8mm Forum: Audio Problem with ST-1200

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Posted by Dan Lail (Member # 18) on September 17, 2009, 05:19 PM:

I am having audio problems with an ST-1200 mag. I was using the speaker out to a matching speaker. While viewing a film the sound died. Inside the back I found the fuse had blown. Also I checked the speaker cord and found that the two speaker wires were pulled and twisted around each other causing a short. I repaired the speaker wire, but now every time I turn the sound on it blows a fuse. The fuse value is 3amp 120v. I replaced it with a 3amp 250v. Help!!

Posted by John W. Black (Member # 1082) on September 17, 2009, 09:51 PM:

Dan,don't increase the fuse value!The reason the fuse is blowing is before the fuse in the circuitry.You could burn out the amp altogether.The speaker wire more than likely shorted out a resistor.Also,for your pot,www.DigiKey.com will have your pot.Good luck.Also,you might want to find an alternative to Leon for your work.He doesn't have a real electrical background.Years ago.he trashed a GS1200 that belonged to a good friend of mine.I've known him for years,a nice guy,but I wouldn't let him work on my stuff.He learned from messing around with projectors.I've done the same,but I would never offer to work on someones machine.In Reel Image,there are a few real repair shops that advertise.Good luck!

Posted by Steve Klare (Member # 12) on September 18, 2009, 06:09 AM:

Resistors usually don't short out when they overheat (they open up), but semiconductors very often do, This is likely something like a transistor, diode or IC. Hopefully it's something easy to find a replacement for these days.

My best guess is a power transistor in the last amp stage.

Posted by Kevin Faulkner (Member # 6) on September 18, 2009, 06:34 AM:

Its the STK Audio output IC which has failed and is extremely common in the ST1200 series of machines. The IC will fail if a 4 ohm speaker is used at high volumes so a dead short will take it out imediately.

I have had to replace this IC numerous times for various people and quite often it's becuae they have wired two 8 ohm speakers to the one output which presents only a 4 ohm load on the amp. What many poeple dont realise is that as the frequency lowers the impedence also goes down and can present nealry a short acroos the IC output and hence the failure.

A direct short as in this case will kill that IC straight away. That IC is now very difficult to get hold of and its a case or scouring the web for someone who may have one in stock.

An STK015 or ECG1323 will replace the IC used in the whole ST1200 range of projectors.

Try this eBay seller in the US:

http://cgi.ebay.ph/ws/eBayISAPI.dll?ViewItem&item=230141731905

Kev.

Posted by Martin Jones (Member # 1163) on September 18, 2009, 07:33 AM:

Kevin,
What's the full code number of the STK in those machines? I have a large inventory of IC's from my TV repair years that I can go through to make available for anyone needing anything; also are there any other rare types that you can think of?
Martin

Posted by Kevin Faulkner (Member # 6) on September 18, 2009, 08:28 AM:

Martin, I think Elmo used a few different variations which are all pin for pin compatible its just the operating voltage hence wattage which varies. They have used the STK11, STK12 and STK15. The 15 will replace the others which is why I listed that one above.

The EGC1323 is the Philips European version of the STK15 which is a pin for pin and electrical equivalent and can be used as a replacement.

The STK range of output IC's were quite popular arround the era of 8mm projector manufacture. They were used in some of Elmo's 16mm machines and also the GS1200. The GS1200 uses the STK339 and the Xenon version uses the higher output STK432.

Those are just a few to start with. I'll see if I can find some others to list.

Kev.

[ September 22, 2009, 10:48 AM: Message edited by: Kevin Faulkner ]

Posted by John W. Black (Member # 1082) on September 18, 2009, 10:54 AM:

DigiKey should have them in stock

Posted by Kevin Faulkner (Member # 6) on September 18, 2009, 11:04 AM:

Doesnt look like it John.

K.

Posted by Dan Lail (Member # 18) on September 18, 2009, 03:30 PM:

Thanks everyone.

Kev, I just bought the STK015 from the link you provided. I will try and solder it in myself. In the meantime I think I will opt not to use the speaker out, but use aux out to an amp. [Wink]

Posted by Kevin Faulkner (Member # 6) on September 18, 2009, 07:14 PM:

Dan, thats the way to go. Replacing that IC should be easy. Let us know how you get on. Kev.

Posted by Dan Lail (Member # 18) on September 18, 2009, 11:18 PM:

What would be a good soldering iron to purchase?

Posted by John W. Black (Member # 1082) on September 19, 2009, 12:40 AM:

A Weller with an extra thin tip would be best for circuitboard work

Posted by Kevin Faulkner (Member # 6) on September 22, 2009, 10:49 AM:

A solder pump to desolder the pin connections from the board is also a good idea.

Kev.

Posted by Mike Minton (Member # 1755) on September 23, 2009, 11:07 PM:

Hi there. Brand new here. ALL of you guys seem to be really cool and helpful to everybody! Thought I'd drop a line. Don't know if it will help or not. I noticed someone was looking for the STK015. I know that NTE Electronics manufactures a lot of replacement chips. This might be one. NTE1027 Here is a data sheet for you to double check the cross reference. http://www.nteinc.com/specs/1000to1099/pdf/nte1027.pdf
If this is a good cross, Newark has plenty in stock.
http://www.newark.com/nte-electronics/nte1027/ic-audio-amplifier-single-sip-10pin/dp/31C2596?Ntt=NTE1027

Posted by Kevin Faulkner (Member # 6) on September 24, 2009, 08:15 AM:

Mike, how did you work out that the above IC was an eqivalent? Is there a list on their web site?

Kev.

Posted by Mike Minton (Member # 1755) on September 25, 2009, 12:02 AM:

I checked my electronic suppliers for that original part. (STK015) Some places like Digi-Key will show a compatible cross when you enter that part number. In this case, I know that NTE makes a lot of replacement chips for older discontined IC chips. Mouser, Newark, and MCM Electronics carry that line. I found this cross on either mouser.com or mcmelectronics.com.

Posted by Dan Lail (Member # 18) on September 27, 2009, 09:37 PM:

Hi, everyone. I must thank Steve Klare for sending me some very helpful tips on changing out the STK-015 IC Chip and Kev fro the diagnosis. It was actually an easy endeavor. This chip is right up front an came out smoothly. Now the ST-1200 is ready for the next weekend where it will perform at the Laurel and Hardy Festival in Harlem Georgia.

Here's another fine mess you guys have gotten me out of. [Wink]

[ September 28, 2009, 02:05 PM: Message edited by: Dan Lail ]

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