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 Post subject: Chassis Ground Question Onkyo Receiver - Fixed
PostPosted: Nov Sun 17, 2019 6:23 pm 
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hello folks,

I completely restored this forsaken vintage onkyo TX-3000. it fixed up perfectly and is quite a good set.

however, the speaker terminal strip is damaged.

I am going to mount two RCA plugs on the rear of the metal panel for the speakers A.

the shell of the new RCA panel mount jacks is ground to them. when I mount them with their nut to the metal panel on the back of the receiver, their negative shell will be to chassis ground.

the schematic uses the "fork" ground symbol to indicate the speaker negative.

question / help: does this indicate that the "chassis metal ground" of the back panel is "ground / speaker negative" for when I mount the panel mount RCA jacks to the metal back of the receiver ?

I DO NOT want to take "speaker negative" to chassis ground if their negative floats above chassis ground.

so, am I correct in believing that "speaker negative" is chassis ground and the speaker negative is NOT above chassis ground ?

so (again LOL), when I mount the new panel mount RCA jacks to the metal back of the receiver, their shell will already be at "speaker negative" ?

I know I'm OCDing a little bit about this, but I sunk some nice time and a few bucks into restoring this poor receiver. also, I absolutely love the warm robust sound from [what I believe to be in darlington form] STK chips.

thanks,

steve


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ch gnd.jpg
ch gnd.jpg [ 148.45 KiB | Viewed 929 times ]

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Last edited by Dutch Rabbit on Dec Sun 01, 2019 9:29 pm, edited 1 time in total.
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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Sun 17, 2019 6:52 pm 
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I don’t think it’s essential to insulate the shells of these RCA jacks from the chassis ground. This receiver does use the chassis itself as ground.

However, because a large amount of audio signal current flows through these points, I do recommend connecting the existing thick wires that previously went to the black ( - ) speaker A binding posts directly to the shells of the replacement RCA jacks. Most RCA jacks come with a grounding washer/solder lug that can be used for this purpose. This provides a lower resistance path compared to only using the chassis itself.

Another way to do this is to use a phenolic plate, or even the plastic portion of the original speaker terminal assembly (if not too badly broken). This can be bolted to the rear panel so that it covers the original rectangular cutout in the rear panel where the original speaker terminal was located. Drill 2 holes in the phenolic or plastic plate to hold the new RCA jacks. This will insulate the shells of both RCA jacks from chassis ground, so that all of the wiring can remain isolated from chassis ground, exactly the same as the original speaker terminal assembly.

-EB


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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Sun 17, 2019 7:19 pm 
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yes, that is a great idea--use a phenolic plate and make my own new terminal plate with RCA jacks.

i can not ground speaker B with RCA jacks.

when using Speaker A+B, it puts A and B in series to decrease the load on the outputs.

Speaker B does not go to chassis ground.

i just realized this after i made my post and verified it at the receiver.

when i only have speakers hooked up to A and i move the switch to A+B, there is no response from speaker A, therefore they DO NOT PARALLEL A+B speaker loads.

looking more closely at the schematic and if i understand the throw contacts of the rotary switch, Speaker B DOES NOT go to chassis ground.

am i correct on that via the schematic ? i guarantee the system puts the A and B in series when A+B is selected on the front panel.

i will either get an orginal terminal strip or make my own.

thanks for the great ideas.

vacuum tube is my thing. i am not a real solid state kind of expert--at all, the basics only :oops: .

steve

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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Sun 17, 2019 8:13 pm 
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Got a picture of the OLD speaker terminal strip? I may have an original, from a 30+ year stash of parts from my Dad's old shop. It would save all that engineering a new one. If not me, others may as well. A WTB ad might also generate an original.....

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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Mon 18, 2019 1:04 am 
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Dutch Rabbit wrote:
yes, that is a great idea--use a phenolic plate and make my own new terminal plate with RCA jacks.

i can not ground speaker B with RCA jacks.

when using Speaker A+B, it puts A and B in series to decrease the load on the outputs.

Speaker B does not go to chassis ground.

i just realized this after i made my post and verified it at the receiver.

when i only have speakers hooked up to A and i move the switch to A+B, there is no response from speaker A, therefore they DO NOT PARALLEL A+B speaker loads.

looking more closely at the schematic and if i understand the throw contacts of the rotary switch, Speaker B DOES NOT go to chassis ground.

am i correct on that via the schematic ? i guarantee the system puts the A and B in series when A+B is selected on the front panel.

i will either get an orginal terminal strip or make my own.

thanks for the great ideas.

vacuum tube is my thing. i am not a real solid state kind of expert--at all, the basics only :oops: .

steve
In this receiver the - terminals of the A speakers always remain connected to ground.

When the selector switch is on A+B then the + of speaker A goes to the - of speaker B and the + of speaker B gets the output signal from the amplifier.

In reality, multiple speaker selector switches probably shouldn’t exist. Switches that connect multiple sets of speakers in parallel run the risk of overloading the power amplifier and power supply, possibly causing permanent damage. Because of that risk, series connection for the speakers A+B mode is very common in receivers. But it only sounds good when both sets of speakers are identical or nearly identical. I can’t think of any time that I ever connected multiple sets of speakers to a receiver.

If your goal is to use this receiver rather than selling it, one alternative is to ignore the broken speaker A terminals, instead connecting only one set of speakers, with these connected to the “speaker B” terminals, then set the selector switch to “B” only (not A+B), and enjoy the music.

