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 Post subject: Collins 75A-1 Product Detector?
PostPosted: Nov Thu 09, 2017 3:34 pm 
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I picked up an electrically nice 75A-1 last month and the AM sound really good. I'm toying with the idea of adding a product detector for better CW and SSB and wondering if anyone out there has a proven design for this receiver.

Thanks...Harry


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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Nov Thu 09, 2017 3:57 pm 
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Harry,

If you can stand an external box a Central Electronics "Sideband Slicer" works very well with the earlier 75A series that lack a product detector and the additional selectivity provided by its phasing detector will be welcome in crowded conditions with the Q multiplier equipped version (Model B) even better.

It is a personal bias but I hate making major modifications to pristine vintage equipment.

If you want to continue with adding a product detector a lot is written about adding a product detector to the R-388 and some of this will prove helpful to you. I use a Model B Slicer with my 51J-3 and it works fine with the 500 Khz. IF. The Hammarlund HC-10 will also work with the Collins 500 Khz. IF by readjusting its conversion oscillator and I suspect the TMC GSB-1 would as well.

Rodger WQ9E


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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Nov Thu 09, 2017 5:57 pm 
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Rodger,

Thanks for the reply. External boxes are an option that I considered and I'm not necessarily opposed to them. With my R388 and 75A-3 I've installed product detectors from Treetop Circuits. The modifications are trivial and can be easily reversed and the detectors work quite well.

For the 75A-1 I'm tempted to try building my own if (a) not terribly complex and (b) I can install without extensive surgery on the receiver.

Harry WE1X


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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Nov Thu 09, 2017 9:32 pm 
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What I've done on a few sets is increase the value of the bfo coupling capacitor. No it's no a product detector but it does help w/ cw and ssb reception. When you start w/adding some of these old pentagrid type pd circuits to a set you start to open a whole can of worms w/the avc levels and timing. I like the kiss method.
GL

Terry


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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Nov Thu 09, 2017 11:43 pm 
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Terry,

Thanks. Do you have a rule of thumb what those increases should be?

Harry


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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Nov Fri 10, 2017 1:02 am 
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X2 or 3.
I did it to a 51J4 I think it had a 30mmfd and I went up to 75mmfd.
B4 I had to keep the rf down so it wouldn't overload the bfo signal.
After could crank it up to about 3/4 up till it started overtake the bfo.
Terry


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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Nov Fri 10, 2017 2:25 am 
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Some receivers injected the BFO at the last IF amp control grid instead of going to the secondary of the detector IFT (as the 75A-1 does). The coupling cap was quite small, say, 2~5pF, or sometimes the cable was just anchored close and direct to the control grid through a 10M resistor (no capacitor). It would be easy enough to try and easy enough to restore to the original design.

Bill Orr in the various editions of the Radio Handbook used to recommend adding a BFO output level pot. It sounds like a good idea, but it would have to be a front panel control to have any usefulness.

Rob


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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Nov Fri 10, 2017 4:05 pm 
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Increasing the BFO injection. Yes.
That could be part of a solution.But many vintage communications receivers disable the AGC or AVC, when the BFO is turned on. ie. the AVC line is grounded. So that function of grounding the AVC line would have to be defeated, unless you want to constantly ride the receivers RF gain control. And depending on the receiver, the grounding of the AVC line would also disable the S meter.

Al


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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Nov Fri 10, 2017 8:04 pm 
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And to ad to Al's comments the AVC must be disabled in many of these receivers because of the AVC detection method. Most AM/CW receivers derive AVC voltage in the detector stage and BFO injection into this stage would cause the AVC to develop a control voltage based upon the BFO injection alone causing it to reduce gain even with no signal present.

Receivers like these would have to be modified if you want to use AVC for CW/SSB; those with a separate product detector don't face this issue because the regular detector stage is used only for AVC detection when the product detector is in use. Quite a few of the vintage receivers without a product detector still allowed for AVC with CW/SSB by using a separate AVC detector stage and some were fairly elaborate like the system used in the Hallicrafters SX-88 with a separate AVC amp providing the IF pickoff point for a dedicated AVC detector and amplifier while the BFO also had an amplifier to improve SSB operation.

