IF not entirely successful…

After half a century one would expect the IF transformers to need a realignment. The method is simple.

  • Disable the local oscillator by simply grounding the grid.
  • Tune to the lowest frequency, i.e. approx. 3.5 MHz.
  • Alternately peak each core of each of the three transformers.

To do the peaking, a signal in the IF passband is required. The IF is about 1682 kHz. With the receiver tuned to 3.5 MHz enough signal leaks through to do the peaking. In the old days I used the local oscillator of another receiver as my signal source when I needed one. The best cost saving is no cost at all.

Some time ago I got an old Trio SG-402 signal generator. Luxury. The analog IM-5284 was connected to the speaker output so I could easily watch signal strength as I tweaked each transformer core. This was kind of a historic moment as this was the first time this millennium that the IM-5284 wss powered from batteries. I’ve mostly been using it for current measurements as I have several digital multimeters that works at least as well for all other measurements.

Signal generator on top of speaker. Meterman 37R in front used for frequency measurement. IM-5248 in background for signal strength measurement.

Alignment should be easy. It wasn’t. Some problems:

  • The signal generator was hard to turn precisely and didn’t stick well to frequency.
  • My body movements around the receiver influenced the measurements appreciably. I need to find a less care free way to introduce the signal into the receiver. See thin orange wire in photo! Some proper cables may be needed.
  • The large analog meter on the IM-5284 is wonderful but a little slow. Patience is required.
  • Minute adjustments of iron cores is difficult and more so when they are old. They tend to stick while you gradually increase torque, until they suddenly loosen and travel too far. Patience, patience, patience.

What I think I learned is that the adjustments needed were very small. With the inaccuracies in frequency etc. I am not even sure I improved anything.

I will try again, but I have to work on the first problem with the signal generator. I will try to devise a larger tuning wheel. For my Satellit 2000 I made an adapter to put on the tuning wheel when very fine adjustments are needed. I think I will try the same. Also, I consider opening the unit because the tuning did not feel smooth. I kind of expect to find a capacitor that hasn’t aged well. Familiar story?

If not entirely successful, try again! 🙂

Sidetracked. Story of insufficient capacity

The previous interesting cheat got me intrigued. Could I improve CW and SSB reception with a simple change? As simple as adding a capacitor in the right place?

I was setting up for aligning the IF and front end stages, but I got sidetracked by this idea.

I thought the best place to insert the BFO signal would be the screen grid of the last IF stage. The best way to get it would be from the anode of the BFO triode. The largest capacitor I had that could take the voltage with good margin was 10 nF. The change was barely noticeable and hard to evaluate as the BFO frequency also changed. I decided to connect 8 of them in parallel.

8 x 10 nF in parallel. To the right is an attempt at making a capacitor from enameled wire. Why? That is a story for another post.

Even that did not make a convincing improvement. It is possible that a much bigger capacitor is needed. Or a different way of connecting it. Or perhaps remove the 5 nF capacitor from anode to earth as that one swallows much of the signal.

Incomplete soldering

While investigating where to connect the capacitor I found an incomplete soldering joint.

For the picture I lifted one leg of the resistor away to show that it was not properly soldered.

I don’t know how long it has been like that. Perhaps since it was built. Tension kept it pressed against the right tube socket leg. Therefore functionality was not affected. I choose to solder it in place.

So that is part of what has kept me busy instead of aligning the receiver. Which I should get around to do.

Interesting cheat

I’ve suspected for a long time that the BFO signal is weak. Today I made an experiment that strengthened that hypothesis but also encouraged me to do something about it.

While listening to a strong station on SSB I have to turn the RF gain way down to prevent terrible distortion. That makes the sound in the speaker rather low, even with AF gain at max. I can get much louder sound on AM.

While listening to a strong local station I turned on an RF signal generator and tuned it for zero beat. I could now turn up the RF gain and listen to loud and clear sound. That’s encouraging and promises that if I could get a stronger BFO signal injected I would significantly improve SSB reception.

BFO schematic. Note that there is no output path! The on/off switch is connected to the 125 V DC supply.

When I first looked at the schematic a while ago I couldn’t find what path the signal takes from the BFO to the IF chain. I suspected a misprint of the schematic. However, the newer HR-10B schematics are exactly the same. You’d expect such a mistake to be corrected. In fact, Heathkit used to deliver errata sheets and updates to their manuals.

