The PMR6A Receiver itself will now undergo the re-incarnation process.
1-18-2020 ~ when you know stuff, you can do stuff! Not a uBitx v.6; but a trip back 67 years!
73's
Pete N6QW
1-17-2020 ~ It is Alive!
More Space Filler.
More Space Filler.
Ham Safety while working with Thermatrons (vacuum tubes).
Most of our homebrewing taking place today uses 12 VDC and that is not high voltage. Although I do know of a case where a constructor had his hand on a 5 VDC 30 amp power supply output -- seems like his ring got shorted across the supply and the ring nearly melted around his finger, which subsequently I think required amputation. So any voltage can be dangerous.
The voltages in the PMR6A under load show 260 VDC on the lead to Pin #3 on the power connector. [There is a clue here for those who read every word...] The problem with the HV is that it can start your heart (defibrillator) or stop your heart if applied too long.
One of the additional steps I will be undertaking is to provide shield covers over the power supply so one does not experience an unwanted jolt! HV kills!
73's
Pete N6QW
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That process will entail a clean up, tube testing, circuit integrity checks and a lot of getting to know the innards of the PMR6A. But that will not happen in one day. So stay tuned.
To have a space filler for this blog since I will be working in the background on the PMR6A Receiver I would like to share a little more about simple rigs that are now nearing over 70 years old!
Enter the WesKit BN-1 Novice Transceiver. You might think of this kit much like the $3.50 Pixie CW Transceivers that are currently on eBay direct from China. Here is a BN-1 in all of its glory!
There are some notable points to observe here and firstly is that it could be operated over the range of 3 to 8 MHz --shades of a Paraset. It was battery operated (a plus for portability) and you could have as high as 180 volts on the plate and as low as 45 volts. Most likely the power output with say 90 VDC on the plate in the less than one watt range. The plate dissipation was one watt and when the two halves are operated in Push Pull the Pout is 2 watts! So definitely QRP.
Yes a CW transceiver was all done with a single tube, a 3A5 which is a dual triode intended as a power output stage in a battery operated receiver. I did see one variant where a 6SN7 was used in lieu of the 3A5 which opens the door to a bit more power output. But the 6sN7 needs more filament voltage (6.3 Volts)
Yes a CW transceiver was all done with a single tube, a 3A5 which is a dual triode intended as a power output stage in a battery operated receiver. I did see one variant where a 6SN7 was used in lieu of the 3A5 which opens the door to a bit more power output. But the 6sN7 needs more filament voltage (6.3 Volts)
One slick advertising photo shows the BN-1 with a loaded coil whip antenna on the box but was never produced or sold that way. I can just imagine the number of hams who were getting postal money orders (the plastic card of the 1950's) and sending off to purchase a BN-1. With that whip antenna it was now a complete rig in a box on 80M CW!
With a large tip of the cap to my friend Bill, N2CQR who recently built and operated a homebrew ET-2 that was not unlike the BN-1 in that it had a one transistor regen and an a one transistor 100 milliwatt oscillator. Bill made two dozen contacts in multiple states. So the BN-1 could likely do the same. Most BN-1 field reviews were not so kind attesting to a poor receiver and low power. But others had remarkable records using the BN-1.
Circuit Forensics
I guess one could argue was this a true transceiver as we think of transceivers today? What we have is a tunable regenerative receiver (1/2 the tube) covering the range of 3 to 8 Megahertz and a crystal controlled modified Pierce Oscillator (the other half) covering the same range. So the true transceiver is only when the receiver is near the Crystal operating frequency. But I guess you could operate cross band with the receiver on one band and the transmitter on another. Unlike the Pixie which forces a received frequency near the transmit frequency.
So lets us look a little more closely at the schematics in our tour of the circuit forensics. Although the two schematics are drawn a bit differently they are the same.
The first thing that catches my eye is the DPDT toggle switch which is the TR switch (on a small metal box). You had to really be deft when you engaged that switch. You guys with 500 pound gorilla hands, if you pounced on that switch you probably would move the received signal location. There are no vernier drives on those tuning capacitors and with a range of 5 Megahertz over the arc of the capacitor --a velvet touch is mandatory. So that could be an issue.
