Reflecting new addition to the project

A vintage ham radio station needs a matching vintage SWR-meter. That is exactly what I got today. It is beaten up and untested. Like its new housemates (DX-60 and HR-10) it could use new knobs. I plan to make new knobs for them all. White knobs may be the closest to the originals. Image shows prototype of black knob on the DX-60. I kind of like it. The orange one is half a joke. What do you think?

HM-11 SWR meter shown along DX-60.

The front is quite nice but the case is dented and and scratched. It would need a paint job to get up to the same standard as the DX-60 it will be connected to. One suggestion though is that I leave it as it is but clean and protect it with Turtle Wax.

You can see slight differences in the nuance of the colors of the paint. Heathkit used several shades of the green color. This has been a source of confusion and frustration for restorers.

Internal components may need replacement, but I hope the galvanometer works.

The meter comes from the estate of LA8KJ, kindly administered by LA3RK who I had the delightful experience of meeting for the first time.

HM-11

It is a very conventional SWR meter with forward and reflected modes and a sensitivity adjustment so that the reflected mode can be calibrated in SWR. The intended frequency range is from 160 meter to 6 meter ham bands. Power levels up to 1 kW. As is common for such meters, don’t expect to use it for QRP levels.

The HM-11 is actually a new appearance of the AM-2. The circuitry is the same but the casing is styled to match the DX-60 and other equipment at the time. I think the technical term for this type of upgrade is SSNW (Same Shit – New Wrapping). Newer versions are HM-15 and HM-102 which are basically newer wrappings still.

The builder can select to build it for 50 ohms or 75 ohms by the choice of a couple of resistors.

LA8KJ

The meter was owned by LA8KJ, Carl Daniel Haa√łen who went silent key 2nd of November 2015 at the age of 86. He was licensed from 1965 but was an active participant in his local group long before that, including a period as QSL manager.

As a young boy he worked at two different radio manufacturers but later found a career outside radio. His sense of humor shines through in this piece he wrote for “Hallo Hallo” (search for LA8KJ).

Thanks!

Thanks to LA3RK, Olaf for excellent service and also some parts he gave me for another project coming up!

Comparing old receivers

I am not finished with the HR-10 yet. A performance test is a little premature. Still, that is just what I did today. With the LA2OLD net running on 80 meter I listened using both the HR-10 and my old Grundig Satellit 2000 from about 1975. I should say, the Satellit 2000 was an expensive receiver at the time. It’s versatility and performance justified the price. HR-10 from the early 60’s was marketed as a “basic receiver” for amateur use only.

Grundig Satellit 2000 with optional SSB 2000 unit in foreground. Note that the uppermost of the three tuning knobs on the right have a fine tuning adapter on. I made this to ease tuning when I need to be extra precise. It slids easily on or off any of the three knobs.

Listening, first on AM then on SSB, the Satellit won hands down. I switched the 80 m halfwave dipole between them to compare. That dipole is of course much better than the telescopic antenna built into the Satellit. Still, the Satelitt with its own antenna did better than the HR-10 on the big dipole. Strong stations sounded much better on the Satellit. Weak stations that were unreadable or even below the noise floor on the HR-10 could be read on the Satellit.

Frequency stability of the Satellit is also much better. Something that is very noticeable on SSB.

Is the HR-10 a lost cause?

NO. I enjoy playing with it. The tuning dials are nice because they are made for amateur bands only.

I think there is room for improvement. Except for realignment that I have mentioned before, I think all electrolytic capacitors need a check and possible replacement. I am postponing that until I get working on the DX-60 so that I can borrow an ESR meter and order many capacitors at the same time.

Also, I think the preamp should be made switchable. One way or the other.

Instability may be curable with zener diodes in the power supply.

Never heard of Grundig Satellit 2000?

In brief:

150 kHz to 30MHz in 20 bands, except for 400-500 kHz. That’s AM with selectable bandwidth (2.5 or 5.5 kHz). 8 of the bands are bandspread BC bands, two of which also includes ham bands (40 and 15 meter). Also 87-108 MHz FM.

