Need some electronics help!

[Ral-Clan]

Hi Amiga folks,

I am a long time Amiga user, but I am actually working on another electronics repair project that I need some advice on from the more experienced electronics people here on this forum.  It's actually similar to the capacitor replacement problem / battery damage problem Amigas have.

I'm fixing a 1992 (Amiga era!) Yamaha QY20 music sequencer.  I got it really cheap, but it doesn't function (I already knew this).  There is nothing on the display and there is a light hum through the headphones, but it is otherwise unresponsive.

I opened it up and noticed that a chip (labelled MAX680) was covered in white stuff and looks suspicious. There was white stuff around the chip and the legs seemed covered in white "fuzz" as well.  This seems to be a voltage converter chip.  The Yamaha service manual says it is a "MAX680CSA (XL660A00)".


(click on this photo to enlarge it)

On the opposite side of the circuit board, above it, are four capacitors.  They are all rated the same - they say "x2 22 16V" on them.  The service manual says these are electrolytic capacitors. These look okay, but I'm thinking they might have leaked and damaged the chip underneath (there are a few "through" solder holes nearby in the board where bad stuff could go through).


(click on this image to see a larger version)

So...I am going to try and replace all four capacitors and the chip.  I am actually a pretty good solderer and and have replaced fine components before when I had instructions to follow.....but I am NOT an electronics expert.  I can do the mechanical work of replacing the components, but have never bought components online before.

So this is where I need some help from the experts here.

1) Can anyone help me identify these components to make sure I am going to purchase the correct replacements?  I think the MAX 680 chip is this one:

http://www.futurlec.com/Maxim/MAX680.shtml

But I'm having trouble finding a source for the capactors.  I don't really know how to read the numbers printed on the capacitor and I don't know what specific type of capacitor I'm looking for.

2) Is there a reliable place to buy these components online?  Preferably one where I can buy individual components?  I have looked on Jameco, but I don't think I can get the chip I need there.  

There seems to be a lot of the MAX680 chips on eBay:
http://www.ebay.ca/sch/i.html?_from=R40&_trksid=m570.l2736&_nkw=max680

They all have the MAX680 number, but the code afterward is not CSA as on the original chip.  I'm not sure if this matters or not.

And I THINK this is the capacitor I want:
http://www.ebay.ca/sch/i.html?_from=R40&_trksid=m570.l2736&_nkw=22UF+16V+SMD+ALUMINUM+ELECTROLYTIC+CAPACITORS

Can anyone confirm I have the right products picked out?

Any other advice is welcome.

Thanks very much guys!

[Ral-Clan]

:nervous:

I think dead components are the least of your worries if this is a multilayer board..

Yeah, but it's worth putting a few bucks into it just to see if it will work.  I think it is not a multi-layer board, based on the service manual.  PS: I am fairly confident I can repair any damaged traces.

[Ral-Clan]

I am curious as to what your procedure is for repairing damaged traces.

If it's a broken trace on the surface of a circuit board, you can take a fine copper wire filament (like from a pair of headphones).  Sometimes you might have to separate filaments in a multifilament wire and just use one strand (if it's a very fine trace).  Cut it to length to bridge the gap. You need to tin both ends of this.  Or if it's a very short "bridge" you can tin the whole length before you cut it off the original wire it came from.

Then you lightly scrape away the varnish/coating on the circuit board to expose some copper/gold trace on either side of the break. If you can tin this, it is good, but hard to do. If the tinning solder won't stick to the traces, you can try brushing some flux on these exposed areas.  The solder might stick then.

Next place the short bit of filament across the gap, with the ends touching the two exposed areas of the traces.  You might need to hold it down in the centre with something sticky so it doesn't roll or blow away.  Unfortunately, when working with pieces of wire this small, metal tweezers act as a heat-sink - so you cannot use them.

Touch the soldering iron to either end of the filament bridge and - HOPEFULLY - it will bond with the tinned exposed traces - repairing the break.