The only items being switched is filament power between the two separate filaments. However on receive the crystal is grounded but on transmit connected to the plate of the transmit half.; but there is no filament voltage supplied to the transmit half of the 3A5. I have no explanation about why it would be grounded on receive.
However as mentioned on transmit, the crystal is connected to the plate circuit to form a critical function in the kB=1 criteria. The Pierce oscillator uses this topology. BUT normally one sees a blocking capacitor in series with the crystal on the plate side.
There are many concerns about crystals used for frequency control. Two concerns with the use of quartz crystals are voltage and current. If say you were running 250 VDC on a plate of the 6SN7 variant, then one side of the crystal would be hot with 250 volts without the cap. The other issue is that huge currents can be developed in crystal oscillator circuits and an excessive amount will shatter your one and only rock.
Perhaps the cap was left out because of cost --a penny here and a penny there. I have no crisp explanation as to why it wasn't installed.
Now a big caution about Crystal currents. I see there are many FT-243 style crystals being sold on eBay today that are essentially smaller HC-25 or HC-49 crystals stuffed inside a FT-243 holder and re-labeled with a new frequency. Read the fine print... It says these crystals are only good for QRP rigs where the current is low. I suspect you put one of those crystals in a 6AG7 oscillator circuit you will see a mushroom cloud of smoke in your shack
The only items being switched is filament power between the two separate filaments. However on receive the crystal is grounded but on transmit connected to the plate of the transmit half.; but there is no filament voltage supplied to the transmit half of the 3A5. I have no explanation about why it would be grounded on receive.
However as mentioned on transmit, the crystal is connected to the plate circuit to form a critical function in the kB=1 criteria. The Pierce oscillator uses this topology. BUT normally one sees a blocking capacitor in series with the crystal on the plate side.
There are many concerns about crystals used for frequency control. Two concerns with the use of quartz crystals are voltage and current. If say you were running 250 VDC on a plate of the 6SN7 variant, then one side of the crystal would be hot with 250 volts without the cap. The other issue is that huge currents can be developed in crystal oscillator circuits and an excessive amount will shatter your one and only rock.
Perhaps the cap was left out because of cost --a penny here and a penny there. I have no crisp explanation as to why it wasn't installed.
Now a big caution about Crystal currents. I see there are many FT-243 style crystals being sold on eBay today that are essentially smaller HC-25 or HC-49 crystals stuffed inside a FT-243 holder and re-labeled with a new frequency. Read the fine print... It says these crystals are only good for QRP rigs where the current is low. I suspect you put one of those crystals in a 6AG7 oscillator circuit you will see a mushroom cloud of smoke in your shack
The transmitter tuning involved the old light bulb trick to show when the antenna was taking juice -- brighter = more fire in the wire! BUT given the link coupling -- this might be an issue with harmonics when operating say on 3706 kHz ( Novice CW frequency in 1950) and the second harmonic would be 7412 kHz. That might get you a post card from an OO.
So using that rig today might not meet the current FCC spectral purity issues on transmit. A Band Pass filter, should you build this transmitter, is much in order. Today around $7 will get you a tube and socket from Antique Electronic Supply in Phoenix, AZ
So using that rig today might not meet the current FCC spectral purity issues on transmit. A Band Pass filter, should you build this transmitter, is much in order. Today around $7 will get you a tube and socket from Antique Electronic Supply in Phoenix, AZ
Because the regen receiver is in fact an oscillator even in receive it too is radiating a signal. Think about those hams running 10 milliwatts on WSPR and being heard hundreds and even thousands of miles away.
A couple of issues with the receiver! Your Sony Walkman 32 Ohm earbuds will not work as the phones must be high impedance "magnetic" earphones and they are an active element in the receiver plate supply circuit! Also the headphone interconnects are insulated "pin type" connections so another issue. Typically the 3.5 mm stereo jacks have a common ground. Ditto for the Key and Crystal using the same type pin jacks.
Let us now look at the back side of the BN-1 box and again at the schematic.
Since this is the back side the receiver portion is along the left side where clearly we can see Coil L1 and L2. On the right side we see L3 and L4 as well as the connections to the pilot bulb (antenna current indicator) and the two wing nut antenna posts. Pretty clear.
BUT and a BIG BUT ... The schematic does not show any connections from the antenna to the receiver! The antenna is always connected to the transmitter coil L3.