-EB


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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Mon 18, 2019 2:16 am 
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this receiver will be a keeper. thanks for all the advice. the series/parallel thing was explained just as i figured it would be.

brian, PM sent for pic of terminal strip.

steve

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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Tue 19, 2019 3:31 am 
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I will post pictures of the terminal strip this week, hopefully Tuesday.

steve

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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Tue 19, 2019 3:45 pm 
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It may not make any difference, but personally if I was looking to obtain a ground for a speaker I would not hit chassis ground directly; I would isolate the outer shell from chassis and return it to ground as near the common point of the main filter caps as I could get. reason is the high current speaker leads would be carrying. If this went near a common point near the preamp stages they could pick up the difference and possibly oscillate or degrade quality. (Think the concept of Single Point grounding.)

You already got the series ground of the B speaker covered so I won't rehash that! Although as previously mentioned I don't like the potential problems differing speaker characteristics could introduce.


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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Tue 19, 2019 4:26 pm 
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CaveRat wrote:
It may not make any difference, but personally if I was looking to obtain a ground for a speaker I would not hit chassis ground directly; I would isolate the outer shell from chassis and return it to ground as near the common point of the main filter caps as I could get. reason is the high current speaker leads would be carrying. If this went near a common point near the preamp stages they could pick up the difference and possibly oscillate or degrade quality. (Think the concept of Single Point grounding.)

You already got the series ground of the B speaker covered so I won't rehash that! Although as previously mentioned I don't like the potential problems differing speaker characteristics could introduce.



That's right in a hypothetical case, but, it already uses chassis return everywhere else, so I don't think this is a serious issue - or, rather, this change wouldn't really solve the problem.

Failing anything else, putting it back the way it was originally would probably be the first solution.

Brett


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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Wed 20, 2019 1:44 pm 
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you guys are absolutely right.

i will either get 4 isolated rca jacks and mount them to the metal rear plate, make a new plate, or get an original terminal plate.

...not going to mess around.

thanks,


steve

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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Wed 20, 2019 1:46 pm 
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brian and rest of posters,

i will post pics of terminals wednesday eve. i fell asleep early last night.

steve

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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Thu 21, 2019 1:52 am 
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here are the speaker terminals.

they are 3 & 5/16 inches from center mounting hole to center mounting hole (green).

the rectangle opening on the back panel is 2 & 15/16 wide by 5/8 high (red and blue).

speaker A and speaker B are identical.

thanks,

steve


Attachments:
SAM_5706.JPG
SAM_5706.JPG [ 104.83 KiB | Viewed 783 times ]
SAM_5707.JPG
SAM_5707.JPG [ 102.67 KiB | Viewed 783 times ]

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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Thu 21, 2019 6:28 pm 
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I looked and don't have one. We did scrap one in the past - I sold all the knobs on eBay in 2002.

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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Thu 21, 2019 9:35 pm 
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thanks for looking

steve

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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Fri 22, 2019 1:26 pm 
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You could just swap the A and B speaker terminal strips, since you're unlikely to ever need the B set anyway.

Personally I never liked RCA jacks for speakers, period. In this case I might even be inclined to replace the A speaker terminal strip with binding post terminals. That would require a new piece of something. (plastic, wood, metal) to cover the old hole and hold the new binding posts. Since the binding posts are isolated from ground, that would solve any possible issue with chassis grounding..... just wire them as the old terminals were wired.

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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Fri 22, 2019 1:54 pm 
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i may have a pair coming from a friend. terminal set B is broken too.

i am not going thru with the " chassis gnd" RCA jack original plan.

i will update.

steve

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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Fri 22, 2019 4:20 pm 
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That’s a relatively common speaker terminal assembly for late 1970’s through 1990 receivers and integrated amplifiers. Not only Onkyo, but many brands. It shouldn’t be difficult to find replacements.

I’ll take a look at my parts stash. I might have an exact replacement for what you need. In a previous career I owned/operated an audio/video/pro-sound repair shop (1975-2000). I still have a garage full of parts. I’ve always been very fond of Onkyo products. They sound good, they last a long time, and they are repairable.

RE: Speaker terminals: During the 1990’s the manufacturers gradually began “upgrading” their speaker terminals, making them larger and more robust. Unfortunately they are still “high failure rate” items, even today. Most often they get damaged during handling and moving the receiver or amplifier, especially when it weighs more than 50 pounds.

-EB


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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Fri 22, 2019 4:24 pm 
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FYI: Downsize the speaker fuses to 2A or 2.5A.

That will reduce the chance of frying the unobtainium STK output devices from accidental short circuits in the speaker wiring.

-EB


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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Fri 22, 2019 4:44 pm 
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Yet another way to tackle this is to replace the original thumbscrew posts with high-quality modern binding posts. From looking at your photos it appears that the rectangular plastic plates are intact.

What usually fails in the OEM item is the female threads in the stamped interior connection lug get stripped out so that the exterior thumb screw cannot be tightened. I suspect the interior female threads were designed to be just good enough to be used a couple of times. And sometimes the entire exterior thumbscrew just gets totally broken off.

I’ve previously fixed these by getting 4 black and 4 red individual binding posts.
Then I drilled out the original female threaded holes to a size that the rear part of the replacement binding post will fit through. The original metal lugs can often be left in place. Just drill out the old female threads in them.
The replacement binding posts are secured by the nuts that come with them.

If you replace all 8 of the original thumbscrews it will look as good as new.
Better than new actually.

-EB


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 Post subject: Re: Chassis Ground Question / Help Onkyo Receiver
PostPosted: Nov Sat 23, 2019 1:53 am 
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hello eb,

thanks in advance for looking. i appreciate it.

great idea on downgrading the fuses. i believe 3A are in there now, factory stock.

my terminals have the metal square that holds the thumb screwpost. the metal part is broken out of the plastic. the plastic is broken too, so i must replace the entire plastic part.

the screwpost and its metal base flops out of the plastic terminal assembly.

steve

edit: sentence structure fixed after typing via mobile device.

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