H3000 help? Getting closer, but still can't pinpoint

[soapfoot]

I've got an H3000 that a friend found in the trash. It powers on and the display is normal--it loads patches, soft keys work. Passes audio in bypass. Input signal does not meter, and doesn't pass audio when engaged.

After much digging, something is pulling the +15v supply rail down, dragging the +5v supply rail with it (both show only a few fractions of a volt at their output). The -15v and -5v rails seem perfect.



Nominally +18v does appear on pin 1 of U7, so the fault appears to be downstream of the regulator.

I've replaced U17 and tantalum C71. The fault still exists, with pins 2 and 3 of U17 showing a short, but would welcome other leads/insights.

For testing, I removed C72 and U20, and the fault persists--this leads me to believe that the +5v portion of the supply is likely fine (is that a fair assumption?). I reinstalled C72 (ceramic) and a fresh 78M05 (both old regulators tested normal once removed from circuit).

My next step was to identify on the schematic all destinations of the +15v supply to look for faults. My preliminary list included 22 ICs and about 15 tantalum caps (I focused on these components as most likely to cause a short).

Some of the ICs are socketed, which made testing without those easy. No result, though--the short persists. I left the unsocketed ones alone for now.

Proceeding to the Tantalum caps, i went through and measured across them in-circuit looking for shorts. Several times (three in particular: C129, C118, C106... schematic excerpts below) showed a short in-circuit, but once removed were verified good (and the short persisted across the empty pads).

As part of basic service, I replaced the large electrolytic filter capacitors, and since I was having issues I replaced the Schottky bridge (CR1) and the 1A bridge rectifier (I knew this was the longest of shots, but since I'm having power issues and I had the parts on hand and it was easy).

On my first examination of the unit, I noticed that a tantalum capacitor in the -15v supply (C74) had blown itself apart. I replaced it before even powering up, and the -15v rail seems absolutely fine, so this is likely unrelated... still, I thought I'd mention it.

I also noticed that at least one of the Murata filters is leaking its dark brown goo all over everything. I cleaned up the board as best I could... should carefully removing these rare/unobtanium parts be my next step? Is there a way to test non-socketed ICs for shorts without removing from circuit? Or is there something else I should try next? Thanks in advance!

 

[soapfoot]

Complete service manual attached

 

[Ike Zimbel]

Yes to removing the filters. It could be a short in one of them. FYI, it's normal for 78/79xx regulators to put out about 0.5v or so when there's a short across their outputs, they're in a self-protect mode. So, you definitely have a short. In my experience, shorted IC's are rare, but do check the soldered ones carefully for signs of cracks or bulging or similar.

 

[soapfoot]

Thanks so much! I'll remove those filters to test now. Looks like someone is making reproductions now, but they're not cheap.

 

[soapfoot]

Yes to removing the filters. It could be a short in one of them. FYI, it's normal for 78/79xx regulators to put out about 0.5v or so when there's a short across their outputs, they're in a self-protect mode. So, you definitely have a short. In my experience, shorted IC's are rare, but do check the soldered ones carefully for signs of cracks or bulging or similar.

Okay, so I removed all the filters one at a time and checked. They're all out now, and I still have a short.

Good to rule that out, but now I have a decision to make... pay about 700 bucks for a new set of filters for a unit I've never heard with a still-undiagnosed problem? Or put the old ones back in only to likely have to replace them later anyway?

I'm leaning toward the latter--I managed to remove them without breaking any of those tiny, fragile pins--but I'll leave them out for now until I can find the short.

 

[mjrippe]

If you have a good multimeter you can often find a short by putting the neg lead on a solid ground point and tracing through the circuit with the pos lead. It may go from >1 ohm to <0.4 ohm or similar when you get close. There is a thread about it here somewhere...

 

[TheJames]

Proceeding to the Tantalum caps, i went through and measured across them in-circuit looking for shorts. Several times (three in particular: C129, C118, C106... schematic excerpts below) showed a short in-circuit, but once removed were verified good (and the short persisted across the empty pads).

Might I suggest you go back to those points and look at nearby components? If you had a strong short near C129, C118, and C106, while those components themselves might not be bad something close to them is.

 

[living sounds]

The filters are only op amps, resistors and capacitors potted. If one of the op amps in there overheated it probably got hot enough to melt away some of the "glue". You can remove that substance and replace the faulty op amp:

https://groupdiy.com/threads/lexicon-murata-filters-de-potted-detailed-picture-included.60993/
The circuit for the filters (or something equivalent) is in the Lexicon M200 manual (the one using the 1.2nf caps:

https://gearspace.com/board/geekzone/57629-lexicon-200-schematics-service-manual.html

 

[living sounds]

Once you solve the power problem, I would suggest to insert a simple passive RC filter instead of the Murata filters and see if the unit passes signal correctly. You can then put the Murata filters in one after the others. Chance is they are not even broken, but simply got too hot to keep the glue in (mounting an H3000 without allowing it sufficient "breathing space", especially in an un-RCed hot studio room might do that).