As it happens the BFO triode is one part of a tube (6EA8) that contains the last IF amp pentode. That is where the BFO signal is supposed to be mixed in. But there is no input for the BFO signal:

2nd (last) IF amp. Screen grid gets clean DC stepped down from the 125V supply line. T3 connects to diode detector.

Best guess is that there is internal leakage in the tube that gets the signals mixed.

So what to do?

One simple idea is to connect a capacitor from the anode of the triode (pin 1) to the screen grid of the pentode (pin 3). The two .005 capacitors may swallow a lot of that signal, but there should still be more left than just internal leakage. I think.

Another idea would be to twist together wires to the anode and the screen grid so that the wires makes a simple transformer. That transformer would also unavoidably become a capacitor. I would have to think through the phasing in that case.

This is where I would like to hear your opinion.

In depth

Manual, including schematic for HR-10(B) Searchable


As you may have noticed, my SB-600 could use some cleaning.

I did not build this one myself. It used to serve on a club station with a now long decommissioned SB-100. After years in dark and dusty storage it got a new life as a partner to my SB-104A. It is not as stylish as the SB-604 would have been, but appearance wise it was an improvement of the non-boxed speaker element I had been using so far.

To clean it I removed the internal HP-23E power supply that I do not need. It is meant to be used with many tube type transceivers. It’s condition is unknown but I didn’t want to harm it with liquids. Turned out it was secured with only three of the four screws and no washers. Not metric screws of course 🙄

I wanted to remove the speaker element and the wooden plate temporarily for the same reason. However the way it is supported is unclear to me and I couldn’t find a manual online. More precisely, I could find SB-600 manual scans online, but they were all missing the critical pages. Do you have a more complete manual? Another reason I wanted to work on it was that I would want to move the front grid back so that the screws do not protrude outside the front edge. Looking at pictures on the internet, this one seems to be modified.

I noticed it is also lacking rear rubber feet, so I will have to fix that. It could also use a paint job to cover the scratches. Unfortunately Heathkit was not very consistent in the colors used.

Anyway, with some effort (and care) it is much cleaner now.

A cleaner SB-600. In background part of HP-23 supply getting capacitors charged by IP-2728 power supply (center right). Charging current measured by IM-5284 (front right).

And it still makes sound.

Microphone candidates

“Anyone can talk, even a parrot”. That is what hardcore CW operators have said. I enjoy the challenge and exhilaration of working CW, but I know how to talk too.

To operate on AM the DX-60 needs a microphone. The previous user (LA4FK) apparently used a hand held mike, but that one is not available. The microphone input socket is of an unknown type without provision for Push-to-talk (PTT). At the time of writing this I have not yet put much thought into Transmit/Receive switching so I don’t even know if I am going to use conventional PTT.

Microphone socket

None of my candidate microphones has a plug that fits. Potential workarounds:

  • Make a short adapter cable. Seems like a PL-259 can be made to fit.
  • Replace the microphone socket on the transmitter
  • Add an extra microphone socket on the rear of the transmitter.

Primary candidate

This beauty was a gift from my friend Per Kristian. I can assure you he is much more of an expert on microphones than I am. I just like the way it looks. I think the nostalgic look is a perfect match for a nostalgic transmitter.

Old looking Shure 55SH microphone
Shure 55SH

There are a few challenges to get this one on the air.

  • Impedance is low. The tube type transmitter prefers a higher impedance. I may have to make a transformer. Potentially built into the transmitter, perhaps with its own input socket
  • I need a cable with an XLR plug for the microphone end. That’s not a plug type I have in my junk box
  • There is no provision for PTT, but there is an on/off switch that could be rewired. That one locks in either position.
  • It needs a stand, either for desktop placement or supported off the rig or wall (saving desk space). The threads for the stand are inconveniently non-metric and placed in front of the XLR socket.

None of those should be show-stoppers. Just hassles. Even if this beauty never gets on the air it serves well as decoration.

Runner up

I have a Yaesu YD-844 microphone, currently fitted with the two-pin+screen plug (Amphenol CDM 80-MC2M) that fits most newer Heathkits. It needs no work, other than matching plug and socket. Impedance is right (actually switchable), PTT is there, and the stand is part of the assembly.


If for any reasons the favorites fail I have a Turner 754 and a Drake 7075 which happens to look suspiciously similar. Those both have the 4-pin plug used by several rigs. According to Turner catalog from 1970 the Turner has a frequency response of 300-3000 Hz. Taylor made for common communication standards.

Before the parrot can get on the air however, we need to work on the DX-60.

