8mm Forum: TQIII Speed Problem Cure

This is topic TQIII Speed Problem Cure in forum 16mm Forum at 8mm Forum.

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Posted by Paul Mason (Member # 4015) on September 02, 2016, 01:00 PM:

Some members may have noticed that I was looking for a green B & H TQIII projector to act as a donor for spares for my 1694 with a speed problem. The 1694 was running very slowly and the lamp voltage warning lights flashed randomly. A fellow member kindly sent me a cheap 1695 with a cracked worm which was ideal.

I replaced the 1694 motor servo IC with a later type circuit board from the 1695. This stopped the warning light problem but the projector still ran very slowly. Racking my brains for some time I decided to check the motor drive belt. It seemed OK- maybe a very little slack?

Nevertheless as I hadn't got a new belt I changed it for the old one from the 1695. This belt seemed almost the same as the other one. Result once I adjusted the speed pots slightly it ran properly at 18 and 24 frames/sec. I guess the old one had stretched enough to cause a problem. Moral seems to be don't overlook the obvious and you can't tell whether a drive belt is OK just by looking at it.

Posted by Paul Mason (Member # 4015) on September 10, 2016, 06:16 AM:

To update, I noticed that I hadn't cured the flashing overvoltage warning lamp - just the speed problem. The warning lamp is connected to the lamp overvoltage sensing circuit board. This is on the left of the motor board in the projector base. Replacing the old board stopped the annoying flashing. You can calibrate the overvoltage threshold but I haven't bothered so far.

Posted by Dave Groves (Member # 4685) on September 10, 2016, 08:00 AM:

Paul, how I envy the abilities of you and other members. I can write a letter to anyone and spell like I was a dictionary, but a mechanic I'm not. I seem to have been born with a brain with all the investigative elements missing. Perhaps next time round it will be different, and then I'll discover all the projectors have died the death and it's all gone digital. Thank goodness there's still a few folk left I can take my machines to.

Posted by Paul Mason (Member # 4015) on September 10, 2016, 11:54 AM:

Hi Dave,

We all have our strengths and weaknesses. I'm a middle-aged electrical engineer with an interest in analogue electronics so I don't know much about (for instance) digital electronics or how to change a worm gear. The TQIII projectors have several circuit boards so if you identify what they do it's fairly easy to change them.

Posted by Terry Sills (Member # 3309) on September 10, 2016, 02:27 PM:

Well done Paul.
A while ago I posted a similar log when I had a problem with fluctuating speed on a projector. An initial cursory examination of the drive belt showed no obvious problem so I investigated all other probable causes which proved fruitless. I went back to the belt, took it off and on closer examination found a section of ununiformity which as it passed over the drive pulley was causing the variation in speed. Changed the belt and Hey Presto!
The moral - always thoroughly check the simple and obvious/ most likely causes first.
Perhaps this is the first law of Physics/Dynamics/Engineering - who knows, but it is something so easily dismissed. we always assume the worst. I wonder why?

Posted by Paul Mason (Member # 4015) on September 11, 2016, 01:12 PM:

Thanks Terry,
I have now changed the motor belt again for a new one. The original belt and the first replacement were almost smooth on the inside. The new belt has serrations for better grip.

Posted by Paul Mason (Member # 4015) on September 16, 2016, 11:56 AM:

I've noticed the noise from the amplifier when the volume seemed excessive and crackly so I replaced the power amp board for a new one. However the pins for the wiring plugs are much closer together on the preamp/volume board needing smaller plugs than the existing ones. I want to avoid disturbing the projector wiring so this needs thinking about. [Frown]

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