B&H 1695 TQ3 running too fast

[Jonathan Wood]

Hi all, I have a Bell and Howell 1695 TQ3 (Grey) projector that is running way too fast, probably around 35-40fps. The speed switch has no effect. Ive tried adjusting the two pots as per the manual but this has no effect. Ive had a quick look inside and the belt looks fine (being a TQ3 it only has the one pulley so I don't see how that can be the problem.) I've noticed small signs of scorching on the end of the 1000mf 35v axial cap on what I think is the fuse board (Its straight in front of you above the motor?) which I will replace. Does anyone have any ideas ? I'm assuming the speed control PCB is in the bottom near the amp ? . Any help or advice will be gratefully received .
best wishes, Jon
(Also posted on the 8mm forum)

[Tony Bandiera Jr]

I would first replace that cap as it has most likely failed. Unfiltered DC feeding control electronics results in strange and bad things happening. If that alone does not solve the issue, time to dig deeper.

Measure the driver transistors/mosfets to make sure they haven't failed shorted. If they have shorted, you'll need to carefully check ALL of the drive control components to make sure they haven't failed and caused the drivers to fail. Replace anything questionable or obviously failed.

I tried a search for a schematic with no luck. If you can take very clear and good sized pictures of the speed control circuit board I can try to guide you to specific components to check out.

Too bad you are so far away from my location, this would be a relatively easy fix for me on my bench. But I think shipping to the US from UK and back would far exceed what the projector is worth.

[Jonathan Wood]

Hi Tony,
thanks so much for taking the time out to reply , it’s much appreciated!

i found the manual here
http://www.film-tech.com/warehouse/manuals/BHTQIII.pdf

i believe the capacitor in question is C301 on page 79, one side does seem to go to the motor and the other to the PCB so your Initial theory could well Be right . I put a meter across it (in circuit ) and it does seem ok but there is definitely evidence of scorching at one end .
EDIT I took a closer look at the board and I think it’s c302 that’s suspicious not c301

Since I made the original post I found a spare PCB in an old machine I’d forgotten I had so it might be best just to swap it out ?
Pic attached

I also have the board with the cap on it so do you think it’s worth just swapping both boards ?
the spare boards are from an older turquoise 1695 , and the diodes look older but I’m hoping it will be the same spec?

yes it’s a shame I can’t get it to you as I’d definitely take you up on that offer but I dread to think how much shipping to the US would cost !

thanks again Tony

[Tony Bandiera Jr]

Ok thanks for finding the manual as well..

Here's my take on it, after reviewing the circuits in the manual:

The "Motor Servo IC-101" (Part ME, see page 7 of the manual) is probably shorted, as it is what controls the speed of the motor in conjunction with the tachometer (ref.pg 63). That said, If the tach leads have an open connection, that could also cause a full speed runaway. Check for continuity on the yellow wires at the points "J108" (Pins 6 and 7 of the servo IC socket evidently) (ref, pg 63) If it is open circuit there, that could easily be the problem.

Do you have a spare of that ME IC-101 in your other machine? If so get the actual part number printed on it and see if replacements are still made.

Can you get a good pic of the scorched capacitor?

And it looks like, based on the several updates in the manual that there are changes from each model year, so I am not 100% confident that a board swap would work. You would have to visually verify that EVERY component on the two boards are identical. Also, since those caps are also old they may be bad too. Better to replace with fresh caps of the right ratings and type.

Don't change the IC-101 "ME" until you are sure there are no other failed components in the speed control board. I have a feeling that the IC is no longer made so a replacement may not be obtainable.

[Jonathan Wood]

Hi Tony,

thought I’d update you !

i found another servo board identical to the one in the machine so I did a straight swap I also changed the suspect cap on the fuse / diode board , all to no avail ! I then changed the whole fuse/diode board and still no joy .

I did try to meter pins 6/7 on the ic but I got open circuit not only on the board in the machine but also on the other 2 ( one of which I believe was ‘new’ and unused .) Not sure where to look for the tach leads ? I’ve attached a pic of the suspect cap but when I removed it , it checked out fine on the meter ?

I’m a bit stumped really so any further advice would be very gratefully received .

cheers

jon

ps I ran the machine a bit longer this time and I kind of starts off roughly normal , albeit a bit unsteady and then it gradually build up speed to around 45 FPS, do you think it could be the motor or maybe the transformer ?

[Tony Bandiera Jr]

Sorry for the lag, very busy out here.

That mark on the cap is from the spot welding of the lead wire to it, so if it tests ok on a cap tester it is fine.

The tach leads are, according to the schematic, attached to those IC socket pins. they run from the motor and are two of the four wires coming out of the motor. The tach could be nothing more that a set of contacts that open and close as the motor turns. Try measuring continuity as you slowly turn the motor by hand and see if it shows a pulsing continuity.

I think the IC could be bad, or there is something else in the electronics acting up. The "starts off roughly normal, albeit a bit unsteady and then it gradually build up speed" tells me that something electronic is overheating and could be causing the problem. Get a can of "Freeze spray" from an electronics supplier, and when the machine reaches max speed, spray components one at a time, starting with the control IC, then any transistors, then diodes, and finally resistors. (Capacitors usually don't fail in a way that would react to freeze spray, but if all the other parts don't react to the spray hit the caps too.) You should find one or mare parts that react to the spray and those would be suspect. If the suspect part is a transistor, make sure you test ALL components associated with it using a meter to make sure they aren't out of spec or bad, BEFORE you replace the transistor.

Hope all this helps, it would be awesome if my remote service call gets this thing going properly.

[Jonathan Wood]

Hi Tony,

thanks so much for your reply , I really do appreciate the time you’re taking out to help me !

I’ve ordered a can of freeze spray and will try your suggestions as soon as it arrives .
I did swap out the complete ic Board for another one and I still got the same results . I just plugged the new board in and left it ‘dangling ‘ I don’t know if this would have had any influence on the results? Maybe the board needs grounding to the chassis to work via the aluminium bracket ?

it would be very cool if your remote diagnostics get it going , you clearly know what you’re talking about !

best
jon