[TMC] GPR-90RXD stuff

Wed Aug 11 12:03:30 EDT 2021

Hi John, Chris and all!

In looking at this more closely, and without some TMC tribal knowledge, let’s just assume that L38 is probably center tapped, even if it isn’t.  There are several vendors that sell 300 uH inductors: Mouser, DigiKey, and Surplus Sales of Nebraska among them.  It would be a relatively simple task to stick two 300’s in series, being careful to make sure windings are both oriented in the same direction, making a 600 uH center tapped inductor.  Chances are it would work just fine, but if not, other values are available that would add up to a total of roughly 585 uH with a tap somewhere other than in the center.  

I’ve sent a photo to Chris, but it’s really hard to see what exactly is going on.  The inductor is the three section pi, and looks like an RF choke. The tap is a green sleeved wire that seems to come from the top of inductor in the photo, but does not. The tap connects to pin 5 of the accessory socket. One end of the coil goes to ground, while the other end connects to C59, the 3 pf BFO coupling cap.    Shielded wires run from pin 5 to the BFO input jack, and to C163, which is near the BFO tube. 

It’s worth thinking about.  

73, George – W1LSB

PS – If anyone wants the photo, drop me a note. 

From: John Poulton [mailto:jp at cs.unc.edu] 
Sent: Friday, August 06, 2021 5:01 PM
To: Chris Bolkan
Cc: George Maier; tmc collector's group
Subject: Re: [TMC] GPR-90RXD stuff

Just for the heck of it, I looked up the drawing for this coil (L38, BFO output coil).  From the parts list, it's TMC part # CL-231, which is also the TMC drawing number.  

Here's the drawing URL (I've scanned most of the drawings for smaller coil assemblies):

https://tmchistory.org/tmc_drawings/drawings_db/cl/cl_200_299/cl-231.pdf

The drawing is singularly unhelpful.  It shows a 3-rank pi-wound coil with an inductance of 585uH (in agreement with the schematic, but not the parts list) with a powdered iron core, but it doesn't even mention a tap or indicate where it might be..  Arghhh.  Maybe the wire type and Q will be useful to you.

Chris, hope you're able to find a photo of the coil.  I'd be interested to know what you find out from it.

73, John K4OZY

On Fri, Aug 6, 2021 at 2:21 PM Chris Bolkan <bolkyboats at gmail.com> wrote:

Hi George,
Thank you! I would appreciate that pic and yes just email directly.
I will look again, but I am pretty sure it is not there because I
traced it all out and ended up capacitively coupling the BFO signal
into the IF. Would be awesome if I am wrong! My goal is to make the
receiver original, and then like everyone else, find an SSB adapter. I
am looking for one of the rack mount adapters though because that is
how I will ultimately mount the receiver.
Best regards,
Chris
KI7NXY

On Fri, Aug 6, 2021 at 11:11 AM George Maier <GeorgeW1LSB at verizon.net> wrote:
>
> Hi Chris:
>
> I'm happy to send you a photo, but not sure it will get through the reflector, so I'll send it to you directly.  Meanwhile, I suspect that the inductor you are seeking (assuming it's L38) may not missing, but is located far from the BFO.  In my radio, L38 is between pins 5 and 6 of the 8 pin accessory power out octal socket on the rear panel while the tap goes to pin 7.  The 3 pf BFO coupling cap, C59, is connected between pin 6 of that socket and pin 7 of the 6AL5 detector.
>
> Hope this helps!
>
> 73, George
>
> -----Original Message-----
> From: Chris Bolkan [mailto:bolkyboats at gmail.com]
> Sent: Friday, August 06, 2021 1:13 PM
> To: George Maier
> Cc: tmc collector's group
> Subject: Re: [TMC] GPR-90RXD stuff
>
> Hi George!
> I am a new member too and I also have an RXD. Mine is missing the tapped coupling inductor between the BFO and where it connects to the IF (at least according to the schematic I have). If you could take a picture of yours I would really appreciate it. I need to see what it looks like so I can figure out what I need to make to take its place.
> This group is amazing and very helpful.
> Chris
>
> On Thu, Aug 5, 2021 at 7:00 PM George Maier via TMC <tmc at mailman.qth.net> wrote:
> >
> > Hi All:
> >
> >
> >
> > I'm a newbie to the TMC list, but an old timer in the hobby; first
> > licensed in 1958 as KN1GXT and K1GXT a year later.  The current call
> > sign was pretty much a mid-life crisis, but that was almost 20 years ago now.
> >
> >
> >
> > Regarding TMC, I've had lust in my heart for a GPR for decades, but
> > never owned one until now.  It's a Canadian made GPR-90RXD (ca 1971
> > vintage) that came with an original TMC cabinet and a matching TMC
> > speaker.  John P tells me that this a rare find so I'm wondering if
> > anyone in the group has seen any other Canadian made receivers.
> >
> >
> >
> > It's actually in very nice shape physically and works rather well
> > after some TLC and a complete alignment.  I'm still cleaning it up
> > here and there, and will probably have the cabinets repainted
> > professionally provided the color can be matched accurately.
> >
> >
> >
> > Does anyone know the actual color chip that was used, or have the paint
> > formula?   Also, the speaker grill cloth must be replaced so I'm searching
> > through all of the web based grill cloth resources, but have not found
> > a really close match yet.  My interior designer XYL has given me some
> > of her fabric catalogs to look through, so may I'll find something
> > there unless someone has a good resource.
> >
> >
> >
> > Last but not least, the BFO injection in this receiver was
> > ultra-wimpy, making it barely useable on CW and a waste of time on SSB
> > .  It's different from other GPR's in that the BFO is injected at the
> > detector while other models have the BFO injected at the grid of last
> > IF tube, except for the RXDS with its built-in product detector.  The
> > RXD is actually more like most Collins, Hammarlund, Hallicrafters and
> > National radios that do not have built-in product detectors, but their injection levels are much higher.
> > Adding 20 pf to the original 3 pf BFO coupling cap made all the
> > difference in the world; it's now a very pleasant receiver on CW and manageable on SSB.
> > Score one for the old guy!
> >
> >
> >
> > Any info or tips will be sincerely appreciated.
> >
> >
> >
> > Thank & 73,
> >
> > George - W1LSB
> >
> >
> >
> >
> >
> >
> >
> >
> >
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