Harman Kardon ST-7 strobe issue

[Briant8]

Hi All,
So I just dove into my first ST-7, replaced tracking wheel, belts, DeOxit on pots, etc...

After much frustration, cussing, cajoling and downright threatening (it never seemed to listen at all, btw) it’s up and running and sounding great.

My issue is that the strobe speed is off.
When I dial in the speed with the RPM app, the strobe is way off.

When I dial in the speed with the strobe, the speed tests out at about 39 RPM (on 33-1/3)

I did try the coarse speed control and I can stabilize the strobe using it, but the actual platter speed is too fast.

Any thoughts are appreciated!

 

[HalfApt]

Where did the TT come from? Sounds like the strobe mirror is set for 50 cycle current.

eg. running the 50 strobe on 60 cycles would produce 60÷50x33.33 = 39.6 rpm (I think).

 

[Briant8]

Ohhhhh....
good thought!

Let me look into how to check that.
Thanks!

 

[HalfApt]

The mirror is square, w/half masked off for 50/60 Hz operation.

The outer half exposed for 60 Hz:



The inner half exposed for 50 Hz:



The mask they used (called "fish paper") is flat/matte black so it doesn't reflect any light itself.

 

[Briant8]

Thank you both, that was the issue.

All is finally well in ST-7 land!

Brian

 

[merder]

The mirror is square, w/half masked off for 50/60 Hz operation.

The outer half exposed for 60 Hz:

View attachment 1710378

The inner half exposed for 50 Hz:

View attachment 1710379

The mask they used (called "fish paper") is flat/matte black so it doesn't reflect any light itself.

Thank you SO MUCH for this info. I just replaced the roller, etc. in an ST-7 and could not figure out why the strobe was a blur... it's fixed now, thanks to you (and I couldn't have done the roller without information I found on here as well). Thank you, everyone!