8mm Forum: Bell & Howell projector running too fast

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Posted by Tassos Laudas (Member # 588) on April 17, 2007, 09:27 AM:

Today something unexpectable occured to a friend's Bell & Howell 21DCR: The motor is running excessively fast and the speed selector doesn't work. A couple of hours before this, everything worked perfectly including the selector itself. After a small pause he started the projector again and a fuse went blown, and it was when he replaced it that problem began... Could it be the transistor nearby the motor's output on the board?
As far as know this model is virtually the Sankyo 700.
Any help would be appreciated!

Posted by Lee Mannering (Member # 728) on April 17, 2007, 11:04 AM:

I had a Bell & Howell that did this and it was a transistor which was sited at the front of the machine on the left circuit board. It was the one which controls the motor speed. When mine went it caused the motor to run at full on voltage and very fast indeed. I am sorry to say I had problems getting hold of the transistor and in the end replaced it from a tatty machine. I caused the problem myself when servicing the machine as I earthed a screwdriver from the transistor to chassis by mistake and had forgot to unplug the mains lead. At least I survived !

I offer this solution on as an example of the fault I had a few years back and it may not be the problem for you perhaps.

Good luck.

Posted by Tassos Laudas (Member # 588) on April 17, 2007, 04:09 PM:

Thank you Lee, in deed there is a transistor just behind the back cover that seems to need replacement. I hope it's the only one to be replaced! By the way, do you remember the exact type as the code number has been erased by the heat? I'd be grateful!

Posted by Rick Skowronek (Member # 385) on April 18, 2007, 02:43 PM:

Hi Tassos,

I can get you a replacement current part number for the transistor if you can find someone who has the schematic for the B&H (Sankyo 700)./ I've replaced a couple of these projector motor transistors and the value is not critical. That is except for appropriate current handling and rated voltage. From my limited experience, they tend to be NPN silicon transistors with a current rating of at least 6 to 8 amps maximum and usually a 100 VDC rating.

Depending on the mounting style on the heat sink determines what would replace it the best. If it's diamond shaped with two screws (one at each end of the diamond) and a solid metal case, the ideal replacement is a 2N3055. It's well over rated and would easily handle this application. If it is a square plastic transistor with a metal backing plate and one screw holding it to the heat sink and has 3 leads coming out the bottom into the PC board, it could be replaced by a TIP3055. Same capability as above but different mounting case type. Just for your search if you have trouble with these numbers the diamond case shape is a TO-3 and the square plastic version is a TO-220 case.

Pretty certain at the 90% level that one of these would do the job. Ease of replacing is dependant on your soldering/desoldering skills. When you buy the replacement insure you pick up a small tube of heat sink compound. This provides a thermal transfer medium from the transistor to the heat sink. always watch for and replace exactly any insulators used on the transistor and mounting screws. The only thing you need to check with our esteemed forum members is that this is indeed an NPN transistor. The replacements are dirt cheap and available most anywhere that sells parts.

Rick

Posted by Tassos Laudas (Member # 588) on April 20, 2007, 06:45 PM:

Rick i'd like to thank you for your message which i've found really helpful. I'll try out your recommendation and let you know wether it works!!!My major concern is wether it's only a matter of a single transistor to be replaced rather than many components...I hope the former!

Posted by Rick Skowronek (Member # 385) on April 24, 2007, 06:04 PM:

Tassos,

Me too. Some of the later models used full power integrated circuits with the associated circuitry and power handling capability all built into one IC. Those would not be replaceable. However, from the circuits I've been privy to, and that's not extensive, most have driver circuitry attached to an NPN transistor for the final control. Here's hoping that's your case otherwise bye-bye projector unless you can find someone with a scrapper that has a good motor control circuit.

Just for your confidence building, most Japanese projectors used the ciruitry/power transistor type. The early Japanese transistors were notoriously overated and could fail without notice. They ran very close to their actual max ratings.

Again, here's hoping that's the case with yours. Could be really cheap or SOL (S--t out of Luck) time.

Rick

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