Boswell's instructions to the letter, and then take all that data into consideration before you decide whether or not to repair this unit.

    Does it have the stink of burnt electronics?
    Do you have the equipment to desolder and test diodes, capacitors, etc,?
  9. johnknag

    johnknag Active Member

    I appreciate the advice, Boswell, I wasn't aware of the parts request system. I've put in a request with Harman for the part. I don't think there's anything wrong with the rest of the circuitry; I think the transformer went over temp because I had it in a rack with other hot components (something the owners manual specifically said not to do...) and it just got too hot in the box with the tubes on. But yeah, there may well be problems down the line.

    In response to dvdhawk, I do have the equipment to test the rest of the unit, and I was hoping to do so if I could find out what the output voltage of that transformer is. Realistically, though, if it's not the transfomer, or if the replacement cost for that part is too high, I might just recycle this one and buy a new one (or another pre...) but money is always a factor so if I can fix this for $20 and a bit of my time, I'd like to do that.

    Thanks for the insight guys, and I'll report back on how my experience with Harmon's replacement parts system goes.
  10. dvdhawk

    dvdhawk Well-Known Member

    I'm glad to hear that you can test at component level. I'd be doing the same, and I'm glad you're keeping one eye on the big picture.
    The great Mr. Boswell will be an excellent guide for your adventure.

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