MX800 left channel fault (2024)

Hi all

Long time reader first time poster – thank you all for this amazing resource.

I have a Yamaha MX800 that has had the left channel die. On opening up there is visible damage to Q315, R367, C309, C311(was smoking) & C313. I have tested all components against the right channel and can’t find anything out besides the visibly damaged components. I have removed the left amplifier board and the right channel runs fine without it (neither channel works with it installed).

A few questions;

• I’m going to replace all electrolytics, and have found 2SA1606 and 2SC4159 as substitutes for Q313 and Q315 (2SA1306 and 2SC3298), is there anything better suited or more readily available out there, and do I need to replace both channels in tandem?
• Given the pattern of damage, are there any other likely damaged components I should be looking to test/replace?
• If I was to replace the output transistors, what would be a good substitute/upgrade to 2SD2053/2SB1362.

I know this is probably a job for a professional but I love tinkering and get as much enjoyment out of reviving old gear as I do listening to it, it is a fairly beat up example so not too worried about keeping it immaculate.

Photos attached - any help appreciated.

Thanks for the photos, I just wish they were not at an angle and were a bit higher res or maybe better focus? The purpose is to visually check the integrity of solder connections and pick out any potential cold/cracked joints/traces.

ro_wilk said:

and have found 2SA1606 and 2SC4159 as substitutes for …

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You need to keep in mind that at least based on the data sheets, 2SA1606 is a 15W part whereas 2SA1306 is a 20W one, it may or may not matter in this case but still. They are equivalent in all other respects, again according to data sheets. Does the 2SA1306 get really hot on the R-Ch? I haven’t searched for a better replacement, someone else might know. Personally I think 2SA1606 will most probably be fine.

ro_wilk said:

Given the pattern of damage, are there any other likely damaged components

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Sounds like you’re not seeing any fuses blow or resistors fry upon power up so, I think it’d be best ( this is what I’d do) to inspect/reflow all the solder joints on the L-Ch, R-Ch, and every other board. This is to eliminate solder issues before you start your repair work. Last thing you need is to wonder if any malfunction is due to a component or it’s solder connection. This is what I recommend to everyone, including myself . Again, someone with more intimate experience with this amp might have more specific recommendations.

With the L-Ch out, it’s also a good chance to check you power supply voltages that feed the two boards (L & R) prior to reconnection, I’m assuming you have a schematic.

ro_wilk said:

I’m going to replace all electrolytics,

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See Also

8 Most Common Yamaha MX825v EFI Problems With SolutionsTroubleshooting Yamaha MX825V EFI Problems: Common Issues and SolutionsYamaha Mx825V Efi Problems: Troubleshooting & Solutions

Please do this after the correct operation of the amp has been checked/established, and in stages so you can double-check nothing has been broken in the process.

Hi Sparkplug, thanks for the help. have been chipping away at this for the last few months and have found your advice helpful.

To clarify, the amp is blowing fuses with the Left channel sub board fitted. but right channel works fine without it installed. Have started using a DBT to test.

I have reflowed every joint i can access on the main and left board (checking carefully for bridges) and have replaced the following (all genuine and from reputable stores);
all electrolytic capacitors on the left channel
D333, D309, D311
Q309 with KSA992
Q311 with KSC1845
Q313 with 2SA1306 (direct replacement)
Q315 with 2SC3298 (direct replacement)
Q317 with 2SD2053 (direct replacement)
R345
R343
R371
R351
R367
R319
R321
R309

And on the main board; Q513, Q515 and R531.

However the DBT still glows with the left channel installed. I checked the voltages on the connecting PINs with the left channel out and they seem consistent between L & R.

So i still have a short somewhere. A few questions;
- How does one check for 'railed' components? I cna't seem to find any shorts to ground but have heard this referenced else where on the forums.
- Does the fact that it only shorts with the L channel board installed isolate the fault to this PCB or could it still be somewhere on the mainboard?
- I have checked all resistors, zeners and power/driver transistors. is there anything else i'm missing? films caps etc?

This was a helpful thread also and a very similar issue to me. https://audiokarma.org/forums/index.php?threads/mx-1000-blown-left-channel-help.989935/

Thanks!

ro_wilk said:

Does the fact that it only shorts with the L channel board installed isolate the fault to this PCB or could it still be somewhere on the mainboard?

