[R-390] R-725, How To Make Your Own
David Wise
David_Wise at Phoenix.com
Mon Aug 22 17:31:40 EDT 2005
I want to apologize in advance if I am bringing up
something that was resolved in the past. If it was,
just say so and summarize the result.
Tom mentions below that most of the mod consists of
rewiring the '390 deck for 6.3V heaters because "the
'390A can't handle the 25.2V load". Might that not
be an issue if you didn't run the ovens? As we all
have sworn :) never to do? I haven't done the math.
If you think this is viable, how about an abridged
procedure that leaves the deck series-wired?
It would be a LOT less work.
73,
Dave Wise
> -----Original Message-----
> From: Tom M. [mailto:courir26 at yahoo.com]
> Sent: Sunday, August 21, 2005 4:14 PM
> To: Mark Huss; Kenneth Arthur Crips
> Cc: r-390 at mailman.qth.net; Steve Hobensack
> Subject: Re: [R-390] R-725, How To Make Your Own
>
>
> Ferrite beads, where? I have the R-725 drawings and there
> are no ferrite beads
> to be found.
>
> Also the tuned circuit IF decks were specially made for the
> R-725. They did
> not use decks from the R-390 (although I have done that
> myself and it works
> great).
>
> Here are the simple steps necessary to roll your own R-725
> style IF deck form a
> junker R-390.
>
> Winter is coming. Everyone needs a good project.
>
> ------------------ cut here -------------------
>
>
> The main difference between a R-725 and the R-390A is that
> the R-725 utilizes a
> custom built IF deck that is very similar in construction to
> the R-390 IF deck.
> It has tuned circuit selectivity instead of mechanical
> filters. The mechanical
> filters of the R-390A created distortion when that radio was
> employed for radio
> direction finding use.
>
> Motorola was awarded a contract in 1956 (476-PH-56-91) to
> prototype the
> R-725. I know of a couple of these Motorola sets exisiting today.
> Packaging of modified sets for quantity DF use were handled
> by Arvin Industries
> and Servo Corporation of America. Approximately three
> hundred R-390A's were
> modified to the R-725 configuration. New IF decks were
> manufactured by the
> modification companies (actually salvaging some of the
> components from the now
> junker R-390A IF decks) and installed in existing R-390A's.
> The new IF decks
> were named "SERIES 500 IF STRIP ASSY." They looked almost
> just like R-390 IF
> decks except that the IF connectors were relocated to match
> the cables and
> connectors in the R-390A chassis. The circuits were designed
> to plug and play
> in the R-390A instead of the R-390. The R-390 IF decks are
> not interchangeable
> in the R-390A (until now that is, after perfoming the
> modification described
> herein).
>
> Notwithstanding DF capabilities, a side benefit of the SERIES
> 500 deck is that
> it provides a smoother sound than does the stock R-390A IF
> deck. Mechanical
> filters are said to "ring" and after a while can be fatiguing
> to the listener.
> The purpose of this procedure is to describe how one may
> "roll his own" SERIES
> 500 IF deck from a surplus R-390 IF deck. Please note that I
> don't advocate
> trashing of a good R-390 to do this mod. The IF deck I
> started with came from
> a Motorola junker. I would urge you to likewise find a
> junker R-390 as a
> source of an IF deck for this project. Make sure that the
> deck is in working
> condition prior to beginning the modification.
>
> The R-390 IF deck was designed to operate with one 25V filament supply
> (unfortunately the 25V supply available in the R-390A is
> insufficient to power
> all of the filaments in the R-390 IF deck). The R-390A deck
> was designed to
> operate with a combination of 6.3V and 25V filament supplies.
> The task
> involved in this conversion is to rewire the R-390 filaments
> to comply with the
> voltages available in the R-390A and provided at the main IF
> deck connector,
> plug and play, without the addition of any new power
> transformers. Each of the
> twelve tubes in the R-390 IF deck must be addressed for full
> compliance with
> the voltages available from the R-390A. We will also drop
> the B+ a bit.
>
> Plug P112 of the R-390A shall be plugged-in to the R-390 IF
> deck at jack J517.
> There is much commonality here, except for the connections
> mentioned herein.