Rodger WQ9E


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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Nov Fri 10, 2017 8:22 pm 
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For a noob like me this is really fascinating. Keep in going as I'm actually learning something.

Harry


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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Nov Fri 10, 2017 9:37 pm 
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It isn't always the coupling cap for the CW oscillator (BFO) that makes the difference in signal strength. Because a low level signal is desired the oscillator tube is often running well below what it could as far as signal strength is concerned. In the National NC 183 the oscillator tube plate resistor is 220k. That was changed for the NC 183D to 100K to increase the plate voltage for the tube. The coupling cap was upped from 10p to 21p. It is the combination of the two that makes the difference in the upgraded circuit. I had trouble with the BFO in my NC 183 and went ahead and changed the two components to match the 183D. It made a big difference.

Norm

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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Nov Fri 10, 2017 10:29 pm 
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There was a small QST article some years back about adding a product detector to the SX-101, but it applies to other sets as well. A somewhat low-hacks mod that you make on a mini-box inside the set. It was discussed here a year or two back and that should reveal the issue # and page. Making a product detector internal mod is going to require finding a way to add a switch somehow. That was distasteful to me so I didn't implement it. If I recall correctly the SX-101 mod used the somewhat useless noise limiter switch for that purpose.

I agree with Rodger here, something like a 75A-1 is a historic set such that it seems a shame to start modifying it, but there are changes that can easily be backed out later if you insist on the mod. I really like my CE Model A slicer although even that needs some mods to the receiver in that you need the receiver's audio stage. A pox on those owners of the 1950's who used their Slicer on their receivers by drilling several holes in the rear panel and installing RCA phono jacks, often with Dymo tape labels! My opinion on mods has gotten to be that since I have a lot of modern ham gear as well, I don't need to try to improve vintage stuff to make it work on SSB or to be Hi Fi. Back decades ago, someone might have had only one receiver and then the mods became attractive.

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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Nov Sat 11, 2017 3:26 am 
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Geoff,

You've given me some food for thought...or pause. If I do anything to the 75A-1 it's going to be (a) minor and (b) easily reversible. Consistent with that I may do a variant of Terry's suggestion with the BFO coupling cap. Since it's a rather small value cap (1pF) I'll make a gimmick capacitor and add that to the existing coupling cap and see what happens. Easy to make, install and remove.

Harry


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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Nov Sat 11, 2017 4:52 am 
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Harry,

You might consider tacking a small trimmer cap in as an experiment so you can try different levels of BFO coupling and then once you figure out what you want replace it with a fixed value cap close to the value of the trimmer. If you use one of the miniature open plate trimmers it is pretty easy to estimate the capacitance based upon the amount of plate closure; of course if you have a capacitance meter you can measure any variable easily.

Try it with both weak and strong signals because sometimes a BFO will try to shift frequency and lock onto a strong signal if the coupling to the detector is too great. A receiver set up this way can make every signal sound like a directly keyed single stage oscillator/amplifier transmitter :)

Rodger WQ9E


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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Nov Sat 11, 2017 10:26 am 
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Another option. The October 1962 issue of CQ described a very simple product detector procedure: inject the BFO at the last IF amp screen grid through a cap (in this case, 180pF), and add a 2.5mH RFC between the existing voltage dropping resistor and the screen grid (to keep it above ground). The author claimed it worked well. Easy enough to try. However, I have to mention that the writer consistently refered to the screen grid as the suppressor grid in his text, yet it's clear he meant to say screen grid. The schematic shows this.

I have scans of the original article if you're interested. This method I have not tried myself.

