Conn ST-11 Strobotuner Mod Help - diyAudio
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Old 1st January 2009, 02:03 AM   #1
labjr is offline labjr  United States
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Default Conn ST-11 Strobotuner Mod Help

The oscilllator in this unit has always been unstable.

I've compared it with a better strobe tuner. It's good but not quite as good as my newer electronic strobe. When I tune with the ST-11 I can hear little beats when I play chords that I don't hear with the new strobe tuner.

I think the problem lies in the oscillator circuit because I'll calibrate it. Tune three or four strings. Check the calibrate mode again and it's a little out. I can't seem to keep it from drifting no matter what I do.

I think the ST-11 can be as accurate as the best new technology if the oscillator can be made more stable.

I'm wondering if anyone with some talent in digital design wants to help me try to improve this old workhorse.
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Old 12th January 2009, 05:12 PM   #2
nhuwar is offline nhuwar  United States
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I know this sound corny but you could try leaving it on for a few hours before you use it. The drift could be caused by it just heating up. You could also try recapping the unit with higher tolerance caps.

I was wondering if it had a crystal oscillator but it does not. If it did you could have used a ocxo but it looks like it uses line frequency as a stable oscillator.





Nick
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Old 12th January 2009, 05:53 PM   #3
labjr is offline labjr  United States
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Quote:
Originally posted by nhuwar
I know this sound corny but you could try leaving it on for a few hours before you use it. The drift could be caused by it just heating up. You could also try recapping the unit with higher tolerance caps.

I was wondering if it had a crystal oscillator but it does not. If it did you could have used a ocxo but it looks like it uses line frequency as a stable oscillator.





Nick
Hi Nick!

Thanks for the reply

I've actually worked on quite a few of these units. They all seem have the same problem. The oscillator circuit is TTL and is unstable no matter how long your warm it up. The oscillator circuit probably needs to be redesigned. It outputs 146.66666 khz when calibrated so it's not an off-the-shelf frequency.

Actually the line frequency is used just as a reference to calibrate the oscillator.

I'm hoping someone can help to devise a nifty circuit that retains the adjustability, yet is very stable. Maybe even with a center detent which locks the oscillator to dead-on 440 HZ concert pitch, since most people want to calibrate it for perfect pitch.

The good thing is that the oscillator circuit is all on one module which is removable. So a new board could be easily installed. Without making permanent modifications to the unit. Thus retaining the original value of this old classic.

There are a lot of ST-11's out there so I think it would be a good update. I just know know where to start. There are probably far more talented people than I, so I thought I would suggest the project.

Thanks
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Old 12th January 2009, 06:12 PM   #4
nhuwar is offline nhuwar  United States
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Well good luck, and sorry wasn't more help.


Nick
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Old 25th August 2009, 10:12 PM   #5
tiziano is offline tiziano  Belgium
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Could you send me the electric diagram of that oscillator please ?
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Old 25th August 2009, 10:30 PM   #6
tiziano is offline tiziano  Belgium
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I got it ... I saw the oscillator ... I think is the section of 72121 and the NPN transistor ... well ... How can you pretend that this stage can be precise and stable ? Sorry but I saw: 10% resistors, instead of 1% or 0.5% .. no crystals ... just RC oscillator ... I ask myself: what should be the real frequency generated by this stage please ? And the motor: is a step motor ? Because I don't think it is ... please correct me if I wrong ... bu what I see it's just something was good 50 years ago ... today you wan have better circuitry ... with step motor and crystal oscillators ... so I have just the following questions:

- frequency of the oscillator ( what should be )
- is it a step motor already ?

Thank you very much
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Old 1st September 2009, 11:47 AM   #7
Enzo is offline Enzo  United States
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It is not a stepping motor, no.

have you measured this drift in oscillator freq? In servicing these, especially as they age, I often have to replace the motor caps. Dry caps could make the motor drive unstable. And keeping fresh oil on the motor shaft also prevents it from dragging. And allow the wheel space back from the window to reduce air drag.

