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 Post subject: Collins 75-A3 70E-12 PTO repair - update
PostPosted: Jan Wed 29, 2014 6:57 pm 
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Wow. I finally got this thing to get some stations after a recap job, and I found many small problems (pots, wiring, reversing mods, dirty ceramic caps and my understanding :shock: ) rather than a smoking gun. I'm still having difficulting getting many stations.

The IF strip is now tuned, and getting signal through the mechanical filters. Through some alignment difficulties I found that the VFO does not track to the dial, in fact on the 160 meter dial setting the VFO 1.955 - 2.955 mc range extends 180 kc wider in travel than the 1.5 - 2.5 mc dial range (130 kc lower than 1.5 dial setting and about 50 kc higher than 2.5 dial setting) So setting the VFO correctly to the dial at either high/low range will just Move all the error down/up range.

(I found this during troubleshooting by bypassing the variable IF and injecting 1.6, 2.5 mc into the mixer tube and finding at what dial position the VFO was mixing to 455kc, max voltage at the diode load test point). Again, the IF strip is correctly, sharply tuned to 455.

So the PTO endpoints are off, and my copy of the manual didn't suggest adjustment, and found out from posters below that later versions gave adjustment instructions.

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Last edited by SmoothOscillator on Jan Mon 19, 2015 2:42 am, edited 3 times in total.

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 Post subject: Re: Collins 75-A3 Alignment 70E-12 VFO tracking.
PostPosted: Jan Wed 29, 2014 8:26 pm 
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I would strongly suggest adjusting the Collins PTO for proper end point setting and "spread". This is covered on the web and in several Electric Radio articles. The caution against opening the case was reasonable 50 years ago when the case seals were still good, the internal desiccant was dry and operational, and Collins had replacement PTOs readily available. None of this is true today and many of us have worked on a number of Collins PTOs. You can actually dry the desiccant as part of the procedure and probably return the PTO to near original drift specs, something I am sure it doesn't come close to meeting in its current form. You also want to properly clean and lubricate it to avoid damage that comes from using a receiver with a dirty/dry PTO mechanism.

These Collins designs with their cam driven multi-stage tuning will never perform as they should if the PTO is seriously out of adjustment.

I have a Collins 310 exciter which is about to go on the bench for restoration and I am sure the PTO end points will be way off as is just about every Collins PTO that hasn't been adjusted in decades.

Rodger WQ9E


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 Post subject: Re: Collins 75-A3 Alignment 70E-12 VFO tracking.
PostPosted: Jan Wed 29, 2014 8:32 pm 
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For what it's worth, the procedure is on page 5-1 of the manual which you may already have. Otherwise here it is (wish they showed pictures). http://collinsradio.org/archives/manual ... Manual.pdf I agree that warning signs about the seals are irrelevant after this many years. Perhaps there are better descriptions of the procedure out there as well.

I've never seen this done and am concerned about the mention of particular tools. Hopefully someone can elaborate on the tools required to unlock the trimmer adjustment and actually do the adjustment.

Curtis Eickerman

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 Post subject: Re: Collins 75-A3 Alignment 70E-12 VFO tracking.
PostPosted: Jan Wed 29, 2014 9:13 pm 
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Someone posted a image of the tool on AMfone, hopefully this site allows a link to it: http://amfone.net/Amforum/index.php?topic=31295.0 Also Bruce W1UJR provides nice photos of his PTO rebuild: http://www.w1ujr.net/rebuild_pto.htm

Rodger WQ9E


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 Post subject: Re: Collins 75-A3 Alignment 70E-12 VFO tracking.
PostPosted: Jan Wed 29, 2014 9:48 pm 
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To make life easier...
Attachment:
pto_tool[1].jpg
pto_tool[1].jpg [ 44.45 KiB | Viewed 8108 times ]
That's not going to be real fun to make. Not impossible given availability of some 0.2" OD tubing with ID of 0.138". The other part just needs 1/8" rod ground to a screwdriver point.

If that nut is really tightened down, tubing might not work unless it is steel.

Curtis Eickerman

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 Post subject: Re: Collins 75-A3 Alignment 70E-12 VFO tracking.
PostPosted: Jan Thu 30, 2014 3:39 am 
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Gents thanks! Interestingly, my hard-copy manual does not mention self-servicing of the PTO. Mine is dated 1953, where the one you reference is 1954 - perhaps they changed their minds about user servicing. Also, the serial number on my PTO is 921, seemingly a relatively low number.

Also, the schematic doesn't show a trimmer within the PTO housing…unless I'm missing something.

Glad to see this copy of the manual, I'm willing to try and adjust this…will have to experiment with tools, and I think it's worth giving this a try. Will report back.