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Probably but not necessarily, the fault could also be from somewhere on the power supply board that feeds power to L-Ch board. I’d check out power supply board wiring/soldering as well. It’s probably a silly short somewhere … LOL

I found the short! Did the C-E continuity test accross the power trannies and noticed a short on a few of the 2SB1362s. I pulled the one closest to the initial damage and tested it, looked all ok except for the C-E short, I must have missed this step on the initial test. With this pulled the apparent short on all the others went away.

I also tested the resistance across all the I/O pins and all is now OL. How immensely satisfying.

Have ordered a replacement SB1362.. watch this space.

ro_wilk said:

Have ordered a replacement SB1362.. watch this space.

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From where ?

Appears well reviewed by other AKers

Just in case.. SANKEN complementary pairs 2SA1492 & 2SC3856 Beefier than the OEM transistors.

I have not vetted Consolidated. Could be legit. But I have vetted B+D . My source for genuine Japanese transistors.

Update here - I put it back together and fired up on the DBT, all looked good with voltages within spec at the pins on each channel, and bias MV not far out between channels- I got it very close to 10mv on each with the pots at a similar level. I was testing the bias (off DBT) at TP R568 when another of the original output trannies on the left channel (SB1362) shorted out. I'm not sure if i accidentally touched something with the probe or if it popped on it's own accord (possibly avoided if i had replaced them all with the Sankens?). Luckily i had another on hand so have replaced and reassembled and it's again working ok on the DBT.

A few questions before I attempt to adjust bias again to ensure i'm doing it right;
- I noticed the bulb in the DBT brightened as I turned up the bias on each channel, i guess this is normal as it would draw more current?
- Can i adjust bias on the DBT? Presumably this will result in lower than actual readings.
- Is it ok to adjust bias when turned on, or best to test and turn off to adjust?
- Are there any other tests i should do to ensure the amp is operating within spec?

Thanks

ro_wilk said:

Can i adjust bias on the DBT? Presumably this will result in lower than actual readings.

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You shouldn’t make adjustments while on DBT as it pulls your voltages somewhat lower than actual and cause inaccurate settings. Once you ascertain there are no shorts and the unit operates normally, you can go off DBT and make any adjustments.

ro_wilk said:

Is it ok to adjust bias when turned on, or best to test and turn off to adjust?

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Any adjustments should be made while the unit is turned ON, follow the SM instructions.

ro_wilk said:

Are there any other tests i should do to ensure the amp is operating within spec?

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Follow the SM, it contains all the necessary procedures.

Use IC clips for monitoring voltages, can’t rely on your hands holding a probe, it often ends in disaster. LOL

You likely over biased the outputs

Sparkplug said:

You shouldn’t make adjustments while on DBT as it pulls your voltages somewhat lower than actual and cause inaccurate settings. Once you a certain there are no shorts and the unit operates normally, you can go off DBT and make any adjustments.

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I concur..

Thank you both.

Ok so I reassembled everything for a second time, tested on DBT, and this time when plugged into mains Q319 popped and shorted out - i don't think it's bias related as i had barely touched the pot at this point (unless it was set too high to start with). The voltage at the left TP (via IC Clips!) was also fluctuating this time, i think maybe Q323 blowing on the first try took something else out with it. Have tested all zener/resistors on the board and trannies Q309-Q315. Only thing I could find is zener D333 had blown open, so replaced that also. Could that have been causing the outputs to blow?

There seems to be something wrong with that Q319/Q323/Q327 bank. Of course it could also be that the replacement output trannies are dodgy. I have one more spare, so will try that before ordering the Sankens.

Thanks again.

ro_wilk said:

Could that have been causing the outputs to blow?

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Yes

Hi guys, have just tried firing it up for the 3rd time, after testing thoroughly on the DBT (no shorts, protection turns off after 4-5 seconds, voltage at the I/O pins, and at the B of all output trannies even and within spec). I also checked the temperatures of all trannies when initially powering on (off-DBT) and nothing was heating up.

This time when adjusting the idle current, Q317 shorted out with R367, R343 and R371 all visibly burnt. I was tweaking the pot back and forth a bit to get it to 10 mv - could this have caused it?

Going to order those Sanken's, replace all output/power trannies and have another crack. Is there any other reason in might work fine on DBT and then chernobyl off it?