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Post by oldheadtech on Jul 1, 2009 10:09:07 GMT -5
Here's a technical question about the synthesizer board on a D201. While doing an alignment per the instructions for the D201 manual, I find that adjusting the 16 Mhz crystals doesn't work out to what I think the book is saying. I'm told (I think) to adjust the trimmer cap C137 to adjust the AM frequency on channel one, but, find that it does nothing at all to change that frequency. However, the trimmer next to it, C136, easily adjusts the frequency to specs. This is the same all the way across the band, the indicated adustment cap is offset by one position. The USB adjustment doesn't work on any channel's trim caps. It looks like all the board's diodes are OK by ohm meter test and the resistors are all within specs. Have you seen this strange offset before? Any clues as to where to look for t he problem? Thanks and best regards!
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Sandbagger
Administrator/The Boss
Posts: 6,245
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Post by Sandbagger on Jul 1, 2009 11:00:15 GMT -5
Here's a technical question about the synthesizer board on a D201. While doing an alignment per the instructions for the D201 manual, I find that adjusting the 16 Mhz crystals doesn't work out to what I think the book is saying. I'm told (I think) to adjust the trimmer cap C137 to adjust the AM frequency on channel one, but, find that it does nothing at all to change that frequency. However, the trimmer next to it, C136, easily adjusts the frequency to specs. This is the same all the way across the band, the indicated adustment cap is offset by one position. The USB adjustment doesn't work on any channel's trim caps. It looks like all the board's diodes are OK by ohm meter test and the resistors are all within specs. Have you seen this strange offset before? Any clues as to where to look for t he problem? Thanks and best regards! C137 is the USB crystal trimmer. C136 is the AM/LSB trimmer, so as you found out, the AM adjusts properly when you tune C136. USB should adjust using C137 for the channel 1-4 crystal. Are you adjusting for the 16 Mhz frequency on the synth board, or are you adjusting for final 27 Mhz carrier signal? If setting for final frequency, you'll have trouble setting the SSB frequency due to there being no carrier present.
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Post by oldheadtech on Jul 1, 2009 13:43:58 GMT -5
Sandbagger, thank you for your reply! I need to say first that I meant to write that it is the 20 Mhz signal I'm looking at for frequency, not the 16 Mhz signal. I also think I see the problem with the adjustment ------ my service manual says that trimmer C137 is the AM/LSB trimmer and C136 is the USB trimmer, which is exactly opposite of what you (and experience) say. Apparently, when my manual was printed, the "trimmer/adjusted frequency" list was reversed in the alignment procedure. The way you say is correct. Here's one other question for you on the subject of adjusting the frequencies, do bad crystals usually result in little response to the trimmers, or, is the cause usually out-of-tolerance resistors on the boards? Some of the trimmers seem to do almost nothing to the frequencies, while others shift the frequencies quite a bit on the same board. Thanks for your courtesy and time in answering these questions!
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Post by 2600 on Jul 1, 2009 15:00:05 GMT -5
Quartz crystals drift out of spec from two influences: Age and use.
If you find that the AM/LSB trimcap goes to one end, and won't reach the target, look for a crack in the solder bond to the screw slot on that trimcap. If its end-to-end range is too small, or erratic the solder bond from the metal screw head to the metallized layer on the trimcap's ceramic disc is broken.
Under some storage conditions, the trimmer caps in a D201 may become stuck and seize. Won't turn. The natural response is to twist harder until you hear a "CRACK!" sound. That's the sound of that solder bond breaking, most often. This broken connection now reduces the cap's end-to-end range of adjustment. But you might need to look inside a dozen D201s to find this problem.
Or maybe just one.
Age alone can cause a crystal to drift. A NOS crystal made in 1976 may or may not still be on frequency, even if it has NEVER been used in a radio. Best explanation I ever heard is that strain can be introduced when mounting the crystal inside the can. Just a little "bowing" of the wire leads inside is enough. Over the years, that strain can relax. The effect is like slacking the string of a violin. The "pitch" where it oscillates will drop.
The "use" factor, or "mileage" refers to the physical stresses inside a quartz crystal that accumulate from use. It is a moving part inside, don't forget. And any moving part will eventually wear out. Not 'if' but WHEN.
Finding that one or two (only) of the 16 MHz crystals won't adjust, that the (intact) trimcap just "won't turn far enough" tends to suggest where the previous owner's home channel or channels were found. Most especially, if the USB (only) trimcap sets on target, but the AM/LSB crystal won't, this suggests that the mode used most by the previous owner was NOT USB on that channel.
A crystal that remains "low", and won't come up far enough to reach the channel center can be spoofed by pulling out one end (ONLY) of the silver-mica cap in parallel with that trimcap. This has the effect of extending that trimcap's effective range of adjustment upwareds a kHz or more. But this can result in a crystal that is sluggish to start up when selected. There will be a tradeoff with this trick, and it may not help if that crystal is weak or has drifted downwards too far.
This advice pertains only to the 16 MHz crystals. The 4.4x0 MHz crystals just have to be replaced when they drift beyond the limit. Or use the VFO.
A D201 that will transmit AM on frequency, but appears to receive AM off-frequency suggests checking the 5.8015 MHz crystal next to V301 is next. It's all alone in a socket next to V301. In the original open-chassis D201, the socket is soldered to V301's socket pins underneath. The callout is "X300". When this one drifts, you're pretty much out of luck unless you have a replacement. This crystal, and this oscillator circuit won't let you "pull" the crystal's frequency by adding a capacitor or an inductor to it. Go ahead and try if you like, but don't expect much joy from this strategy.
The crystals that are used the greatest number of hours tend to fail first. Doesn't explain all of the failures we see, but it's a big factor. And the more "miles" the radio has on it, the bigger a factor it will be.
A frequency counter is a perfectly good way to do this, but I prefer to use a communications monitor that lets you hear/see a zero beat with the frequency you're measuring.
For that matter, just tuning in the radio's internal frequency that's being adjusted lets you 'hear' the process. Any old shortwave or general-coverage ham receiver/transceiver will do this, so long as it can receive SSB or CW. A trimcap that is noisy or erratic will produce a beat note that jumps around when you disturb it. Hearing a smooth and unbroken "whoop" from the speaker as you spin a trimcap serves to confirm that it is undamaged, and less likely to drift after being set. A trimcap that is 'touchy" and does random things when you touch it will never be stable in the long run.
73
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Post by windhund on Feb 22, 2021 19:13:37 GMT -5
I'm so glad I found this. This is my first Tram D201 restoration and I just started the alignment process. I'll make a note of it. Whew! Attachment Deleted
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