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avatar: MErath
MErath
8 posts
Replied to MErath's post on January 16th, 2014

We had to replace a optical sensor and when plugging the cable back in, reliaze they werent labeleld is to whihc one is TACH 1 and which is TACH 2. 

While spinning up a type 50 rotor by hand, the tachometer shows ~150 rpm.

On board # 2, TACH 1 TP = ~300mV p-p, while TACH 2 TP = 1.32v p-p. The TACH one signal is erratic and weak. I swapped with a noather board but no improvement. Sensors are new. Is there another section in between to check out?

I am assuming the spindle is TACH 1 , while the overspeed disc is TACH 2. I can unplugg one and trace it through. But what  signla should i see on TACH 1?

The rotor has a good disc. Tested with a type 19 which has a new disc. still has the same result.

avatar: dpkleessr
dpkleessr
1122 posts
Replied to MErath's post on January 16th, 2014

The way to know which connector goes where is really quite simple.  The connector with the ORANGE wire goes to the overspeed pickoff.  The pickoff that sits underneath the edge of the drive spindle is your TACHOMETER signal and the other obviously is for the OVERSPEED signal.

You are incorrect with your assumption about what the TACH 1 and TACH 2 signals represent.  The TACH 1 signal is the raw signal coming from the tach pickoff and the TACH 2 signal is the tach signal after it has been digitized by the electronics.  You need a good 1.0VDC peak to peak signal as you SLOWLY rotate the spindle at the TACH 1 test point.  The adjustment of the TACH signal may be a two part process.  First, you try to adjust the height or distance between the bottom of the spindle and the TACH pickoff to achieve the proper signal level.  This is done by lossening the allen head screw that holds the position of the entire black plastic assembly that sits underneath the spindle assembly.  You do not want or need a rotor in place to make this adjustment.  You may also see an extra variable resistor screw head that was added on the BOARD #2.  IF it is there, it can be used in conjunction with the vertical positioning to get the proper TACH signal level.  You will also know that the potentiometer has been added because there will not be any black stenciling on the plastic cover to indicate what the function of the pot is.  If not then you can only work with the vertical positioning to get the correct signal level.  After you get a good TACH signal, then and only then do you try to adjust the OVERSPEED signal.

Since you did not say which pickoff you replaced I will speak generally about the pickoff replacement.  The distance between the pickoff sensor and the disk on the rotor is critical to getting the proper voltage swing.  That's why Beckman service engineers have a special tool with an overspeed disk on it that makes this adjustment much easier than trying to do it with a rotor in the way.  There is a small screw that holds the OVERSPEED pickoff to the black plastic piece so that is how you can adjust the height of that pickoff while monitoring the OVERSPEED signal test point.  You are looking for a 3.0VDC signal peak to peak here that should change as you SLOWLY rotate the rotor.

I hope that this helps with your task.

Don

avatar: MErath
MErath
8 posts
Replied to dpkleessr's post on January 20th, 2014

Fantastic feedback, Thanks.

I adjusted the height of the inner most sensor to give maximum Vout on TACH 1 to .99 V but at low it was 0.06. I adjusted the pot just under UNIJ until V hihg was 1.1V this giving me just abouve 1v p-p.

After I had sent my last email, I foudn the system was only getting to 3000rpm and the lower base lamp for board 7 wasnt lighting. I pulled the board to find a power resister had release its magic smoke. I replaced the board and was able to get up to 9000RPm with no further TACH issues. but still getting SPED errors with a good disc. Even after adjusting the optical sensor.

On our second centrifuge, I have 1.2v p-p on TACH, and can spin up to 10K but when it gets under 50rpm, TACH started to fail. Now it constantly gives me TACH error.

If you have any other suggstiong to lok at , it would be of great help. I've been off since thursday,s o I plan to go down and get some more measurements. I may check boards #5 & 6# as well. Perhaps im getting SPED error because the system is unable to spin up to speed?

Michael

avatar: trong.ha
trong.ha
67 posts
Replied to MErath's post on October 28th, 2019

Hi, got a "TACH" error, i replaced board #6 and the error message go away, but then i get another problem unit cannot pick up speed (no more than 500rpm)

Trong

avatar: trong.ha
trong.ha
67 posts
Replied to trong.ha's post on October 30th, 2019

i got it fixed by replacing board #7.