Need some electronics help!

[hairy]

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?

Yes, that's correct.

Unless your multimeter is NOT a digital one, don't worry too much about range and polarity, it will simple display opposite sign or display over voltage. Nevertheless, try to use the black probe on GND and the red one on V+ and V- pins. Mine has ranges like 200mv, 2V, 20V, etc., in this case the correct range would be 20V. Just choose the first available range that's higher than the voltage you're going to measure.

Just looked at the datasheet again, we're lucky since all three pins are located on three corners, this way even if the chip is SO package, it should be farly easy to push probes over the pins from the respective outer side, without the risk of making shorts.

This is the complete pinout:

1 C1-
2 C2+
3 C2-
4 V-     <= red probe here should read something near -10
5 GND   <= black probe here for both measures
6 Vcc
7 C1+
8 V+     <= red probe here should read something near +10

The line on the chip is the "front" of it. Looking toward the front, pin 1 is the top-left one, pin 4 the bottom-left, 5 bottom-right, and 8 top-right.

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

[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.

[hairy]

One last detail crossed my mind: due to the way those kind of regulators work internally (by slicing the 5V power and then rectifying/multiplying to both polarities), the external capacitors play a fundamental role in obtaining correct DC levels on the outputs.

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.

Being able to check caps in circuit was suggested, and that would be really cool, maybe for the display issue too.
But I don't have the faintest idea on how that ESR meter works. The only thing you could try with a simple multimeter would be to check if any capacitor failed to "short", though I think it's very rare: I had many of them blown on a PC gfx card, and the thing still worked albeit much more unstable, sign that they were failing to "open".

[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.

[TCMSLP]

It was me who mentioned checking the capacitors in circuit.  An ESR meter will a) check the actual useful capacity of a capacitor and b) check the (effective?) series resistance.  Essentially, it'll tell you if a capacitor is good or bad.  

http://en.wikipedia.org/wiki/ESR_meter

It's a very useful tool to have but does cost about £70 here in the UK.  I'd suggest borrowing one - I'd loan you one myself but it's the one bit of test gear I haven't yet bought.

I'm assuming the 'regulator' is actually some sort of switched power supply, generating +/- rails from whatever the input voltage is.  In this case I'd expect the capacitors to be quite fundamental, plus quite likely an inductor or two.

However, I imagine failed capacitors will simply prevent the regulator from getting up to voltage.  The only exception to this would be if they were internally shorted.  You can test for this by placing the multimeter on 'resistance' (or conductivity test) mode and placing the probes over each capacitor.

You'd expect (assuming the kit has been powered off for some time) each capacitor to initially show a very low resistance (practically a short, 'continuity test mode' would probably initially emit a beep), this will slowly (or quickly depending on cap size) increase as the capacitor charges from the current supplied by the meter.  Eventually you should read a very high resistance.   If the resistance continues to stay low or the continuity test continues to emit a beep then something isn't right.

However, this test will be effected by other components in circuit so can only provide a rough indication.  An ESR meter would be the best bet.

Whilst on continuity or resistance mode, you can test diodes.  They should read high resistance (no conductivity) one way, low resistance (conductivity) the other.  Again, results can vary greatly due to the surrounding circuit so results can be vague or change (as surrounding capacitors charge/discharge etc).

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


Steve / TCM

[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).

[Tenacious]

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 batter this one has in it).

Not in a power supply circuit, like yours.  TCMSLP gave a pretty good description of what you should see testing for the various components.  Sometimes you can "lift" only one leg of a 2 legged component, without removing it entirely for test.

You can damage the circuit under test IF it is powered and your meter is in current mode while you are trying to read voltages.

You can damage the meter IF the circuit is powered and the meter is in resistance mode.

In general, if electronics get wet, but, are not powered ON until after completely drying out (and conductive residue removed), they will usually survive.  If powered ON while still wet, destructive currents can have pathways to sensitive components far from the power supply, wiping out many components.

[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.

[TCMSLP]

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.

The data sheet here (http://datasheets.maximintegrated.com/en/ds/MAX680-MAX681.pdf) indicates which capacitors are related to which charge pumps (the + or -).

I'd perhaps be tempted to lift the legs of the outputs (or cut tracks) and ensure the basic voltage pump is working correctly;  There could be something else downstream causing problems.   However, if you haven't replaced the capacitors, looking at the datasheets, this would be the next step.  

I wouldn't give up just yet

[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.