 

[Ike Zimbel]

If you have a good multimeter you can often find a short by putting the neg lead on a solid ground point and tracing through the circuit with the pos lead. It may go from >1 ohm to <0.4 ohm or similar when you get close. There is a thread about it here somewhere...

Lots of good ideas in this thread. I will just add that while tantalum caps are known for failing short, other caps can also fail short, so you should check ALL de-coupling caps, regardless of whether they are electrolytic, ceramic, tantalum or film.

 

[soapfoot]

I think I've found it! C148 (10µF Tantalum) shorted.

With it removed from the circuit, no more short on U17.

I've replaced it, but in checking other stuff I cracked two of the .1µF axial ceramics, so I'll have to place an order.

 

[Whoops]

I think I've found it! C148 (10µF Tantalum) shorted.

With it removed from the circuit, no more short on U17.

I've replaced it, but in checking other stuff I cracked two of the .1µF axial ceramics, so I'll have to place an order.

I came late to the party, but while reading your first posts I wanted to advise you to replace All the Tantalum caps, without even thinking about it, replace all, shotgun method.

I'm glad you found the culprit, but now that you found it, I advise you anyway to replace all the tantalum caps. You already had 2 that failed, one that blown in the negative rail and one that was shorted in the positive rail. If 2 failed the others will fail also sooner or later, if they are not bad already.

Have fun with it, hope you make it work, keep us posted

 

[living sounds]

I've often read the opinion that dry tantalum caps are OK if they have a sufficient voltage margin. They will usually only go bad if an overvoltage due to a malfunction somewhere else occured. If I remember correctly the H3000 uses 35V tantalums on the +/- 15V rails, so they should be OK (as long as the above holds true).

 

[Whoops]

I've often read the opinion that dry tantalum caps are OK if they have a sufficient voltage margin. They will usually only go bad if an overvoltage due to a malfunction somewhere else occured.

I have replaced many shorted Tantalum capacitors over the years in equipment were overvoltage never occurred.
Some old Tantalum types/brands just became unreliable over the years.
As most of the time they fail short circuit, having a Tant fail can be quite destructive and you want to avoid that as much as possible

If I remember correctly the H3000 uses 35V tantalums on the +/- 15V rails, so they should be OK (as long as the above holds true).

How can you state that the caps “should be ok”?
2 tantalum capacitors already failed, no signs it was overvoltage, so how do you know others will not fail soon?

We don’t, we can only guess the Tant capacitors in the unit became unreliable after all these years and they will continue to fail.
New Tantalum capacitors will make the unit more reliable

 

[Script]

I would eventually replace the tants, that is if unit is worth a repair.

 

[living sounds]

That is simple not correct, I have replaced many shorted Tantalum capacitors over the years in equipment were overvoltage never occurred.
Some old Tantalum types/brands just became unreliable over the years.
As most of the time they fail short circuit, having a Tant fail can be quite destructive and you want to avoid that as much as possible




How can you state that the caps “should be ok”?
2 tantalum capacitors already failed, no signs it was overvoltage, so how do you know others will not fail soon?

You don’t, we can only guess the Tant capacitors in the unit became unreliable after all these years and they will continue to fail.
New Tantalum capacitors will make the unit more reliable

I wasn't talking about the above case, of course. In a unit where tantalums already failed I agree - replace all of them. On the other hand, my H3000 is a really early (old) one and the tantalum caps as well as the Murata filters are fine. But it looks like my unit always received enough ventilation (though some of it apparently was cigarette smoke... ;-).

 

[soapfoot]

I’ve already replaced quite a few of the tantalums in the course of troubleshooting; might as well do the rest.

In my unit they’re 25v parts on 15v rails

 

[FIX]

When they short, they are like a piece of wire. If their is enough current, they will flare like a small torch. Always check them first. They are a better cap, but when they go, they can cause a fire at most and a hole in the PCB at least. I just never use them because of that.

 

[Ike Zimbel]

It should also be noted that there's a big difference between using tantalum caps for power supply decoupling, as in this case, where they can and will fail short, and using them for signal coupling, like in the Neve 1073 et al. Has anyone encountered shorted tants in the latter application? I don't think I have.

 

[soapfoot]

Any disadvantage to using aluminum electrolytic caps to replace the tantalums?

plenty of 10uF with 2.5mm lead spacing seem to exist