8mm Forum: My GS-1200 Just Bit the Dust

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Posted by Claus Harding (Member # 702) on October 20, 2011, 10:23 PM:

I like the GS, but....this is getting bad.

The machine has never inspired the trust in me the ST-1200HD does, in terms of just running and not acting up.

I had a trailer reel on when all of a sudden, the transport stopped with the lamp on. Blob on screen, me lunging for the light switch...I checked everything, film wasn't jammed, and hit it one more time.
5 Seconds and bang, once again.

I took film out, ran the machine empty. After about 10 seconds, it did it again, it waited about 3 seconds and started back up. Then it did it again, waited, then ran slowly.
At that point the acrid smell of burnt electronics was suddenly filling the air, so I shut it down.

The speed issue has always been there with this one.
Sometimes the variable speed wouldn't work, or work only in the "slower" direction; sometimes the machine itself was sluggish....I guess Leon will get Christmas money from me [Frown]

Has anyone had this problem with their GS? Any suggestions as to what is happening? I imagine whatever central board runs the projection speed has gone bad. The rewind was working fine, but then that is a dedicated little motor which presumably runs "on its own time."

I did not need this...I was about to send my ST-1200 up for a bit of regular maintenance, and now the Big Boy has gone down.
It makes a man think of having just STs with two-bladed shutters.

Claus.

Posted by Larry Arpin (Member # 744) on October 21, 2011, 12:48 AM:

It sounds like a problem I was reading about so I have copied the solutions:

Things you need to check are the ESS switch for dirty contacts, main drive belt for slippage and speed control switch.

The other possibilities are worn motor brushes or a fault on the speed control board.

I have seen the very same symptoms as yours on GS's before and most often its caused by 3 dioides in series with the neg connection of the motor. These 3 diodes are in series with the neg connection to ground and its across these that the protection circuit monitors the motor voltage. These diodes run very hot and on some machines dry joints eventually form giving the sort of intermittent fault you are describing. You will very often find that the machine will run for a while and then die as the diodes get hot. When the diodes cool down again the machine will once more try to, or run.

The diodes are on the main board by the rectifier blocks and smoothing capacitors.

To see if this is the case put the machine on its front so that you can view the underneath of the main circuit board and check round the centre of the board. If these diodes are suffering from dry joints you will find a patch on the board which is scorched and the solder looks crystalline.

When this occurs I remove the old diodes and replace with higher wattage (1.3W) versions and make sure that all old solder is removed. In some cases this scorching has damaged the tracks in which case a slightly different approach must be used to get round the burnt track problems. This can only be determined from the extent of the damage.

I hope this gives you an insight into the problem and gets you out of trouble. Check the easy things first.

LA here: Hope I haven't violated any rules. Just trying to help out.

Posted by Claus Harding (Member # 702) on October 21, 2011, 01:45 AM:

Larry,

First off, thank you so much for your detailed explanation; I can change belts but have little/no experience in hunting for bad electronics, but your post made me want to go in and see if in fact what you describe is the case.

I will start with all speed-related switches and go from there.

Perhaps time to break out the old soldering iron and get daring... [Big Grin]

Claus.

Posted by Martin Jones (Member # 1163) on October 21, 2011, 04:35 AM:

Larry,
Definitely no rules violated!!! Your kind of accurate and detailed information based on REAL experience is exactly what helping other people is all about.
Information and advice based on surmise and "urban myth", (and that which is in no way related to the problem in hand), is all too common, and sometimes difficult to spot until the damage is done!
Martin

Posted by Steve Klare (Member # 12) on October 21, 2011, 03:36 PM:

Hi Claus,

While you are being daring please keep mindful that the cool thing about diodes is they operate differently inserted in a circuit one way than the other...

-and spectacular events often happen when they are installed backwards!

Posted by Graham Ritchie (Member # 559) on October 21, 2011, 05:10 PM:

Years ago I had the GS judder and the feed stopped. I found that the rewind motor on the front arm has to be turning when fwd in selected. If the front motor stops for any reason the tension between the film going into the top sprocket increases big time...not nice. What I did was to clean the commutator "gets gummy" and those two small brushes also carefully scrape clean between the copper "segments" on the commutator. I did both take up and rewind motors in place by carefully unscrewing the motor rear cover and removing it with its brushes. Anyway its been years now and "touch wood" its been running fine. [Cool]

Claus never sell your ST1200 projectors those machines are much much more reliable than the GS1200. [Smile]

Graham.

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