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Author Topic: Yamaha G100 MA Board adjustment/input signal level  (Read 347 times)

telephone

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Yamaha G100 MA Board adjustment/input signal level
« on: January 30, 2021, 10:58:28 AM »
Greetings all, and thanks for adding me to the board.
I recently acquired a G100 210, series2, and am in the process of replacing the electrolytics. I have a question regarding input signal level when adjusting the MA board. On page 4 of the service manual it calls for a -6db 1khz signal. I’m having a hard time figuring out how to translate that. A friend of mine who’s fairly fluent in radio transmission came up with a figure of 1.417V p-p. Considering the relatively low signal level of a pickup, that number seems rather high. Can anybody offer some guidance? Thanks for taking the time.

Aaron

Forgot to mention, this might be one of my favorite amps. Over the decades I’ve been thru several high dollar tube amps as well as a number of offerings from Quilter. The G100 is a remarkably good sounding unit.
« Last Edit: January 30, 2021, 11:19:50 AM by telephone »

g1

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Re: Yamaha G100 MA Board adjustment/input signal level
« Reply #1 on: January 30, 2021, 02:45:57 PM »
On the 'general specifications' page of that manual, it states that 0dB is referenced to .775VRMS.  So it is using the dBu reference.
So -6dBu is 388mV RMS or 1.1V peak to peak.
( http://www.sengpielaudio.com/calculator-db-volt.htm )
Your friend will have used the dBV reference, which is close but not correct for this application.

You are applying the -6dB signal to a connector elsewhere in the circuit, not to the guitar input.  So general guitar pickup output level is not relevant to this section of the circuit.

g1

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Re: Yamaha G100 MA Board adjustment/input signal level
« Reply #2 on: January 30, 2021, 03:03:06 PM »
Also, for the center voltage adjustment, rather than setting pin5 to be 45V, I would set it to be half of what is measured at pin6, as it may not be exactly 90V.

telephone

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Re: Yamaha G100 MA Board adjustment/input signal level
« Reply #3 on: January 30, 2021, 04:33:29 PM »
Thanks for the clarification. It is greatly appreciated. I now see where I got confused regarding test conditions.
Just to be crystal clear, per step 2, I'll be reading the waveform at speaker output, which will also be connected to an 8ohm dummy load, yes?

As well, thanks for the tip regarding center voltage adjustment. I wouldn't have known otherwise.
« Last Edit: January 30, 2021, 04:37:31 PM by telephone »

g1

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Re: Yamaha G100 MA Board adjustment/input signal level
« Reply #4 on: January 30, 2021, 07:33:23 PM »
Yes, for step 2 you are looking at the waveform on the scope.  At the speaker out with 8ohm load resistor.
After last adjustment (idle current), I would go through all 3 adjustments a second time as they can interact. (do not repeat step 1)

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Re: Yamaha G100 MA Board adjustment/input signal level
« Reply #5 on: January 30, 2021, 08:35:34 PM »
Alright, you've answered all my questions. I can't thank you enough!

telephone

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Re: Yamaha G100 MA Board adjustment/input signal level
« Reply #6 on: February 14, 2021, 11:51:33 PM »
Thanks again for your input @g1.
After receiving the appropriately rated dummy load I finally got around to performing the MA board adjustment. Waveform looks good.
Regarding center voltage adjustment; according to the manual (step 3 of MA board adjustment), I'm adjusting pin5 voltage in the absence of test signal, yes? Pin6 is reading 108V as opposed to 90V. Should I be worried about that much of a deviation? Per your recommendation I've adjusted center voltage to 54V.
« Last Edit: February 15, 2021, 12:26:16 AM by telephone »

g1

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Re: Yamaha G100 MA Board adjustment/input signal level
« Reply #7 on: February 15, 2021, 01:33:41 PM »
Correct, no signal for center voltage and idle current adjustments (load should be connected).

From the schematic it shows the main filter cap to be 160V (2200uF) so the positive supply rail being 108V instead of 90 should not be a problem.  But double check it's 160V rated to be sure.  The older version used 100V cap I think, or someone could have replaced it with a lower voltage rated cap.

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Re: Yamaha G100 MA Board adjustment/input signal level
« Reply #8 on: February 15, 2021, 03:50:46 PM »
Thanks again for the confirmation. The 2200uF caps are replacements rated at 200V. Looks like I'm all done with this one. Much appreciated!
« Last Edit: February 17, 2021, 09:05:12 PM by telephone »