Re: {Collins} R388 Readings



The VFO can be fixed. There is a tutorial, maybe more than one, on the CCA web site on repairing the VFO. The loss of linearity is very common. The deviation from band to band is from crystal drift. The fix is to replace the crystals. Note that each crystal is used for two bands and a couple are also used on harmonics so, replacing most crystals will fix at least two bands. Collins devised a clever scheme to minimize the number of crystals needed. If you can tolerate the deviation from band to band keep the present crystals or replace only the couple of worst ones. However, I do suggest adjusting the VFO. The main difficulty is getting to the adjustment. The VFO can be adjusted while in the receiver but IMO its best to remove it. This is not trivial since you must remove the front panel. The explanation for the loss of linearity that seems to be most accepted is that the ferrite core of the VFO changed characteristics over time. Collins seems to have known the material was not entirely stable but have been unable to find better material at the time. There is a trimmer coil that was intended to get the VFO exactly on the nose but the drift may be so great as to be beyond the range of adjustment. The recommended solution is to remove a turn from the trimmer coil. I think it may be possible to accomplish the same thing by adding a shorted turn adjuster but have not tried this. The VFO is adjusted by juggling the exact position of the core and the trimmer so that the end to end span is exactly right. Since the core will no longer run on exactly the same part of the coil the linearity may still not be right and it may be necessary to adjust the corrector stack. I have never done this job but others have and there are hints about how to go about it. A properly adjusted VFO will be exactly on the mark everywhere in its range. Don't be concerned about duplicating the original hermetic sealing. Evidently it didn't work very well to begin with and whatever change was caused by moisture getting in has already long been done. I have never seen any data on the drift rate of old VFO's after they have been adjusted. I suspect that the drift slows down with time so that probably most of it has long ago taken place. The linearity of the dial from end to end is NOT affected by the band crystals. Whatever off-set they produce is constant throughout each band and is compensated for by the movable pointer (fiducial to Collins). There are other things in the VFO that can affect the span and linearity, I think mainly bad capacitors and possibly mechanical problems caused by old lubrication. Once you do a job on a VFO its likely it won't need attention again for a very long time.


----- Original Message ----- From: "Philip" <ka4koe@xxxxxxxxxxx>
To: "Richard Knoppow" <dickburk@xxxxxxxxxxxxx>
Cc: "Collins@xxxxxxxxxxxxx" <collins@xxxxxxxxxxxxx>
Sent: Saturday, July 20, 2013 6:51 AM
Subject: {Collins} R388 Readings


OK

Here are the readings for the VFO on the 2 MHz band, MHz and amount off in KHz

1.5/+3.5
1.6/+3
1.7/+2.5
1.8/+2
1.9/+1
2.0/0
2.1/-1
2.2/-2
2.3/-3
2.4/-4
2.5/-4.5

Not that far out, it seems. I've seen much worse.

Here are the deviations, center of each band, and MHz, and error in KHz

1/+3.5
2/0
3/+3.5
4/0
5/+4.5
6/+1.5
7/+3.5
8/0
9/+4
10/+0.5
11/+4.5
12/+1
13/+6
14/+3
15/+3.5
16/0
17/+3.5
18/0
19/+3.5
20/+2
21/+4
22/+1
23/+3
24/0
25/+5
26/+2
27/+3.5
28/0
29/+3.5
30/+3

Most common deviation is +3.5. It would seem the easiest "fix" would be to slide the KHz dial on the shaft -2 KHz and leave as is. What say you? Thanks.
Philip KA4KOE





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