Amek Purepath DMCL Schematic Request

[OwenM]

Hi there

Its a bit of an outside chance as these units are a little niche, I'm sure not many made and I can't find anything online. They are really so nicely built, and for maintainance you can pop off the top and bottom panels and work on the board while it still sits super securely in the 1u rack frame, if I can work out what exactly I need to do it it should be a breeze!

Anyway, here's hoping!

O

[The Dude]

Maybe try to contact Neve, since it was apparently designed by Rupert :

https://rupertneve.com/contact/

Otherwise, what's wrong with it? If one channel is bad, use the good one for reference. If neither channel works, I'd be checking power supply voltages. You should be able to figure it out looking at regulator part numbers, etc. Probably the usual +&- 15V supplies, Maybe 5V for switching circuits, etc.

[OwenM]

Thanks! I've just emailed them. I tried Amek directly but they pointed towards Harmon Kardon in the UK last time I had a look at this and they weren't much help, I sent it to them and they told me it was all fine...

Last time I popped the top on this I was really novice with electronics, and I didn't want to touch it as it was so bloody expensive. Now I feel, looking around, that I could probably work on it no problem. Its really well labeled all round and in a very easy-to-understand layout.

It has a few faults;

-Channel 2 distorts a little on input at all signal levels, sometimes! The main thing this preamp has going for it in the studio is ULTIMATE CLEAN gain, so thats a bother!

-The digital card just gives out noise (the card is an extra and not really where you are putting your money with this unit but would be nice for my setup if it worked)

-When you first engage channel 1's LP filter it gives out a sin wave at the frequency the filter is set at. A bit of wiggling and it behaves as normal!

-Lastly, and what has led me to posting, now when you switch it on theres no power and just a whine coming from the PSU area.


Each amp has 4 big electrolytics that are all bulging on mine (all 8 of them in total). And there is a bulging electrolytic in the power supply too. I'm gonna replace all those but thought while I learn the unit it would be cool to try and fix everything!

I do think, once I've got those in, then I MAY be able to figure it out, but I may well be making a new thread here in the process

Thanks for the help as always!

EDIT

It's hard to capture in a photo just how NEAT this thing is compared to other stuff I've worked on

[The Dude]

Yep. If there are bulging caps, that's a good place to start. C297 near the PT looks bulged for sure. Check power supply voltages with a scope (if you have one) to make sure they are not only up to supply voltage, but clean. It looks like at least some of the supply voltages are labeled on the board itself. That digital board won't like poorly filtered power. I'd also check, clean, re-seat all of the ribbon cables. They are often the cause of problems in these units- especially intermittent ones.

[OwenM]

Yep. If there are bulging caps, that's a good place to start. C297 near the PT looks bulged for sure. Check power supply voltages with a scope (if you have one) to make sure they are not only up to supply voltage, but clean. It looks like at least some of the supply voltages are labeled on the board itself. That digital board won't like poorly filtered power. I'd also check, clean, re-seat all of the ribbon cables. They are often the cause of problems in these units- especially intermittent ones.

Yup, I've got C297 on order and will follow with the others if it powers up after replacement. I think I can see the way the graphic on the left corresponds to the test point P1 on the right, they've just left out the ground labels on the right near the test point but the rest corresponds to the label on the left of pic. I have a scope, bought under the guidance of you fine peoples here and used it in anger a few times so I'll do that once I get the new PSU cap in.

Great info, thanks, I'll do the ribbon cables and get the new caps in and see!

[OwenM]

Well, I replaced all the bulging caps, that happened to be the larger Jamicon ones, with nice high temp Nichicon ones and from a bit of limited testing it seems to have fixed everything. The digital card works now even on a rough, unclocked, test. Major achievement! I've never had this unit working like this since I've owned it and I've sent it to various 'techs' over the years who have sent it back to me saying 'no problem'. Seems silly that all my planning and workarounds over the years would have been alleviated by just one of them checking the bloody caps, that were so easy to replace on this unit.

That said, since the install it did once fail to power up and give out the whining sound from the PSU. May not have sussed it yet, but repeated power ups since have worked just fine so hopefully it was just a new cap forming artefact? Maybe? Anyway, its working better than it has in years so I'll get using it and report back if any problems arise again!

[OwenM]

Ok, so this morning I had a thorough test and it's much better, but still not right.

Secondary symptom I hope... the digital card output sounds ok, but is a bit high-passed compared to the sound on the analog out and is very quiet. But it works! Thats a big plus.

The unit won't clock to an external source, but running on its own clock it's fine. I guess the bad sound MAY be just because I'm running it unlocked. For various unrelated reasons I can't select my sound card to be the slave at the moment.

The digital board has a slow flashing red LED on it, though I could find nothing about what that may be on the net. If its suspected relevant I'll double check the part etc. Its next to an IC.