Setting up for alignment

Bottom front: Heathkit SB-600 speaker, HR-10 receiver. On speaker: Trio SG-401 signal generator. Rear Heathkit IM-5284 multimeter. Notice alignment tool behind the dial needle on the receiver!

I apologize for not using a Heathkit signal generator for this work. That would be most fitting. Especially the IG-5280 RF Oscillator that not only was a low cost entry level model, just like the receiver. It also uses the same case as the IM-5284 multimeter and they are stackable. In my defense, I found an SB-600 speaker for the occasion. Not bad?

I built the IM-5284 years ago for the purpose of aligning my SB-104A that I was building at the time. The rest of the 52xx series would be nice to have, but out of economic reach at the time.

I now have several digital multimeters. They are more accurate, more convenient, smaller and more versatile than the IM-5284. Most are autoranging and none use batteries as quickly as the IM-5284. Most of them are cheaper too. So I use them a lot. The old IM-5284 gets used when I need to measure current, especially for a long period as it can do so without batteries.

For peaking circuits, an analog meter beats the digital ones. There will be a lot of peaking when I align the HR-10. So the IM-5284 may get batteries for the first time in more than a decade. Besides, it’s a Heathkit so it has a VIP card when it comes to working on the HR-10.

BFO alignment

My strategy for getting the BFO frequency right is simple. Tune a carrier for max using AVC and the S-meter. Then with the front panel BFO tune in the 12 o’clock position, adjust the coil slug for zero beat. Using a Heathkit alignment tool of course.

Easy said. There was no well defined peak. The IF filters are rather broad, which makes sense for a AM receiver. Turns out the RF gain setting changes the Local Oscillator frequency dramatically. Also, at high RF gain settings the BFO does not appear to work properly. But I did my best. It was way off when I started. Now it seems to be quite ok. The proof of the pudding comes when listening to CW and SSB on 80, 40 and 20 meters. USB and LSB settings appear symmetrical. I could listen to qsos on all of those bands.

There was no amateur activity to be heard on 15 or 10 meters. But I could hear AM broadcast that was definitely not transmitting in those bands. This was probably a case of image or other spurious signals. Keep in mind that the local oscillator uses frequency doubling for these two bands, increasing the number of spurs. Better front end filters may help. Aligning them is next on the list. Or may be the IF filters first?

What are you doing here?

HR-10 is an all tube receiver. Or all “hollow state” as some like to call it. So I didn’t expect to find a transistor in there. Or two. Or a printed circuit board for that matter. But this is what I found:

Unexpected find. A PC board populated with two transistors and other components.

Situated inside the front end unit, close to the band pass filters and band switch, this is most likely a preamplifier. Two transistors in the preamp may seem like overkill and risking intermodulation and overload problems. It would make sense to either make it selectable or use it only for the higher bands, especially 15 and 10 meters that suffer from bad sensitivity in this receiver. The lower bands, especially 80 meter is filled with a high noise level and higher risk of overloading. But it seems to be permanently connected. When I get some experience with this receiver I will contemplate making the preamp switch selectable.

Based on component count and documentation I got with the equipment, this is a preamp described by W6DYD (William J. Onesky. Now SK) in CQ magazine August 1968. I couldn’t find this online after more than 60 seconds of searching. If you have a link, please comment below!

Thanks to N3RYB for contributing research for this post.

Partial reassembly and test

So the capacitor is smoother, the dial string assembly is working again. Time to put the capacitor back in.

Removing the capacitor wasn’t that hard. It was fixed in place with three screws, only one of which was non-trivial to access, and gravity was on my side. Getting it back in was another story. When the receiver was originally built, the capacitor was mounted before the big clumsy front end with the multi wafer band switch, multiple fragile trim-able filters, tiny leads, walls and screws. I have to carefully steer the screwdriver in between all that to reach the screw. If I could get the screw there in the first place. Lots of frustration getting the screws to reach their holes before gravity snapped them. Then I remembered the simple but very efficient Heathkit nut starters that came with some of their kits. Of course I had saved them:

Top two white are alignment tools for hexagonal inset, typically coil slugs. The red ones are nut starters with different dimension in each end. One has been converted to an alignment tool for screwdriver type slots, typically found on ceramic trimmer capacitors. This conversion was part of the building process and as everything else described in the excellent Heathkit instructions. One of the nut starters have been cut in two as per Heathkit instructions. The yellow one is a newer alignment tool. The point of an alignment tool vs. just a plain screwdriver or hex driver is that it contains little or no metal, avoiding influencing the circuitry under alignment.

