All the way to Mercury and back for a Line 6 DT-50…

A flicker of hope for those DT-50 owners who have suffered a blown power transformer (or two)…

If you read the forums, even on Line 6’s own support pages, you will quickly recognize that something went quite wrong with the DT50 amplifier. It has also mysteriously disappeared from their catalog without explanation leaving its little brother, the DT25, behind.  I find it especially odd, since the owners of DT50 amplifiers speak so well of them.  Let me relate my recent experience with a customer’s DT-50 and the long road to putting this amp back into service.  I’m not a big fan of modeling amps in general, but I will happily admit that the latest generations have really improved in terms of real, usable tones that actually approach what they claim to mimic.  I’m also not a big fan of overseas contract manufacturing on so many levels, but especially on quality control. Questionable tooling, non-existent quality systems, using cheap hand labor to replace more reliable automated assembly, rampant intellectual property theft, and ongoing counterfeit parts supply chain issues all leave me feeling the best amps are still made in the west. Things can go wrong, and do.

The DT-50 has, if you have read even a few forums, been plagued with a very high failure rate of power transformers. To Line 6’s credit, many were replaced under warranty, and (in a slight nod to the scope of the issue) they continue to offer replacements to some customers who have, by now, recently fallen out of the warranty period. They even offer, in some cases, to pay postage one-way.  The caveat here, is that, again if you follow the posts, many of these replacement transformers are ALSO failing some months later. The transformers that are failing are made by Chuang Meei in Taiwan, part number 11-30-0044.

Shipping an amp is an expensive, if not risky proposition, so I was not surprised when my customer told me he’d rather have the repair done locally, and preferably without using another Chuang Meei part.  The owner had not had much luck talking to Line 6 initially, and the offer to repair it out-of warranty apparently came a bit later in their conversations. It was also suggested that he might bring the unit to a Line 6 service center, and he was duly given a couple of contacts in Michigan, although not in our area.  When contacted, those shops were backlogged on repairs or “unable to get that transformer right now”, he was told. This customer really liked this amp, and had it integrated with a few other Line 6 products in a seamless way, and he was not about to give up easily.  Replacement parts are not something Line 6 sells much of – only about 18 items are available via their store, mostly knobs and pots and logos, unless you are a “service center”.

Having heard rumors that Mercury Magnetics actually make a DT-50 power transformer replacement, he contacted a MM distributor, JMI Music in Fort Worth, Texas (www.jmimusic.com; 817.284.5877).

While not a catalog item, it turns out that they DO make one, and told him they could wind him one on special order. So, order placed, and a tranny was delivered about two weeks later.  It is a model BOG-DT50-P and cost in the neighborhood of $218.  Not cheap, but MM has an excellent reputation for made-in-USA quality. I was disappointed to find that, when asked, MM specifically did NOT guarantee that the color codes of the unit would match up with the original, even though it was apparently specially-wound for the DT-50. That meant we had to verify every circuit and ground to make sure it would install correctly.

There are 15 wires on this unit. Line 6 again disappointed us by repeatedly insisting, in multiple calls, that a schematic was not available due to “non-disclosure agreements” (not the lack of an NDA, as in, with me, but an NDA itself, leading me to think perhaps I was dealing with a contract service center and not an actual Line 6 tech department), and further, they would not/could not even verbally identify, by phone, just the color codes of the wiring of the original DT-50 transformer. (So those risky wire colors are secret now?)  Again, none of this would be a big deal, but MM effectively had told us the colors may not match up.

Here’s what we did to prove it one way or another.  Using an LCR meter, and knowing the original PT had a least a shorted primary, we shorted the primary of the new MM unit and made AC impedance measurements across all the secondaries and noted them down on the MM transformer’s supplied pinout documentation. We then measured the DT-50’s shorted unit’s secondaries and compared values. We found that it was pretty obvious which windings were which based on relative resistances at a test frequency of 120 Hz. To double-check the work, I followed those connections back to the PCB traces and to each of the diodes and filter caps to verify that the voltage of each secondary was compatible with it’s proposed power supply circuits. No surprises found here, folks.  In this case, everything matched up perfectly.  Well, except one thing.

