I had originally intended to publish a more extensive article, but have moved on to other projects. I am sharing here some tips that you may find useful. I always coveted the Heathkit Twins, but could never afford them. A friend was cleaning out his basement and passed these along at a reasonable price. I now had the opportunity to enjoy a dream from the past.

These are the early filter type sideband rigs from the sixties, so be aware of their limitations. The spurious output performance is better than the Heathkit Marauder, the previous SSB rig. Unless the trap was adjusted carefully, the spur on 20 meters was a problem. Drift was greatly improved by the new LMO in the twins. The Marauder occupied a monster Apache cabinet and was similar in appearance to the Apache. The SB401 and SB301 are significant improvements over the early versions. In particular, the transmitter is much easier to switch from split to transceive. A small reed relay to switch the VFO around would correct this in the early 400. The 300 is an excellent receiver, especially if it has the accessory AM and CW filters. The 301 was friendlier to the 6 and 2 meter converters, but no other significant changes.

The SB303 was an all solid state receiver, the last in the series. I plan to do a write up on this project later, as this is part of my permanent collection.

Compactron tubes are used extensively in the transmitter. These are getting scarce. Stock up on them while you still can. The receiver has only one compactron. It can be replaced with a 12AU7 and 6AU6, if you are clever.

The transmitter was improved by inserting 10 Ohm resistors at various B+ points. It did give the kit builder a “second chance” if there were shorts. The resistors smoked to alert the owner of errors. Perhaps you could recover from them if you killed the power quickly. More significantly, the 82K resistor on the terminal board underneath in the front was changed to 2 Watt. You should check this before power up. As a minimum, the CW & Tune Drive adjust will not work right if the value goes up. If it goes down in value, the balanced modulator diodes will fail and other ugly things will occur.

The antenna relay (not the main control relay) also develops a permanent magnetism that causes it to hang up in transmit mode. I have seen people put thin pieces of plastic on the pole piece to fight this. The easiest way to correct this is to use a powerful tape head demagnetizer to reduce the residual magnetism. Then swap the wires to the coil of the relay; it will probably magnetize again, but it will take quite a while. If it happens again, just swap them back.

Follow the manual during alignment. Be sure to perform the neutralization carefully. Power output will be 100 Watts, but lower on 15 and 10 meters for sure. If you get 75 Watts on 10 Meters, call it good. Use the grid drive meter to determine if the power is not up to snuff. If you are not getting enough grid drive, changing the finals will not correct it. Low B+ on the low voltage or resistors that have drifted in value can cause the exciter to not deliver enough drive to the finals to operate properly.

Operating the Heath Twins as a complete station requires the construction of three cables each exactly 24" long from RG-62/U coaxial cable (90 Ohms) exactly as described in the SB300/301/303 receiver manual. Yes, the transmitter connection is described in the receiver manual. The cable parts came with the receiver, not the transmitter. Go figure. Anyway, the impedance is not relevant. The shunt capacitance per foot is very critical, since it can detune the circuits which are used to link the receiver to the transmitter. The crystal HF oscillator in the receiver is used to drive the transmitter. The USB and LSB BFO crystals in the receiver are used to operate the transmitter. In the SB400, you may find it advantageous to remove the BFO crystals EXCEPT THE CW crystal, to prevent a beat note on receive. The CW crystal is required to operate CW and Tune modes. I do not know if the 401 requires this, but try it if you have problems. The LMO (VFO) is linked between the two rigs for transceive, and is the ONLY oscillator switched to go to split frequency operation. In a pinch, I have heard that some people will disassemble a junked AM car radio antenna, which often uses a similar coax cable. I have a quantity of the correct coax on hand, which I have often mailed to those who need this item.

The LMO sometimes develops a jump or warble as you tune in a station that is annoying in CW or SSB modes. This is a link to fix that problem: http://nebula.wsimg.com/d6a8106142d2492dec285d95be47aaf6?AccessKeyId=D1250C433DB440D6B60D

NOTE: The SB400/401 often comes WITHOUT the crystal pack. You do not need ANY crystals (except the CW crystal) installed to operate the transmitter, if you are using it with a Heath SB300/301/303. For troubleshooting purposes, to operate the SB400/401 as a stand alone unit, simply "borrow" one or more crystals from the SB300 until you can get the SB400 running. Then connect them as described above. Note also that you have to re-align the HF oscillator coils as described in the manual as well as some other adjustments. Since the crystals for 29.0 to 29.5 and 29.5 to 30 MHz covered territory for AM and FM, many did not install the crystals in a SSB/CW only rig. Again, the receiver came with the whole set; use the cables described above and you have no problem.

UPDATE -I have heard one report for VFO changing frequency when switching to transmit. I have never encountered this myself. I found the voltage regulation built in OK. The person told me he added another VR tube. I am not sure that the power supply capacitors, resistor values drifted, or something else may have been off. But this is added as a cautionary note, in case you have problems.

Good luck with your restoration. These have nice audio on transmit. The ALC circuitry works better than the transceiver SB100.

           

GUEST UPDATE 4/19/2014 - I was studying the schematic for the SB-300 and found an error on the heterodyne crystal oscillator switch wafer labeled BS1R. The common connection for crystals Y6, Y5,Y2 and lastly Y1 should be grounded. Right now without any connection, one side of all of these crystals are floating. They do have it correct for Y3, Y4,Y7 and lastly Y8. This is just a schematic drawing mistake and obviously the wiring of the receiver is correct since the receiver works!

73,
Joe, W3GMS

UPDATE 7/12/2016 - One more Heathkit Twins glitch is addressed at: http://www.boatanchor.com/heath/SB300-SB301%20Voltage%20Regulation%20.pdf. I have not personally tried this modification myself, but it appears to be carefully researched and implemented. Please keep in mind that the SSB equipment of this day was not very stable; people were using SB-10 adapters driven by DX-100 VFOs that were barely stable enough for AM, marginal for CW, and totally unacceptable for SSB. Comparing first or second generation SSB gear to modern rigs is just not reasonable.