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Old 2nd March 2019, 11:02 PM   #31
Telnet100 is offline Telnet100  New Zealand
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Morning All --
While poking around with the Dvm, I have discovered something else that does not look quite right, and so before I do an all-up Performance Test, I would rather wait for advice, as I do not want to risk any of my work up to this point.
There is an Op-amp, IC3 on the drawing, which looks like it may help with DC restoration of the Output Rail. - (I could be wrong on this.)
On one Channel, the Output voltage of this is .444 Volts, which agrees perfectly with the Akai Schematic. On the other Channel, the Output voltage is only .058 Volts which seems a little low.
Looking at the Collector Output voltages of the Driver Devices on that Channel, TR9=+4.21v and TR10=minus 4.22v.
The Other 'Good' Channel measures TR9=+4.22v and TR10=minus 4.24v, so nothing really amiss here. The 'bad' Channel input voltages to the Boards I made are pin 12=+1.58v and Pin 10=minus 1.58v, so even, great. The Input Voltges to the 'good' Channel on the boards I made are pin12=+1.58v and pin 10= minus 1.61v?
Thinking that the TA75558s Op-amp may have been damaged in the previous history, I carefully replaced it with the correct new one, but those output voltages remain exactly the same.
i don't really want to go poking around too much in the High Gain Front End, as there is always a chance of slipping with the probe and damaging something down the line.
If necessary I could solder in some test points to alleviate this risk --
Does this point to something out of calibration in the Front End?
Both Channel Output Rail Voltages hover around plus/minus 2 or 3 millivolts, so I can't see much wrong here, but of course the DC Gain of that Op-amp is probably high as there is no DC Feedback path around it, only AC Feedback C28, which I have replaced in both channels. I have also checked the Values of those 4 resistors around the input to the Op-amp, all good. Power supply rails to the Op-amp measure at plus and minus 15.2 Volts, with no apparent noise on them. When Powered down with NO Psu Voltages, everything I measure comparing the two channels looks practically identical, so this is a head scratcher --
I just thought I could field some advice before going any further --
Thanks So much for any and all help so far!
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Old 3rd March 2019, 10:34 AM   #32
Mooly is offline Mooly  United Kingdom
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STK1080ii Devices
The opamp is used as a DC servo and the fact your DC offset at the main amp terminals is just a couple of millivolts is proof enough its all working correctly.

The differing opamp output voltages are simply the correction voltages needed to bias the amplifier to zero volts DC offset. Any imbalance in the amplifier itself will need higher correction voltages. When the opamp runs out of range (hits the rails) the correction stops.

Yours is fine
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Old 3rd March 2019, 10:50 PM   #33
Telnet100 is offline Telnet100  New Zealand
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Hi Mooly
Thanks for that. I was just concerned that something might have been amiss because all the other voltages are so alike to the Akai Schematic. That was a Good Explanation of how it's supposed to work. I have Corrected that Akai Schematic re the connection of that TR10 Collector and it's voltages, which I have attached to be of help to others.
OK, now I can go ahead and do some performance testing --
Many Thanks for All!
Attached Files
File Type: pdf Corrected Akai AM-U60 Drawing.pdf (966.6 KB, 20 views)
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Old 15th March 2019, 04:52 AM   #34
Telnet100 is offline Telnet100  New Zealand
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Hello, All --
Did an all up Performance Test after playing it for a couple of hours.
The result was 75 V p-p into 8 Ohms, which equates to 87.5 Watts RMS, measured with 1 Khz Input; that is just short of Akai's 88 Watts. That was with only One channel Driven.
No sign of any sort of instability, so that's good.
if Any One wants the Artwork for my PC boards, just ask.
Thanks for all help received; I'm really thankful for that.
Attached is the Output, just short of clipping, as viewed on the 'Scope.
Attached Images
File Type: jpg 75v Pk-Pk @ 1 Khz Sine Wave.JPG (331.6 KB, 72 views)
File Type: jpg The Fan Cooled Load Resistors.JPG (346.5 KB, 74 views)
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Old 15th March 2019, 05:22 AM   #35
Telnet100 is offline Telnet100  New Zealand
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Hello, All

Can Anyone explain how I can start a New Thread -- Can't find the Start Thread button --
Thanks, and Kindest Regards -- Telnet100
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Old 15th March 2019, 12:46 PM   #36
Mooly is offline Mooly  United Kingdom
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That's odd. I'll ask, as we had a similar case reported a week or so back and that needed the account resetting.
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Old 15th March 2019, 01:31 PM   #37
Pano is offline Pano  United States
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I've rest your account. Can you see it now?
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Old 16th March 2019, 03:53 AM   #38
Telnet100 is offline Telnet100  New Zealand
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Hello
Thanks for that -- No, Still can't see -- Maybe I have to go back out and Log Back in to see it? -- I'll try that now --
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