Rane MP24 -- Smoking Resistors on Powerup...

[rs_]

Hi, I bought an early Rane MP24 DJ Mixer today for a pittance since it needs repair.

Here is the Service Manual for my exact model:
http://www.rane.com/pdf/old/mp24lcsch.pdf

It powers up, and Power LED D26 (as seen on last page of Service Manual) lights up -- but resistors R102 and R104 start smoking after a few seconds. (R102 and R104 are associated with Phono Input 1. They can be seen on Page 1 of the Service Manual on the PCB Layout, and on the first page of the Schematic Views, which are towards the end of the document.)

I replaced the power rails C34 and C35 (last page of Service Manual) since I had some replacements handy. The diodes in the Power Supply (note that this unit does NOT have the external Rane Power Supply) test fine.

I tried removing all the ICs (they are socketed) associated with the smoking resistors, and the smoking persists.

I'm new to this kind of thing but wanting to learn. What is the best place to start investigating?

Thanks for any suggestions,
rs

[Enzo]

I can't blow the layout up enough to read the numbers without it turns to blur.

Are you sure you are looking at R102 and R104? R104 for example is a 100k resistor. Even if it had 18v dead across it, that amounts to less than 6 milliwatts of dissipation, not enough to heat the part. COnsidering the ICs run on +/-18, that is about all we can get across it. Same with R102, 88.7k in the feedback loop of an op amp, just really hard to imagine a way to get it to burn up.

In case there is a typo, follow the copper trqaces from say R104 and tell me what the two ends connect to.

[rs_]

Hi, yeah -- the blur is making it challenging.

Here are the smoking resistors. All of them can be seen on the first page of the schematic.

1) If you look at IC Z10 on the PCB layout and head due "northeast", there are three resistors arranged vertically. The topmost one does not have a number designation and is simply labeled "100".

2) If you look at IC Z12 and head due "northeast", there are three resistors arranged vertically. The topmost one does not have a number designation and is simply labeled "100".

3) If you look at IC Z5 and head due "west", there are two resistors arranged vertically. The bottom one does not have a number designation and is simply labeled "100".

Does that help you? Thanks so much.
rs

[g1]

Those resistors are not shown on the schematic, however they are referred to in note #5 on page 7 of the manual.
You said pulling the associated IC's does not stop the problem, so I would guess the 47uf's are maybe tantalum and shorted.
Try removing those caps, (circled and marked 47 near each of the affected resistors on layout drawing). If the resistors stop burning, replace the caps (if the caps are bad they will probably also read a low resistance on your meter).

[rs_]

Sure enough, removing the caps stopped the burning.

I removed the (3) affected caps, then once I had a fourth popped up, and then once that cap was removed then no more burning.

What would have caused such a thing (provided this was the true affliction and not a symptom)? Some kind of short or power surge?

Thanks,
rs

[g1]

What is the voltage coming out of the 18Volt regulators (Z16 and Z17)? Should be + and - 18 volts. If it were too high (bad regulators) it could be cooking the caps.
Are they tantalum caps? (look like beads rather than small cans) If they are tantalum and 4 have already gone I would replace them all with can type electrolytic.

[Enzo]

What g-one said...

You have the pair orf 100 ohm resistors at each IC per the note. If you loook, nearby will also be two circles with a 47 in them. Those are the caps.

I hate tantalums, they have no tolerances for anything outside their specs.

But don't kid yourself into thinking there will always be a THIS caused THAT when you find a bad part. Some voltage surge 10 years ago could have weakened those tantalums. An afternoon in the trunk of the car on a HOT AUgust day might have cooked them, and they failed months later. YOu will never know why, just that they failed and need replacing. It is rarely something anyone "did" to it.

[rs_]

Voltage appears fine.

The caps were electrolytics and I will replace them with the same. Are the values super-crucial?

I'll replace them and provided everything tests fine, I'll call it fixed.

Thanks, rs

[Enzo]

SO each IC has a 100 ohm in series with each power rail. You want them the same, but if you had no 100 ohms, I'd have no problem with 120 or 150. Same with the caps. 47uf, lacking those, I am sure 22uf 33uf or 100uf would probably work as well. This is Rane which tends to engineer a lot, most similar products wouldn't isolate the ICs at all.

[rs_]

That was awfully nice of them to over-engineer it.

I've been extremely impressed with the build of this thing in general. Somehow I thought it was "cheesy" because of the prices they go for. The faders are nice and it uses ALPS Blue Velvet pots for the 4-Band Program EQ.

I bought a broken one because I had some extra Blue Velvets and nice opamps lying around...and I'm going to perform this mod:
Mischpult Modifikationen - Seite 2 - DJ - Deejayforum.de

[Enzo]

yes, compare the nice Rane product to some cheap Numark DJ mixer. No comparison.

There is a chain of nite clubs over here that has discos, and they used to buy some DJ mixers from Biamp. I thought they were nice mixers too. Biamp got out of that game a long time ago though.

[Mark Black]

One of the real killers for the wonderful old MP24's are the lovely ALPS 80mm slide pots, no longer available AFAIK. Definitely over-engineered - in a good way.

Oh by the way - check/reflow all the solder joints at the PCB interconnect pins & sockets. That was good advice I got from a Rane tech many years ago and proved to be valid. Along with that I usually found cracked solder joints at many of the pots and jacks, and occasionally a jack pin that was broken off and making intermittent contact within the solder joint.

[g1]

Very good quality mixer.
One other common fault with them was muting Fet's. The linked schematic shows them as MPSA17 (bipolar) but many had Fet's (2N5638 ?), probably the earlier versions.
Anyway, a bad mute Fet could pull down the control voltage so all mute Fet's would turn on and all outputs would be muted.

[rs_]

Yeah, one of the other parts that needed replacing was a PCB interconnecting Molex connector. Rane sent me one for free. They also suggested that I remove some FETs on the Master Output entirely. Maybe it was the ones that caused the muting problem. My new rotary faceplate is about to arrive:

[zeus1]

Voltage appears fine.

The caps were electrolytics and I will replace them with the same. Are the values super-crucial?
Thanks, rs

Values are not much crucial you can use near values. But when replacing resistors try to replace them from original material types otherwise there might be noise problems.