So how does the received get from the antenna to the receiver. Often in a regen receiver you would have a third coil winding that essentially link couples the off the air RF to the tank circuit of the regen. I thought initially if that if coils L3/L4 were close enough to coils L2/L1 then we would have mutual inductance link coupling. But it is clear that they are too far apart for that to happen.
Here is where we can get fooled. I keep looking at that circuit diagram as two tubes. Like Bills ET-2, where there were two separate FET devices, the 3A5 is two triode tubes in a single tube shell. Not two separate tubes!
I am theorizing that in the signal path from the antenna to the regen receiver is the inter-electrode capacitance of the 3A5. Thus within the 3A5 itself the antenna on receive is connected to the receive half through a small capacitance internal to the tube.
From the NJ7P Tube Data Base we have data for each triode so we can assume there likewise is a small interelectrode capacitance between the two triodes. That value in the 1 to 2 PF range would give "light coupling" between the antenna and the regen. The light coupling is probably good for a smoother regeneration and with moderate band condition some signals would be heard!
Each Triode
Source ........................................ RCA RC-29 - 1975
Input ......................................... 0.9 pf
Output ........................................ 1.0 pf
Grid to Plate ................................. 3.2 pf
A couple of issues with the receiver! Your Sony Walkman 32 Ohm earbuds will not work as the phones must be high impedance "magnetic" earphones and they are an active element in the receiver plate supply circuit! Also the headphone interconnects are insulated "pin type" connections so another issue. Typically the 3.5 mm stereo jacks have a common ground. Ditto for the Key and Crystal using the same type pin jacks.
Let us now look at the back side of the BN-1 box and again at the schematic.
Since this is the back side the receiver portion is along the left side where clearly we can see Coil L1 and L2. On the right side we see L3 and L4 as well as the connections to the pilot bulb (antenna current indicator) and the two wing nut antenna posts. Pretty clear.
BUT and a BIG BUT ... The schematic does not show any connections from the antenna to the receiver! The antenna is always connected to the transmitter coil L3.
So how does the received get from the antenna to the receiver. Often in a regen receiver you would have a third coil winding that essentially link couples the off the air RF to the tank circuit of the regen. I thought initially if that if coils L3/L4 were close enough to coils L2/L1 then we would have mutual inductance link coupling. But it is clear that they are too far apart for that to happen.
Here is where we can get fooled. I keep looking at that circuit diagram as two tubes. Like Bills ET-2, where there were two separate FET devices, the 3A5 is two triode tubes in a single tube shell. Not two separate tubes!
I am theorizing that in the signal path from the antenna to the regen receiver is the inter-electrode capacitance of the 3A5. Thus within the 3A5 itself the antenna on receive is connected to the receive half through a small capacitance internal to the tube.
From the NJ7P Tube Data Base we have data for each triode so we can assume there likewise is a small interelectrode capacitance between the two triodes. That value in the 1 to 2 PF range would give "light coupling" between the antenna and the regen. The light coupling is probably good for a smoother regeneration and with moderate band condition some signals would be heard!
Each Triode
Source ........................................ RCA RC-29 - 1975
Input ......................................... 0.9 pf
Output ........................................ 1.0 pf
Grid to Plate ................................. 3.2 pf
[One of our very astute blog reader's emailed me after this was initially posted and made me aware that there was perhaps even a bit more sophistication to this circuit... lurking there in the bushes. If we think of L3 / L4 as the front half of a Band Pass Filter coupled with a very small capacitance (internal to the tube) and in turn that L1 / L2 is the other half of the BPF. This then would indeed add some front end selectivity. So having the transmitter tank tuned close to the receiver operating frequency makes for a nice way of achieving a BPF and keeping crud out of the receiver circuits. Bravo WesKit!]
Thus today's filler has covered the WesKit BN-1 Novice two band CW Transceiver that likely does work but in itself might not bring thrills but real challenges.
(The WesKit BN-1 was sold through Western Radio in Kearney, Nebraska and if you mated that rig with a Gotham Vertical --you could work the world!)
73's
Pete N6QW
(The WesKit BN-1 was sold through Western Radio in Kearney, Nebraska and if you mated that rig with a Gotham Vertical --you could work the world!)
73's
Pete N6QW