Here's another idea: take the signal off the screen grid of the last IF amp through a small cap and send it to the BFO control grid. Use the BFO as a regenerative detector. Add a simple filter to the plate to remove the RF (the usual 47pF/47K resistor/47pF pi network should work fine) and send the audio through a cap to the 500K volume control. You can tune the BFO to select upper or lower sidebands. The AVC operates normally. This circuit I have tried, it's part of the National NC105/121. It does work fairly well, although very strong signals will overload it. In the case of the NC105/121, this circuit was stupidly located just inch or so from the PS transformer, hence there is an annoyingly high level of hum. I've also tried this method with an external regenerative receiver tuned to 455KHz: it worked surprisingly well, keeping in mind the context: a way to improve CW/SSB reception without making major circuit additions. You just have to be careful about hum: the control grid of a regenerative detector is very high Z, very sensitive. It was not for nothing that OT regen builders kept their PS on a separate chassis, several feet away.

Rob


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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Nov Sat 11, 2017 5:56 pm 
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In many cases the simplest way, though somewhat intrusive, is to replace the BFO
oscillator with a 6BY6 or 6BE6. Or, on an SP600, replace the BFO buffer amp
with a 6BY6 or 6BE6. I tried do that to my SP600 with one of those Vector
"tower" tube extenders that allows building circuits on it. It got the B+ from
the plate pin with a filter the remove the IF signal, and the AF was fed out
with coax. It worked fine but there was no reason for me to keep it since
my other boatanchors work great on SSB (i.e. R390A (minor mod), SX-88 (minor mod),
NC400, and RA17c12.)

One big note to ALL boatanchors with product detectors and using AGC:

I find AGC on CW or SSB often very annoying, without this trick.

At least where I live, on all bands through 20 meters, the white noise on the antenna
is FAR above the sensitivity level of the radio. This means that the AGC delay does not work,
and even signals a little bit above the noise cause annoying noise pumping.

That can be gotten rid of with a variable attenuator in the antenna lead, with no
increase of noise, by attenuating the signal so the antenna noise is above the receiver noise
by only 3 dB or so. Then weak signals do not activate the AGC. The Racal has one built in,
but only 10 dB steps.


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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Nov Sun 12, 2017 2:48 am 
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I also like the idea of using a pentagrid BFO/product detector. Any recommended designs?

I know they don't have the best reputation, but the next step up gets fairly involved.

Rob


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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Nov Sun 12, 2017 2:48 pm 
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rsingl wrote:
Harry,

You might consider tacking a small trimmer cap in as an experiment so you can try different levels of BFO coupling and then once you figure out what you want replace it with a fixed value cap close to the value of the trimmer. If you use one of the miniature open plate trimmers it is pretty easy to estimate the capacitance based upon the amount of plate closure; of course if you have a capacitance meter you can measure any variable easily.

Try it with both weak and strong signals because sometimes a BFO will try to shift frequency and lock onto a strong signal if the coupling to the detector is too great. A receiver set up this way can make every signal sound like a directly keyed single stage oscillator/amplifier transmitter :)

Rodger WQ9E



Rodger,

I have a few trimmers lying around, but from their size I suspect they're too large. Will start with the gimmick next week and report back.

Thanks...harry


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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Mar Tue 06, 2018 8:57 pm 
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I have a 75S-1 that came with a rewired tube socket (V-12 / 6V6 audio output). Story has it that it's a Prod Det. mod, where after rewiring the socket and adding some parts, you break off the index of the tube and reinsert it a couple of pins off-axis. Does ANYBODY know about this mod? Does anybody have DOCUMENTATION on it? The mod was removed for rewire to original, but the cabling from the MAN/AGC/CW switch is hard to ID (which is which). I'd like to put it to either original or modded state. Thanx in advance. Tom


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 Post subject: Re: Collins 75A-1 Product Detector?
PostPosted: Mar Wed 07, 2018 4:09 am 
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There has been mention of needing another switch. Something I've done in the past is to replace one of the front panel pots with a unit having a "Pull" on/off switch.

Another part to swap out but you get your switch.

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