Click on the thumbnail images to see a larger image. Use the back button to return to this page.
I was looking for an SB-300 on these pages but now one has found me. Now that I have finished working on the Heathkit DX-60B transmitter I have found the "Basic Ham Receiver HR-10B" is too crude to use these days and I was looking to replace it with a Heathkit SB-300 or an SB-301. I wanted to be able to interface the receiver to the SB-610 and SB-620 accessories and the HR-10B will not work in this configuration without extensive modifications to the 610 and 620. I put out a request for an SB-300 on our local BoatAnchor net one Tuesday night and VK5LB decided I had a better use for the one he had, so we did a deal.
I am now looking for the following Heathkit rigs to add to my collection:
Read about the latest additions to the Heathkit collection - SB-110B and HP-23B
Rummaging on the internet I have found many articles on what is wrong with these Transmitters and how to improve them. The two most serious problems were poor audio quality and inadequate modulation due to the use of a crude Carrier Control Modulation scheme.
Remember they were very cheap, about the half the price of most other similar transmitters of the day and this was made possible by not using High Level Modulation and its expensive modulation transformer and output tubes.
On a web site by Bob (K4TAX) I found a circuit for a rebuilt modulation stages that fixed both problems. This circuit was a refinement of one by another Ham in the USA, Ed (KS3K). These fellows improved the frequency response, increasing the value of the coupling caps and adding negative feed back. The biggest advance was to supply the cathode of the modulation tube from the Negative Bias supply. This allows greater depth of modulation to be achieved.
Other articles talked about adding Press To Talk provision and Antenna change over relays. The Mode switch would not last long if it was used to change from transmit to receive all the time. As with all the simple devices made in the USA in the 60's and 70's the mains wiring was far from safe, being on the mode switch with all the other switching circuits.
I have added a small 12 Volt transformer and regulator that is on all the time, and three relays:
Some fo the photos above were taken before the second round of modifiations talked about below.
While I was at it I rebuilt the negative bias supply with a full wave bridge instead of half wave rectifier and I replaced the ancient rectifier diodes and filter caps. I found it was easier to remove all the parts from the audio and power supply sections and start again, with more modern components. I have produced a circuit diagram of my modifications with Kicad.
I put the project away for a year and when I got back to it in February 2012 I found I had made a slight blunder with the switching and had not finished fitting the cathode load resistor for the modulator tube. With these little errors fixed and some reworked crystals fitted, I connected a dummy load and powered the DX-60 on, twidling the knobs as it said in the book and on the modification web page. The crystals I have used are modified original ones with modern 3.579 and 7.159 crystals fitted. Power output seems to be about 35 Watt.
I had originally tried to put a buffer for an external frequency counter in the VFO but because of the problem on the 80M band I decided there had to be a better way. I eventually decided on "sniffing" a little bit of the RF from near the power output tune via a probe and a coax cable. If you look closely at the photos above you will see how I have done this. There is a long machine screw mounted on the chassis via a ceramic insulator, it is next to the output tube. So far it works, and has the added benefit of also sniffing the frequency on all the bands, 80 to 10 Metres. In addition to the BNC for the VFO input I have have also added a 12 Volt DC output for the frequency counter and a socket to power an expernal antenna changer over relay from the PTT circuit. The frequency display I am using is a digital dial kit designed by KD1JV and available from Hendricks QRP kits.
When I first tested this item I found it seemed to have a serious problem. The output frequency on the 80M position was in fact doubling as can be seen in the first image below.If you compare that image with the next one taken with VFO in the 40M position it is quite clear what is happening.
I spent quite a bit of time on this problem and evenually decided the buffer / multipler circuits in the transmitter were designed to handle this.
When I connected it to the DX-60 transmitter I found the oscillator signal into the first stage of the DX-60 was about half that produced by the crystals so I doubled the HT supply in the VFO by adding a second OD2 voltage regulator tube in series with the original and adjusting a few other circuit values.
I have also modified the VFO to have a Press to Talk provisions using another relay and a tune position. The small transformer on the chassis is used as a 6 Volt to 12 Volt auto transfomer so I can use 12 Volt car dash board LED lamps for the dial lamps to reduce the heat inside the unit.
This receiver was in quite good condition but I do not like the RCA connectors for RF leads and the un-earthed 110 Volt power chord. As this kit was sold DownUnder it had a 115/230 Volt power transformer and when I was removing the original two pin power connector from the back of the chassis I discovered the 0.005uF RF bypass capacitors accross the power mains entry were only rated at 150 Volt AC, must have been very good quality to survive such a big over load.