After testing with a multimeter to ensure continuity across the repaired trace, you can re-insulate the trace by "painting" the repaired area with clear nail polish or shellac.  Give it several coats.

That's it.  It's very tricky.

I've had success with this method with reasonably sized components.  i.e. 8-bit era stuff. When it gets extremely fine (like nowaday's electronics) it's obviously more difficult (sometimes very much so), but it's doable.  I also use a 5x and 10x jeweller's loupe.

[Ral-Clan]

The Max680 you're looking at it is not correct. The one in the photo is SOIC, you're looking at DIP chips.

Do you have an SMT rework station? I use an Aoyue 8208 and it's cheap but awesome - I'd strongly recommend against too much SMT work with a soldering iron

Thanks!  This is precisely the kind of blunder I was hoping to avoid.  Like I said, I can replace the parts without to much difficultly as long as I get some help identifying the correct part.  So I'm grateful.

Yes, I don't plan to replace any more than I've stated above - it's too difficult to attempt much more SMT work by hand.

[Ral-Clan]

The corrosion looks like it is from water with the power ON.  You have probably already tried simply cleaning all of the corrosion with a fast drying, non-residue solvent and a tooth brush.  Try to work the bristles behind the pins and under the chip to get it all.  All of that fuzz may be conducting small currents between the pins of the chip that aren't supposed to be there.

If cleaning is not enough, I would definitely replace the chip first and wait and see about the capacitors.

Thanks for the tip.  I do suspect that this electronic device has been exposed to water....perhaps a house flood.  There was a small amount of silt inside (not too much, just a trace) which make me suspect this.

I only paid $20 for this item, knowing it was dead - taking a gamble to see if I could fix it.

I did not realise that the crud on the circuit legs could conduct current.  I have cleaned it but I haven't tested it since.  I will have to do so.

[Ral-Clan]

Have you tried, referring to the pinout on the datasheet, to measure the +10V/-10V output from that chip using a multimeter?

Here's where (embarassingly) I have to profess my ignorance.  While I am a fairly good soldering and tinkering, and have repaired electronics and swapped parts where I had instructions, I am not an electronics expert (only knowing what I have learned myself).  I have a multimeter (a decent one) but I do not know how to conduct the measurement as described above.  Could you give me a step-by-step?

Am I correct in that, with the device powered on, I would touch one multi-meter probe to the circuit leg, and the other to ground to test if the proper voltage is coming out of that leg?  The multimeter would be set to measure DV voltage.  Is this correct?

That's fascinating to hear the QY20's samples were used in GMPlay for the Amiga - great Amiga connection!

I appreciate all the help I am getting on from members contributing to this thread!

[Ral-Clan]

Sorry if it's a bit pedantic, but at least I hope it's understandable!

Perfect!  Pendantic is exactly what I need.  I will test and get back.

PS: I did try testing it after simply cleaning the oxidization off the chip and it still wouldn't start up.  There is a little audio "thump" in the headphones when powering up, and when powering down the LCD display randomly shows black lines on the screen for a split second.

Okay, off to test the voltage of the chip with a multi-meter.

[Ral-Clan]

Okay, I've tested the MAX680 chip as directed.

The results were not to spec.

With the Multi-meter set to DC and 20V, and the black probe on pin 5 (ground), this is what I got:

PIN 4 (V-): -1.65V
PIN 8 (V+): +4.96V

That seems way out of spec to me (as it's supposed to generate 10V -/+ from these pins).

I think it's time to buy a replacement MAX680 and see if that fixes things.

[Ral-Clan]

If you're going to replace both the caps and the IC, you could start with the caps and recheck the voltages before replacing the IC also.

Well, I had resolved to just replace the regulator.  It's on it's way from Digikey and I've already removed the original.

