A-1000 Rehab: DC voltage in signal path SOLVED

Johnny_Law

AK Member
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SOLVED on May 13, 2023!!! See here!

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This has been waiting to get some TLC from me for years. I wanted to make this thread to start the process of going through it. The report from the previous owner was turns on, no sound. Taking a guess - power supply / protection. Hopefully no blown output transistors. But plan to do a full recap.

First, I need a couple things! I've been looking for the correct replacement silver button cover for my A-1000's Tuner input button for almost 2 years. Anyone got one? Thanks in advance for any leads!

IMG_20190219_182042.jpg

NB614520 Button A1000.jpg

Second is the power cord - I heard there are cords of different capacitance used? I am not sure which this has. Mine was purportedly ripped in half because of a car door mishap!

To be updated as soon as I find a power cord and get it to the bench ...
 
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While the part numbers are different, I wonder if a button from an A-500 wouldn't work. I see them pretty regularly on ebay getting sold as defective for low money. (Knob Ass'y, Silver: A-500 = NB610190, A-1000 = NB614520).

Monday when I get to work I'll ask my friend with the A-500 to measure the width/height of one of the buttons and will report back.

Cheers,
James
 
While the part numbers are different, I wonder if a button from an A-500 wouldn't work. I see them pretty regularly on ebay getting sold as defective for low money. (Knob Ass'y, Silver: A-500 = NB610190, A-1000 = NB614520).

Monday when I get to work I'll ask my friend with the A-500 to measure the width/height of one of the buttons and will report back.

Cheers,
James

Thanks James! Unfortunately the A-1000 / A-2000 buttons are unique in that they are metal, instead of the plastic buttons used in the A-700 / A-500, and others. And actually, @OMGCat! has already taken mercy on me and give me a button cover set from an A-700. So that's the contingency plan over here ... in the meantime I am hoping for a metal, silver button cover to emerge from the ether someday.

I see your location is Munich - beautiful city - I was out there for a conference last year, stayed on Maximilianstraße.
 
Thanks James! Unfortunately the A-1000 / A-2000 buttons are unique in that they are metal, instead of the plastic buttons used in the A-700 / A-500, and others. And actually, @OMGCat! has already taken mercy on me and give me a button cover set from an A-700. So that's the contingency plan over here ... in the meantime I am hoping for a metal, silver button cover to emerge from the ether someday.

I see your location is Munich - beautiful city - I was out there for a conference last year, stayed on Maximilianstraße.

OK! Good Karma on OMGCat! At least you have something to start with. The silver faced stuff shows up pretty regularly over here, I'll keep my eyes open and give a shout if I see something useful.

As far as large cities go, yes Munich is a pretty nice place to live. And it's not far from the mountains and South Tirol, which makes it even better. If you make it over here again let me know, we could meet up for a beer. Lots of great Biergarten's here.

Cheers,
James
 
Mr. OMGCat! hooked me up with a button for an A-950 (Japanese version of A-1000) I'm currently reviving too, very kind.

The relay driver / protection section on this amp uses small devices which can fail, I bump them up to TO-92L or TO-126 packages (with higher V and Ic) on daily driven amps. Easy to check if this is the culprit by checking DC offset before the relay (as I'm sure you already know).

Also be careful with the bias transistors, they are easy to bump when working on this amp and the legs are usually brittle from heat cycling. Mechanical problems here can take out an output stage, I replace the stock 2SD-400 with TO-126 packages (OnSemi BD135-16S, can be screwed directly to heatsink with thermal paste, noting orientation / pinout).

I also find it's worth going over all power amp solder joints, easy enough if recapping. The little "BBQ board" near the volume pot and 1x cement resistor (along with 3x same on phono board + diodes they shelter) are worth resoldering too.

The silver button caps appear to be sanded to a brushed finish then anodized clear (no dye), you could probably do this to your black one and get a close match.
Going a step further, if they come off easily the selector ones could all be re-anodized clear as a batch for consistency. Just an idea if you get stuck finding one.