Optional SSB 2000 unit containing BFO and a product detector I actually understand (unlike this one). Also has a switchable audio filter that is useful for SSB and good for CW. A switch to disable AVC and an MVC potmeter which we would usually call RF gain.

Twisted progress

I have made a significant improvement. I will get to that but not without first boring you with some peculiarities from long time back.

You may remember the paper material I got. One reader called it a “time capsule”. Included was the book “Radiobyggeboken” (The radio construction book). Look at what I found in that book:

V2 on the left is last IF amp and detector. V3 to the right is the BFO. But what is the schematic symbol on the connection between the two transformers (top between MF2 and MF3)? Text explains it as a pair of wires twisted together to form a capacitor

May be capacitors were expensive in the 60’s. May be the builder was frugal. Or maybe this was common practice. I don’t know. The idea was new to me. Though I have to admit I once made a variable capacitor using cardboard and Aluminum foil.

It’s fun reading about almost forgotten tricks. This one hit home extra because I have been working on BFO injection to the IF in the HR-10 receiver. See my mostly failed experiments!

Product detector rant and mystery

Of course, the proper thing to do would be to build a product detector into the receiver. An example can be found on this blog post about improving the HR-10.

I can hardly think of the word “product detector” without thinking back to were I was introduced to it. That was in the Danish book “Vejen til sendetiladelsen” which I read in preparation for my license exam. I passed the exam thank you, despite reading a foreign book and never understanding the schematic of that product detector. I didn’t have Internet or search engines at the time, but I have now spent some time trying to find a similar schematic and an explanation. After some effort I found a close match:

From a nice RF Cafe blog where it is fig. 11. That blog seems to be a reprint from February 1972 Popular Electronics.

This is almost identical to what I found in the Danish book. The differences are irrelevant to my lack of understanding it. The similarity undermines one hypothesis; that there was an error in the schematic.

My problem is that I can’t understand how this works, or even how significant level of signal remains on the audio side. As I read the schematic, the point between the two diodes will quickly become positively charged, stopping almost all further signal propagation or mixing. A little will get through because of tiny reverse currents and capacitance in the diodes. But that should be minimal. Nothing in text or drawing suggests the breakdown voltage of the diodes are involved.

Did I finally find the explanation after all these years? Nope. The text in the blog/original article matched the Danish text so closely I am convinced the Danish writer had this article by his side when writing.

I now rely on my readers being smarter than me to explain the workings of that detector once and for all.


Should I modify the HR-10 with a product detector? I hesitate. This is not the best receiver I have. If I wanted to build a better receiver I would not use the HR-10 as a starting point. It has a high value as a really nice vintage receiver. I have fallen in love with this charming box with all its shortcomings. Putting much effort into making it closer to a modern radio seems hard to justify. But I could still pick some of the low hanging fruit. That was the idea behind my attempts at injecting more BFO power via a simple capacitor from the BFO to the screen grid of the IF amp. The lack of dramatic success may be because I tried to inject to the screen grid instead of the control grid. Or because I didn’t use enough capacitance.

What if I (reluctantly) connected the capacitor to the control grid instead? It would have to be a much smaller capacitor. Still, the IF resonant circuit would be detuned a bit and a little power lost to ground. Schematics in this blog post. I decided to try, accepting that I would have to realign the IF again. But I planned to do so anyway once I get to improve the signal generator (SG-402).

I felt like trying the idea from Radiobyggeboken. Meanwhile I had discovered this idea was not unique at all. SM7NDX mentions doing something like that on his HR-10. Unfortunately details are sketchy.

Come on let’s twist again

I just tried randomly. No calculations, no objective measurements.

Twisted enameled wire serves as a tiny capacitor connecting BFO to IF last amp.

I have no idea if this “capacitor” is close to the sweet spot or not. My ears can tell me this was a major improvement.

OK. A little dressing is needed etc. but I have (almost) picked one low hanging fruit. I have a twisted feeling of accomplishment.