> In general, you will be converting 25V series connections
> into 6.3V (herein
> referred to as 6V) parallel connections for most tubes, and moving the
> connections of the BFO/PTO/ballast tube series to a different
> connection point
> in jack J517. The 6V filament supply shall be provided to
> the R-390 deck by
> pin 20 of P112 from the R-390A.
>
> General instructions: Refer to the schematic for the
> original R-390 as the
> "before" schematic. Use the best soldering technique you can
> in this limited
> access space. Don't insulate or bind any wires until
> instructed to do so.
> You will be utilizing some of the new 6V supply connections
> more than once.
> Make sure you can recognize your new wires. I used black
> wire for the 25V
> supply, red wire for the 6V supplies, and green wire for new grounds.
> The first task is to install a B+ dropping resistor to better
> match the 180V B+
> that the R-390 IF deck is expecting. To do this, locate
> inductor L503 under
> the IF deck. This will be found snapped into a holder right
> above pin 2 of
> J517. Disconnect one end of the coil, and install in series
> with it (the
> equivalent of) a 470 ohm 2 watt resistor. This will tame the B+.
> The first tube circuit we'll work on is the ballast tube circuit.
>
> V508 (5749) and RT512 (3TF7) These must be supplied by the
> R-390A 25V filament
> supply. To do this, sever the connecting wire at pin 8 of jack J517
> (underneath the deck) to free this slot up (hint: save access
> to the connector
> end of the wire as you will use it to wire supply to V509).
> Then, sever the
> connection at pin 2 of RT512 and wire this pin to pin 8 of
> J517 of the R-390
> deck with a long piece of new wire. The filament return
> connection remains
> unmodified. This modification will make the BFO/PTO/ballast
> tube series
> connections identical to the R-390A 25V filament supply
> connections. As
> mentioned above, this 25V supply is insufficient to supply
> the remainder of the
> tube filaments, thus the need to employ the 6V supply for this task.
>
> The following 6V tubes shall have filaments wired from the 6V
> R-390A supply.
> The filament pins of these tubes are pins 3 and 4. Don't
> sever any connections
> unless instructed to do so. The modification will use as
> much existing R-390
> IF deck wiring as possible (and thus may seem a bit screwy to
> you until
> finished).
>
> V504 (6BJ6) 6V will come from its existing connection at pin
> 4. Ground will
> come in the next step.
> V503 (6BJ6) Wire pin 3 of V503 to pin 4 of V504 for 6V
> supply. Ground V503,
> pin 4.
> V502 (6BJ6) 6V supply will come from an existing connection
> at V503, pin 3.
> Ground V502, pin 3.
> V501 (6BJ6) Sever ground connection at V501, pin 3 and wire
> pin 3 to V502, pin
> 4.
> V505 (6AK6) 6V supply will come from existing connection at
> pin 4. Ground will
> come in the next step.
> V506 (6AK6) Ground pin 4 of V506. Wire pin 3 of V506 to pin 4
> of V505 for 6V
> supply.
> V509 (6BJ6) Locate the free wire which was cut from
> underneath J517, pin 8, and
> connect it to the 6V filament supply at J517, pin 20.
>
> The following tubes are 12AU7's wired in various series
> schemes in the R-390.
> They must be rewired according to their 6V option for use in
> the R-390A. Note
> two of the connections require dropping resistors on the 6V
> source of V507 and
> V510 to obtain the desired 5.3V filament voltage.
>
> V511 Sever ground connection at pin 5. Connect pins 4 and 5
> together for 6V
> supply, ground pin 9.
>
> V507 Sever connections at pins 4, 5 and 9, including the two
> resistors (one
> 120 ohm and one 22 ohm). Wire 6V supply from your previous
> work at V505,
> through the deck opening for variable capacitor C525, to pins
> 4 and 5 through a
> 3.9 ohm, 1 watt resistor. Ground pin 9.
>
> V 510 Sever connections at pins 5 and 9. Ground will come
> from existing pin 4
> connection. Remove 120 ohm resistor between pins 4 and 9.
> Wire pin 5 to pin
> 4. Wire 6V supply from J517, pin 20 to V510 pin 9 through a
> 3.9 ohm, 1 watt
> resistor.
>
> There are no changes to any of the other connections in the
> R-390 deck.