The mounting feet of the new transformer do NOT line up with the original threaded PEM-nut inserts in the chassis. They are about ¼” off in one dimension.  Just enough that if I drilled 2 new holes, they’d be compromised in strength as they would be really close to the PEM-nut inserts. A better plan would be to swap out the mounting brackets from the original transformer and use them instead on the MM. Fortunately, the core spacing of the main bolts did match, so this actually works out pretty well. All four bolts must be completely removed to do this, and some new spacer washers placed in the top 2 bolts so the end-bells still retain a cooling air gap of about 2mm at the top (see pics) like it has at the bottom.  Pay attention to the MM paper and plastic insulators on the screws and get them all back into place, it’s tricky, but they are there for a reason. These pics show the Chuang Meei Feet now sitting under the MM iron:

Once that was accomplished, it was a simple matter of bolting it in place and splicing the matched-color wires together using the pre-tin, then sweat together, then heatshrink method, to keep the original molex-type connectors rather than soldering the new leads directly to the molex headers.

Note that the center-tap of the B+ for this amp goes to ground. MM placed a flag sticker warning label on the B+ green/yellow wire about this. In some rectifier designs, shorting this to ground will result in a short-circuit through the diodes, but in the DT-50 on the bench here, it was used and grounded.

 

Before I connected the secondaries back up, I applied 12.5 VAC (via the variac) to the power input of the amp and verified I had 1/10^th of the expected AC voltages on the each of the secondaries,  before I risked applying the wrong voltages to the PCB circuitry. Everything looked dead-on, so we connected all and powered up the amp. In this case, the output was a perfect 50 watts at clip into 8 ohms, the bias was very close to spec, and after all functions were verified a solid burn-in was completed. We then re-dressed all of the leads safely as per the original wire dress.  As a result, one VERY happy customer took his beloved amp home knowing that this problem should never return.

 

Power transformers can fail for a number of reasons. Often, an internal, self-resetting thermal fuse is to blame when it fails to re-set. Other times, arcing across windings is to blame, insulation issues, or a shorted output tube connected without an HV fuse will take out the smaller-gauge B+ winding by drawing too much current. I like to do post-mortems on parts whenever possible. In this case, we tore the original unit open and found that one side of the 120/240 primary was simply shorted out. Hopelessly shorted, with no signs of heat or smoke. Perhaps a winding error, or a tolerance issue. Transformers get quite hot, and copper expands and contracts a lot with temperature cycling. It has to be accounted for in the design. To see an occasional tranny go bad is to be expected.  To have a whole group of them go bad is either faulty design or faulty manufacture, bad tolerance stack-up, poor materials or a combination of these.

 

I suspect Line 6 is acutely aware of what happened, but they are not talking about it and not being very helpful, either. If this case is typical, and they expect to keep their (once-loyal) customers, they should be providing better customer support for their products, especially at this price point of equipment ($1800 list). Simply sending in the transformer alone for an exchange/replacement would have been a low-cost $35 (OEM price) solution they should have offered (only 4 screws and 3 connectors to remove/replace).  Leaving the customers hanging or offering a $65 shipping bill and waiting weeks to get an unimproved replacement is not a good response to an obvious quality issue. My guess is that they simply did not have enough trannys to go around (some threads recalled being told they were out of stock) or the whole lot were so suspect that they just stopped DT-50 amp production altogether and tried to limit economic damage while they could. Sourcing from Asia is always risky.  Now you know.

 

I’m basically putting all this out there so other people, who until now cannot get their amps fixed, can know there IS a solution, and this is what it takes. Whether it’s worth it to you, well that depends how much you like the amp. For me, I‘d have to say it is one of the best-sounding modeling amps I’ve ever heard, and worth making it right. By the way, if you are brave, and just have to have one, I noticed Line 6 has “Refurbished” DT50’s quietly available via their online store for the time being, at $1,039.00.

I do wonder, at that price, which power transformer do they have in them?

 

Dan , MTA, LLC