Need to resurrect vintage Yamaha amp

gmcman

Would anyone happen to have experience with older Yamaha amps, as far as servicing the IC boards?

I found a MX-1000 that obviously has been hurt and had a repair attempt. Looks like a good overload on one channel and there are some resistors that were replaced and some that should be replaced.

I don't know what to look for or what to check, but am competent enough with a DMM and a soldering gun that with some aid...I could get through this.

Unit powers up, but is in "Protection". I'll get some pics up prob tomorrow but any help is appreciated.

Looks like all the higher dollar caps are good, power transformers are good, just has a short somewhere and I feel it's a simple fix...albeit tedious, man there are a gazillion components stuck to these boards. :exclaim:

[Deleted User]

Hello,
Step number one is to try and find the amp's schematic and service manual if possible. It's kind of like if I were driving to Denver, I know it's "out west somewhere", but having a good road map would be helpful.
Cheers, Ken

Gavin.Wright

If the amp is in protection I would expect shorted output transistors, in addition to open emitter resistors, shorted pre-drivers etc... That being said, Ken is right, you need a schematic to continue. Yamaha MX1000 pwr sm.pdf

gmcman

I have the schematic and its geared for the experienced technician for sure. I can see where everything is however I'm not 100% on how to check the components...especially if there is live current present.

The manual has complete schematics but lacking in the troubleshooting dept.

I will get some pics up tonight.

Gavin.Wright

Most schematics do not explain specifics on troubleshooting unless there is some type of "test mode" built into the device. It sounds like this project may be a bit too much to tackle without some help - do you have a friend that may be more comfortable with the troubleshooting process? Perhaps they could help you check some voltages and try to pinpoint the issue.

heiney9

Based on the above statements, best to leave it to a trained technician. If you don't have the ability to check components you really have no business trying this type of repair. I doubt very much it's a simple resistor or cap. As Gavin stated, it's likely a whole bunch of components are toast.

H9

gmcman

I do have someone that can help, however it would be via telephone.

I have measured the DC offset and I have a good 1.7mV on the right side, which looks perfect, the left however is the culprit. Definite over-voltage which likely tripped the safety. I do get an occasional relay click when pressing the speaker selector, but like you all have stated there is surely a lot to check.

I have messed with caps and resistors but never troubleshooted transistors. Are transistors serviceable parts and will it alter the sound being from a vintage nature?

mhardy6647

Is this what you have?
http://sportsbil.com/yamaha/mx-1000-s.pdf
http://sportsbil.com/yamaha/mx-1000-sm.pdf

H9 is right (imagine that!) - the dang soiled state stuff is complex, touchy, and parts are often difficult/impossible to find. Easy to do more harm than good. Problem could be trivial or could be profound. Best left to either a pro or an amateur with relevant experience.

Egocentrically speaking... I don't mind working on vacuum tube stuff, which is simple, straightforward, fairly forgiving of eff-ups, and for which parts are generally available -- but I eschew troubleshooting ss stuff.

FWIW... generically speaking, the likely problem areas include:
* dirty/bad relay or other component in the protection circuitry
* power supply (supplies)
* shorted driver device(s)
* shorted output device(s)

Whaddya check? Look for burned up stuff, check actual component values (out of circuit), voltages

There ARE vintage Yamaha experts treading the boards; check AK and you can ID some (one of the best, unfortunately, is now retired).

[Deleted User]

There's a very helpful book, "How to Test Almost Everything Electronic" by Delton T. Horn on the TAB publishing label. I've probably gone to this book up a hundred times when trying to figure out why something's not working. I recommend it highly.

gmcman

Tough to multi-quote on this phone but yes mhardy....that's the one. I do have a bug in someone's ear at AK and this weekend we're gonna get into this thing. It also took a blow to the top corner above the speaker terminals which were also pushed in about 1/2 inch or so. This is probably where it all started and the subsequent repair did little or nothing.

I will get it apart more tonight and check the speaker board. If I need to put $200-300 into it I will still be ahead of the game, hopefully not though.

Thanks Ken for the reference...I will check it out.

gmcman

Well I tore into this with the aid of someone who knows these things backwards and forward. A few phone calls later and we were able to get a good grasp on some damage. Definately has a few bad transistors and a couple cracked solder joints. Seems the damage is isolated to the left-channel board and I'm going to replace all the transistors since I need to pull them anyway to get the most accurate reading, this board really took some current for sure.

Overall it's more than i've tackled before on one single circuit board repair but I feel confident enough to get it done. Hopefully I won't need any specialized test equip but will cross that bridge if it comes.

I'll get some pics up later.

gmcman

Whatever caused the initial problems, it appears the party responsible for poor soldering techniques created other issues along the way...shocker.

I have the resistors, all the transistors, and a few diodes to replace the boogered components and the cruddy carbon comps that don't need to be in there.

It seems when the transistor at Q313 was replaced, the base leg was stretched over to a jumper and from what I can tell the base needs to be isolated...not 100% sure though. the rest of the caps in this sub-board check out so hopefully with the transistors installed everything will come together...or at least narrow down any more issues.

Sam Jobs

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