EE Minimax DAC repair

Hello Gang,

Although I don't consider myself a Newbie when it comes to electronics or fiddling with technology (been doing it for over 20 years), I seem to be scratching my head as to what went wrong with my OpAmp swap. As per Doug's article, I tried the OPA2604 (dual) in U1/U2 and AD797B (single) in U6/U7 then fired up my previously working DAC. There was little or distorted output from the unit, which I then shut off, inspected interior connections, and tried again; the unit would not start up. Based on visual inspection, there does not seem to be anything obvious wrong with the unit - the fuse and power line are fine (I tried another just in case) no "leakage" or "carbon trails" on the board or otherwise.

Would anyone have suggestions as how to troubleshoot / fix this or know of a qualified / reputable repair person that can?

Thanks in advance.
I have the same exact op-amps in mine. When I first installed them, (before I repowered it up), I noticed I missed the prongs on one side of the op-amp chip. It, at first, looked perfect, but I got a little closer and saw what I did. Make sure you have all the pins pushed in and that none of them have "folded" under the chip itself.

I would pull them all back out and try again. The pins are really small and very easy to bend under or miss the holes completely, like I did.

Also make sure they are oriented the correct way.

For repair Bill at Morning Star Audio, (the importer can help).
Thanks Mofi,

I pulled out the chips to inspect, then made sure they were seated correctly, but no dice - I still can't get the unit to power up.

Bill had no other suggestions, either. I was hoping someone here might have more info (I think Bolder had some options for this unit at one point) or know someone who could fix....
Have you tried placing the stock opamps back into the EE DAC?
Did you insure that you oriented the Opamps in the correct direction? Do the stock Opamps work when placed back in their original location? Did you turn the tube button off if you aren't using a tube?
NJS and Morganc - thanks for the input...

Both original OpAmps and proposed, regardless of position / orientation, do not change the outcome - unit no longer powers up.

I'm assuming this is a power thing, but can't rule out the board - anyone have wiring diagrams, specs, or similar for these?

Daveaudiony, I have been thinking this over, frustrated that you are having a bad experience. It troubles me that someone tried a method I suggested and is having a problem.

Imo, little or distorted output means the unit will need repair. It sounds like one of the Opamps was inserted in 180 degree reversed position from proper alignment. I know, because it happened to me one time.

I have two of the EE Minimax DACs, an original in my office and the Plus unit in my listening room, and have been Opamp rolling both of them over time. I have rolled so many opamps, literally dozens and dozens of times, in both units that eventually I mistakenly placed one in backwards and that was it, the unit died. I simply had the Opamp not oriented correctly with the very small indicator of directionality in the right place - so the Opamp was inserted wrong, and the unit went PPHHHHTT... a small sound of something frying was emitted, and the unit died.

As frustrating as it is to believe that such a seemingly small action as putting in the Opamp reversed could kill the unit, this is consistent with what you are describing. So, I believe that you inadvertently placed one of the Opamps wrong. The indicators of directionality on the IC Opamps are maddeningly small, and easily missed, which is why one must exercise extreme care when aligning them. I do not intend to insult your experience or intelligence, but I know of no other reason why the unit would fail as it did. Similarly to your experience, there were no obvious signs of damage to the board when my unit died.

In the article I mention that I had switched positions - that is, tried a single in the place of a dual, or vice-versa - occasionally of a few Opamps without harm to the unit. I now strongly suggest NOT to do that, as I was informed that there are some modded units which may be adversely affected by it. I know of no instances in which a unit was damaged from it; the objection was theoretical, and I followed up with a warning addendum to my article.
However, from discussion with designers regarding the distinct purposes of the pins of the Opamps I believe reversed placement would cause major damage to the circuit, rendering the unit inoperable.

Check to see if the yellow light at the center of the circuit board is illuminated. When the Opamp incident happened, the yellow light died, showing the integrity of the circuit was compromised. If that yellow light is not lit I think you definitely have a repair happening.

Other good suggestions have been mentioned including deselecting the tube button, as when you remove the tube the entire unit falls silent if it is not turned off. I don't even know how to approach the possibility of there being a "bad" Opamp inserted; I think the likelihood of it is slim. Some people have bought Opamps from alternative sources and some have bought ones with longer legs and trimmed them down. I have no comment on that except I would think it more safe to do the swap precisely as I indicated.

Now, here is some encouragement; get it repaired and then as crazy as it may sound, do not sell it, but rather consider moving on to the Burson or DEXA discrete Opamps, as they are worlds better! They are absolutely worth the extra money. I have been using the DEXA Opamps with scintillating results. They are larger and easier to handle when rolling as well. Both the Burson and DEXA brands are far simpler/easier to work with both in terms of placement and identifying the orientation of the Opamp in relation to the circuit board. You absolutely must observe meticulously placing them with the half circle to the right as you face the unit. Otherwise, there may be another repair coming. :(

I urge you not to give up on it because there is a lot of change/improvement available via the discrete Opamps. I would not even consider going back to the IC Opamps, but move directly to the discrete Opamps.

