Several years ago, I purchased this MR-18 at a hamfest. I did not have any documentation such as a schematic. Requests to the usual web based groups turned up nothing. Apparently this set is very scarce. Powering it at the DC input marked at 13.5 volts on the back provided barely any noise. The AC input did nothing. Since the set had a couple of scarce four-legged N-channel dual-gate field effect transistors as well as other FETS and transistors, I did not want to service it without a proper schematic. The set sat on my shelf for quite some time.
Comparison to the GR-78
While doing some research on Heathkit, I spotted information on the somewhat less rare GR-78, a Heathkit portable general coverage shortwave receiver offered beginning in 1969 to the last ad in the Winter 1977 catalog (below). The GR-78 catalog picture immediately reminded me of the MR-18. I located the GR-78 schematic and a partial manual so I could compare the two thinking that the circuits might have some common elements.
Heath's first direction finder kits were the DF-1 and DF-2 which did not have a sense antenna. Heath followed up with the MR-21/ 21A which incorporated a sense antenna allowing selection of the proper null for direction finding. The MR-21 only covered three bands; VLF, broadcast and a single shortwave band to 3.45 MHz. The MR-21 was replaced by the MR-18.
Using the GR-78 schematic, printed circuit diagrams, and voltage charts, I started on the MR-18. The set had three loose wires. I quickly determined that those had originally led to a missing battery. Both the GR-78 and MR-18 use a 500 mAh 9.6 volt Nickel Cadmium rechargeable battery permanently wired into the sets. The one that had been cut out of the MR-18 had not leaked before removal. The three cut wires had been connected to the positive, negative and center tap on the rechargeable battery.
With 9.6 volts from the bench supply fed to the positive and negative wires, the set showed some life on the upper end of one of the shortwave bands but was nearly dead on the other bands especially on the lower end of each band. Putting my finger on the sense antenna connector caused some noise increase on each band. I was relieved that the N-channel dual-gate FETs were functioning.
Heathkit's alignment instructions
Heath has alignment instructions for the GR-78 implying that the coils for the RF, mixer and antenna sections on the bandswitch circuit boards had been preset at the factory. Only the trimmer caps for each band were to be adjusted in the absence of an accurate signal generator. I found that the tuning accuracy of the one short wave band that was partly working was off considerably. Had someone tried to align the set while it was not working well? I started the alignment with that band adjusting the trimmer caps first and then the coils on what had been the dead lower end of the band. That made the set come alive with excellent sensitivity across that entire band. It was now obvious that the alignment had been severely compromised. The other three bands also came alive after alignment. The set was indeed a hot performer on all its bands with proper alignment.
So what had happened to the set?
I'm convinced that an original owner had removed the dead rechargeable battery to keep it from corroding the chassis. A later owner had tried to power the set from its AC cord or its 13.5 volt DC connection as I had done at the start. Neither means of powering the set would have worked properly. The AC connection or the 13.5 volt connection are merely there to charge the battery. A light bulb and a resistor in series limit the charging current. Trying to power the set with just the charging circuit would have limited the actual operating voltage to a very low level, not enough to pull in stations. I assume that the set had then been misaligned in an attempt to fix it. Of course, it is also possible that the set was never aligned properly to begin with.
Powering the set properly
Both the GR-78 and the MR-18 can be powered by an external well-regulated 9.6 volt source. I decided to keep with Heath's design with an internal Nicad battery pack. The junk box yielded a shrink-wrapped pack of 8 rechargeable AA cells rated at 600 mAh. I opened the heat shrink pack to connect to the center tap so that the original Heath AC charging circuit would work. I had to reshape the battery pack to fit into the radio's original battery location. I used two zip ties to attach the battery pack and color coded wire nuts to connect the three battery pack wires to the cut wires inside the radio. The result worked quite well.
Using the set
While direction finding is simple with a modern GPS, it's interesting to use a more primitive technology. Beyond direction finding, the MR-18 is a fun set to use for shortwave listening and back-up use in the 40 and 80 meter ham bands. The lack of bandspread makes SSB somewhat difficult but careful tuning does make it possible. The set is a true joy to use on the broadcast band. The sensitivity is excellent and the direction finding antenna can be used to null out directional interference as well as to separate two stations on the same frequency.
I listened to the local airport beacon at 250 KHz to try out the sense antenna and pick the proper null. The radio's complete portability is an added plus since the set can be used away from the AC power lines and radio frequency noise sources. This radio will see a lot of use.
List of other Heathkits I have repaired
Here is an index to my other Heathkit projects.
Date 1-22-13, updated with catalog info 5-20-13
The previous project on the bench was a portable Hewlett-Packard HP-410B VTVM