[TMC] GPR-90RXD - L38 deep dive UPDATE

George W1LSB GeorgeW1LSB at verizon.net
Thu Aug 12 17:57:56 EDT 2021 

You’ve probably figured this out by now Chris, however the tap is 25% or so into one of the pi sections, not between the sections.

73, George



> On Aug 12, 2021, at 4:27 PM, Chris Bolkan <bolkyboats at gmail.com> wrote:
> 
> 
> And I am unclear where the tap is. is it in the middle of a coil or at the 1/3 separation
> 
>> On Thu, Aug 12, 2021 at 1:23 PM John Poulton <jp at cs.unc.edu> wrote:
>> Congratulations, George!  What an interesting result...  Any idea what fraction of the Pi-winding turns you'd taken off by the time you'd removed 26 turns?  Just curious about the turns ratio on the transformer...
>> 
>> 73, John K4OZY
>> 
>> 
>>> On Thu, Aug 12, 2021 at 12:33 PM George Maier <GeorgeW1LSB at verizon.net> wrote:
>>> Problem solved!    After breaking the tap due to my own clumsiness, I removed L38 from the radio and looked at it closely.   The tap was definitely in the pi section closest to the grounded end of the inductor.   With nothing to lose at this point, I proceeded to unwind the last pi section to find and hopefully repair the broken tap, and there it was only 26 turns later.  Unwound one additional turn, cleaned the wire ends, reassembled the tap, and put the 26 turns back on the inductor.  
>>> 
>>> If you’ve ever taken turns off a pi section you know what happens when you try to rewind it; it’s never as neat, no longer sticks together, and looks messy.   A small piece of heat shrink tubing over the pi sections made everything neat and strong enough to use again. 
>>> 
>>> After passing the continuity checks, the inductor was re-installed in the receiver.  It now works as it should have originally; BFO injection is fine and very useable. 
>>> 
>>> The bottom line, especially for you Chris, is that you can make a suitable tapped inductor out of standard pi section type just by unwinding turns as noted above and creating a tap.  I’ve seen 500 uH inductors in a few places, and one of those would probably work.  A 600 wound be nice to find, but probably not easy to find.
>>> 
>>> Photos of the repair are available if anyone wants them.
>>> 
>>>  
>>> 
>>> 73 and thanks to all for your comments and suggestions.  John; you nailed it!
>>> 
>>>  
>>> 
>>> George – W1LSB
>>> 
>>>  
>>> 
>>> From: John Poulton [mailto:jp at cs.unc.edu] 
>>> Sent: Wednesday, August 11, 2021 1:52 PM
>>> To: George Maier
>>> Cc: Chris Bolkan; tmc collector's group
>>> Subject: Re: [TMC] GPR-90RXD - L38 deep dive
>>> 
>>>  
>>> 
>>> Hmm.... In looking at the schematic, it struck me that this inductor is used as a step-up autotransformer, transforming the low-impedance, lower swing, BFO output to a higher impedance, higher-swing signal at the injection point.  If you separate the thing into two inductors, the magnetic coupling between the windings won't be nearly as strong as in a single inductor (maybe...?), so the current induced in the upper-most winding will be reduced.
>>> 
>>>  
>>> 
>>> I also wonder why TMC chose to make the thing a 3-winding PI.  Perhaps the tap is 1/3 of the way up?   Or, 2/3..? That would make more sense, given the physical structure of the coil.  Any way to tell exactly where that extra lead is connected to the coil?  If the 1/3- speculation is right, it'll be at a point between 2 of the pi sections.
>>> 
>>>  
>>> 
>>> The whole design seems a bit ad-hoc, really..  Wish it were possible to talk to one of the engineers who worked on this variant!
>>> 
>>>  
>>> 
>>> 73, John
>>> 
>>>  
>>> 
>>> On Wed, Aug 11, 2021 at 11:35 AM George Maier <GeorgeW1LSB at verizon.net> wrote:
>>> 
>>> 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 pseudo tap somewhere other than in the center. 
>>> 
>>>  
>>> 
>>> I’ve sent this photo to Chris, but it’s really hard to see what exactly is going on.  The inductor is the three section pi in the center of the photo, and looks like an RF choke. The tap is the green sleeved wire that seems to come from the top of inductor, but does not. The tap connects to pin 5 of the accessory socket, while the ends connect to
>>> 
>>>  
>>> 
>>> Worth thinking about. 
>>> 
>>>  
>>> 
>>> 73, George – W1LSB
>>> 
>>>  
>>> 
>>> <image001.jpg>
>>> 
>>>  
>>> 
>>> 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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