Image

" width="464" height="1024" alt="vfo1"></a>

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 Post subject: Re: Collins 75-A3 Alignment 70E-12 VFO tracking.
PostPosted: Jan Thu 30, 2014 3:57 am 
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My guess is the biggest problem is the home made tool has to be sturdy enough to loosen what might be a now stubborn to loosen lock nut. It might be more feasible to start with a crude but sturdy version of the first tool that could be made by grinding a relief into a standard screwdriver blade of the correct thickness-just leave the outer edges to engage the slots. This doesn't provide the hollow shaft needed for the tightening after the adjustment but it shouldn't require a lot of torque to re-tighten it sufficiently so your working hollow tube version won't need to be nearly as sturdy opening up the possibility of using easy to machine materials.

Rodger WQ9E


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 Post subject: Re: Collins 75-A3 Alignment 70E-12 VFO tracking.
PostPosted: Jan Thu 30, 2014 5:26 pm 
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I looked over the dimensions and will try the screwdriver approach. I may remove the PTO and do an inspection, and figure out the adjustment tools, then reinstall and do a dry run to make sure it'll work...

I'm just very pleased that this thing can be adjusted...my manual didn't allude to that!

This is my first dual conversion radio, and I've learned a lot!

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 Post subject: Re: Collins 75-A3 Alignment 70E-12 VFO tracking.
PostPosted: Feb Sun 02, 2014 7:41 pm 
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I made the tool from a solid fiberglass rod. Notched both ends and bored one. Coat with epoxy.

Later I ran across a R-388 with the tools included.

Carl


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 Post subject: Re: Collins 75-A3 Alignment 70E-12 VFO tracking.
PostPosted: Feb Sun 02, 2014 10:40 pm 
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Unless I misunderstand your original post, you may have bigger problems than tweaking the endpoints on the VFO. the adjustment may offer 10's of KHz of adjustment, but you said you had 180 KHz of endpoint error. Although I'm familiar with the R-390A VFOs, and not the 75A amateur VFO, I doubt an end point adjustment will solve your problem. I'm not sure what the problem could be. Maybe one of the internal temperature compensating caps has failed?

Ed W2EMN


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 Post subject: Re: Collins 75-A3 Alignment 70E-12 VFO tracking.
PostPosted: Feb Sat 15, 2014 4:15 am 
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Well, here are the pictures of the PTO - I finally got it out. The seal was broken and looks like there's an obvious cap replacement.

Interesting also that there's a second adjustment screw on the back of the PTO, that adjusts this compensator mechanism rotating an iron slug collar slightly as the unit travels…looks like something factory set.

I'm not sure what the procedure is here. DJED postulated that there are temp adjusting caps in there? Could they be the two 10 pf parallel yellow jobs?

Image


Image


Image

" width="800" height="600" alt="PTO1"></a>

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 Post subject: Re: Collins 75-A3 Alignment 70E-12 PTO endpoints off - Pictu
PostPosted: Feb Sat 15, 2014 6:02 am 
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Very interesting to see inside one of those cans, especially since I have a couple of Collins receivers. I'm curious; what are those sausage-like things in pic. #2?

Chris


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 Post subject: Re: Collins 75-A3 Alignment 70E-12 PTO endpoints off - Pictu
PostPosted: Feb Sat 15, 2014 8:36 am 
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Hopefully you did not loosen the cam stack. In most cases the end point spread is corrected with the tools shown above, on the corrector coil, and that's it. The cam stack is for linearity only and in my experience about 9 times out of 10 stays correct. You can't set it with the can off and it's an afternoon's job even if you know what you are doing and have a scrap can cut with a slot and the special tools to move the stack shims.

You appear to be not measuring the end point spread correctly. Since the PTO is out of the radio, the spread is most simply calculated with a frequency counter and counting the turns. 180 kHz would be a major problem, I don't see how it could get that bad.

The description in the manual as far as reinstalling the PTO is a good way to tear it to pieces if you are not very careful. There are no end stops which protect the slug and leadscrew and if you are not careful you can install it so that the PTO is at the end of its travel but the dial assembly is not quite there, and then even slightly forcing the tuning knob can damage the PTO. I suggest setting the main dial to the middle of the range and then injecting a signal at the antenna port, and tuning the signal in with the pto shaft. This is a bit of a fiddle but safer than the way the manual describes. Then get everything lined up once the PTO is in place. There are several turns of overtravel at each end on the PTO (more or less.)

Note that there is an article by Bill Orr from a 1970-something CQ or Ham Radio describing how to rebuild and reset the PTO. It is for the PTO in the 51J-3 and 51J-4 but the process is basically the same other than there are different schemes of corrector coils on various PTO models.

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 Post subject: Re: Collins 75-A3 Alignment 70E-12 PTO endpoints off - Pictu
PostPosted: Feb Sat 15, 2014 3:04 pm 
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As Geoff stated 180 Khz. is very unusual. If it is that far out I would suspect either the moving core was damaged (not likely unless the cap replacement person took the entire PTO apart) or the external windings were disturbed. There is one suspicious looking area under the mica cap that looks like the turns might have been moved but I can't tell from the photo.

The corrector stack is a major pain if someone has changed it so don't touch it unless someone has already tried and failed miserably. I have done a couple of them and I believe someone wrote up the procedure in Electric Radio. Setting up a screwed up corrector stack isn't rocket science but it is very tedious; it is very much like setting up the L/C vertical delay line in an early Tektronix 500 series scope. Every adjustment affects the point you want to change but additionally impacts adjacent calibration targets so it is a long iterative process if you want it to turn out as it should.

