Film-Tech Forum ARCHIVE: Help with Kodak Pageant AV-126-TR Amp

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Posted by Patrick Curran (Member # 9925) on 02-03-2018, 03:08 PM:

I was rebuilding the amp for my Kodak Pageant projector and I accidentally turned the 500k bias potentiometer thinking it was the volume control. I looked at the service manual for it, but it didn't give very specific instructions on how to make sure it's in adjustment or not. So far, I've just set the pot to the lowest setting using my multimeter, but I really don't know how far to set it from there.

Posted by Steve Guttag (Member # 268) on 02-03-2018, 06:38 PM:

It has been awhile since I'd worked on amplifier circuits, to be quite honest.

However, what bias is doing is to, in a class A amplifier, is to place the output in the middle of its range (hence Class A amps run hot since they are ALWAYS on). In a class B or AB (what you have), the goal of bias is to eliminate/minimize crossover distortion (when the positive and negative alternations exchange. It should be continuous. So a suitable sine generator and O'Scope and looking at the output at that point should let one know if they are there and how it is shifted would let one know direction to turn the adjustment. Note, the instructions essentially specify that you obtain SMPTE P16SL (400Hz tone at about full modulation though it is really just for balancing two machines for equal output) so if you have some, you can check the output voltage spec too.

The schematic of the amplifier does list voltage points. IF your meter is COMPLETELY ISOLATED (battery powered, not plugged into the wall to get any ground reference), then I would measure Vce of Q-4 (Test Points 5 and 8) and adjust until I go their desired values (at least as a starting point). Vce (V5, 8) seems to be -14.7VDC (no signal on the input).

That is where I would start and then check with actual signal and finish based on what the scope said (if anything).

Note, whereas you probably have dried out capacitors in there and the resistors are likely in the 10% range (capacitors in the 20%), the voltages you will read at the test points are going to be approximations but it would be worthwhile to go through it and check each test point to see if any are notably out of range.

Posted by Mark Gulbrandsen (Member # 72) on 02-18-2018, 01:41 PM:

Apply a LOW LEVEL 15khz signal to the amp and attach a scope on the output to you have just a watt or two output. Adjust for minimum crossover distortion in the middle of the sine wave as seen in this image. Let the amp idle for an hour and be sure it's not running too hot. If it is back off on the bias slightly, slowly over another hour till it's no more than just warm. Those amplifiers are not class A and should not run very hot. I used to service hundreds of Pageants every year back in the 70's for the local school district, they are very easy to fix. Alternatively, you can use a distortion analyzer to do this and see if you can make it meet specs.