VenusAstronomer wrote: ↑Fri Sep 27, 2019 3:49 pm
Hi all -
I have an old-style Newtonian scope (14.5 inches), solid fiberglass tube, on a steel fork equatorial base, set in concrete, in an observatory. I live in Florida - LOTS of dew! The control box (which I have schematic for) was made for a place where there is no dew, so things have shorted out a few times over the years. Well, this time, by golly, I want to not just replace the shorted out capacitor, or whatever, I want to get this thing done right, in a watertight box. This SHOULD be a pretty straightforward thing, for someone who knows what they are doing! (As in - not me!) I have tested both motors (RA is 60 hertz AC; Dec is 12v DC), and when power is applied to them directly they work, so it's just the controls to the wires that are my problem (the accursed controller box). The RA motor, being 60 hertz, tracks along, with the help of fine adjustments, using a timing chip and diode, both of which have shorted out in the past. Fine adjustment centering in RA is done by either speeding this up, or "stalling" it for the appropriate amount of time. Dec centering, with the DC motor, is done by running the motor one way or the other. (I have the schematic for the box.)
Along with the tracking needing to be fixed, the field centering also needs work. The controller for this had been jury-rigged to 2 focuser hand-boxes, taped together. The previous owner used this, but I have just used the slip-clutch feature to manually center things - which is seriously AWFUL, especially with a big, heavy scope! I would like to get field centering control on a paddle type box. Seems like that wouldn't be too much to hope for in 2019!
Any suggestions on who (in Florida) I might be able to get to do this, or if I might be able to get this done myself with a little advise?
Hi VenusAstronomer,
Nice scope. The control circuit is pretty simple and should be pretty easy for someone with basic electronics skills to diagnose and repair. Because it has mains voltage floating around in there you need to be very careful. If you want to go to the expense of sending it to Australia, I can fix it for you. However, because your
RA drive is driven by a synchronous motor, you will never be able to have a high speed slew for centering. Synchronous motors can only be speeded up to about twice their native speed before they start to lose torque and stall and unless they are a special type of synchronous motor, they cannot be reversed.
However, if you want to get a better more useable drive system, I would suggest that you replace both
RA and
Dec with a dual axis micro stepper motor control or servo motor circuit which will give you high and slow speed slew in
RA and
Dec on one hand paddle. If you add encoders to a servo system, you could possibly even upgrade to a
goto system. There are off-the-shelf systems for doing this however because you have an
ATM scope, you will need to do a custom mechanical linkage which will involve a bit of work.
The POT you refer to doesn't tweak the power, it adjusts the frequency of the oscillator circuit. Synchronous motors speeds are only governed by the AC frequency. voltage fluctuations only affect torque not speed. If you run the whole thing off a DC supply, even a DC supply that plugs into AC, then mains fluctuations won't affect the speed or torque. Circuits like that are susceptible to temperature changes.
First things first -
To get your drive system working again in its current function, I can talk you through some easy tests that need no electronics knowledge.
1. Try powering the circuit from both the AC input and the DC input? Does one work and the other not?
2. There are three fuses in the circuit. These are the first thing to check. Disconnect completely from power before removing. If you have a meter you can test them but for a couple of dollars, you can easily replace all three of them. I have coloured them yellow in this schematic. The three fuses may have external screw off access points or they may be mounted in small spring clip brackets on the circuit board.
3. The two transformers are very unlikely to have failed.
4. The 556 timer oscillator (RED) is a very robust circuit. It is possible but very unlikely to have failed in my experience.
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EVERYTHING BELOW THIS POINT WILL NEED SOME BASIC ELECTRONICS KNOWLEDGE TO LOCATE, TEST AND REPLACE.
5. The wires from the handbox to the 556 (PINK) carry the resistances (ALSO PINK) that determine the oscillator speed. Resistors and capacitors in a circuit like this rarely fail. However, if any of those wires have a bad connection, the circuit won't oscillate, no oscillation, no 120vac output.
6. The power amplifier circuit amplifies the oscillator through the output transformer to produce 110v ac. Possible because this is providing the grunt(power) to the transformer it could have failed. if just one of the TIP29C power transistors(BLUE) have failed, the circuit won't work.
So try step 1 first. Then 2.
Amateur astronomer since 1978
...................Web site : http://joe-cali.com/
Scopes: ATM 18" Dob, Vixen VC200L, ATM 6"f7, Stellarvue 102ED, Saxon ED80, WO M70 ED, Orion 102 Maksutov, ST80.
Mounts: Takahashi EM-200, iOptron iEQ45, Push dobsonian with Nexus DSC, three homemade EQ's.
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