Rodger WQ9E


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 Post subject: Re: Collins 75-A3 Alignment 70E-12 PTO endpoints off - Pictu
PostPosted: Feb Sat 15, 2014 3:39 pm 
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The sausage looking things are a desiccant to absorb moisture.

I'll do a bit more research, and didn't touch the adjustment mechanism.

Was my method for calculating endpoint error correct?

Quote:
(I found this during troubleshooting by bypassing the variable IF and injecting 1.6, 2.5 mc into the mixer tube and finding at what dial position the VFO was mixing to 455kc, max voltage at the diode load test point). Again, the IF strip is correctly, sharply tuned to 455.


While It's out I'll check the resistors and caps that are there. It looks like one .01 uF cap has already been replaced (held in place by the white silicone). I'm thinking I can then power this up on the bench (out of radio) and see how it operates. If I still have problems I'll look at 540 pf + 10 + 10, and hesitate to do much with those.

Thanks all.

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Last edited by SmoothOscillator on Feb Sat 15, 2014 3:52 pm, edited 1 time in total.

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 Post subject: Re: Collins 75-A3 Alignment 70E-12 PTO endpoints off - Pictu
PostPosted: Feb Sat 15, 2014 3:49 pm 
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Before you go any further I would carefully characterize the frequency versus dial position using either a frequency counter (best) or a calibrated external receiver if you don't have a frequency counter. Measure the frequency of the PTO directly rather than received frequency in the 75A-3 to prevent other externalities from confusing PTO measurement. Either use Excel or pencil and paper to graph frequency versus every 90 degrees of shaft rotation throughout its range. Direct measurement will ensure that you are measuring the actual PTO frequency and the output graph will show the distribution of discrepancies which will provide clues as to what is wrong.

The PTO doesn't need to be in the receiver for this, just provide jumpers to its power connectors and an output to your counter (or a small stub antenna for receiver pickup).

Rodger WQ9E


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 Post subject: Re: Collins 75-A3 Alignment 70E-12 PTO endpoints off - Pictu
PostPosted: Feb Sat 15, 2014 3:55 pm 
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Rodger - that's a great suggestion. I'll find a suitable knob to put on the shaft and give this a shot.

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 Post subject: Re: Collins 75-A3 Alignment 70E-12 PTO endpoints off - Pictu
PostPosted: Feb Sat 15, 2014 8:42 pm 
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The usual way is to make a homebrew knob with a scale around the skirt, and a fixed pointer such as a bent piece of stiff wire. Just be careful to not stress things near the end stops when you turn the knob. Generic instructions are to pick a spot near one end of travel and do the number of turns specified for your model PTO, writing down the frequency at the start and then when you reach exactly the last turn. Then you can see if the range has "spread" or contracted. It doesn't matter what frequency you start or stop at, it's the number of turns and frequency high minus frequency low that matter. Adjust the corrector coil and go again, until the frequency difference from high to low end is what the book says for the number of turns. On certain PTO's you will run out of range on the corrector and have to undo a turn from that coil, but I have only seen that necessary on the 51J series last generation PTO's.

Even just this job is a bit tedious but once it's done you should never have to do so again.

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 Post subject: Re: Collins 75-A3 70E-12 PTO repair - update
PostPosted: Jan Mon 19, 2015 3:07 am 
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All - I changed the title of this thread as my PTO rebuild is complete.

I took way too much time looking up articles and trying to figure out the tooling for the endpoint adjustment and the solution was to take the coil out from the inside of the can. A locknut tool I fashioned from a short piece of brass tube that I filed down, and it worked long enough for the repair.

I mis-measured in the previous post and although my endpoints were off by about 30kHz, it was linearity that was also off.

I replaced caps and resistors, except for a few of the caps within the can. After the rebuild, I mounted this in my cheap jig, lashed it to the radio, and began correcting for endpoints, going back and forth a few times. I had 14 1/2 turns of travel to work with. Once complete, I took the plunge as linearity was off in some cases. It was a snap to do, it just took an hour or so. The absence of the can cover reduced the oscillating frequency by about 50kHz...so had to tweak, put can on, read result, remove can, re-tweak, etc. but results were very close.

The hard part was rebuilding the circuitry inside the base of the can - tight quarters. Other than that the adjustments were a snap.

After a realignment, and further readjustments I was getting a lot of stations, and some CW! PTO was the problem all along! :mrgreen: I have some more minor work to do, but almost there.
Thanks all.

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Image

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 Post subject: Re: Collins 75-A3 70E-12 PTO repair - update
PostPosted: Jan Mon 19, 2015 3:12 am 
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Congratulations! Enjoy the feeling of success.

I just finished doing a quick assembly of a SSTRAN AMT-3000 AM "rebroadcaster" and now I can get to a recently acquired 51J-4 which I hope doesn't have any PTO issues :)

Rodger WQ9E


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