But the main, and first problem that may clear up others potentially (since just the PSU cap fixed most of the digital cards problems!) is that on power up I get a buzzing whine from the PSU area sometimes. It sits there, no power, just whining. Video attached. I scanned for where the sound was coming from and it seems either the transformer TR5 or the ceramic cap C293 next to it. I know ceramics can be noisy, I'm unsure wether it is the cap itself misbehaving or it's being 'resonated' in some way to make it much louder than it usually would be.

I know SMPS's can be noisy and this points to a few areas to look at, but I'm new to that sort of tech! Chopsticking around didn't seem to be able to make a difference.

Eventually (30s) the unit powered up and the whine stopped. Power cycling again caused the whine to appear for a half second before it again stabilised. Once it had done this a few times it started powering on as normal.

Mods; This has shifted away from a schematic request, please move if you feel necessary!

Link to the sound - https://youtu.be/cok57O_L8iA


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[The Dude]

That sounds like another bad electrolytic to me. It's on it's way out and starts to hold after a few power cycles or after it slowly charges. If you have a cap tester test all of the PS caps. If not, you'll have to shotgun replace them. I'll bet it's worse when fired up cold as opposed to after being run for a while.

[OwenM]

Sorry just got back to this, of camping for my UK 'Summer' hols . Ok cool, I can do that as there's not too many. I only have a small ESR meter, a DIY component tester kind of thing that gives ESR, reads capacitance and shows the voltage loss %. It's not a very accurate device but shows a ballpark reading. I'll take out the 9 caps, looking for bulging ones first and see what they say, I'll probably just replace them all considering the age of unit and the relative cost of doing so being pretty low compared to its working value. Yes exactly, once I've got it working the problem disappears for the immediate future then appears again the next day!

[Enzo]

once I've got it working the problem disappears for the immediate future then appears again the next day!

That doesn't sound like a bad component, that sounds like a faulty connection, which can include such things as cutout contacts in insert jacks and such.

[nosaj]

[OwenM]

That doesn't sound like a bad component, that sounds like a faulty connection, which can include such things as cutout contacts in insert jacks and such.

Hmm, I know what you mean, having chased down many of those sorts of problems in the past, but this feels different. It's just in the power supply, and I'm not touching or moving the unit while power cycling it, if I just keep turning it off and on the problem works it's way out (booting faster each time) and the unit powers up and the whine noise stops. Theres nothing plugged into the various audio sockets, nothing apart from the power cable and the fault happens directly in the PSU, before reaching any of the ribbon cables or non-solder contact points etc. The PSU board solder joints look fine. Given the units age and the fact that so far 9 electrolytics are confirmed bad I think I am suspecting the suggestion The Dude proposed is the first thing I'll look at. If you leave it whining for 30 seconds it seems to stabilise and turns on. Worth changing those electrolytics anyway as the rest of em were bulging when I opened it!

[OwenM]

Well I think I've found my culprits, well at least until I unearth the next problem...



They were next to a particularly hot part of the board, they tested bad on my proper dmm as well just double checked on the diy esr!

[OwenM]

Ok, so I decided to recap the PSU, it's a 20 year old expensive and heavily used unit that runs real hot. Spending £15 on caps made sense! So they are in and since then the PSU has always powered up, no more whining and hanging when powering on. I think it was probably the two little ones I found to read particularly bad, but some of the others, especially the red Dubilier ones, were starting to drift away.

Neve also put me the way of an Amek expert who worked at the company in those years and still works as a service tech on their desks, amongst other things, and he was kind enough to send me a schematic, attached below for anyone who may need it!

AMEK PUREPATH DMCL SCHEMATIC.pdf

Now onto the inevitable question! I seem to have fixed the most pressing issues, not turning on, distortion on chn 2 and digital card non existent. Now the two remaining problems are the output on the digital card is very low and it doesn't seem to want to clock to anything (though I haven't got much to test that with), and theres a little resonant hump when I first engage the low pass filter, thats not a bother really though.

I've scoped around and all the voltages are pretty much there to .2v on the 5v source and about .5v on the larger sources. Theres an unusual pattern on most of the voltages sources though, clearer on the larger 21v and 17v test points. It's essentially about 40mV of ripple peak to peak, drawing the pattern shown in the photos below. Does this look weird, or can ripple sometimes look like that? Should I be bothered about this much ripple? I've seen those that are seeking less than a mV and those that say 100mV is no bother! And if I have some excess ripple what else may cause that in a power supply after the caps? I'd like to address this first and hope the digital card plays nicely once it's getting nice clean power!

I'm a little nervous because the all Dubilier ecaps I have came across so far had started to go bad, and there are about 100 of them in this thing!

Thanks as ever for anyone enthused to answer!