If I put the new MAX680 regulator in without changing the capacitors - and the capacitors are indeed bad - would it damage the new regulator?
The reason I say that, is that this surface mount stuff is very tricky to remove, and I'd rather not remove more components than I have to.

If faulty capacitors won't likely damage the regulator, I'll install it first.  If it doesn't fix the problem, I'll replace the capacitors.

While I am waiting for the parts from Digikey to come, is there anything that I can check simply with a multimeter?  I see there are lots diodes and resistors on the board.  I'm guessing that these are things I can check with a multimeter without damaging anything.  Is that correct?

It would be great if it's simply the regulator problem.  If the actual ROM chip has been damaged, then that's going to be impossible to replace.

[Ral-Clan]

It was me who mentioned checking the capacitors in circuit.  

I'm only an amateur - someone who has more experience with repairs may be able to offer more useful advice
Steve / TCM

Thanks for all the tips.  I'll give it a try.

It would be great to have all this testing hardware.  Unfortunately, I'd quickly spend far more than a working Yamaha QY20 would cost on eBay.

Too bad there wasn't someone nearby that had such a piece of kit.  But I will use the multimeter test and try and interpret the results.

The advice on this thread is helping clarify a lot of the "blurry" knowledge I had.

Just one question, can the voltage a mutimeter supplies in continuity mode (resistance mode) damage a component?  I'm assuming not, as I think it is very low (can't remember exactly what battery this one has in it).

[Ral-Clan]

So I got the package from Digikey with my parts.  I was shocked at how fast they shipped it from the US to Canada (something like three days).

Funny thing was, I only ordered the tiny, tiny MAX680 and a four foot length of heat shrink tubing just to have around, and they shipped it in this HUGE four foot long triangular cross section box full of a bundle of paper padding longer than my leg and almost as thick!  Oh well, great shipping but I wonder how they can make a profit!

The only problem about the chip was that they sent me the wrong part.  Instead of the MAX680, and 8-pin chip, I got an Analog Devices AD7819YR (a sixteen pin chip).

It's some sort of 8-bit A/D sampling chip.  Here's the datasheet:
http://tinyurl.com/b2dzp44

I called Digikey and they are shipping me the proper chip and I don't have to return the original.  I wonder if the A/D converter chip it could be used for a fun project.

On another note, I tried using a multimeter to test the diodes, but the readings were too hard to read (probably affected by other components on the board).  A lot of diodes read no continuity either way - they can't all be bad so I figure it was probably just problems me trying to read them while they were installed on the board.

[Ral-Clan]

I finally got the MAX680 chip in the mail - once again DigiKey was SUPER fast with shipping to Canada.  I couldn't believe it (two days).

I installed the new chip.  Unfortunately, it didn't fix the problem :-( .

Strange thing is that I measured the voltages on the pins that we discussed above.  They are now different.  Giving me +0.45V and +6.5V.  Very odd, and even more odd that the pin that is supposed to give negative voltage is giving positive voltage.

So, I guess this QY20 has bigger problems than that single chip (or just that the problem lies elsewhere or in multiple faulty components).  It's nothing I can see, nor test due to not having the equipment needed to diagnose this.

But I will keep the QY20 around.  Perhaps one day my electronics knowledge will increase the point I can repair it.  I can at least try and swap parts out one by one as I find replacements salvaged from other gear (I won't be buying any more replacement parts).  

Maybe ONE day it will revive!

Thanks, everyone, for the help.

[Ral-Clan]

Did you try replacing the capacitors?  The data sheet for the MAX680 shows the 4 (?) capacitors are directly related to the generation of the appropriate voltages.

Okay, the capacitors will arrive in a few weeks (shipping from China, apparently) so I will try replacing them then.

As long as it insn't a problem with any of the Yamaha custom chips (which would be very hard to source) then eventual replacement of all the capacitors, diodes & resistors will hopefully find the solution.

A bit of blind trial and error, but this might be long term project as I find free replacement parts in salvaged components.