I need to know if this is normal or not. When aligning the 737, things go fine until T5. T5 couples the mixer into the ceramic filters. I find adjusting it has almost no effect on the signal amplitude, even though I'm right down near the noise where I peaked everything else. Anybody done one of these or similar?
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Assuming that you're aligning with a distortion analyzer in the "modern"/1970s manner, tuning for minimum THD+N at progressively lower RF levels, this isn't necessarily unusual. That transformer is very low Q compared to the ceramic filters, so doesn't show peaks very well. You'd be safe and correct to return the core to its original position and call it good. You are measuring everything important in this case.
All good fortune,
Chris
All good fortune,
Chris
Still suspicious, I applied a signal to the collector of the mixer transistor, the input for T5, and did a sweep, looking at the output of T5 before the ceramic filters. What feeble peak there is resides at about 10.5 MHz and moving the slug top to bottom doesn't change much. It also doesn't change THD or anything else in the alignment. If nothing else, I'd expect the peak to be closer to 10.7 MHz. As reluctant as I am to pull it and investigate further, I think it's probably time. Just never ran across a "does little to nothing" adjustment before!
I have a SX780 that uses the same. You need a strong signal and a signal meter or voltmeter on the signal level post.
Adjust for highest level on the meter.
Adjust for highest level on the meter.
OTOH, you're observing a difference of 2%, possibly within the tolerance of the sweep generator's cal, and certainly within the batch tolerance of ceramic filters, which are sorted into groups and color coded for center frequency. Interesting to investigate, but probably just normal tolerancing.
All good fortune,
Chris
All good fortune,
Chris
OK, after much agonizing and pondering, I'm concluded T5 is working correctly. At the frequency and level specified, it shows enough peaking to be convincing. Not dramatic, but consistent with other adjustments. Moving on now!
T5 seems to require doing exactly what the alignment procedure says- peak it at 60 dBuV signal using the front panel meter. What led me astray was trying to get too fancy!
Ok, sign me up for the loony bin. The last step of the alignment of the MPX section is, wait for it, adjust T5 for minimum THD. It has no effect whatsoever at any signal level or measurement setup I can come up with. Maybe they built it this way or maybe T5 has a problem, but distortion is low and everything else is OK, so this may have to go home the way it is. My confidence in their procedures would be higher if they hadn't mixed up which pot was amplifier offset and which one was bias current!