> Prior to installing the modified R-390 deck in your R-390A,
> you must check your
> work.
>
> Perform continuity checks from J517, pin 8, with RT512, pin 2.
>
> Perform filament supply and ground continuity checks as
> follows. Note: There
> will be more than one ground connection at various tube
> sockets, but the
> filaments should have continuity exactly as shown.
>
> Filament supply is checked from J517, pin 20.
>
> V501, filament, pin 3; ground, pin 4.
> V502, filament, pin 4; ground, pin 3.
> V503, filament, pin 3; ground, pin 4.
> V504, filament, pin 4; ground, pin 3.
> V505, filament, pin 4; ground, pin 3.
> V506, filament, pin 3; ground, pin 4.
> V507, filament, pins 4 and 5; ground, pin 9.
> V509, filament, pin 4; ground, pin 3.
> V510, filament, pin 9; ground, pins 4 and 5.
> V511, filament, pins 4 and 5; ground, pin 9.
>
> Now insulate any bare connections, and use mini-tie wraps to
> secure the new
> wires to sturdy nearby points.
> To enable final installation of the deck in your R-390A,
> you'll need to make
> two adapter cables. These cables shall consist of jumpers
> (RG-59 is OK,
> approximately eight inches in length) with BNC's on each end.
> You'll also need
> two adapters of the type found on the back of the frame of
> the R-390A at the IF
> OUT jack (AMPHENOL 47200). This will provide crossover from
> MB connection
> (R-390A standard)to BNC (R-390 standard). Connect P-218 of
> the R-390A to J-526
> of the R-390 IF deck with one of the cables. Connect P-213
> of the R-390A to
> J-525 of the R-390 IF deck with the other cable. It is a
> good idea to label
> these cables.
> Install the deck in your R-390A. You will notice that the
> screw holes are the
> same as for the R-390A IF deck, however the screws of the
> R-390 deck are of
> larger diameter. I did not change these screw as they are
> captive into the
> deck. The BFO, BANDWIDTH, and power connector of the deck
> will hold it in
> place, however I would not install it in a Jeep this way.
> Changing these
> screws is optional.
> When you turn on the power, make sure your dial lamps light
> up normally. If
> they don't, you have a filament supply problem so turn off
> the set immediately
> and troubleshoot.
> For great sound, instead of using the built in audio deck, I
> prefer to tap the
> audio from the diode load jumper at the back of the set.
> Through a 0.1 uF or
> larger capacitor, feed this signal into your line audio amp
> of choice, and
> enjoy the tuned circuit audio of the new R-725, errrr,
> R-390A with tuned
> circuit IF. You'll get the smooth sound of the R-390 and
> R-725, but have the
> parts availability and support common to the R-390A for the
> balance of the set.
> I've used my modified IF deck in two different R-390A
> frames, and it worked
> equally well in both. In my opinion, it makes the long term listening
> experience much more enjoyable.
> If you have any questions about the mod, please feel
> free to write.
>
> 73 DE N5OFF
>
>
>
>
> --- Mark Huss <mhuss1 at bellatlantic.net> wrote:
>
> > Basically an R-725 is an R-390A modified for HF DF using an
> array of
> > vertical antennas and measuring the phase delay of the wavefront.
> > Because of the phase shift in the mechanical filter across
> its response
> > curve, they substituted an R-390 IF deck for the R-390A IF
> deck, then
> > added a whole s***pot full of ferrite beads to suppress
> leakage normally
> > present in the R-390A. the result is the quietest and most
> sensitive
> > R-390A made. In the shop I worked in, they always had at
> least 6dBm
> > better signal to noise ratio.
> >
> > Kenneth Arthur Crips wrote:
> >
> > >How does the R-725 differ from the R-390A. I can see the
> RF/IF deck is
> > >different is that It or are there other differences.
> > >
> > > Ken
> > >
> > >
> > >
> > >Kenneth A. Crips, W7ITC, Cheyenne,
> > >Wyoming. with The Chew Crew; Beezie,
> > >Chic', and Cowboy, Irish Terriers who
> > >are Red House ninjas with 10th degree
> > >Black belts in Bed Fue, Mooch Fue,
> > >Chew Fue, and Con Artist Fue
> > >
> > >_____________________________________________________________
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> > >
> >
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