Nut starters and/or alignment tools came with kits that needed them. In some cases a nut starter was modified to an alignment tool. I built several kits so I have saved several nut starters and alignment tools. Some of these will come in handy later when the receiver shall undergo realignment. Now that I have published pictures of them I need to secure them well. These are highly sought after by people who didn’t take care of theirs.

Screws fitted perfectly to the old nut starter and I could easily get them in. Thank you Heathkit for your attention to detail and completeness!

Once the capacitor was in place the dial string had slipped. I got it straightened up – eh – wound up again. Some careful adjustments and the test could start. It was a pleasure running the dial all over the band.

It was dead silent

For style I connected an SB-600 speaker. Heard nothing. Not on any band. AF and RF gain at max. I tried with headphones in case speaker had a problem. Just a faint white noise. A weak pop when switching BFO on or off. Checked the antenna cable. Tried the 41 meter broadcasting band where strong AM signals are to be found. Nothing. Reseated all tubes (with power off of course). Looked for foreign object debris or loose connections. Switched on the AVC. Equally silent, but the meter pegged. That got me scratching my head. Could something overlay the AVC line? I started inspecting the schematic hoping to find a plausible way for that to happen. Or at least what kind of values to expect when doing measurements. Traced the AVC line to the accessory socket. Then it dawned on me.

The HR-10 needs the mute pin shorted to make sound. Either you let the transmitter take care of that, or you use a plug in the accessory socket with a short lead. I had removed that plug to make it easier to handle the receiver when working on it. Found and inserted the plug. Voila! Sound! Tuned around and found signals. CW, SSB, RTTY, FT8. The first time ever this receiver heard FT8 signals. I can say for sure FT8 was not around last time this receiver was in use. The AM sounds were ok. The others didn’t sound so well. I blame the BFO for that, but age, lack of alignment may also be factors.

I will work on the BFO next. At some point I will have to check all alignment. All electrolytic capacitors should be checked. I may have to borrow an ESR meter for that. I expect some or all to need replacement.

Strings attached

HR-10 flywheel during work.

When I first put my hand on the HR-10 receiver I noticed the tuning didn’t work. First thing to suspect is something mechanical, especially the dial string with all its pulleys and stuff. As soon as I opened the receiver I noticed the stuck variable capacitor and didn’t look any further. I’ve made some progress on the capacitor. Let’s get to that further down. Preparing for reinstalling it, I took a closer look at the dial string mechanism. Working on such mechanisms can be very frustrating at times. Fortunately this one is much simpler than those in my Grundig Satellit 2000.

The flywheel was mounted the wrong way and not tightened at all. It is not critical to the operation of the radio so might have been that way since it was built. However the string was not wound the correct way around the tuning knob axle, which itself was rich on oil. This leads me to suspect that the user was in the process of repairing it but never finished.

Frustrating story short: Now it’s in place (string attached and all) and seems to operate the way it is supposed to. So what happened to that pesky capacitor?

A more docile capacitor

Nostalgic 90 W soldering iron

I used the weekend to get myself a new soldering iron. Well, “new” is not describing it well. It’s older than the HR-10. At 90 Watts it emits 3 to 4 times as much heat as the one that couldn’t unsolder the contacts on the capacitor. Neither could this one. Not even the two combined with plenty patience added.

So I did as best as I could using plenty alcohol and time. Rinsed in water, dried with compressed air and added bicycle lubricant through a syringe. Also, importantly, I forced the ends of the frame apart. That seemed to reduce friction quite a bit. Not sure how it could have become compressed in the first place. The frame is strong and not exposed to any considerable forces inside the cabinet.

The net result anyway is a significant improvement. I’d like to see it even smoother, but this isn’t too bad. Importantly, there are no dents in the travel anymore.

Photo taken before most of the cleaning.

Remounting in the radio could happen any day now. I am looking forward to the day this receiver is operating well with no strings attached 🙂


After more than half a century things get dirty. With the lid still off the HR-10 receiver I made a cleaning effort of flat surfaces inside. I used alcohol and a cloth. I hope that will also reduce the smell. I carefully pulled each tube and gave each hole in the tube socket a drop of contact cleaner. Just to reduce the risk of hard to troubleshoot problems.

Also, in the “an ounce of prevention” strategy I planned to loosen all ground lug nuts and treat them to some of that contact cleaner. Until I realized there is no such thing as a metric nut in this unit. My toolbox is wanting on nut drivers.

I tightened the set screw on the band switch. Now it feels much better.