Best wishes!
 
So here's a question (and I think @OMGCat! knows the answer) - but is there a specific type of power cord that I should get as a replacement? The manual simply states 10A 125V ... any other critical specs?

Power Cord A1000.jpg
 
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It's a two lead but with a sheath rather than the normal two wires stuck to each other in the middle. I still have one around here somewhere that I can drop off once I locate it in the parts pile.
 
It's a two lead but with a sheath rather than the normal two wires stuck to each other in the middle. I still have one around here somewhere that I can drop off once I locate it in the parts pile.

Ah, got it - thanks! Yeah, if you are ever heading out this way, that'd be cool!
 
Ok here we go ... initial inspection. Case removed, cleaned some dust off, and snapped some pics.

Bird's eye view from the top. First impression, BEEFY!

IMG_20190914_090704.jpg
Poking around ... hmm, some signs of heat. Will have to investigate.

IMG_20190914_090713.jpg

IMG_20190914_090720.jpg

And looks like some suspicious glue / possible corrosion under some caps.

IMG_20190914_090855.jpg

Underneath, looking pretty good (other than black heat marks on underside of that little board w/ all the resistors on it).

IMG_20190914_091548.jpg
 
-Checked all OPTs, no shorts
-Power cord soldered on (thanks again @OMGCat! )
-Plugged into DBT, dim. Doesn't seem to be in protection mode ...

... but further poking around leads me to believe it is in protection. No audio output at all, totally dead. Now that I've discovered no shorted OPTs, onto the power supply / protection circuits!

Pic of the OPTs.

IMG_20190914_090729.jpg
 
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Clean up that glue damage.And carefully inspect the corroded leads and jumper wires integrity.
 
Do you hear a relay "click" shortly after power on ?

No click. And no voltage at all at the terminals - definitely in protection.

I just measured voltage at L101 and L102. Finding some small voltage at L101 but -30V at L102. Lifted leads on the OPTs on that side, none shorted ... dang this service manual is hard to read. L102 got cut off between pages 11 and 12. Which OPTs are upstream of L102?

Clean up that glue damage.And carefully inspect the corroded leads and jumper wires integrity.

Definitely part of the plan! I do spy some corrosion. Will attack w/ white vinegar and perhaps baking soda/water, and measure to confirm integrity underneath.
 
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It is abundantly clear to me that you need a good complete usable service manual.

E-mail Norman at Analog Alley and tell him you need a complete Yamaha A-1000 service manual. If he has one .. It looks like he does.. the are super Hi Q color pdf files for around $13. Norman is my go to for Yamaha manuals.

http://www.analogalley.com/m_yamaha.htm
 
In the meantime ... the PDF manual I have (which appears to be the only free PDF version available online), it doesn't even indicate which OPTs are in which physical locations. Am I missing something? How do I know which OPT is which?

And, there is only a signal path with reference voltages traced for the left channel, not for the right. That's annoying.

That said, I have significant DC voltage at all but one of the 2SA1333/2SC3370 emitters. I have lifted the legs of the right channel (L102-side) OPTs but the -30V remains both at L102 and at the emitter pads with lifted OPT leads.

If only I knew which OPTs were TR128, TR140, and TR158, as these are my suspects from what I can see in the service manual. L102 is testing continuous to the emitter pads of two OPTs, I just (1) don't know which two they are and (2) because the service manual is missing L102, I cannot tell if that continuity is correct (I assume it is to TR158, but not to TR140 unless TR158 is shorted B to E, which it isn't according to my multimeter ...).
 
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There are only 4 output transistors total.. TR157 and TR159 for the left channel associated with L101.
TR158 and TR160 for the right channel associated with L102.



See post # 16..:thumbsup:
 
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The rest of those MT-200 transistors TR155/156 and TR161/162 are APS rail switching transistors. Not final outputs.

APS auxiliary power supply
 
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