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Yamaha newbie A-700 resto thread

AFAIk the zeners act as voltage regulators. Above their stated "zener voltage" they allow reverse current, effectively diverting all voltage above that level on that rail to ground.

It is important to find a data sheet and find the exact sub-class of the zener. The zener voltage will be shown as a range for each sub-class. I then calculate the nominal value from that for ordering purposes.

I found that it is getting harder and harder to find a lot of sub-classes of through hole zeners, the last time that I ordered some from Mouser they were back ordered for months.

Just in case it is what is needed, I am attaching the HZ series datasheet here. HZ zeners are pretty common in Yamaha amps I think.

Cheers,
James
 

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Soundscape said:
I'm not a current flow expert either. I know diodes allow current flow in one direction. Zeners are current controlled diodes. So if there's uncontrolled currents happening, I chase diodes. A transistor can be looked at as two diodes.
Long story short regarding chernobyl repairs. In my experience the semiconductors are the problems & the smoking parts are just symptoms. The tranny's & diodes are the most sensitive and fail first, creating excessive currents that BBQ's resistors, caps, etc.
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That was a good article, thx for sharing. The Pioneer handbook had a good outline of failures like I had. I think what caused my issue was originally setting the bias on DBT (oops) which I now know is a no no, and that caused one of the outputs to fail open and deliver +B to where it shouldn't have been... Smokey Smokey pop pop, lol...

I saw an older thread for a different Yammy amplifier where someone had a similar Chernobyl event and even after finding a faulty part and "fixing" the problem, it was suggested to them they should go ahead and replace all semiconductor devices on the blown channel, in case some are still testing good but got stressed during the catastrophic failure. Not sure if it would apply to me here, though.
 
analogwisdom said:
I think what caused my issue was originally setting the bias on DBT
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I'll be honest with ya'll. Let's say my amp is supposed to be biased at 10mV. I set it to 5-6 mV on the DBT and listen to music on it for a few minutes before moving it to a regular power outlet. I think there was a fault in your piece.

To be more of a redneck... I use the diode checker jones multimeter feature alot when troubleshooting. In circuit tests too. Short circuited diodes & tranny parts really stand out. Open trannys are more tricky and I usually have to pull them to find opens.

Seems transistors & zeners fail first. Then the regular diodes after that in my experience.
 
Soundscape said:
Every chernobyl'd piece I've worked on had a blown pre-driver in it. That would be TR128 & TR130 on yours.
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TR130 was what caused the biggest smokeshow when the chernobyl event happened, it was like a little firecracker... I posted a pic way back:

analogwisdom said:
View attachment 3139481
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Thank God for the Yamaha engineers who decided to put the fusing resistors in the circuit, because they saved a lot more stuff by being in the line of fire, lol.
 
analogwisdom said:
TR130 was what caused the biggest smokeshow when the chernobyl event happened, it was like a little firecracker... I posted a pic way back:



Thank God for the Yamaha engineers who decided to put the fusing resistors in the circuit, because they saved a lot more stuff by being in the line of fire, lol.
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So the yellow highlights on the print are swapped parts? I'd swap D130 no matter how it tests.
 
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Soundscape said:
I'll be honest with ya'll. Let's say my amp is supposed to be biased at 10mV. I set it to 5-6 mV on the DBT and listen to music on it for a few minutes before moving it to a regular power outlet. I think there was a fault in your piece.

To be more of a redneck... I use the diode checker jones multimeter feature alot when troubleshooting. In circuit tests too. Short circuited diodes & tranny parts really stand out. Open trannys are more tricky and I usually have to pull them to find opens.

Seems transistors & zeners fail first. Then the regular diodes after that in my experience.
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EDIT First paragraph:
If the troubled channel looks like it's biasing normal, like the good channel does. I'll watch the DC offset at the speaker terminals for a bit with nothing connected. I like to see less than 20mV in general. Kinda depends on the piece. If it still looks good after 10 minutes, then I'll hook up the speaker wires for some music on beater speakers. After that my confidence goes up that the amp is fixed. Then I'll set the bias all the way down to zero & plug into regular power, no DBT, no speaker wires & watch the DC offset again. Then I'll set final DC bias level.
 
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^ Disclaimer:
I've only done bench work for 4-5 years as a hobby. My background is transmission lines & easy electrical ckts. Fixed most of my pieces though. Usually pro's chime in when I say bad info. They haven't been too vocal yet, lol.
 
Soundscape said:
EDIT First paragraph:
If the troubled channel looks like it's biasing normal, like the good channel does. I'll watch the DC offset at the speaker terminals for a bit with nothing connected. I like to see less than 20mV in general. Kinda depends on the piece. If it still looks good after 10 minutes, then I'll hook up the speaker wires for some music on beater speakers. After that my confidence goes up that the amp is fixed. Then I'll set the bias all the way down to zero & plug into regular power, no DBT, no speaker wires & watch the DC offset again. Then I'll set final DC bias level.
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Sounds like a good ritual to go through, thanks for sharing. I need to get a set of beater 8 ohm speakers. My beater pair are some old HH Scott from the 70s and they're 4 ohm.

You work(ed) in utilities? Me too! I'm on the electric distribution side.
 
Bratwurst7s said:
AFAIk the zeners act as voltage regulators. Above their stated "zener voltage" they allow reverse current, effectively diverting all voltage above that level on that rail to ground.

It is important to find a data sheet and find the exact sub-class of the zener. The zener voltage will be shown as a range for each sub-class. I then calculate the nominal value from that for ordering purposes.

I found that it is getting harder and harder to find a lot of sub-classes of through hole zeners, the last time that I ordered some from Mouser they were back ordered for months.

Just in case it is what is needed, I am attaching the HZ series datasheet here. HZ zeners are pretty common in Yamaha amps I think.

Cheers,
James
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Those might work ok but here's a data sheet for the HZ-L series. What's the difference? The -L designation means low noise series, like the originals.
 
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BZX are supposed to be low noise also. MTZ ans MTZ-J don't state anything.
Often you are limited to what you can find at all for a particular sub class.

James
 

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Bratwurst7s said:
500mW.

I just wanted our friend Soundscape to get enough to read. :biggrin:
I'm not so sure that you can buy HZ or HZ-L any longer. Mouser doesn't have any.

Cheers,
James
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I couldn't find them. The replacement I have coming today is NZX6V2, with the "B" rating the same 5.8 to 6.1 listed for HZ-6 "C1" in its datasheet. Not gonna pretend like I figured that out on my own, you can probably guess who I got it from.

I do appreciate the sharing of datasheets and articles, though. I do want to learn what to look for and be able to figure out appropriate substitutions on my own eventually.
 
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