ELMO ST-180 Running Fast Fault

[Bren Jacques]

Hi all,

I'm currently repairing an Elmo ST-180 Super 8 sound projector. I'm currently trying to trace a fault with the motor running flat out and the speed control makes no difference. I've checked over the motor control board which only the 2200uF capacitor was faulty. For good measure I have replaced all of the electrolytics as I have the values to hand. The Toshiba rectifier diodes test okay as do the transistors (no shorts).

I did suspect the IC but after replacing the fault still exists. Without wanting to go on a component swapping spree I thought I'd ask here if anybody has had the same fault and pinned it down to a particular component? Also if anybody has the schematic for the board then that would be great, I know they exist but I have been unable to find a copy online.

Cheers
Bren

[Steve Klare]

Hi Bren,

I don't know ST-180 extremely well, but there is a speed sensor mounted on the motor of the ST-800 which tells the motor control circuit how fast the motor is spinning. If this is for some reason disabled, the motor control will "think" the motor is stopped and go absolutely full-throttle trying to get it up to speed. If the motor is healthy the frame rate will go through the roof.

Maybe something like that?

I have a schematic for ST-600 (should be close enough) and it shows what I would take to be some sort of inductive pickup next to the motor and feeding into the controller IC.

Do you have a schematic? I'd share the file if you don't.

[Bren Jacques]

Thanks Steve, it hadn't crossed my mind that there'd be a speed sensor on the motor, on the ST-180 there are two trim pots which I presume are for silent and sound speed setting. I wouldn't have expected them on the board if there was a sensor too but I could of course be very wrong. I presume the motor sensor is similar to a hall effect type picking up a magnetic field. Yes if you could share the schematic that would be excellent, even if different it's likely to share some similarities with the ST-180 I'm sure.

[Bren Jacques]

Just to update, the motor does indeed have a sensor mounted at the bottom end. I've checked the windings of the coil and there is resistance, the poles look in good order with no damage too so I have no doubt to believe that this section is not working properly. I'll check the continuity to the board and then check the boards tracks to ensure there are no breakages.

[Chip Gelmini]

What about HTZ adjustments.

I am not familiar with this machine. But in the ST1200 there are two pullies reflecting proper hertz using one of the belts. If the st180 is a DC machine, then I doubt the belt would be adjustable. Note that the st1200 is fully AC and therefore is a whole different ball game.

[Bren Jacques]

Hi Chip, yes the ST-180 runs a DC motor. After looking at the schematic I think it's likely one of the transistors is breaking down and shorting when exposed to the line voltage. I've experienced this before where a short won't measure on a multimeter but when a working voltage and load are applied the short develops.

[Kevin Faulkner]

There were 2 types of motor control board for the ST600. The ST600 & 800 are more or less the same machine electronically. The first Motor control used 3 transistors and the later used 2 transistors and an IC ((Sony M51970L). This very same Motor control board was also used in the later Stereo GS800 so one can suppose that it was also used in the ST800. If you've substituted the 2 transistors and the IC and changed all the capacitors then it only leaves a circuit break somewhere on the circuit board. You might have to try checking all the resistors. On the circuit below C203 is very critical and can cause problems if out of tolerance. 0.47uf 50V. One other thought is that the 18/24fps switch is not working or has a broken connection somewhere. Its such a simple circuit with not a lot to go wrong really.

[Kevin Faulkner]

Correction: Looking at that circuit again I don't think it will be the switch but it would definitely be worth checking the speed control pot for the 24fps speed in case its open circuit.

[Bren Jacques]

Thanks Kevin, I did notice my board is ever so slightly different with having an extra electrolytic fitted, a 22uF 10v, the rest of the circuit looks identical though. I will give all of the resistors a check and make sure they haven't drifted in value. I do hate just blanket changing components in the hope of repairing a fault but in this case there doesn't seem to be a definitive answer when following the circuit. As I previously said I think it's likely that one of the transistors is shorting when under working conditions and the D525 will give a very good path to the power supply line which would be the first suspect if this theory proves true.

[Kevin Faulkner]

Yes looking at that diagram if TR202 is leaky it could turn on TR201 with the same result. It did occur to me that if diode D203 were to become short circuit you would again get full power to the motor.
Lets us know what you find Bren.

[Bren Jacques]

Hi all,

Thought I'd update the thread. I still haven't got very far with the Elmo control board but I have noted the differences on the schematic today. This has been drawn exactly as my board stands in original condition so I presume everything should be correct. Notable differences are an extra electrolytic capacitor (22uF 10v), I've drawn this at the RHS of the circuit next to the Zender diode. Also note connections to pins 2 & 4 are reversed on this example. I've also added some dots to note circuit joins at either end of C205, on the original circuit these aren't here leading you to believe that C205 goes from the motor sensor straight go R207 with no further joins.

Some bits did lead me on a bit of a wild goose chase until I'd worked out the differences but hopefully anybody with a faulty board can use this updated schematic with some confidence.

Now I have a good schematic I can start investigations into the problem and hopefully find the cause relatively soon!

[Bren Jacques]

Further update - Problem Solved! I decided to see what sort of voltage I was getting from the sensor on the motor and the answer was absolutely none. Previously I had measured the resistance of the coil and I know it was good. This time around it gave an O/C measurement. A close inspection of the solder tag shown that the wire from the coil was wrapped around but never soldered. This was probably fine for many years but in its age and tarnished state it had lost its connection.

Soldered back up I now have a working speed switch, I will test the projector and make adjustments as necessary.

[Steve Klare]

Well,

The good news is that motor control IC Mitsubishi M51970L seems to be plentiful on EBay.

That could be failed and driving those transistors hard.

One indication of the health of the IC should be 2.0V between pins 3(+) and 5(-). It looks like that's a reference voltage supplied by the IC.

MITSUBISHI M51970L

[Kevin Faulkner]

Glad you found the fault Bren. Steve you have to be very careful ordering spares off ebay these days especially if they are coming from China. They are into sanding the original number of components and re stamping with the number you are ordering. There are just so many fake components now on ebay.
For this reason I only order electronic components from reputable suppliers based in the UK. Yes they may be a little more expensive but at least they work and don't fail after a few day or not work at all.

[Steve Klare]

Hi Kev,

I completely agree with using established well known sources, whenever possible. I only go the eBay route with something that's painfully obsolete and not in mainstream catalogs, and fixing these machines I've faced that a few times. The pre-amp ICs I got on eBay saved my ST-1200HD, for example.

Where I work, we face the issue of counterfeit components all the time, and it's particularly nasty there because they may impact safety and downtime is very, very expensive. The parts that do turn up counterfeit are often very expensive ones with safety agency certifications, unfortunately both the parts and certs. are cheap fakes! The good fakes aren't obvious until smoke starts pouring out.

What we have in our favor here is there isn't huge demand for these old components, so it makes more sense for the counterfeiters to go after modern, high sales volume parts and leave us alone.