Restoration pictures of WWII Hallicrafters BC-610-E Radio Transmitter


Restored power supply chassis with all new Belden 20 kV rubber wires, and new H.V. rectifier filament transformer from W2AMI.
All components were thoroughly cleaned to prevent H.V. arcing.


The restored power supply chassis.  Silver spray paint was used to cover up the MFP paint and simulate the original plated steel chassis colour.
A relay cover was made from a cast aluminum utility box and painted with black wrinkle paint.
This is a 2,000/2,500 volt power supply with a choke input filter that can deliver well over 0.6 amperes of dc.  The full wave rectifier consists of two RCA 866-A mercury vapour rectifier tubes.  The overload relay activates when the current exceeds 0.7 amperes, disconnecting the primary ac voltage to the power supply and activating the reset relay which holds the overload relay open until manually reset from the transmitter control panel.


The power supply chassis is inverted and set on top of the inverted cabinet, just as was done in the Hallicrafters production line.
1/4" bolts, lock washers and nuts secure these assemblies together.


The power supply and cabinet combination is then put right side up and lifted onto the rolling cast iron base (two people required).


Front view of completed power supply, cabinet, and rolling cast iron base.  The black wrinkle finish and silver power supply chassis trim
look exactly like the factory built HT-4/BC-610 transmitter pictures.


Christo, my father, installing the heavy 104 lb power transformer onto the power supply chassis wih the aid of a winch and support system.


The completed power supply.  The rolling base has an opening underneath (with the cover plate dicarded) so that the power transformer can be bolted to the chassis by reaching underneath to secure the hardware.  This was quite difficult.  If the power transformer had been bolted to the chassis before the chassis was set onto the rolling base, it would not have been possible for me to lift the chassis-cabinet combination onto the rolling base.   The power supply was then tested.  Unfortunately a problem was found.  The overload reset relay coil was open, and did not hold the overload relay open after a momentary overload current.  However, it was fixed by removing the relay cover, and holding a soldering gun to one of the reset relay coil terminals and feeding in solder.  After several tries, the coil suddenly had proper continuity.  This procedure is learned from experience.  Transformer and relay coils rarely open in the middle, but in fact, right at their solder terminals most likely due to improper enamel insulation cleaning and/or the use of corrosive acid core solder in the factory.  The lesson here is, test everything before assembling!!  But in this case I was lucky to have resolved the issue without having to remove the relay - which would have meant taking the power tranformer off  again and removing the chassis from the rolling base to get access to the relay mounting hardware.


The freshly painted modulator chassis (in light cobalt blue, an exact match to the MFP paint colour) installed into the cabinet.
The modulation transformer was mounted afterwards to keep the weight to a minimum while lifting the chassis in (two people required).
Interlocks were bypassed with jumper wires, and a test of the modulator idling was successful.  The 100TH modulator tubes show a nice
orange plate colour.  Unfortunately the push-pull 2A3 audio driver stage is not capable of driving the 100TH's properly. It is fine for voice work at 90% modulation, but totally inadequate for high fidelity audio with 125% positive modulation capability.  When the grids of the 100TH's are driven directly by a low impedance cathode follower stage, this modulator can produce over 400 W of 50-10,000 Hz high fidelity audio through the stock modulation transformer, at 2,000 V plate voltage. Note that the "F" and later models use a modulation transformer with higher primary impedance which restricts the maximum power output to considerably less power.


The RF deck was a disaster - it turned out that 3 of the band switch ceramic wafers were cracked.  Fortunately I had replacement wafers from a Multi-Elmac mobile transmitter band switch that were identical in contact arrangement, minus the shorting feature of the unselected contacts.  Many power resistors were either open or of high value, and the 807 plate choke was burnt black.  All had to be replaced.  Inspection of the schematic diagrams showed that the "D" model of this transmitter incorporated a 1000 ohm grid leak bias resistor in series with the 250TH grid choke for additional class C bias and the 4700 ohms grid leak bias resistor for the 807s was not present.  Both of these differences would result in significantly higher rf drive capability over the "E" model, and a higher efficiency of operation for the 250TH tube.  Most likely field use in war conditions resulted in abuse of this extra drive power and may have caused premature failure of the 807's when mistuned.  For controlled conditions in amateur radio use, however, there are many advantages to using the "D" model circuit, and it was decided to restore it this way.  This picture shows the completely restored RF deck, wired back to factory model "D" condition.


The RF deck installed on the top of the cabinet.  The transmitter is taking on its final appearance!


The front of the transmitter, with power on, standby mode.  Truly a wonderful sight!


Looking down into the top of the transmitter RF deck.  The Eimac 250TH power amplifier tube, and the 100TH modulator tubes below on the modulator deck, all glowing nicely.  An 80 meter tank coil and crystal tuning unit for 3525 kHz are installed, ready for testing.


The transmitter fully loaded into a dummy load is putting out 600 watts of carrier at 2500 V plate voltage and 310 mA plate current, CW mode.  This is much more than the 450 W specification, thanks to the additional grid bias resistor added to the 250TH grid circuit and lots of RF drive voltage from the "D" version circuit!  The 250TH plate shows a nice orange colour.  Efficiency is 77%.  It can run continuously like this.  In fact, BC-610's were sometimes used as AM broadcast stations for military service entertainment.  There is no reason why the transmitter can not be run at 2,500 V plate voltage even in the AM phone mode, and provide at least 500 watts of carrier output, with over 125% positive modulation capability with the correct audio driver. The CW/PHONE switch had to be replaced, a 4PDT Cutler and Hammer switch was found for the job. Someone had operated this switch with the transmitter plate voltage on - something that must never be done. The power transformer ac primary winding taps are connected to this switch directly, in series with the main plate relay.


Rear of transmitter during the 600W carrier output test.


Links:
Schematic diagram and parts list information