There may well be tons of these out there, but I doubt there is much of a market for a replacement board. COnsider that for the price of said board and the labor for a tech to install it, you could have a brand new tuner, more accurate and stable than the old Conn, easier to use, and with a warranty.

Yes, I realize this is an ancient thread.
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Old 1st September 2009, 12:11 PM   #8
tiziano is offline tiziano  Belgium
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Quote:
Originally Posted by Enzo View Post
It is not a stepping motor, no.

have you measured this drift in oscillator freq? In servicing these, especially as they age, I often have to replace the motor caps. Dry caps could make the motor drive unstable. And keeping fresh oil on the motor shaft also prevents it from dragging. And allow the wheel space back from the window to reduce air drag.

There may well be tons of these out there, but I doubt there is much of a market for a replacement board. COnsider that for the price of said board and the labor for a tech to install it, you could have a brand new tuner, more accurate and stable than the old Conn, easier to use, and with a warranty.

Yes, I realize this is an ancient thread.
Hi Enzo indeed you're right ... but just wanting this instrument, for fashion, and just to have fun ... it's interesting to change this oscillator with another one more stable.
All the problematics you told are true ... but consider that Peterson still producing turning wheel tuners for professional use ... with a series of motors and so ... it means that some kind of precision is reached ... in order to compensate little changes in rotation, it's needed to have some kind of feedback that here in the ST-11 there is not ... as a turn counter made by otpical disk for instance. At that time, there were photocells and lamps .. so that it was possible since early '70s ... but I don't find this ... the engine rotates supposing that it's own speed stills constant all the time ...
Then I have even a question: what's the frequency of the oscillator ?
And also: what is the ratio of this synchro motor ? I mean: If I drive it with 50 Hz, how many turns per minute I will get ? 500 ? 5 000 ? 600 ? 6 000 ? Not clue ... those data will help to improve this circuitry maintaining the old fashion estetic ... what do you think ? Is it possible to get those data somewhere ?

here where to buy an high precision oscillator with the correct frequency: http://www.icmfg.com/oscillators.html

Last edited by tiziano; 1st September 2009 at 12:22 PM.
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Old 2nd September 2009, 01:30 AM   #9
Enzo is offline Enzo  United States
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One could buy a higher freq oscillator and divide it down. But i think the first step is to accurately quantify the instability in the existing circuit. Just how much is there really? And is the power supply stable or does it drift?

Unless the existing dividers are also unstable, all you really need is a master oscillator to replace the RC one. Then use the existing divider matrix to get the individual speeds.

I brought up things like mechanicals and motor caps because I don;t automatically assume speed instability is caused by the oscillator. It may well be, but I don't assume that as a starting point.

And while we are at it, all those ICs are in sockets, are they not? And over the decades some of the socket contacts may be getting resistive.

If you want it as a project fine, then marketing is irrelevant.

I have several of the old 12-wheel COnn 6T5s here. A couple even work. Talk about projects. Tube driven tunig fork oscillator as the basis.

Peterson is shifting to LCD screens with a graphic representation of the wheel pattern. A moving pattern on the screen works the same way as the image on the spinning disc.

I have no idea what RPM the wheel spins on each note. Take an existimng unit and measure. it may be drifiting, but you should be able to empirically get the data you want.
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Old 5th September 2009, 10:29 PM   #10
tiziano is offline tiziano  Belgium
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You right but I cannot get an existing unit to make the misures I need ... you right for the new electronics devices also Turbo Tuner is a solution extremely valid but as I told: the fashon of this object is higher ... the problem is to have more precision than what it has really and ... for me it's an hobby ... to get fun rebuilding oscillator ... but ok it looks alike a "industrial secret" so I will not do ... and I will simply merely buy a new generation one :-/ ... sometimes enthusiasms are frozen in this way
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