8mm Forum: ST 180 Motor Speed Control

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Posted by Mark Norton (Member # 165) on September 01, 2008, 03:16 AM:

Have an ST180 which only runs very fast, no motor speed control.
On investigation I found the capacitors and the big IC M51970L on the motor control board were smoking like a steam train when the motor was run!

So I replaced the capacitors and the IC M51970L, as soon as switching the motor to FWD the IC went pop and burnt through at pin 5 again. Did not leave it running long enough for the capacitors to start smoking again.

Any help would be appreciated

Posted by Martin Jones (Member # 1163) on September 01, 2008, 04:03 AM:

Mark,
I believe that this is the same machine as ST-600 M; perhaps Kevin would confirm this? Or let me know identification number (MEC-XX?) on the board. If so,I have circuit diagram I can copy to you (email is best).

Martin

Posted by Mark Norton (Member # 165) on September 01, 2008, 04:23 AM:

Thanks Martin,

The board is MEC - 16.

Posted by Kevin Faulkner (Member # 6) on September 01, 2008, 07:52 AM:

That certainly looks similar to the board in my my ST600D but I'm sure the motor control board in the std ST600M is just a handful of transistors instead of the later IC.

Mark, If you are getting instant failure of the IC it would suggest a short circuit.
It could be one of the capacitors but I would also check those diodes and transistor for shorts. There is probably nothing special about those diodes and they could probably be replaced with 1N4002 diodes available from Maplin. I will look at the board in my machine for you but in the meantime stick a meter across each of the diodes, capacitors and transistor to check for shorts. Let me know what number is on the transistor and I'll see if there is a suitable equiv for you in case you need it.
Is that a power transistor on those wires? I bet that is the problem! Just remembered that one and yes they do fail.

Kev.

Posted by Martin Jones (Member # 1163) on September 01, 2008, 08:33 AM:

Mark has the later IC-based board; the original had two on-board transistors and a seperate power transistor. His board has an IC controller plus two on-board transistors for power output to the motor. I've sent him the circuit diagram for this board, which bears the same bord ID no. MEC-16 as the original board.

Martin

Posted by Mark Norton (Member # 165) on September 01, 2008, 10:17 AM:

Thanks Martin and Kev, i'll let you know how I get on. I got the IC M51970L from a search on EBay funny where things turn up. lucky for me I ordered two.
Martin, thanks again for the circuit diagram, even though it is for the original board it will still make things a lot clearer for me.

Posted by Martin Jones (Member # 1163) on September 01, 2008, 11:13 AM:

Mark,
Not clear what you mean "for the original board"? The one I sent is for the newer IC board that you have got.

Martin

Posted by Mark Norton (Member # 165) on September 01, 2008, 02:10 PM:

Sorry Martin, I was confused. In my excitement I didn't read your email properly. I wondered why you said the only layout I have is for the original board, didn't notice that you also have a diagram of the PCB layout.

Posted by Mark Norton (Member # 165) on September 02, 2008, 06:20 AM:

Checking through, the Transistor on the circuit diagram marked 2SA640 has shorted.
Any ideas for an equivalent? If not I've managed to find the exact replacement but i would have to order from America.

Posted by Kevin Faulkner (Member # 6) on September 02, 2008, 06:37 AM:

Just as I said in my email to you earlier. Mark, get a BC214L from Maplin. This has the same leadouts etc so will go straight in place of the original.

Kev.

Posted by James E. Stubbs (Member # 817) on September 02, 2008, 11:21 PM:

Since we're on the subject of ST-180E's... Mine has no sound. Seems to me the amp board is totaly dead. No speaker sound headphones nadang thing... Leon Norris suggested that the OP AMP IC is likely bad. I've not actually checked it yet, but I will. When I get around to doing that are there any other things I should check out to solve the no sound issue?
Thanks,

Posted by Kevin Faulkner (Member # 6) on September 03, 2008, 12:16 PM:

One other thing you could check is to plug an eternal speaker into it and see if you then get sound. I have seen the external speaker socket give problems due to dirty contacts. It turns off the internal speaker when you insert a plug into the socket. If the contacts get dirty you can loose connection to the internal speaker.

Kev.

Posted by Martin Jones (Member # 1163) on September 03, 2008, 12:53 PM:

Posted by Steve Klare (Member # 12) on September 03, 2008, 01:00 PM:

I've had the same contact-oxide problem from the headphone jack. I had no sound so I thought I'd try to isolate which section of the circuit was bad by listening with headphones. When I tried I got sound. When I unplugged them I got sound through the speaker too...

Posted by Martin Jones (Member # 1163) on September 03, 2008, 01:26 PM:

If your'e wondering about the blank post, I've removed it because it was wrong. If the suggestions so far do not help, check the internal speaker itself and the plug and socket (internal,RCA) connecting it to the machine. If none of these help, and the motor runs OK, you have a problem in either the amplifier DC supply or the amplifier itself. A certain amount of elimination of causes and, only possibly, diagnosis can be achieved by email if you want to try.

Martin

Posted by Kevin Faulkner (Member # 6) on September 04, 2008, 05:52 PM:

James, Just reading back over the posts with regard to no sound and I missed an important question.

If you play a sound film and turn the vol control up do you get movement of the VU meter?

If yes is the answer then its a bad connection to the speaker as the meter is driven by the output IC.

If not then its another problem such as blown output IC or or power supply. Do you hear any hum form the speaker at all?

Kev.

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