Feel free to conatact me via the Audiogon message system if you wish. :)
I have no particular familiarity with the Minimax, but as an electrical design engineer, and after looking at the pin configurations of these devices (shown on page 3 here and page 5 here) I believe Doug is absolutely correct. Inserting either of these op amps oriented the wrong way is a disaster, which will almost certainly destroy the op amp, and probably damage other devices on the circuit board that connect to it, and possibly also those that supply power to it.
10-15-12: Daveaudiony
Both original OpAmps and proposed, regardless of position / orientation, do not change the outcome - unit no longer powers up.
This seems to say that you have tried both the original and the new op amps with both orientations. If so, all of them are almost certainly destroyed. If you are not certain that you used the correct orientation when you first re-tried the original op amps after noting the problem with the new ones, it might pay to obtain replacements (for either or both sets) and try again, in the off chance that other devices on the board were not damaged.

Also, be sure to follow good anti-static practices when handling these devices. As a minimum, that means touching a grounded metallic structure immediately before handling them, avoiding contact between your fingers and device pins (perhaps using a DIP puller to handle the devices), and not working where carpets or other static-prone surfaces are present.

-- Al
I'm just getting ready to order the DEXA op amps for mine. I'll report back after I have a good grasp on their sonic signatures.

As I stated above, I have the same op amps you tried and they do indeed sound very good.

Sorry you are still having trouble.
Hello Douglas, Al, and Mofi,

Thanks for your time and energy.

Although I recall seeing the "squares" on the NE corners of the OpAmps after initial inspection, I can no longer rule out the unlikely. I have no illumination from within the unit - board, diodes, or otherwise - so if we assume one or more of the OpAmps were installed incorrectly, what is the likelihood that the entire unit, as opposed to simply the OpAmps, is salvagable (vs replaced)?

I realize more trial and error might be necessary to answer this, but my time to do so grows short (honey-dos, kids, etc.). Is there a qualified technician / repair shop that might be able to further diagnose / fix this - I have been unable to locate one?

I have no illumination from within the unit - board, diodes, or otherwise - so if we assume one or more of the OpAmps were installed incorrectly, what is the likelihood that the entire unit, as opposed to simply the OpAmps, is salvagable (vs replaced)?
I don't think it's possible to say with a high degree of confidence whether or not the board has been damaged, especially without having detailed knowledge of the design. My guess is that it has been, unfortunately, but I think there is some chance that it may not have been, despite Doug's experience in which it was damaged.

My one suggestion at this point is that you perform the check for illumination of the yellow LED with NONE of the suspect op amps installed. In other words, with those sockets empty. If it then illuminates, I think it would provide some degree of confidence in the integrity of the rest of the board. If it still does not illuminate, without having specific knowledge of the design and the function of the LED it would probably not be possible to draw any conclusions either way.

I have no repair shop recommendation to offer.

-- Al
There would be a good chance that Bill O'Connell of Morninstar Audio, the Distributor of EE products, would have a competent repair facility close by if you sent the unit back. My guess is that the diagnostics would be much less time consuming, and the parts perhaps immediately available if the repair facility is tied in with Morningstar, i.e. working with them on regular basis.

If you get the unit repaired and switch to the discrete Opamps I think it will win you over despite the repair.
Hello again Al, Douglas,

Al -

No LED light. I tinkered a bit more and found that there is power coming to the board, but nothing after that (unable to trace circuits beyond taps to see where problem is). It's beginning to look like a paperweight....

Douglas -

I contacted Bill a few weeks ago about this and asked if he knew of any repair technicians that might be able to help / fix. His first response was to check power line and fuse, which I told him were fine. He then asked which OpAmps, I responded, and on his third email he stated

"...It is really hard to tell what was messed up when folks tinker with the innards of the DAC. It seems unusual that it won't even fire up. maybe a short in the power supply somewhere. Sorry I can't be of more help."

So regardless of the problem, it seems there is no one available to fix this unit, which is unfortunate. It looks like I won't be able to try those discrete Opamps you speak of.

Thank you.
I just sent Bill a link to this thread. I'm sure he can clear up the confusion.
Sorry to hear that, Dave. I still think it would be worthwhile, though, to order replacements for the parts in inexpensive form, and give them a try. Per Doug's articles the LME49710 and LME49720 should be suitable, and can be ordered from Digikey for a total of not much more than $10 for the 4 parts:

LME49710 (for U6 and U7)

LME49720 (for U1 and U2)

Hope that helps.

-- Al
I'm just not sure what I can do for you. Tech repair is expensive,time and trouble shooting to find what went wrong in your op-amp swap. Then replacement parts and putting it all back together.

I have a couple new in box older DAC's not the DAC PLUS that I could let you have for my cost.Probably would cost about the same as doing a repair or a few dollars more. As I saved a couple of these just in case something went wrong with somebody's original DAC if my customers. It is an original DAC correct? Have to ask, did you purchase from me or second hand? Let me install the Dexa's though if considering throwing them in yourself.

Hope that helps,
Hello Al and Bill,

Al -

Thanks for your optimism. At this point I think I'm going to shelve the project, perhaps attack with new vigor at some point in the future. I've already spend quite a bit of time trying to figure this out and it's probably best to take a break and put some energy into other things.

Bill -

Thanks for your offer. Since it sounds like buying new is more practical than trying to fix (if possible) we then venture into "which one do I want" territory. I think there can be a case for this unit, the DAC Plus, or a number of other units (Metrum seems to be getting some A'Gon press). The problem for me is that my "discretionary spending" is not what is was a few years ago, and my original hope with the Opamp upgrade was to tie me over until things change (financially), then see what's on the market.

Perhaps we can speak again in a few months?

Thanks to all.