I also replace the antenna socket and the one below it that had been wired for an SB-620 connection, the three connectors that brought out RF signals to a transmitter, all with BNC connectors. I also replace the speaker and Anti Vox connectors with 3mm phono sockets, most of my equipment uses these as speaker connectors. Fitting the phono connectors was quite easy, I removed the parts of the RCA connector from the backing and reamed the hole to take the new connector.
This is a good receiver, the only thing missing is a noise limiter or better still, a noise blanker. Recently Horst VK5HY picked me up to go to a NERC meeting and he was quite excited about the difference a long lost audio noise blanker made to the problem he has with neighbors Solar Inverters. He rummaged in cupboards and boxes and found this item he had bought quite a few years ago. Now he can chat with his friends in the "old country" in day light. I remembered I was given a Timewave DSP-9+ some years ago and there was no sign of life from it, not even one LED came on when 12 Volt was applied. I found a manual on the web and soon found the 5 Volt regulator was shorted and when I replaced that the little box showed signs of life. My next question was how to I prove it works, I do not usually have a bad noise problem at my QTH and I put it away happy in the knowledge I had fixed it. (See my web page on my experiments with Audio DSP devices, my noise level has gone up a lot as of 2016/2017)
I had been using the SB-300 for some weeks to monitor activity of on 20M at night and there was quite a bit of noise on some of the signals. Slowly it dawned on me this would be an ideal candidate for the trying out the Timewave box. I wanted to connect it independant of the audio system in the SB-300, perhaps simply accross the volume control. The only problem, the SB-300 is a valve radio and this control is 500K and the input impedance to the Timewave box is either 20 Ohm or 20 KOhm. The solution to an "old valve man" like me was to add an extra valve, a 12AU7, using half as a low gain amplifier and the other half as a cathode follower. There is plenty of room on the chassis for this modification and I could add a gain control into these stages so I could set the optimum input level to the Timewave. The output from this amplifier makes use of the "Spare" RCA connector on the back panel. I can now wind up either the volume control on the radio of the volume control on the Timewave box and see what difference it makes. I am still surprised how well it works. I am concerned how HOT the power transformer in the SB-300 runs and I have put a 12 Volt fan on top of the cabinet over the transformer with double sided tape and will do something more perminant perhaps next time I take the radio apart. I think the transformers in the Heathkits were not designed for 50 HZ mains and my extra 300 mA of heater current does not help. If I replace the dial lights with LED's that will help.
I have two of these in the shack and when I needed more loud speakers for more radios I decided they took up too much shelf space with only one 9 by 6 inch speaker in each. I made new craft wood baffles and fitted two five inch speakers to each. I also fitted 3mm and 6.4mm jack sockets for each speaker rather than fixed leads.
Every picture tells a story, looking at the last image above you will notice the original clock mechanism is worse for wear but it also has bigger problem that is not obvious. It is designed for a 60Hz mains frequency and here DownUnder it runs 20% slow as our electricity supply is 50Hz. I looked at the "flip type" mechanisms from several clock radios but when I first looked at this idea they were either too good to destroy the radio or the motor was open circuit.
I then remembered I was given a National Semiconductor clock module by the local destributor sometime in the 1970's. This came complete with even a suitable power transformer and was a sample for manufacturers to experiment with. This was in the days when radios were still being made in Australia. As I had long since lost the original paper work an extensive internet search eventually turned up a site in South America with details of many of these modules. I have since found a 240 Volt 50Hz flip clock mechanism to in and it will look the part again.
This is quite a good receiver for the days when it was produced but the lack of a product detector for resolving SSB signals and its image responce due to the 455KHz IF frequncy means it is not much use today for serious Ham Radio use. I have been offered an SB-300 and will purchase that soon.
I have mounted a dycast box on top of the HM-102 to switch it and the ATU to the various BoatAnchors I have in the shack. The four position BNC switch was given to me by a friend in Queesnland and I have added a rotary switch wafer to the front of it so I can control external circuits associated with each BoatAnchor rig if I need to. The input to the Automatic Antenna Tuner (LDG AT-200Pro) has a two position coax switch to select either the more modern rig (TS-50S) or the collection of boat anchors via this four position switch. One output from the ATU is connected to a G5RV, the other to a TET-Emtron TEV-4 verticle four band antenna.
Progress photos of the 6 Metre AM rig under construction using parts of a DX-60 and an HR-10
I have a plan to build/modify another BoatAnchor with this item .It is in very good condition, all I have done to it is replace the two electrolytic capacitors and fitted a standard four pin microphone socket to the font so I can have PTT control over the unit.
Back to the shack equipment pages
Back to the Ham Radio pages
Back to the Domestic Radio pages
Back to opening page
Written by and Copyright, Phil. Storr © Last updated 06th february 2017
