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PLL Capstan Light on A810

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Hello all,

Yesterday the PLL capstan light on my A810 would blink occasionally after I turned it on. Prior to about 5 days ago the machine had not even been plugged in about a year. (I know, shame on me. ...for not using it, and also for not exercising it during long periods of non-use). By the time I got ready to record it had quit doing it. I've had this machine for 6 years and put 700 hours on it. This has never happened before. In fact, it has always been 100% reliable.

Today I noticed it blinking during a record process. When the light blinks it means the capstan is not up to speed, and of course, this ruins the recording. I normally give the machine time to warm up before recording, but today it only had a few minutes of warm-up time. After the machine had been recording for about 15 minutes it quit doing it completely. It has been on all day, setting next to my computer desk, and it has not blinked at all after it got up to operating temperature.


Does anyone have an idea of what it is that is trying to fail here? Hopefully, it could be something as simple as the capstan motor needs to be lubricated. Any thoughts?

It could be due to an aging capacitor in the capstan control circuit that needs extra time to charge or, as you suggest, a moving part that needs CLA. First download the service manual and check the section on capstan service, and then maybe give the machine a fair amount of run time to see if the cap might reform.

Hopefully Ki Choi will be checking in, as he may have some more specific suggestions.

Doc, thanks for your help. Today I fired up the A810 and it had a couple of hours to warm up while I was doing other things. There was no tape loaded and I did not start the capstan manually, I just let it sit while all the circuits warmed up.  When I did load tape and the capstan was activated there were no problems. I'm not a tech compared to what you or Ki know, but I do readily understand when someone explains, and your remarks about an aging cap that needs extra time to charge and running the machine to reform the cap were quite helpful.

Studer has a service bulletin that says my capstan motor is good for 4,000 hours and up to 10,000 hours if it is re-lubricated. It also says there is a big difference in service requirements between vertically mounted & horizontally mounted machines. That makes a lot of sense and mine is vertical.

I've had the op/serv manual since the day I received the machine and have referred to it a lot over the years. But yesterday all I found in it were schematics and how to remove the assembly. I was unable to find a functional description of the capstan.

Today I found this:
"A capacitive sensor detects the movement of a toothed ring that is rigidly coupled to the capstan shaft, The change in the capacitance of the sensor causes a frequency modulation at the input of the capstan motor control."
I'm guessing if the problem gets worse that finding that sensor would be a really good place to start troubleshooting.

Ki Choi:
Sorry for being late to the post.

There are simple problem areas to look for quick fix fist:

One is to look at the capacitive techo sensor output from the capstan motor. Its functions are described in the section 3.5.9 in the manual.

Second is as Doc B had suggested - there are three electrolytic caps (c12 100uF 16VDC, C16 10uF 16VDC, C18 2.2uF 16V) in the capstan control board need to be replaced.  Even the youngest A810s with serial numbers >6000 are still more than 30 years old.

However, most likely reason for losing PLL signal that I had been experiencing was due to failure of L1 and L2 variable inductors in the capstan control board from machines with serial numbers older (smaller) than ~4000s region.

One of the things that are less than Studer-like was to use these cheap variable inductors with very questionable quality.  These old inductors used some plastic like material to hold the ferrite core in place after being tuned to 5.5Mhz +/- 500Khz.  I found most of the ferrite cores to be out of place due to the holding material that had gone soft from age and no longer holds.

Since I hadn't been able to locate these variable inductors, the only solution was to temporarily position the ferrite core to its proper position and fix it with epoxy glue.  As it dries, I have to double check with a digital scope with counter capability to make sure it holds 5.5Mhz signal.

Hope it helps.


Hi Ki,
Excellent! What an amazing post...thanks much.
I've been using the machine every single day for the last 2 weeks, no problems.
Normally the machine warms up before capstan is engaged.
Yesterday I started capstan right after power-up. After the initial spin-up time it blinked a few times then was steady.
Today I did the same thing...no blinking at all, but the room may have been warmer.
So, whatever problem exists is minimal at this time.
Looks like I have a newer machine- serial # is 6143.
I will copy & paste your info into my Studer folder for future reference!!!
Thanks again,


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